Journal of Magnetism and Magnetic Materials

Published by Elsevier
Print ISSN: 0304-8853
Publications
Optical observations of 100 nm metallic magnetic nanoparticles are used to study their magnetic field induced self assembly. Chains with lengths of tens of microns are observed to form within minutes at nanoparticle concentrations of 10(10) per mL. Chain rotation and magnetophoresis are readily observed, and SEM reveals that long chains are not simple single particle filaments. Similar chains are detected for several 100 nm commercial bio-separation nanoparticles. We demonstrate the staged magnetic condensation of different types of nanoparticles into composite structures and show that magnetic chains bind to immunomagnetically labeled cells, serving as temporary handles which allow novel magnetic cell manipulations.
 
This paper presents a novel application of magnetic particles for biosensing, called label-acquired magnetorotation (LAM). This method is based on a combination of the traditional sandwich assay format with the asynchronous magnetic bead rotation (AMBR) method. In label-acquired magnetorotation, an analyte facilitates the binding of a magnetic label bead to a nonmagnetic solid phase sphere, forming a sandwich complex. The sandwich complex is then placed in a rotating magnetic field, where the rotational frequency of the sandwich complex is a function of the amount of analyte attached to the surface of the sphere. Here, we use streptavidin-coated beads and biotin-coated particles as analyte mimics, to be replaced by proteins and other biological targets in future work. We show this sensing method to have a dynamic range of two orders of magnitude.
 
The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2CP of adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse-echo sequence. This effect is commonly used to measure the concentrations of a variety of small molecules. We perform extensive Monte Carlo simulations of water diffusing around SPIO nanoparticle aggregates to determine the relationship between T2CP and details of the aggregate. We find that in the motional averaging regime T2CP scales as a power law with the number N of nanoparticles in an aggregate. The specific scaling is dependent on the fractal dimension d of the aggregates. We find T2CP∝N-0.44 for aggregates with d = 2.2, a value typical of diffusion limited aggregation. We also find that in two-nanoparticle systems, T2CP is strongly dependent on the orientation of the two nanoparticles relative to the external magnetic field, which implies that it may be possible to sense the orientation of a two-nanoparticle aggregate. To optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is best to have aggregates with few nanoparticles, close together, measured with long pulse-echo times.
 
We demonstrate a single-step facile approach for highly water stable assembly of amine-functionalized Fe(3)O(4) nanoparticles using thermal decomposition of Fe-chloride precursors in ethylene glycol medium in the presence of ethylenediamine. The average size of nanoassemblies is 40±1 nm, wherein the individual nanoparticles are about 6 nm. Amine functionalized properties are evident from FTIR, thermal and elemental analysis. The saturation magnetization and spin-echo r(2) of the nanoassemblies were measured to be 64.3 emu/g and 314.6 mM(-1)s(-1), respectively. The higher value of relaxivity ratio (r(2)/r(1)=143) indicates that nanoassemblies are a promising high efficiency T2 contrast agent platform.
 
In magnetic drug delivery, therapeutic magnetizable particles are typically injected into the blood stream and magnets are then used to concentrate them to disease locations. The behavior of such particles in-vivo is complex and is governed by blood convection, diffusion (in blood and in tissue), extravasation, and the applied magnetic fields. Using physical first-principles and a sophisticated vessel-membrane-tissue (VMT) numerical solver, we comprehensively analyze in detail the behavior of magnetic particles in blood vessels and surrounding tissue. For any blood vessel (of any size, depth, and blood velocity) and tissue properties, particle size and applied magnetic fields, we consider a Krogh tissue cylinder geometry and solve for the resulting spatial distribution of particles. We find that there are three prototypical behaviors (blood velocity dominated, magnetic force dominated, and boundary-layer formation) and that the type of behavior observed is uniquely determined by three non-dimensional numbers (the magnetic-Richardson number, mass Péclet number, and Renkin reduced diffusion coefficient). Plots and equations are provided to easily read out which behavior is found under which circumstances (Figures 5, 6, 7, and 8). We compare our results to previously published in-vitro and in-vivo magnetic drug delivery experiments. Not only do we find excellent agreement between our predictions and prior experimental observations, but we are also able to qualitatively and quantitatively explain behavior that was previously not understood.
 
The Quadrupole Magnetic Sorter (QMS), employing an annular flow channel concentric with the aperture of a quadrupole magnet, is well established for cell and particle separations. Here we propose a magnetic particle separator comprising a linear array of cylindrical magnets, analogous to the array proposed by Klaus Halbach, mated to a substantially improved form of parallel-plate SPLITT channel, known as the step-SPLITT channel. While the magnetic force and throughput are generally lower than for the QMS, the new separator has advantages in ease of fabrication and the ability to vary the magnetic force to suit the separands. Preliminary experiments yield results consistent with prediction and show promise regarding future separations of cells of biomedical interest.
 
Acute rejection in organ transplant is signaled by the proliferation of T-cells that target and kill the donor cells requiring painful biopsies to detect rejection onset. An alternative non-invasive technique is proposed using a multi-channel superconducting quantum interference device (SQUID) magnetometer to detect T-cell lymphocytes in the transplanted organ labeled with magnetic nanoparticles conjugated to antibodies specifically attached to lymphocytic ligand receptors. After a magnetic field pulse, the T-cells produce a decaying magnetic signal with a characteristic time of the order of a second. The extreme sensitivity of this technique, 10(5) cells, can provide early warning of impending transplant rejection and monitor immune-suppressive chemotherapy.
 
Magnetic twisting cytometry (MTC) measures cellular mechanical properties, such as cell stiffness and viscosity, by applying mechanical stress to specific cell surface receptors via ligand-coated ferromagnetic beads. MTC measures simultaneously the rotation of approximately 50,000 beads attached to 20,000 - 40,000 cells. Here we show direct evidence of heterogeneous bead behavior and examine its consequences in the interpretation of cell mechanical properties.
 
We report integration of an InAs quantum well micro-Hall sensor with microfluidics and real-time detection of moving superparamagnetic beads for biological applications. The detected positive and negative signals correspond to beads moving within and around the Hall cross area respectively. Relative magnitudes and polarities of the signals measured for a random distribution of immobilized beads over the sensor are in good agreement with calculated values and explain consistently the dynamic signal shape. The fast sensor response and its high sensitivity to off-cross area beads demonstrate its capability for dynamic detection of biomolecules and long-range monitoring of non-specific binding events.
 
Optical detection of the frequency-dependent magnetic relaxation signal is used to monitor the binding of biological molecules to magnetic nanoparticles in a ferrofluid. Biological binding reactions cause changes in the magnetic relaxation signal due to an increase in the average hydrodynamic diameter of the nanoparticles. To allow the relaxation signal to be detected in dilute ferrofluids, measurements are made using a balanced photodetector, resulting in a 25 μV/√Hz noise floor, within 50% of the theoretical limit imposed by photon shot noise. Measurements of a ferrofluid composed of magnetite nanoparticles coated with anti-IgG antibodies show that the average hydrodynamic diameter increases from 115.2 to 125.4 nm after reaction with IgG.
 
We use dynamic susceptometry measurements to extract semiempirical temperature-dependent, 255 to 400 K, magnetic parameters that determine the behavior of single-core nanoparticles useful for SQUID relaxometry in biomedical applications. Volume susceptibility measurements were made in 5K degree steps at nine frequencies in the 0.1 - 1000 Hz range, with a 0.2 mT amplitude probe field. The saturation magnetization (M(s)) and anisotropy energy density (K) derived from the fitting of theoretical susceptibility to the measurements both increase with decreasing temperature; good agreement between the parameter values derived separately from the real and imaginary components is obtained. Characterization of the Néel relaxation time indicates that the conventional prefactor, 0.1 ns, is an upper limit, strongly correlated with the anisotropy energy density. This prefactor decreases substantially for lower temperatures, as K increases. We find, using the values of the parameters determined from the real part of the susceptibility measurements at 300 K, that SQUID relaxometry measurements of relaxation and excitation curves on the same sample are well described.
 
Magnetization curves of SiMAG-TCL particles acquired by susceptometry (0.53 mg, ~) and relaxometry (1 mg, '). The saturation magnetization detected by relaxometry is two orders of magnitude lower than that of susceptometry, because only a fraction of the nanoparticles exhibit detectable relaxation times. 
Magnetite nanoparticles (Chemicell SiMAG-TCL) were characterized by SQUID-relaxometry, susceptometry, and TEM. The magnetization detected by SQUID-relaxometry was 0.33% of that detected by susceptometry, indicating that the sensitivity of SQUID-relaxometry could be significantly increased through improved control of nanoparticle size. The relaxometry data were analyzed by the moment superposition model (MSM) to determine the distribution of nanoparticle moments. Analysis of the binding of CD34-conjugated nanoparticles to U937 leukemia cells revealed 60,000 nanoparticles per cell, which were collected from whole blood using a prototype magnetic biopsy needle, with a capture efficiency of >65% from a 750 µl sample volume in 1 minute.
 
Magnetic drug delivery has the potential to target therapy to specific regions in the body, improving efficacy and reducing side effects for treatment of cancer, stroke, infection, and other diseases. Using stationary external magnets, which attract the magnetic drug carriers, this treatment is limited to shallow targets (<5 cm below skin depth using the strongest possible, still safe, practical magnetic fields). We consider dynamic magnetic actuation and present initial results that show it is possible to vary magnets one against the other to focus carriers between them on average. The many remaining tasks for deep targeting in-vivo are then briefly noted.
 
Comparison of the normalized velocity profiles v z /<v z >, v θ /<v θ >, and ω/<ω> for the annular channel with the ratio ρ i of inner to outer radii of 0.5, and v/<v> for the parallel-plate channel.  
Quadrupole magnetic field-flow fractionation (QMgFFF) is a separation and characterization technique for magnetic nanoparticles such as those used for cell labeling and for targeted drug therapy. A helical separation channel is used to efficiently exploit the quadrupole magnetic field. The fluid and sample components therefore have angular and longitudinal components to their motion in the thin annular space occupied by the helical channel. The retention ratio is defined as the ratio of the times for non retained and a retained material to pass through the channel. Equations are derived for the respective angular and longitudinal components to retention ratio.
 
The delivery of noscapine therapies directly to the site of the tumor would ultimately allow higher concentrations of the drug to be delivered, and prolong circulation time in vivo to enhance the therapeutic outcome of this drug. Therefore, we sought to design magnetic based polymeric nanoparticles for the site directed delivery of noscapine to invasive tumors. We synthesized Fe(3)O(4) nanoparticles with an average size of 10 ± 2.5 nm. These Fe(3)O(4) NPs were used to prepare noscapine loaded magnetic polymeric nanoparticles (NMNP) with an average size of 252 ± 6.3 nm. Fourier transform infrared (FT-IR) spectroscopy showed the encapsulation of noscapine on the surface of the polymer matrix. The encapsulation of the Fe(3)O(4) NPs on the surface of the polymer was confirmed by elemental analysis. We studied the drug loading efficiency of polylactide acid (PLLA) and poly (L-lactide acid-co-gylocolide) (PLGA) polymeric systems of various molecular weights. Our findings revealed that the molecular weight of the polymer plays a crucial role in the capacity of the drug loading on the polymer surface. Using a constant amount of polymer and Fe(3)O(4) NPs, both PLLA and PLGA at lower molecule weights showed higher loading efficiencies for the drug on their surfaces.
 
Colloidal nanoparticles of Fe(3)O(4) (4 nm) were synthesized by high-temperature hydrolysis of chelated iron (II) and (III) diethylene glycol alkoxide complexes in a solution of the parent alcohol (H(2)DEG) without using capping ligands or surfactants: [Fe(DEG)Cl(2)](2-) + 2[Fe(DEG)Cl(3)](2-) + 2H(2)O + 2OH(-) → Fe(3)O(4) + 3H(2)DEG + 8Cl(-) The obtained particles were reacted with different small-molecule polydentate ligands, and the resulting adducts were tested for aqueous colloid formation. Both the carboxyl and α-hydroxyl groups of the hydroxyacids are involved in coordination to the nanoparticles' surface. This coordination provides the major contribution to the stability of the ligand-coated nanoparticles against hydrolysis.
 
Variation of the magnetic moment m (dots) of the porous g-Fe 2 O 3 /Pt-nanocomposite with time t during electrochemical cycling in different potential ranges  
Combined magnetic and electrochemical measurements of porous g-Fe 2 O 3 /Pt-nanocomposite upon in situ charging in 1 M KOH electrolyte with a scan rate of v of 0.5 mV/s at a constant magnetic field of 5 kOe. (a) and (b) Cyclic voltammograms (CVs) measured in situ in SQUID-magnetometer. The numbers indicate the sequence in which the CVs were recorded. Each potential range was scanned three times (compare Fig. 3). (c) and (d) Magnetic moment m (after linear drift correction) as a function of the applied potential U measured simultaneously with CV. (e) and (f) Plots of m as a function of the accumulated charge Q ((e) belongs to (a, c); (f) to (b, d)). Q¼ 0 corresponds to the first data point of m. Linear fits of the mÀQ behaviour in the potential range À 650 mV to À 350 mV (e) and À 150 mV to þ300 mV (f) are plotted as faint lines visualizing the different slopes in the two charging regions (see text, s 1 , s 2 ).
Variation of the magnetic moment m (dots) of porous g-Fe 2 O 3 /Pt-nanocomposite with time t during electrochemical cycling with different scan rates v (in the sequence: 0.5 mV/s, 0.1 mV/s and 1 mV/s) in the potential range from À 650 mV to þ 300 mV at a constant magnetic field of 5 kOe. The potential U (line) was measured versus a Au quasi-reference electrode. The data for v¼ 0.5 mV/s (left-hand side) are identical to those in Fig. 3 (right-hand side). Note: For better demonstration of the correlation between U and m, the direction of m axis is inversed.
SQUID magnetometry combined with in situ cyclic voltammetry by means of a three-electrode chemical cell opens up novel potentials for studying correlations between electrochemical processes and magnetic behaviour. The combination of these methods shows that the charge-induced variation of the magnetic moment of nanocrystalline maghemite ([Formula: see text]-Fe2O3) of about 4% strongly depends on the voltage regime of charging. Upon positive charging, the charge-induced variation of the magnetic moment is suppressed due to adsorption layers. The pronounced charge-sensitivity of the magnetic moment in the regime of negative charging may either be associated with a redox reaction or with charge-induced variations of the magnetic anisotropy or magnetoelastic coupling.
 
Magnetostatic Maxwell equations and the Landau-Lifshitz-Gilbert (LLG) equation are combined to a multiscale method, which allows to extend the problem size of traditional micromagnetic simulations. By means of magnetostatic Maxwell equations macroscopic regions can be handled in an averaged and stationary sense, whereas the LLG allows to accurately describe domain formation as well as magnetization dynamics in some microscopic subregions. The two regions are coupled by means of their strayfield and the combined system is solved by an optimized time integration scheme.
 
Magnetic particle imaging (MPI) is a powerful new research and diagnostic imaging platform that is designed to image the amount and location of superparamagnetic nanoparticles in biological tissue. Here, we present mathematical modeling results that show how MPI sensitivity and spatial resolution both depend on the size of the nanoparticle core and its other physical properties, and how imaging performance can be effectively optimized through rational core design. Modeling is performed using the properties of magnetite cores, since these are readily produced with a controllable size that facilitates quantitative imaging. Results show that very low detection thresholds (of a few nanograms Fe(3)O(4)) and sub-millimeter spatial resolution are possible with MPI.
 
Any single permanent or electro magnet will always attract a magnetic fluid. For this reason it is difficult to precisely position and manipulate ferrofluid at a distance from magnets. We develop and experimentally demonstrate optimal (minimum electrical power) 2-dimensional manipulation of a single droplet of ferrofluid by feedback control of 4 external electromagnets. The control algorithm we have developed takes into account, and is explicitly designed for, the nonlinear (fast decay in space, quadratic in magnet strength) nature of how the magnets actuate the ferrofluid, and it also corrects for electro-magnet charging time delays. With this control, we show that dynamic actuation of electro-magnets held outside a domain can be used to position a droplet of ferrofluid to any desired location and steer it along any desired path within that domain - an example of precision control of a ferrofluid by magnets acting at a distance.
 
Magnetic elastomers have been widely pursued for sensing and actuation applications. Silicone-based magnetic elastomers have a number of advantages over other materials such as hydrogels, but aggregation of magnetic nanoparticles within silicones is difficult to prevent. Aggregation inherently limits the minimum size of fabricated structures and leads to non-uniform response from structure to structure. We have developed a novel material which is a complex of a silicone polymer (polydimethylsiloxane-co-aminopropylmethylsiloxane) adsorbed onto the surface of magnetite (γ-Fe(2)0(3)) nanoparticles 7-10 nm in diameter. The material is homogenous at very small length scales (< 100 nm) and can be crosslinked to form a flexible, magnetic material which is ideally suited for the fabrication of micro- to nanoscale magnetic actuators. The loading fraction of magnetic nanoparticles in the composite can be varied smoothly from 0 - 50% wt. without loss of homogeneity, providing a simple mechanism for tuning actuator response. We evaluate the material properties of the composite across a range of nanoparticle loading, and demonstrate a magnetic-field-induced increase in compressive modulus as high as 300%. Furthermore, we implement a strategy for predicting the optimal nanoparticle loading for magnetic actuation applications, and show that our predictions correlate well with experimental findings.
 
In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4°C and 7°C above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B(0). Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B(0). Results are presented for the expected temperature increase in small tumors (~1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002 to 0.01 solid volume fraction) and nanoparticle radii (1 to 10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful The goal of this work is to examine, by means of analysis and simulation, the concept of interactive fluid magnetization using the dynamic behavior of superparamagnetic iron oxide nanoparticle suspensions in the MRI environment. In addition to the usual magnetic fields associated with MRI, a rotating magnetic field is applied transverse to the main B(0) field of the MRI. Additional or modified magnetic fields have been previously proposed for hyperthermia and targeted drug delivery within MRI. Analytical predictions and numerical simulations of the transverse rotating magnetic field in the presence of B(0) are investigated to demonstrate the effect of Ω, the rotating field frequency, and the magnetic field amplitude on the fluid suspension magnetization. The transverse magnetization due to the rotating transverse field shows strong dependence on the characteristic time constant of the fluid suspension, τ. The analysis shows that as the rotating field frequency increases so that Ωτ approaches unity, the transverse fluid magnetization vector is significantly non-aligned with the applied rotating field and the magnetization's magnitude is a strong function of the field frequency. In this frequency range, the fluid's transverse magnetization is controlled by the applied field which is determined by the operator. The phenomenon, which is due to the physical rotation of the magnetic nanoparticles in the suspension, is demonstrated analytically when the nanoparticles are present in high concentrations (1 to 3% solid volume fractions) more typical of hyperthermia rather than in clinical imaging applications, and in low MRI field strengths (such as open MRI systems), where the magnetic nanoparticles are not magnetically saturated. The effect of imposed Poiseuille flow in a planar channel geometry and changing nanoparticle concentration is examined. The work represents the first known attempt to analyze the dynamic behavior of magnetic nanoparticles in the MRI environment including the effects of the magnetic nanoparticle spin-velocity. It is shown that the magnitude of the transverse magnetization is a strong function of the rotating transverse field frequency. Interactive fluid magnetization effects are predicted due to non-uniform fluid magnetization in planar Poiseuille flow with high nanoparticle concentrations.
 
In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4°C and 7°C above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B(0). Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B(0). Results are presented for the expected temperature increase in small tumors (~1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002 to 0.01 solid volume fraction) and nanoparticle radii (1 to 10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful selection of the rotating or sinusoidal field parameters (field frequency and amplitude). The work indicates that it may be feasible to combine low-field MRI with a magnetic hyperthermia system using superparamagnetic iron oxide nanoparticles.
 
A microfabricated magnetic sifter has been designed and fabricated for applications in biological sample preparation. The device enables high-throughput, high-gradient magnetic separation of magnetic nanoparticles by utilizing columnar fluid flow through a dense array (~5000/mm(2)) of micropatterned slots in a magnetically soft membrane. The potential of the sifter for separation of magnetic nanoparticles conjugated with capture antibodies is demonstrated through quantitative separation experiments with CD138-labelled MACS nanoparticles. Capture efficiencies ranging from 28-37% and elution efficiencies greater than 73% were measured for a single pass through the sifter.
 
This study demonstrates a method for improving the resolution of susceptibility magnitude imaging (SMI) using spatial information that arises from the nonlinear magnetization characteristics of magnetic nanoparticles (mNPs). In this proof-of-concept study of nonlinear SMI, a pair of drive coils and several permanent magnets generate applied magnetic fields and a coil is used as a magnetic field sensor. Sinusoidal alternating current (AC) in the drive coils results in linear mNP magnetization responses at primary frequencies, and nonlinear responses at harmonic frequencies and intermodulation frequencies. The spatial information content of the nonlinear responses is evaluated by reconstructing tomographic images with sequentially increasing voxel counts using the combined linear and nonlinear data. Using the linear data alone it is not possible to accurately reconstruct more than 2 voxels with a pair of drive coils and a single sensor. However, nonlinear SMI is found to accurately reconstruct 12 voxels (R(2) = 0.99, CNR = 84.9) using the same physical configuration. Several time-multiplexing methods are then explored to determine if additional spatial information can be obtained by varying the amplitude, phase and frequency of the applied magnetic fields from the two drive coils. Asynchronous phase modulation, amplitude modulation, intermodulation phase modulation, and frequency modulation all resulted in accurate reconstruction of 6 voxels (R(2) > 0.9) indicating that time multiplexing is a valid approach to further increase the resolution of nonlinear SMI. The spatial information content of nonlinear mNP responses and the potential for resolution enhancement with time multiplexing demonstrate the concept and advantages of nonlinear SMI.
 
Single particle magnetization and size measurements of micron and nano sized, magnetic particles were made using a previously described device referred to as Cell Tracking Velocimetry, CTV. Three types of commercially available, and commonly used, magnetic particles were studied in this report. While the CTV instrument provides individual particles measurements, the average magnetization and size measurements were found to have reasonable agreements with reported values from instruments which measure bulk values. In addition, the CTV instrument, using electromagnets, can also determine magnetization curves, which also proved to have reasonable agreement with other published studies. Given that magnetic separation and analysis technology is dependent on the quality of the magnetic particles used, studies such as this one using CTV provide not only average data, but also provides information with respect to the distribution of the properties such as magnetization and size. For example, the spread of the data in magnetic and settling velocities were found to be predominately due to the size distribution of the analyzed particles.
 
Ultraacoustic and magnetic investigations have been performed in the Gd100-xYx polycrystalline system with 0≤x≤12 at%. The unexpected first-order phase transition at the temperature Th≈248 K, which is slightly below the Curie temperature, is revealed in all studied alloys except pure Gd. The surprising feature of this transition is not only its presence, since it is accepted that there is no phase transformation in this temperature region in this system for x≤10, but its weak nonmonotonic dependence on x, unlike the Curie temperature. The analysis of the temperature dependence of the ultraacoustic and magnetic properties allows one the supposition that the magnetic phase transition at Th is of the order-to-order type from collinear ferromagnetic structure to ferromagnetic helicoid structure.
 
In this study, we investigated that both magnetic switching and tunneling magnetoresistance (TMR) ratio of MTJs with the NiFeSiB free layer. The junctions were fabricated by a photolithographic patterning procedure and ion beam etching. A magnetic field of 100 Oe was applied during deposition to induce the uniaxial magnetic anisotropy in ferromagnetic layer.The NiFeSiB layers were used to substitute for the traditionally used CoFe and/or NiFe layers with the emphasis being given to obtaining an understanding of the effect of the amorphous free layer on the switching characteristics of the MTJs. Ni<sub>16</sub>Fe<sub>62</sub>Si<sub>8</sub>B<sub>14</sub> has a lower saturation magnetization (Ms: 800 emu/cm3) than Co<sub>90</sub>Fe<sub>10</sub> and a higher anisotropy constant.
 
MFM images of tracks written in ME and MP tape have been obtained. The analysis of the images concentrated on the track edges. A track written with signals of 0.5 mu m wavelength overwrites a part of a track written with a wavelength of 1 mu m The sharpness of the edges was derived from MFM results. It can be seen that the MP sample shows smaller changes in sharpness of the edge with an increasing write current than ME tape. In ME tape, the region between the lambda = 0.5 mu m and the = 1 mu m parts of the track is much wider than the original lambda = 1 mu m edge.
 
Summary form only given R<sub>2</sub>Fe<sub>17</sub> compounds (where R is a rare - earth element) crystallize in two structural: hexagonal (with Th<sub>2</sub>Ni<sub>17</sub> type and P6<sub>3</sub>/MMC space group) and rhombohedral (Th<sub>2</sub>Zn<sub>17</sub> type and R3m space group). The type of structure depends on the size of the R atom.
 
This work is devoted to the description of the orbital and magnetic structures of pure and charge-ordered manganites, caused by crystal and charge structures and the Jahn–Teller effect. The model under consideration is based on the orbital dependency of the magnetic interactions in Jahn–Teller crystal. The orbital structure is considered dependent upon the Jahn–Teller distortions of the coordination of the Mn3+ ions and upon the crystal field. It is shown that exchange interactions forming the magnetic structure of the charge-ordered manganite could be described in the superexchange approximation because of localization of the charge carriers. For pure manganite and for charge-ordered phase, the exchange parameters dependencies and the single-ion anisotropy dependency, upon the angle of orbital states mixing, are drawn. Therefore, it is shown that the orbital structure of the charge-ordered phase is formed mainly by cooperative Jahn–Teller distortions and, taking into account the charge superstructure, causes the non-trivial magnetic structure.
 
Four manganite samples of the series, (La1/3Sm2/3)2/3SrxBa0.33−xMnO3, with x=0.0, 0.1, 0.2 and 0.33, were investigated by X-band (∼9.5 GHz) electron paramagnetic resonance (EPR) in the temperature range 4–300 K. The temperature dependences of EPR lines and linewidths of the samples with x=0.0, 0.1 and 0.2, containing Ba2+ ions, exhibit similar behavior, all characterized by the transition temperatures (TC) to ferromagnetic states in the 110–150 K range. However, the sample with x=0.33 (containing no Ba2+ ions) is characterized by a much higher TC=205 K. This is due to significant structural changes effected by the substitution of Ba2+ ions by Sr2+ ions. There is an evidence of exchange narrowing of EPR lines near Tmin, where the linewidth exhibits the minimum. Further, a correlation between the temperature dependence of the EPR linewidth and conductivity is observed in all samples, ascribed to the influence of small-polaron hopping conductivity in the paramagnetic state. The peak-to-peak EPR linewidth was fitted to ΔBpp(T)=ΔBpp,min+A/Texp(−Ea/kBT), with Ea=0.09 eV for x=0.0, 0.1 and 0.2 and Ea=0.25 eV for x=0.33. From the published resistivity data, fitted here to σ(T)∝1/T exp(−Eσ/kBT), the value of Eσ, the activation energy, was found to be Eσ=0.18 eV for samples with x=0.0, 0.1 and 0.2 and Eσ=0.25 eV for the sample with x=0.33. The differences in the values of Ea and Eσ in the samples with x= 0.0, 0.1and 0.2 and x=0.33 has been ascribed to the differences in the flip-flop and spin-hopping rates. The presence of Griffiths phase for the samples with x=0.1 and 0.2 is indicated; it is characterized by coexistence of ferromagnetic nanostructures (ferrons) and paramagnetic phase, attributed to electronic phase separation.
 
The magnetic and magnetocaloric properties of polycrystalline La0.70(Ca0.30-xSrx)MnO3:Ag 10% manganite have been investigated. All the compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the Insulator-Metal transition temperature (TIM) and Curie temperature (Tc) are observed at 298 K for x = 0.10 composition. Though both TIM and Tc are nearly unchanged with Ag addition, the MR is slightly improved. The MR at 300 K is found to be as large as 31% with magnetic field change of 1Tesla, whereas it reaches up to 49% at magnetic field of 3Tesla for La0.70Ca0.20Sr0.10MnO3:Ag0.10 sample. The maximum entropy change (\DeltaSMmax) is 7.6 J.Kg-1.K-1 upon the magnetic field change of 5Tesla, near its Tc (300.5 K). The La0.70Ca0.20Sr0.10MnO3:Ag0.10 sample having good MR (31%1Tesla, 49%3Tesla) and reasonable change in magnetic entropy (7.6 J.Kg-1.K-1, 5 Tesla) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.
 
We report on the structural aspects and superconductivity of the La1−xPrxCaBaCu3O7−δ system (RE: 1113) with x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 1.0. X-ray diffraction studies show that Pr substitutes isostructurally in LaBaCaCu3O7−δ (La: 1113) until complete replacement of La. The superconducting transition temperature, Tc, determined from AC susceptibility measurements decreases with x. The relative Tc depression due to Pr in the LaBaCaCu3O7−δ system is less in comparison to that found for La site Pr substituted LaBa2Cu3O7 system. Normal state DC magnetism of the Pr substituted samples show a Curie–Weiss paramagnetic susceptibility with deviations below about 10 K. The non-superconducting compounds (x=0.7, 1.0) display Schottky-type anomalies which are attributed to crystalline electric field effects (CEF). The related entropy change for x=1.0 sample amounts to ΔS=7.75 J/mol K. Rietveld analysis on neutron diffraction pattern of x=0.0 sample shows that 46 at% of the La is occupying the Ba site, in normal Re: 123 unit cell. A similar situation is suppose to occur in case of PrBaCaCu3O7, for which the refinement of Ba and Pr sites is near impossible due to their similar scattering cross-sections for neutrons. Ba site sitting Pr does not contribute to the Pr-4f hybridization with Cu–O conduction band, giving rise to a lower TN and less effect of Pr on superconductivity of La: 1113 system than for La: 123. The results strengthen extended Pr-4f hybridisation with Cu–O conduction band.
 
Bond lengths (,~) for the compound Nd3Fe27.sTi1.5
Volume (Vphas e) relations, number of nitrogen atoms in the unit cell (N) and experimental relative volume increase values (AV/V); V1:5, V1:12, ~2:17H, V2:17r~ and V3:29 are the unit cell volumes of the corresponding compounds; in the case under study Npha~ ~ 1 is the number of
A series of alloys with the composition Nd3Fe27.5Ti1.5−yMoy (0 ≤ y ≤ 1.5) and the Nd3(Fe,Ti)29-type structure have been synthesized. The X-ray diffraction pattern of these compounds can be indexed in the monoclinic symmetry with lattice parameters of , , and β = 96.930(2)° and the structure can be described in the A2/m space group (Z = 2). This is a minimal non-isomorphic supergroup of P21/c. With this description the number of Fe sites is eleven. The Curie temperature depends on the Ti, Mo concentration and increases from 400 to 437 K with increasing Ti. The room temperature saturation magnetization for the y = 0 sample is 139.6 A m2/kg and the anisotropy field 3.7 T. After nitrogenation a lattice expansion of 5.3% is observed. Four N atoms per formula unit and their probable sites (4i and 4f) are predicted. An empirical model is proposed for the calculation of the number of the N atoms for all phases having a superstructure relation to the 1:5 phase. The Curie temperature increases by 280 K reaching 663 to 712 K depending on the Ti:Mo ratio. The room temperature saturation magnetization of the y = 0 sample becomes 170.6 A m2/kg and the anisotropy field 8.0 T after the nitrogenation process. X-ray powder diffraction patterns of magnetically aligned powder samples of the parent and the nitrided compound indicate the presence of an easy-cone-type anisotropy. The calculated room temperature anisotropy constants, K1 = −1.80 MJ/m3, K2 = 1.86 MJ/m3 for the parent y = 0 sample and K1 = −5.17 MJ/m3, K2 = 3.57 MJ/m3 for the nitrided one, confirm this assumption.
 
Inelastic neutron scattering experiments were performed on U(Ru1-xRhx)2Si2 with x=0 and 0.02 in order to investigate the variations of the antiferromagnetic (AF) fluctuations with doping Rh. We have found that the energy of the magnetic excitation observed at Q=(1,0,0) in the hidden order (HO) phase markedly decreases from to with increasing x. In addition, the staggered susceptibility χQ estimated from the inelastic peak develops in the HO phase at x=0.02, while it is suppressed at x=0. These results indicate that the AF fluctuation in the HO phase is significantly enhanced with doping Rh.
 
The crystalline structures, magnetic properties and magnetocaloric effect (MCE) of MnCo1−xGe alloys (0.02⩽x⩽0.2) have been reported. The crystalline structures of MnCo1−xGe (x⩽0.06) alloys are mainly of TiNiSi-type phase, and Ni2In-type structure dominates for x>0.06. With decreasing Co concentrations the saturated magnetization of these compounds decreases. Large low-field magnetic entropy change −ΔSM of about 2.3 J/kg K in MnCo0.94Ge alloy has been obtained for a magnetic field change of 1 T. Moreover, it is found that TiNiSi-type phase exhibits larger −ΔSM than Ni2In-type one. For MnCo0.94Ge alloy, considerable low-field refrigerant capacity (RC) (∼460 mJ/cm3), low coercivity and easy synthesis make these alloys potential candidates for near-room temperature magnetic refrigerants.
 
In boron-substituted melt-spun Sm(Co,Fe,Cu,Zr)7.5-type alloys a nanocomposite microstructure and high coercivities in both as-spun and short-time annealed ribbons can be obtained. In the present study three different compositions, namely Sm(Co0.73Fe0.1Cu0.09Zr0.04B0.04)7.5, Sm(Co0.70Fe0.1Cu0.12Zr0.04B0.04)7.5 and Sm(Co0.70Fe0.1Ni0.12Zr0.04B0.04)7.5 have been examined in order to investigate the influence of composition on the magnetic properties and the microstructure. Melt-spun ribbons have been obtained and annealing has been followed under argon atmosphere for 30–75 min at 600–870 °C. For the as-spun ribbons the TbCu7-type of structure and fcc-Co as a secondary phase have been identified in the X-ray diffraction patterns. For the annealed ribbons above 700 °C the 1:7 phase transforms into 2:17 and 1:5 phases. The TEM studies have shown a homogeneous nanocrystalline microstructure with average grain size of 30–80 nm. Coercivity values of 15–27 kOe have been obtained from hysteresis loops traced in non-saturating fields. The coercivity decreases with temperature, but it is sufficiently large to maintain values higher than 5 kOe at 380 °C.
 
In this study, magnetic and magnetocaloric properties of Pr0.68Ca0.32−xSrxMnO3 (x=0, 0.1, 0.18, 0.26 and 0.32) compounds were investigated. X-ray results indicated that all the samples have a single phase of orthorhombic symmetry. The orthorhombic unit cell parameters increase with the increase in Sr content. Large negative magnetic entropy changes (−26.2 J/kg K at 38 K and 5 T for x=0 and −6.5 J/kg K at 83 K and 6 T for x=0.1) were attributed to ultrasharp metamagnetic transitions. The peak value of ΔSm decreased from −4.1 J/kg K for x=0.18 sample to −2.4 J/kg K for x=0.32 at 1 T magnetic field.
 
The crystal and magnetic structure of polycrystalline $La{}_{0.5-x}Nd{}_{x}Ca_{0.5}MnO{}_{3}$ (0.0 \ensuremath{\le} x \ensuremath{\le} 0.5) samples have been investigated using magnetization, resistivity, transmission electron microscope, and neutron diffraction techniques. The samples are isostructural and possess orthorhombic structure in \textit{Pnma} space group. On lowering of temperature, the samples exhibit CE - type antiferromagnetic structure coexisting with a weak ferromagnetic ordering. The charge and orbitally ordered antiferromagnetic phase is weakened by the growth of ferromagnetic phase. The evolution of structural distortions and magnetic structure at low temperature as a function of Nd doping exhibit a strong correlation with A - site disorder ($\sigma{}^{2}$). Comment: To appear in J. Magn. Magn. Mater
 
We have studied the magnetic, magnetocaloric and magnetotransport properties of RSn1+xGe1-x(R=Gd, Tb, Er; x=0.1) series by means of magnetization, heat capacity and resistivity measurements. It has been found that all the compounds crystallize in the orthorhombic crystal structure described by the centrosymmetric space group Cmcm (No. 63). The magnetic susceptibility and heat capacity data suggest that all the compounds are antiferromagnetic. Large negative values of {\theta}p in case of GdSn1.1Ge0.9 and TbSn1.1Ge0.9 indicate that strong antiferromagnetic interactions are involved, which is also reflected in the magnetization isotherms. On the other hand ErSn1.1Ge0.9 shows weak antiferromagnetic interaction. The heat capacity data have been analyzed by fitting the temperature dependence and the values of {\theta}D and {\gamma} have been estimated. Among these three compounds, ErSn1.1Ge0.9 shows considerable magnetic entropy change of 9.5 J/kg K and an adiabatic temperature change of 3.2 K for a field of 50 kOe. The resistivity data in different temperature regimes have been analyzed and the dominant contributions have been identified. All the compounds show small but positive magnetoresistance.
 
Samples of La0.7Ca0.3Mn1−xGaxO3 with x=0, 0.025, 0.05 and 0.10 were prepared by standard solid-state reaction. They were first characterized chemically, including the microstructure. The magnetic properties and various transport properties, i.e. the electrical resistivity, magnetoresistivity (for a field below 8 T), thermoelectric power and thermal conductivity measured each time on the same sample, are reported. The markedly different behaviour of the x=0.1 sample from those with a smaller Ga content, is discussed. The dilution of the Mn3+/Mn4+ interactions with Ga doping considerably reduces the ferromagnetic double exchange interaction within the manganese lattice leading to a decrease of the Curie temperature. The polaron binding energy varies from 224 to 243 meV with increased Ga doping.
 
Temperature and pressure dependence of magnetic properties in the NdMn2−xFexGe2 series of solid solutions (0.1⩽x⩽1.0) are reported. The (P, T) magnetic phase diagrams are determined on the basis of the AC magnetic susceptibility measured in a weak magnetic field. The measurements were carried out under hydrostatic pressure up to 1.5 GPa in the temperature range 80−430 K. The reported data show that in the studied series of solid solutions, a drastic change in magnetic properties takes place in a narrow dilution parameter range (0.4⩽x⩽0.5). While taking into account the magnetic properties, the studied range of Fe content could be divided into four regions. Only in the case of x=0.3 and 0.4, the external pressure significantly influences the magnetic properties of the samples.
 
Optical absorption spectra, transverse Kerr effect, resistivity and magnetoresistance of polycrystalline films La1−xSrxCoO3 (x=0.15;0.25;0.35) are studied. The temperature dependences of optical and magneto-optical properties were found to have specific features which can be caused by changes in spin state of Co3+ ions. The results obtained support the phase separation scenario in doped cobaltates. Magnetic interaction between different phases is controlled by the number of Co3+ (intermediate-spin state, S=1) ions in the orbital ordered state.
 
Study of structural, magnetic and transport properties of the polycrystalline La0.8Pb0.1□0.1MnO3 (LPMO) and La0.8Pb0.1Na0.1MnO3 (LPNMO) samples has been carried out. The X-ray diffraction shows that both samples crystallize in a rhombohedral structure with the space group. Ferromagnetic and insulating states are found for the LPMO sample, whereas a ferromagnetic behavior below 247 K and insulator–metal transition at 180 K are observed for the LPNMO sample. The temperature dependence of magnetic susceptibility at higher temperature for both samples reveals the presence of the Griffiths phase above the Curie temperature. The thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T3/2, in accordance with Bloch’s law. The magnetization does not reach complete saturation up to a field of 6 T. Moreover, analysis of cell parameters indicates that the structure of LPMO is more distorted than LPNMO whereas additional results from the unit cell volume and the Curie temperature combined with previous results point to a lower average radius 〈rA〉 (the average ionic radius, related to the one-electron bandwidth) and higher variance (the variance, measuring the quenched disorder) for LPMO reference to LPNMO. This result is in agreement with the enhancement of Griffiths phase characteristics and the low value of the spin stiffness constant D for LPMO. Finally, we show that the magnitude of the magnetic inhomogeneity depends on the random substitution in the A-site cations.
 
The mixed-valence manganites Pr1−xNaxMnO3 have been investigated by neutron diffraction, electric transport and magnetic measurements. Similarly to related systems with divalent alkali earths, the increasing monovalent sodium substitution decreases the Jahn–Teller deformation of the MnO6 octahedra, lowers the resistivity and changes gradually the magnetic ordering from the layered type antiferromagnetism (x=0) through spin-canted arrangements (x∼0.05) to the pure ferromagnetism (0.10⩽x⩽0.15) with TC∼125 K. The samples with ferromagnetic ground state are not metallic below TC but show appreciable magnetoresistive effects in a broad temperature region. The electronic localization at low temperatures is further enhanced in the sample with the maximum sodium content x∼0.2. Electron and neutron diffraction evidences that Pr0.8Na0.2MnO3 exhibits a commensurate charge and orbital ordering of the Mn3+/Mn4+(1:1) kind below Tco=215 K, followed with a transition to the antiferromagnetic arrangement of pseudo-CE type at TN=175 K, analogous to that of previously studied Pr0.65Ca0.35MnO3. In addition, Pr0.8Na0.2MnO3 undergoes below ∼50 K a spin reorientation and, simultaneously, ferromagnetic clusters in the charge-ordered matrix are formed. By application of external field of 2–5 T below Tco, the insulating charge-ordered antiferromagnet is transformed to a metallic ferromagnetic state which is persistent below ∼60 K, i.e. temperature close to the spin reorientation transition.
 
Nanocrystalline γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds were synthesized by using a citrate precursor route. It was observed that lattice constants for γ′-Fe4−xNixN (0.2⩽x⩽0.8), decrease with increasing Ni atom concentration. The atmospheric oxidation of γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds result in the formation of Fe-oxide layer at the surface of the ultrafine particles in addition to pure nitride phase. The local magnetic structures of the iron atoms in the nitride materials are quite similar to those found in the case of dilute alloy systems. The role of Ni substitution in γ′-Fe4N as γ-Fe4−xNixN is investigated with regard to crystal structure and magnetic properties of the ultrafine nitride materials. The results suggest that the average magnetic moment per iron atom in ultrafine γ′-Fe4−xNixN compounds is affected by the Ni atom concentrations, superparamagnetic relaxation, presence of oxide layer and randomly canted spin structure at the particle surface. The observed larger values of coercivities in the ultrafine materials are on account of the reversal of magnetization by spin rotation mechanism.
 
Top-cited authors
John Slonczewski
Ivan K Schuller
  • University of California, San Diego
Helmut Kronmüller
  • Max Planck Institute for Intelligent Systems
J M D Coey
  • Trinity College Dublin
Josep Nogues
  • Catalan Institute of Nanoscience and Nanotechnology