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Multiferroics - Science topic

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In doped multiferroic composites of BTO-CFO, i observed increase in dielectric constant by the application of magnetic field. However, in some cases, a decrease in dielectric constant was observed by applying magnetic field. Which factors may be responsible for such behavior?
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I am replying to this question as my working research field in same,
In the case of magnetodielectric(MD) it is true that some composite shows increased MD i.e. positive MD effect some shows negative magnetodielectric effect. The MD effect depends on a numbers of factors such as type interphase coupling between the phases in the composite materials, microstructures of the two phases, inter-diffusion effect at the interfaces, conductivity differences of the two phases, change of conductivity with magnetic field, presents of defects porosity, magnitudes of local fields etc.
Depending on the above mentioned parameters , the dielectric response or the permittivity of the composite material should have to be analyzed by applying a magnetic fields.
So, depending upon the nature of permittivity change occurring in the composite material negative or positive MD effect can be predicted.
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We have measured the magnetic field dependence of the ordered magnetic moment of Tb and Mn in multiferroic TbMn2O5 by single crystal neutron diffraction. The Mn moment does not change significantly with magnetic field but Tb moment does. How can I calculate the expected field variation of Tb moment from crystal field effects?
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For Polarization vs Electric filed loop measurement of multiferroic materials, is It necessary to make both top and bottom electrode of same materials..? and what will be the effect if both side electrodes are of same or different...
What is the reason of getting wide flattered loop like inclined horizontally...
What is the Difference in behavior of the characteristics curve of PE loop of pellets and thin films of same materials...
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Dear Subhasri Subudhi ideally electrode size should be same as our pallet size, because it may effect polarization at low frequency. At low frequency time is more so there will be contribution from space charge polarization because of electrode.
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What affects the polarization value w.r.t electric fields in pellets, thick films and thin films of multiferroic materials ?
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Dear Subhasri,
Electric polarisation of pellets, films or thin film basically same. All depends on the field strength. The difference in these three different dimensions are only due to the geometrical structure and the contact surface.
Pellet : Pellets are generally of circular structure, sometimes rectangular. The fringing out of electric field depends on the surface geometry and surface area ( continuity of E and D). The thickness is another cause of fringing. For films, the thickness is very less ( about 100 nano meter for thin films) and fringing out of Electric field is very less.
Unless, the bulk dielectric property is changed due to reduced dimension( nano property) , there will be no change of dielectric polarisation for these three cases. ( I am considering the samples are homogenous)
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N Das
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Generally for multiferroics sample pellets, we make metals surfaces on both side to make electrodes just like two parallel plates and measure the PE loop. But in case of PE loop of multiferroic thin films, if the area of electrodes of both sides of the film is different, then does the sample show the required characteristics loop?
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Dear Subhasri Subudhi,
The surface area of two electrodes must be of same area.
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N Das
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Recent publications mention that the magnetic and ferroelectric phases in a multi ferroic material couple through the piezoelectric phase of the ferroelectric. How does that coupling occur if the dielectric is paraelectric and not ferroelectric? Also how would you measure the magnetodielectric coupling constant of a paraelectric/ferrite multiferroic material? Thanks
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Dear Somnath Sengupta,
If I consider piezoelectric polarisation or any type of electric polarisation, that orients the electric dipoles in a particular direction.
Magnetism is due to the atomic magnetic moments and their orientation in a lattice.
So, due to the electric polarisation of the material, the electric dipoles are oriented .
This may change the magnetic dipole orientation in the lattice .
Another point is AC effect.
It is obvious that change in electric polarisation, changes electric flux inside as well as outside the material.
So, according to Faraday's Law of induction, there will be an induced magnetic field and hence may cause a magnetisation of the material.
But, all these are general statements.
Electromagnetic coupling can't be discussed without details of the material.
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N Das
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I want to know how we can relate magnetostriction with Applied magnetic field mathematically? Or if there is any way to derive it using mechanical equations?
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There are some classic thermodynamic models to discuss the influence of an external magnetic field on magnetostriction. And we can discuss the influence of external magnetic field on magnetostriction in ferromagnetic materials with different initial stress states.
【1】Shi Pengpeng. One-dimensional magneto-mechanical model for anhysteretic magnetization and magnetostriction in ferromagnetic materials. Journal of Magnetism and Magnetic Materials 2021, 168212. https://doi.org/10.1016/j.jmmm.2021.168212
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Kindly tell me any one? what type of the few things during choosing the grains?
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Muhammad Umair The standard error is proportional to the reciprocal of the square of the number of particles examined. In order to get the mean within 1% standard error then 10000 random particles need measuring. If the x90+ part of the distribution is required to a standard error of 1% then those 10000 particles need to be in the x90+ part of the distribution. You’ll need to distinguish between aggregates and agglomerates from single particles and also remember that you’re looking at a 2D representation of a 3D particle and shape is a 3D issue…
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Hello everyone,
I want to know during PLD, where from bound charges come at the surface of ferroelectric materials? I want to know their origin, their charge either positive or negative, and their screening process (i.e. how to screen) and their effect on polarization of material. Moreover I also want to know what is depolarizing filed and what is its origin and how it could effect ferroelectric materials? A good ferroelectric material should have high depolarization field or low and why?
The last thing I want to know that these bound charges/depolarization field could exist only in PLD film preparation or also can be in hydrothermal/solgel film synthesis ?
Kindly anyone who is expert explain it.
Thank you
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Hello,
First of all, the bound charges are inherently generated by the ferroelectrics, by its definition, so I guess what you ask is the origin of screening charges. Screening charges comes from band bending, surface reconstruction, and absorption, etc, their characteristic dynamics are diverse.
If the bound charges are not fully screened, then comes the depolarisation field. Usually this leads to domain reconfiguration. The ferroelectrics will reduce its depolarisation field to reach minimum energy. So it should be not large if the system is stable, but it is not a criteria of whether it is good or bad.
Screening process do exist in all the preparation techniques.
Please refer this review for details.
Kalinin, Sergei V, Yunseok Kim, Dillon D Fong, and Anna N Morozovska. “Surface-Screening Mechanisms in Ferroelectric Thin Films and Their Effect on Polarization Dynamics and Domain Structures.” Reports on Progress in Physics 81, no. 3 (March 1, 2018): 036502. https://doi.org/10.1088/1361-6633/aa915a.
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Dear Colleagues
I am working on Multiferroics. In the course of my research, I have to etch SrTiO3 entirely (not only TiO2 terminated surface) during the process to fabricate hall bars. Most of the literatures I have gone through so far have not regarded SrTiO3 as principal layer, but as substrate. Can anyone please help me, based on their experience, how can I effectively etch SrTiO3 after lithography?
Thanks a lot
Abu Naushad Parvez
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Dear Abu Naushad Parvez,
this article may be useful for you
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Research and development of lead-free piezoelectric materials are presently the hottest topics in the field of piezoelectricity. Also a huge advantage of PZT and related systems, is that this one family, with minor modifications, is suitable for nearly all applications.
Environmental and health issues related to Pb-based materials are obvious. According to RoHS, any homogeneous component containing more than 0.1 weight % of lead is subjected to restrictions ; and hence, the fact that the best piezoelectric materials contain lead up to 60 weight %, may in the future, seriously hamper their use in everyday applications.
my question is that "why people looking for replacement for PZT ( lead zirconat titanate ) which contain Pb up to 60% of weight because Pb is considered toxic or dangerous for life . While the alternative lead free piezoelectric KNN, BNT , BNN , BST , BT , BZT , all these alternatives also might contain toxic or hazard element for example Barium is toxic and dangerous for environment "
Why people want to replace lead? The alternative are toxic also? so why the replacement of hazard element with another hazard one ?
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Piezoelectric materials based on lead have held several applications such as automobiles, microphones, sonar, resonators, medical imaging/diagnostics, printers, ultrasonic motors, wearable devices, smart structures, medical implants, etc.) for over 50 years. The dominance of PZT-based ceramics is due to their superior piezoelectric response, which ultimately ensures an unmatched efficiency in the direct interconversion of electrical and mechanical energy. Beyond this superior piezoelectric response, lies a level of toxicity that threatens the position of PZT as the leading piezoelectric ceramic, and has sparked urgent global efforts to identify environmentally benign substitutes. A fundamental issue that emerges with the recognition of lead toxicity is the need to find surrogate materials in the myriad of products in which PZT plays a major functional role. Potassium sodium niobate (K x Na1−x NbO3 or KNN hereafter) is a potential Pb-free replacement for PZT[ 4 ] and for room temperature applications in particular looks promising. Material replacement in existing products has many obstacles, such as substitution costs, price ratio, and in some instance the end user's propensity to change. Consequently, for material substitution to be viable: (i) the benefit of implementing a novel and untested material must be worth the risk of abandoning the well-established current materials; (ii) the cost of substitution must not exceed the overall benefits; (iii) the costs of renovating production equipment and processes is acceptable; (iv) the implications of substitution are manageable in a wider systems context; and (v) institutional, legal, social, and environmental consequences can be overcome.
For more information, you can read this article.
Ibn-Mohammed, T., et al. "Are lead-free piezoelectrics more environmentally friendly?." MRS Communications 7.1 (2017): 1-7.
Regards
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One can study the multiferroic materials in theoretical perceptive too. For which, you have to understand the formation of the crystal structure. From which we can calculate the magnetic coupling in Fe3+ ion using crystal chemistry method. Then changes in the lone pair effect can also be determined. From these result, it is possible to justify the multiferroic ordering exists in the given materials. for detailed analysis please go through the article: L.M. Volkova · D.V. Marinin, J Supercond Nov Magn (2011) 24:2161–2177. DOI 10.1007/s10948-011-1178-5
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What is the possible reason for obtaining two dielectric loss peak in multiferroic composites?
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If it is a polycrystalline material then could originate from grain boundaries and grains itself. Since in that case your sample could be considered as a mixture of two phases with different electrical properties.
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Doing the CASAXPS fitting software for Fe doped BaTiO3-xFe XPS data, it gives the At% . What is meant the atomic ratios of Fe2+:Fe3+ in a particular concentration of xFe in BaTiO3.
For me it comes around Fe2+:Fe3+ as 49:51. whether it may mean there is no creation of oxygen vacancy or it just simply gives the quantification of particular xFe in BaTiO3?
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You give no margins of errors in 51:49 for Fe2+:Fe3+ (is it too hard to use the suffixes? Or use Fe(II) and Fe(III)..). Typically in XPS we use margins of 15% or more for peak heights, areas, and derived compositions. I see your 'result' as being no different to 50:50 or 49:51.
Be very wary of any fitting software. In differing circumstances almost anything can be fitted.
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The coupling coefficient identifies the existence of two or more ordering in single phase. But how can I calculate and justify it?
Can any body provide any eBook link or reference paper related to this topic?
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Thanks a lot
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Science does not stand still. New opportunities for research keep appearing and, as a result, new findings and discoveries happen hand to hand with artifact discoveries. These discovery some time with considerable controversy in the literature, sometimes at unusually impoliteand unprofessional levels. Some time artifact discoveries also surprised the world of science.
1) Different groups presents different results on same material and trying to prove each other results as wrong. Is it not sicietificy sound if these groups exchange specimens before they claim the work of others is simply wrong?
2) In some cases materials have been considered to be with ground breaking discovery when the data can be interpreted more simply via other well-known mechanisms. Is it not import to look wider before claims a breakthrough discovery?
3) In some cases the experimental results are true, despite theory implying that this is not possible. Is it appropriate to reject a experimental output just because theory doesn't exits which can explain it?
4) Controversy and attention on a new anomalous phenomenon such as Room Temperature Superconductivity.
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You are right Dear Prof. Dr Aga Shahee. Anyway, thank you so much.
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Ferromagnetic ordering breaks the time-reversal invariance irrespective of nature and type of ferromagnetic ordering. Does anti-ferromagnetic ordering also breaks the time-reversal invariance irrespective of nature and type or one can observed breaking of time-reversal symmetries in some AFM state (Like Neel State) and its preservation on other states?
In AFM state, is staggered magnetization only responsible for time revers symmetry breaking or any other intrinsic effect can also lead to time revers symmetry breaking?
In the ferromagnetic state, where the magnetic moments have spontaneously chosen to point in one particular direction, time reversal effect inverts the magnetization, so it would have a microscopically-observable effect. We thus say that ferromagnetism breaks time-reversal symmetry. What about AFM (M =0), is time-reversal symmetry broken in all case just because of change of sign of their staggered magnetization due to time reversal effect or time-revers symmetry breaking will depend upon type and nature of AFM state.
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Dear Prof. Dr Aga Shahee
To my knowlegde the expression for the free Gibbs energy in antiferromagnets is given approximately by the following expression Eantif , and it has to be time reversal invariant always---pp. 25 & 170 in [1] & also pp 167 in [2].
Eantif = a M1.M2 - 1/2 b [(M1.n)2 + (M2.n)2] - (M1 . M2).H (*) where a is the exchange constant & b is the anisotropy constants, n is the anisotropic axis, H the external field & M1 & M2 the magnetization vectors which are given by the sum of all magnetic dipoles inside the sublattices of the antiferromagnet (*) pp. 250 of [1]
I quote Profs. Kaganov & Tsukernik book pp. 25 & 170-171:
"...The energy cannot change sign under time reversal (in these cases energy is said to be invariant under time reversal) This is clear from the expression for the ellergy of a free particle E = mv2/2. Under the reversal: t ---> - t & the sign of the velocity v is reversed, while that of v2 is not..."
[1] M.I. Kaganov & V. M. Tsukernik, "The Nature of Magnetism" Science for everyone, Mir-Moscow, 1995.
You can also check:
[2] Eletrodynamics of continuous media by Acad. L. Landau & E. Lifshitz, ch V-#48 pp 167, eq 48.2, Pergamon 1984. They use the phi thermodynamic potential free energy.
CC. Prof.
Behnam Farid
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I have deposited the polycrystalline thin film of BiFeO3 multiferroic thin film using Pulsed laser deposition. XRD curve depicts the (100), (200) and (110) orientations.
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you can make use from the attachment files
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A transition from ferromagnetic structure to flat spiral structure was observed in a groups of transition metal alloys. The space group is Pnma and the magnetic Mn atoms occupy 4c position. How to understand the occurrence of spiral structure in such compounds?
It is know that Dzyaloshinskii-Moriya interaction due to spin-orbit coupling is responsible for the spiral structure in some multiferroics oxides. However,  this interaction just occur in the sample without centrosymmetry. In my sample, the Pnma structure is centrosymmetric, and this interaction cannot occurs. Is there some other interactions or models to explain the spiral magnetic structure in centrosymmetric transition metal alloys?
For transition metal alloys, the interaction should be itinerant or direct interaction?
Sometimes, the electrons localize and delocalized partially. In this case, should the interaction be RKKY?
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We are sharing some thoughts on this question in a preprint coming in a few days. In short, magnetic frustration is enough for sprial (or even skyrmion) formation in centrosymmetric systems; and in particular, RKKY is a typical mechanism for realizing such sprial orderings.
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We have acquired FMR curves dI/dH=f(H) for Fe3O4 superparamagnetic nanoparticles. I need to calculate their effective magnetic anisotropy Keff. How is this possible? Thanks in advance 
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@ Rasbindu V. Mehta can you please share the paper/method for calculating the effective anisotropy constant using field cooled and zero field measurements.
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Respected researchers,
please share the role and applications of magneto-dielectric effect in a multiferroic material. is there any possibility to verify the energy storage capacity using magneto-dielectric effect instead of P -E loop.
Thanks in advance
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Hello Robin,
your correct. In fact there is much current research in this direction, for example:
though of course the list is much longer.
Best regards
Gareth Monkman
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In case of Multiferroic Perovskite materials, the Current-voltage curves sometimes don't pass through origin and show diode type characteristics due to polarization. But in illumination condition, i have observed that with the interaction of photons some materials show lower current value than dark condition. What would be the physical significance of giving low current value?
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Dear Subharsi,
welcome!
I believe you. One normally observes that when displaying the illuminated I-V curve inn the first quadrant that the reverse scan of the I-V curve will be under the forward scan. This means that polarization voltage lower the terminal voltage for the same current in the reverse direction of the scan.
In order to share in the discussion of the results one has to examine your I-V curves.
Have a nice day!
Best wishes
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Both multiferroics and super-capacitors are separately the area of interest in current era. Is their any way for multiferroics to be used in super-capacitors for any purpose ?
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I think they don't have specific limitation for being used as super-capacitor cathode material. They are mostly oxide ceramics and oxide ceramics are interesting cathode materials for pseudo-capacitance applications.
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We know multiferroic property is the combination of the at least two ferroic properties among- (anti)ferromagnetic, ferroelectric and ferroelastic. In multiferroics, electric field controls magnetization as a result magnetoelectric coupling is found in that materials. How do this coupling originate? Is there any references?
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Dear Researcher You may go through these papers to have clear understanding of M-E coupling and control in both qualitative as well as quantitatively:
2. Nature Materials volume6, pages13–20 (2007)
3. 10.1126/science.1113357
4. Nature Materials volume7, pages425–426 (2008)
First try to understand that how do ferroic orders are arsing or combing in a single phase materials.They may be due to either charge,spin,orbital or lattice .Coupling among themselves are governed by very basic physics laws or symmetry breaking.
coupling and control may be of two types as commonly mentioned in literature Type-I or Type -II.
Kindly ref DM interaction basics .
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Measurement of magnetoelectric current (density) seems to be very popular to investigate the magneto-electric response of multiferroics. It'll be grateful to know how it is measured and about the protocols.
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Apart from ferroelctric photovoltaic effect, the multiferroic materials is enhance the power conversion efficiency, like a magnetic or ferroelastic materials. why? and how?
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The group has developed a new class of materials comprising elements such as bismuth, iron, chromium and oxygen. These "multiferroic" materials (materials that exhibit at least two ferroic orders) absorb solar radiation and possess unique electrical and magnetic properties. "Thanks to their semiconducting band gaps and their polarization induced electrical fields, these materials demonstrate an alternative pathway to achieve charge generation and separation, enabling the development of a new generation of photovoltaic devices," says Professor Federico Rosei, Director of INRS-EMT; UNESCO Chair in Materials and Technologies for Energy Conversion, Saving and Storage (MATECSS); and Canada Research Chair in Nanostructured Organic and Inorganic Materials.
Link
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i am working on BiFeO3 multiferroics where Sm and Gd are doped at Bi -site and Sm=0.1 fixed while Gd varied at step of 0.025. Magnetic properties are continuously increases while Dielectric and microstrain shows zig-zag pattern. plz provide suitable answer.
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thanks a lot K. karthik and F. Grabner
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I have temperature dependent dielectric data. How to calculate relaxation time using Vogel-Fulcher relationaship?
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Thanks Dinesh ...
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How to calculate real and imaginary parts of dielectric constant from impedance measurement?
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The dielectric constant signifies the efficiency of dielectric material to store the electrical energy and the dielectric loss indicates to loss of electrical energy in the form of heat energy. In ac fields permittivity becomes complex quantity and has real and imaginary parts. The imaginary part (εi) is associated with dielectric losses and the real part (Ɛr) an indication of the degree of polarization. The greater the degree of polarization, the greater the value of Ɛr. The dielectric constant is a function of frequency. As frequency increases, the value decreases due to polarization mechanisms no longer being able to follow the rapidly changing field. Imaginary part is always positive and represents loss factor or energy absorbed. The measurement of the real part of relative permittivity, εr is generally done by measuring the charge in capacitance of a capacitor by the introduction of the dielectric between its electrodes. The imaginary part or the complex dielectric constant εi is found from the measurement of tan δ, which is the loss factor of the dielectric. [tan δ = εi/Ɛr ].

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Hello,
What is your suggestion for detecting the frequency of soft mode in thin film? (less than 20 nm)
I need to see the change of frequency as the temperature decreases.
Thanks
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Dear Kashir,
A soft mode is most often associated with a structural phase change of a crystal; on this subject, you can refer to the Landau theory of phase transition ...
If the parameter that changes is the temperature, the frequency of the soft mode tends to zero when the temperature approaches the critical temperature (temperature from which the change of structure takes place); the soft mode can be observed in Raman scattering or in neutron scattering ...
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I am getting negative dielectric loss at high frequency (2MHtz) which become positive beyond 150 degree Celsius.
Is it due to interface problem between electrode and ceramic pellet or something else?
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After a certain temperature the conductivity of the ceramic pallet becomes very high. In this case the test circuit behaves like inductive and system displays negative capacitance and loss factor.
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Which aqueous medium is the best for stable and homogeneous DISPERSION of ceramic (PbTiO3) particles for TEM sample preparation ? if particle size is in 500 nm range what precautions must be follow?
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Hi Chandra Your question hints at some potential problems if you use TEM. TEM requires a thin section, typically < 100 nm. What information are you trying to get from visualization of your system? You state your particles are in the 500 nm range. Thus you'd have to prepare your samples usually by microtoming a thin section of your material in, say, an epoxy resin (and thus you'd need an organic solvent to disperse the particles). Imagine that all your particles are monodisperse (absolutely identica), identical in shape and you slice sections from them. If they're spherical to start you'll have a collection of discs ranging from virtually 0 nm up to the true diameter of the sphere (500 nm in your case). If they're cubes then you'll end up with anything from triangles to octagons. Thus you'll see an apparent distribution from a monodisperse starting material! See attached for an example.
At the sizes (500 nm) you quote, I'd be looking at laser diffraction for a quantitative particle size distribution and SEM for some guidance as to shape, degree of agglomeration etc.
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I have prepared Y2NiMNO6 nanoparticle and did magnetic measurements. From M-T , it follows as literature,i.e , ferromagnetic below 70 K , and paramagnetic above 70K but but surprisingly from M-H plot at 300 K , the plot look like paramagnetic but it has certain coercivity. Can anyone explain what could be the reasons behind it?
I am attaching the plots.
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The hysteresis of the M Vs T curves clearly shows that you have the formation of magnetic clusters with a blocking temperature of 75K. However it seems for M Vs H curves that a very weak ferromagnetic component remains up to 300K. The origin of this weak ferromagnetic component may be coming from a secondary phase or to the fact that some clusters are not blocking due to a size distribution i.e. not all the clusters have the same size.
Best regards,
Pedro
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What has been done so far...
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Materials with broken symmetries are always interesting and emerging new science and technology.
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Is it possible to observe the ferroelectric domain through FESEM or HRTEM? how the magnetic domain will be observed. what about MFM magnetic force microscopy? 
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I use the Trinocular microscope to observe the domains it is best and useful microscopy technique.
A trinocular microscope has two eyepieces like a binocular microscope and an additional third eyetube for connecting a microscope camera. They are therefore a binocular with a moving prism assembly in which light is either directed to the binocular assembly of the microscope or to the camera.
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The figure shows σac (T) curves for the samples Bi1-xSmxFeO3 (x = 0; 0.05; 0.1; 0.15; 0.2) at a frequency of 1 MHz. How can we explain the displacement of the conductivity curves for substituted samples in the region of lower temperatures in comparison with the initial BiFeO3. What can be associated with a strong increase in conductivity near 300oC?
Thanks in advance for your comments!
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Dear Alikhanov,
Infact, Prof. Sriniwas is saying same thing, what I was suggesting; there is anomalies what is appearing a non-dispersion region (about to zeroline). So better to re-plot the low temperature regime(even reciprocal plot can give you some better inside). It will more clear if you take a few more frequency data.
with regards
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In recent study of different spintronics system, people are studied that the ferroelectricity in type-II mutiferroics is produced by spin current. I want to know to clear the origin of spin current and how it produced the ferroelectricity in the type-II mutiferroic systems at the magnetic ordering point.
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You should read the following article to get some initial knowledge about spin current.
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I performed experiment of a multiferroic system (BiFeO3). I got different curves but how should I know which one is correct
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Dear Priyanka Verma,
First of all, you must understand ST circuit (and its electronics) used for PE loop tracer. It is not only the RC combination rather the applied electric field is also important. If your sample is ceramic then density of your sample also play critical role here.
Coming particular to your material, it gives lossy loops, you have to be very careful and optimize each parameter to make sure you did not get artifact. you can compare your result with previously reported loops and then make your choice.
Remember you should remove/minimize the contribution of electronic circuit in your result because it (ST circuit) is potential divider like arrangement.
regards
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The room temperature M-H curve shows ferromagnetic behaviour. I need to fit that M-H curve with the equation 2 given in picture to fin out the ferromagnetic component of the sample.Can anyone please provide the step by step procedure for fitting in origin or excel?
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Origin allows to create a user defined function for fitting. To do that, go to Analysis - Fitting - Nonlinear Curve Fit. In the Setting tab of the fitting window select 'New' from the Category list. The Function Builder will open where you can construct your function step by step. After saving the new function, it will be available in the User Defined category. Try to set reasonable values for the initial parameters, otherwise the fitting might easily diverge in case of the complex non-linear functions.
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Piezoelectric force measurement -PFM
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But I saw some some reports that ZnO oxide that conclude ZnO is a ferroelectric material.
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I need to correlate the ionic state of Fe with the magnetic properties of Fe doped ferroelectric material. My way of synthesis is solid state reaction method.can I correlate the reason for magnetic properties of that ferroelectric material by taking core level spectra of Fe 2p and parent material site Ti? through XPS we do only surface characterization so is it useful or not? Can we confirm the incorporation of Fe ion in the parent Ti site through shifting of Ti corresponding BE peaks?
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XPS is used to look at the surface composition and oxidation state of species present in the top 5 - 10 atomic layers. For a multi-compoennt system the surface composition can never, ever be the same as the bulk composition. You'll find that the major part of this surface is oxide and carbon. You would need to argon ion etch (or split a crystal as suggested above) to expose the bulk. Are you considerig XPS simply because you have such a unt in your facility? Your magnetic properties and their characterization is the real problem to focus on.
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Hello,
How can we detect ferroelectricity in a material? Or better to say : How to prove whether a material is ferroelectric or not?
Observing a peak in "Dielectric Constant vs Temperature" graph is enough or this is just a necessity?
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According to theory of polarization, the ferroelectricity is a form of spontaneous polarization.
The actual experiment to prove the ferroelectricity is Polarization vs electric field (PE) graph.
For ferroelectric material will show a hysteresis loop in PE plot.
"Dielectric Constant vs Temperature" graph is not enough for this.
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Hello,
I need to study a good review paper on "multiferroics" (published recently) specially with a short explanation of the reason of ferroelectricity in each case. Better to say, I need to gain a general picture of achievements on this field up to now.
Thanks in advance.
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Please follow the link.
Hopefully it will work for you.
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I want to detect the band gap narrowing in Fe doped BTO(BaTiO3) through valence band XPS (VBXPS) characterization. The operator is asking which element do you want to detect in BaTiO3 for VB XPS? I didn't understand that because for the whole structure of Fe doped BaTiO3 we have one band gap along with impurity band gap. Can anyone please suggest the initial steps we need to consider before VB XPS spectra has to do?
How to find out the fermi energy for any ferroelectric material to give the binding energy range for the analysis?
Please suggest books for XPS and valence band XPS.
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Honestly, I also don't understand why they are asking that. You are correct -- there is one valence band (with or without band tail).
Do you have any information on the system being used for VB XPS? Is it a synchrotron source, and they are selecting an x-ray excitation energy according to an elemental edge, e.g. oxygen edge? Is it an x-ray source with more than one anode, such as a dual anode source with Mg and Al anodes, and they are asking which anode you want to use?
For good VB measurements with XPS, I would be concerned with how narrow the line width can be with the x-ray source being used, along with the limits of resolution of the analyzer. But those are going to depend on the specific instrumentation. See the table at the bottom of the web page linked below to get an idea about fixed-energy, elemental anode sources and the line widths they produce. If the analysis is being done at a synchrotron, then there are likely more options to choose from.
I suggest asking the operator what will produce the narrowest line width and what the limits of resolution of the analyzer are. Those two things will greatly impact the quality of your VB XPS data.
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I got XRD data of La and Mn doped BiFeO3 multiferroic powder. How i can refine its crystal structure. 
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Dear  Kumar,
The refinement of crystal structure using powder XRD pattern can be easy refined using Rietveld Refinement method. The Rietveld refinement can be performed with the help of softwares like FullProf Suite, Jana2006 etc. which are freely available. There are few steps in the process of refinement. For more detail you can contact me by mail on dineshiitbhu@gmail.com.
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Hello everyone. I wonder if anyone can help me with this issue, or even a better understanding of it.
I make deposits of BiFeO3 thin films (100 nm) using a polycristalline BiFeO3 target. The technique used is RF magnetron sputtering.
As you may know, BFO needs annealing to reach the multiferroic desired phase. By using XRD diffraction, the initial post-deposit state seems to affect the post annealing result. The first two deposits I make, each of 40 mins long sputtering one after another, are identical, and reproducible each day. However the third sample I try to make, also for 40 mins long after the first two, is not even close to the first two. This is also reproducible each day. I tried resting the target from plasma heating for an hour between each sample deposit, but the results were not any better.
Thank you in advance for any answer.
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Argon with some addition of oxygen usually works fine. The exact amount of oxygen depends on deposition system, you could start from 10% and decrease it until the film becomes clearly deficient with oxygen (colour changes, the lattice constant measured with XRD increases).
Concerning basics... Well, usually when oxides are deposited, some oxygen in the chabmer is necessary to avoid significant oxygen deficiency (for some oxides it may lead to deposition in wrong crystallographic structures, formation of different oxides, and even decomposition of the material). Oxygen by itself is a bad choice for sputtering due to negative ions in the plasma that bombard the growing film, so a proper choice is some neutral gas (to avoid formation of other chemical phases like nitrides when nitrogen is used for sputtering) with addition of oxygen. The heavier the neutral gas you use, the better transfer of composition from target to substrate, so Xe or Kr are the best gases. But, they are rather expensive - for mass-production, at least :) So typical solution is argon, heavy enough to efficiently sputter all materials. Part of oxygen: to my own experience, 5% of oxygen don't affect neither the (argon) plasma, nor the surface and structure of the growing film. 1% of oxygen for some oxides is insufficient for complete oxygenation of the film, but usually even 0.1% is quite enough.
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In some ferrite -ferroelectric composites sometimes jumps are visible in some ferrite -ferroelectric composite sample, but not for all kind of compounds. What may be the possible reason?
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Thanks @Sajid
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This question is motivated by theroetical investigation we have done, trying to highlight ME coupling in some known Multiferroics. Indeed, despite carefull considerations of symmetry and considering the usual paths that such phenomenon should follow namely, magnetic field induced atomic displacement (triggered by Spin-Orbit Coupling in the structure), we could not show any measurable ferroelectric component resulting from applied magnetic fields (in our ab initio calculations). Experimentally, this is clearly observed for materials such as Magnesium Oxide (MgO) by the difference between ferroelectric Hysteresis different from the applied magnetic field and no Mag. field cases.
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About intrinsic ME effect, may I suggest Phys. Rev. B 95, 214406 (2017).
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I am going to study the dielectrics properties of materials (ceramics ). since I am new to this area so can someone suggest me some best books on dielectrics..!! like B.D cullity , Stephen Blundell for magnetism..!! 
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M.E. Lines and A.M. Glass "Principles and Applications of Ferroelectrics and Related Materials"
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I want to understand the basics of multiferroic materials. I am in my 2nd year of under-graduation. I am doing a project entitled 'Characterisation and fabrication of thin films of gallium ferrite'. 
I want to understand from basics, what is the magnetoelectric coupling, how is memory stored using this coupling.
Basically I want to know the physics behind this multiferroicity.
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@Bartlomiej Thank  you very much sir.
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All known EM antennas contain electrical current inside at radiation. What if someone mixes together dynamic electric field generated near ferroelectric piece and dynamic magnetic field near a ferrite? May EM wave be created at some angle and phase shift between the fields? Analogous method may be applied using a piece of 1-type multiferroic (inside it).
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Dear Alexander,
to be exact, there is not a causal relation between the changing of one field and the curl of the other. Both phenomena appear just simultaneously, as W. G. V. Rosser pointed out:
But I do not understand how you intend to produce a changing electric field not accompanied by a magnetic field: If the charge is resting, we have a static field. In order to get a changing field, the charge has to move. The Biot-Savart law says that moving charge (= current) is accompanied by a magnetic field. So, a changing electric field is generally accompanied by a magnetic field.
In your experiments with changing electric fields, the amount of charge on the plate or on the ball is changing. Consequently, there must be an electric current and a nonzero magnetic field. Your result is that your Hall sensor did not register any magnetic field. But this proves only that the sensor is too insensitive or too slow or both to register the magnetic field which arises according to Biot-Savart.
There is one case of moving charge without magnetic field, and you kind of hint at it in connection with the ball: If the charge is evenly distributed on a spherical shell, and if the radius of the shell changes which is equivalent to evenly distributed radial currents, there appear longitudinal waves. However, these waves are not waves in the electric or magnetic field but in the vector and scalar potential fields, A and Phi! Since the vector field A remains without curl, and B = curl A, there is no B field. Furthermore, E = -Laplaceop.(Phi) - d A/d t, but the effects of the waves on these terms cancel each other exactly. So, there is just a static E field.
By the way, I wonder whether anyone has ever tried to verify the existence of these longitudinal potential waves in an experiment similar to the Aharonov–Bohm setup? The outer static E field can be easily cancelled by a charge of opposite sign in the center of the shell.
Anyway, all experiments I know of (including that of Monstein, with nice details), intended to produce longitudinal waves, suffer necessarily from the fact that tangential currents are flowing on the surface of the spherical shell (or something similar) thus generating ordinary radiation fields.
The only way to produce a changing electric field not accompanied by a magnetic field would be to create charge of only one sign, and that seems to be impossible.
As to your experiment with a rotating permanent magnet: In every coil resting in the neighborhood of the magnet, and being so oriented that the magnetic flux through the coil changes, a voltage will be induced according to V = E * length of wire. This principle is employed in millions of bicycle dynamos. In my opinion, your experiment just shows that a flat rectangular piece of metall used as a kind of a monopole antenna isn't a suitable sensor for curl E.
I hope I did not misunderstand important points in your article; for the moment, my view as stated in my first answer isn't modified.
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I am looking at confirming the presence of magnetoelectric coupling in doped-BTO thin films. I have confirmed the presence of ferroelectricity and ferromagnetism independently using the triangular waveform voltage and SQUID magnetometry.
To confirm the magnetoelectric effect, all I need to do is perform standard C-V measurements, with and without a magnetic field, and note any change in capacitance, according to the linked journal.
I have the capacity to perform C-V measurements using a Keithy semiconducter analyser, however I don't have the ability to performing the probing with and without the magnetic field.
Can anyone recommend how to best perform these measurements? Is there a probing station capable of performing these measurements, or is there an affordable way to make my own DIY probing apparatus?
Best wishes
David Coathup
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I have designed a magnetoelectric setup capable of measuring ferroelectric properties (C-V, P-E, I-V…) at different temperatures (77-500 K) under applied magnetic field (up to 1 T). The magnetic field is produced by a custom-developed magnet. Our electromagnet contains two copper coils, and soft iron in their core to focus the magnetic flux on the sample position. One of the coils is mounted on a rail so as it can be shifted to modify the spacing between the two magnetic poles. The magnet is controlled with a bipolar power supply (properly chosen to fit the characteristics of the coils). All sample fixtures are made of nonmagnetic materials. A Gaussmeter fixed close to the sample position displays the magnetic field value. Note that this instrument is meant for static measurements, i.e., the magnetic field is swept at low-frequency.
Glad to share our experience with you and best of luck for your experiment.
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Pervoskite based photovoltaic devices are similar to light dependent resistors. It does not generate current as in the case of conventional semiconductor based solar cells. Any Comments on this?
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Dear Sabramanian greetings i must recognize that such Q is so intersting. LDRs & SCs both convert radiation into electricity, but in LDRs resistivity could vary when material is illuminated (then mainly used in sensors); SCs in contrast are based on varying charge carrier mobility of semicond. (then mean life time,..) thus carriers should separated , not recombinated a second time. The 2principles are quite different. appls of SCs as known are clean renewable energy sources. Brthr Sabramanian welcome as a 4th collaborator!
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Analysis type (i) at Room Temperature, type (ii) at Low Temperature and type (iii) at High Temperature. Which one to prefer? (or) Does it depend on the application of sample?
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I strongly second Oleg's notion that a ferromagnet is not defined by the presence of hysteresis in an M(H) loop (although in many cases there will be one).
To try coming back to the questian as it was raised: do not choose some temperature, be it high or low. It is the temperature dependent behavior which will allow you to make conclusions more than anything else. I would recommend you to find elementary literature on the processes of magnetometry involving "field cooling" and "zero field cooling" as a starter.
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Can we simulate M-H or P-E loops if we give the composition of the multiferroic as input or can we model them?
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Thank you Dr. Sertkol..I would loike to try out MATLAB..however...can you suggest the constituent eqns to model the parameters and composition?
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I have multiferroic TbFeO3 poly crystalline sample. I have put silver paste on both sides of the rectangular sample and baked at 1000 C for 1 hour. I have made I+, V+ contacts on one side of the sample and I-, V- on one side of the sample. The sample dimensions are: length = 6.4 mm, breadth = 4.1 mm and thickness = 1.47 mm.
I am measuring capacitance using LCR meter which is remotely controlled by LabVIEW program. I have put the sample in PPMS chamber and controlling both PPMS and LCR meter using LabVIEW program. The program is not showing any error and measurement is going smooth.
I am wondering why am I getting capacitance value of the sample as an oscillating wave?
Here I have used AC voltage level = 2.5 V (ALC ON)
Bias voltage = 1 V, AC frequency = 5 kHz.
Please comment.
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Is it possible that your sample cracks at elevated temperatures?
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I am doing PFM domain imaging of BiFeO3 sample.. I want to know how to do precicely calculatind the percentage of 71, 109, 180 degree domains in the system. If any one can give suggestions... wil be helpful... thank you
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The output signals in PFM (Phase and amplitude) are quite arbitrary. However if there are 71,109 and 180 degree domains present within the scan there should be than three different sets of contrast (in phase), each one having its positive and negative values of phase (up and down orientation; given if the phase is properly centred before scannong). If you can get access to your data in some platform like matlab than the following steps can work:
1. Suppress the negative values (preferably set to zero) 
2. Identify and replace the pixel values within each contrast group by a unique no. (e.g. 1,2,3 for the three different contrast levels) by using appropriate signal intervals.
3. Calculate the percentage of each contrast group by dividing the no. of pixel with the particular contrast marking (e.g. no. of '1' and likewise) by total no. of positive pixels replaced. This should give you the percentage of that particular contrast group, and likewise for the other groups as well.
If you are not so familiar with matlab, than I would suggest you to use WSXM (open source software). In this program you can open PFM images of any format and by using the flodding option calculate areas of scan with specific contrast levels (could be choosen by 'Find Itervals' option), Later you can use different areas to calculate the respective percentages. 
I hope this helps.
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I have to perform a high pressure X-ray absorption measurements on bismuth based sample at APS synchrotron beam line. I don't know how to load the sample inside the diamond anvil cell for XAS measurements.
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Dear Dr. Yesudhas,
I am afraid I do not quite understand the sense of your doubt concerning the sample size and thickness. You will need a microscope in order to measure the sample size and thickness. The cross size is not critical (provided it is larger than the total X-Ray beam focus size). The thickness is far more critical. If the sample is in powder form, you will have to prepare a pellet  and verify that it is homogeneous in composition and thickness. I would not advice you to try any XAS experiment with a non compacted powder because the results will be unreliable.
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for example a multiferroic composite has space group Amm2+Fd-3m
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There is an option to add more than one phase to the refinement. So you just add an additional phase and refine it the usual way but with a seperate set of parameters for each phase. No mixing or anything funny is needed.
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I am looking at performing ferromagnetic measurements of thin films using a Vibrating Sample Magnetometer. As my department does not possess one, I am having to travel quite a distance to use one, so I wish to familiarise myself with the processes as much as possible before hand.
I am familiar with the basic concept of how VSM measures the magnetic moment of the sample using the induced current in the pickup coils, however, I am unfamiliar with how it specifically measures Ferromagnetism. Also I have not had much luck finding comprehensive literature covering the measurements specifically of thin films samples. I have read that they are limited to a thickness of 50nm, but have been unable to find a reason why.
If anyone could provide any answers to the above queries, or links to any detailed literature who those new to the field of VSM, I would be extremely grateful.
Best Wishes
David
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Dear David,
while you might not think so, your question is still very broad and it is not so easy to come up with an answer which should suit your needs (maybe but for the quest for literature) and not be utterly lengthy.
Maybe you will find more literature when you include SQUID magnetometry in your search. While the sensitivity is not the same, many considerations should take the same direction.
Most important is to realize what exactly it is that you wish to find out. There are so many kinds of measurement protocols you could want to apply...
With thin films, there are some typical issues, though. Usually the mass or volume of the magnetic (-ally interesting) material is many orders of magnitude less than the substrate carrying the film. Even a small amount of dia- or paramagnetic susceptibility from the carrier may easily amount to the same magnitude (or even much more) of a magnetic moment than the moment you're interested in. In such cases, I would say it is mandatory to make accurate reference measurements on empty substrates of same dimensions as the specimen(s) you're interested in. Might effectively double the time required to accomplish the desired task. The magnetometer cannot tell apart where a magnetic moment is coming from. Only you can.
Very important: take massive care that there is strictly NO contamination from any undesired material. Using the wrong (iron based) tweezers just once may spoil your sample for good. 
Also frequent with thin films of magnetic materials: surfaces are easily corroded (i.g. oxidized, or have a different oxide stoichiometry at or near the surface). Thinking about some suitable cap layer might be worthwhile but could also change the characteristics of the film. The better your films are characterized by additional means the higher the chances that you will learn something useful from magnetic measurements.
Finally, do search on RG for previous questions involving VSM & SQUID magnetometry, there are quite a few. These should be instructive in getting ideas about what could possibly go wrong (because this is usually when people seek help here - unlike you). Some typical issues are sample size (stick to the specified volume) and (in-) correct sample mounting.
What is specific about measuring ferromagnets? Quite generally, you have access to mS(H,T), where mS is the total specimen magnetic moment. Unless you measure major magnetization loops, there will be a memory with respect to the "magnetic history" of the sample. Depending on what you'd like to do, it might make sense to think about how to obtain a well defined magnetic state (->, saturation, thermal demagnetization, AC-demagnetization...). You might want to look for the ordering temperature, shape anisotropy, hysteresis, measurement of (partial) magnetic reversal etc..... which all reveal something about the ferromagnet. As I said before, among the most important things is to actually know what type of information you are looking for. You will most probably not be able to do everything that could be done, so you need to judge what is important. You might want to ponder that thoroughly with knowledgeable colleagues around you or at least the folks whose magnetometer you will use.
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I'm working on the canted antiferromagnetic multiferroic materials like BFO, DMO thin films. If i want to know the individual spin orientation on the surface of the thin films, whether it is compensated or not, defects at the surface, roughness induced change in spin configuration etc, please suggest me the techniques which can be used to do the same... thank you??
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If by "individual" you imply "at the single atom level" then you're asking for a lot. With very smooth surfaces, spin polarized STM (SP-STM) can be powerful, but I see potential difficulties: it is highly nontrivial to characterize the chemical nature of the sites you're probing, it essentially probes the surface layer [unless sub-surface defects produce identifiable signatures, which again requires extremely good surfaces to be recognizeable]. An then, it is sensitive to "only" the spin component along the direction in which the STM tip is magnetized. Doing a sequence of measurement with different magnetization orientations of the tip to gain vectorial information has been done in a number of cases [e.g. in the study of skyrmions] but that probably belongs to the top-notch applications of SP-STM.
On top of that, multiferroics are bound to be insulators (ferroelectricity!) and the STM needs some current flow to work...
Other techniques bear other advantages (using polarized x-rays e.g. [-> magnetic x-ray dichroism]  can facilitate obtaining element specific information) but spatial resolution is not to be obtained at the atomic or single defect level. In fortunate cases one may be able to reslove domains and characterize them to some extent. The kind of instrument required would then be a photoelectron microscope, installed at a synchrotron radiation facility.
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Dielectric constant was measured in sintered pellets of the compound with silver paste as electrodes using a Wayne-Kerr impedance analyzer. Note that the tan d peaks correspond to the points where k crosses the ordinate. What could be the explanation for this phenomenon? Contacts were ohmic as evidenced by same current in both forward and reverse applied voltages. 
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Sample, wiring to sample can contribute to stray inductance especially at high frequencies.
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I am just starting out with rietveld refinement. I have been trying to get a good fit for my data of the compounds Bi0.8La0.2Fe(1-x)Zrx using the software Fullprof. But in all my 4 compositions (x=0,0.02,0.05 and 0.1) there seem to be some intensity mismatch of the structural model and observed data. I am attaching the pcr file for the composition Bi0.8La0.2Fe0.98Zr0.02 and also the starting structural model. A picture showing the fit of the data is also attached. As can be seen, the peaks around 2theta = 22, 46 etc. show positive deviation (calculated value is higher) while some peaks have lower calculated value. My obtained chi^2=9.2. How can I improve the fitting ? I am thinking preferred orientation is not the cause since the same trend is seen in all four of my samples. 
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Actually, the fit you show in your picture is quite good.  Given inherent noise and other issues such as particle size of actual material, I honestly doubt you would get  a better fit.  
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I am investigating the multiferroic properties of doped Bi4Ti3O12 thin films on platinum coated silicon substrates. As part of my investigation, I am looking at minimising the film thickness.  One of my initial experiments on thickness control involved using cross sectional SEM to measure the film thickness of samples deposited for different time periods, but otherwise processed using an identical methodology. Samples deposited for 1 hour, 2 hour, 3 hours and 3hours and 18mins(control sample) all show ideal grain flat grain boundaries between the BTO and platinum, and also the platinum, titanium oxide and silicon. However the thinnest sample, deposited for only 30 mins, shows a roughened substrate, as seen in figure 1.
At first, I believed this may be due to damage when the sample was cleaved in half, but another phenomena occurred later.
To investigate the electrical properties dependence on thickness, I fabricated thin 4 films using the same process again, with deposition time controlling thickness. The films were 50nm (24mins), 100nm(49mins), 200nm(98mins) and 380(186mins) thick . Gold contacts were sputtered onto the BTO surface, so electrical contact could be made between to the top and bottom of the film by placing probes on the Au and exposed Platinum substrate.
As seen in figure 2, an almost linear relationship can be seen between 100nm and 380nm, however a huge drop can be seen in the 50nm Sample.
A drop in capacitance could be explained by an increase dielectric contribution between surface states, but roughening of the substrate as seen earlier could also affect the crystallinity of the film.
 I plan to performing cross sectional measurements to confirm the roughening, but am still unable to explain this why this occurs. Could anyone explain why the Platinum films roughens specifically when a thinner film is deposited on it?
Best wishes
David Coathup
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Dear Professor Sreenivas,
thank you very much for you continued support with my work, I am incredibly grateful for your insight. I am aware of the difficult of sputtering from a stoichiometric target, however due to are sputtering apparatus using a cryopump, I am unable to sputter using a O2/Ar gas mixture for health and safety concerns, however I been able to overcome this issue with the annealing in ambient atmosphere.
The films I have produced show good ferroelectric properties, albeit slightly less than that recorded in the journals. This drop could be due to the novel doping, but is also likely to to the lack of optimization in film thickness and post deposition environment, the latter of which will greatly effect the oxygen vacancies in the film.
I have access to AFM, and will attempt the experiment you mentioned. A theory I have discussed with a college is that the absorption of atmospheric impurities into the Pt or the layers beneath may cause the roughening. However, thick BTO layers prevent his, which is why  the roughening only occurs in the samples with the thinnest BTO layers. Does this sound pausible to you?
Many Thanks
David Coathup
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From the temperature dependent dielectric measurement of a multiferroic material we observe a dielectric anomaly around 520 K, which is actually close to the antiferromagnetic Neel temperature. The frequency was 200 kHz. However, from DSC measurement we observed a transition at around 570 K (exothermic reaction). The heating rate was 10 degree C per min. Can anybody please give me any clue why in DSC measurement the transition was observed at higher temperature or why dielectric anomaly was observed at lower temperature? Thank you very much in advance.
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There is another point which should be thought about: DSC measurements measure thermal processes (dependent on temperature, possibly on time and therefor on heating rate) whereas dielectric measurements record dynamic processes  (dependent on frequency. The two resulting transition temperatures may not be comparable. [This is at least true for relaxation processes like the glass transition process.] I don't have enough knowledge about the processes you talk about but the exothermic character of the DSC event indicates a chemical process (like oxidation) rather than a (second order) transition. Repeated measurements should help to decide whether these are reversible or irreversible events. Good luck. Gunther Höhne
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From the temperature dependent dielectric measurement of a multiferroic material we observe a dielectric anomaly around 520 K, which is actually around the antiferromagnetic Neel temperature. The frequency was 200 kHz. However, from DSC measurement we observed a transition at around 570 K (exothermic reaction). The heating rate was 10 degree C per min. Can anybody please give me any clue why in DSC measurement the transition was observed at higher temperature or why dielectric anomaly was observed at lower temperature? Thank you very much in advance.  
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Dear Sir Professor Behnam  Farid,
Thank you very much once again for sending me this useful paper. I read it carefully and now facts are much more clear to me. many thanks once again for your detauils explanation.
Best regards, Basith
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i am using keithley 238 with probe station to measure the i-v charactereristics of a ferroelectric thin film.
Is it possible to correlate the ferroelectric properties of a thin film by using i-v analysis..?
is it possible to quantify the same ?
how can i convert current in to polarization ? (by means of some integration method i heard)
thank you in advance....
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Not reliably without additional information and that is necessarily physics related information.If you integrate current over time then you obtain charge. So far so good - but what does this actually mean?
It could be a measure of polarisation but whether that conclusion is adequate depends on whether it makes sense physics -wise.
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I need to simulate piezomagnetic, piezoelectric and optical response of a composite multiferroic material to an applied electric field. Which COMSOL modules do I need?
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Check out this paper. Might help.
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How can I select a capping agent to control the size and shape of BFO multiferroic nanoparticle?
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The particle size and shape are adjusted by the process method and parameters.
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crystal
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its a charge complex multiferroics. but lot of work has been already done on that. u can try doping with this.
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I study the ferroelectric photovoltaic using the Bi2FeCrO6 material.
Bi2FeCrO6 is multiferroic material with ferroelectric and ferromagnetic properties.
Ferromagnetic properties is result from antiferromagnetic exchange coupling.
Due to the difference of the magnetic moment between Fe-Oxygen, Cr-Oxygen, Bi2FeCrO6 have the net magnetic moment. 
Compared with BiFeO3, because of the ferromagnetic properties of Bi2FeCrO6, band gap of Bi2FeCrO6 is smaller than that of BiFeO3.
So, Photovoltaics using the Bi2FeCrO6 has better properties.
Can you explain why ferromagnetic properties make band gap smaller?
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Ferromagnetism is due to time reversal symmetry and ferroelectricity is due to spatial symmetry. Nature won't allow both the symmetries exists simultaneously, that's why we have scarcity of perfect multiferroic material. (The best example to possess reasonable co-existence is still BiFeO3 and the magnetic property is due to G-type AFM and not FM). For your question, band gap is not due to magnetic property. In-fact the ferromagnetic ordering arise from Fe-O-Fe and Cr-O-Cr exchange mechanism in Bi2FeCrO6, which the former will order in FM and later  in AFM and resultant mechanism will lead to FM/AFM in addition to Fe-O-Cr super exchange mechanism. As these double perovskites are usually insulators due to its structure, resulting in band gap. For understanding the bandgap you should study the theoretical background of the materials. 
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As we know that materials having more than one order parameters are called multiferroics. For ferromagnetic materials it is "spontaneous magnetization". Like this what is the order parameter for ferrotoroidic materials.
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If you wish to know more then please read:
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Multiferroics, Dielectric, Solid State physics, Condensed matter physics
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Kindly see the attached, and see if it has useful information for you
regarding Zn dopant into Bismuth ferrite.
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Thanks in advance for your replies.
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