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ABSTRACT: Together with impaired production of erythropoietin and iron deficiency, the decreased lifespan of red blood cells (RBCs) is a main factor contributing to the chronic anaemia observed in haemodialysis (HD) patients. Atomic force microscopy is employed in this work to thoroughly survey the membrane of intact RBCs (iRBCs) of HD patients in comparison to those of healthy donors, aiming to obtain direct information on the structural status of RBCs that can be related to their decreased lifespan. We observed that the iRBC membrane of the HD patients is overpopulated with extended circular defects, termed 'orifices', that have typical dimension ranging between 0.2 and 1.0 μm. The 'orifice' index-that is, the mean population of 'orifices' per top membrane surface-exhibits a pronounced relative increase of order 54 ± 12% for the HD patients as compared to healthy donors. Interestingly, for the HD patients, the 'orifice' index, which relates to the structural status of the RBC membrane, correlates strongly with urea concentration, which is a basic index of the uraemic milieu. Thus, these results indicate that the uraemic milieu downgrades the structural status of the RBC membrane, possibly triggering biochemical processes that result in their premature elimination from the circulation. This process could decrease the lifespan of RBCs, as observed in HD patients.
Nanotechnology 11/2012; 23(48):485101. · 3.98 Impact Factor
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ABSTRACT: The structural and magnetic properties and spin dynamics of dextran coated and uncoated
-Fe2O3 (maghemite) nanoparticles have been investigated using high resolution transmission
electron microscopy (HRTEM), 57Fe nuclear magnetic resonance (NMR), M¨ossbauer
spectroscopy and dc magnetization measurements. The HRTEM observations indicated a
well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The
combined M¨ossbauer and magnetic study suggested the existence of significant interparticle
interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies.
The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those
of the bulk material, indicating the same hyperfine field values at saturation in accord with the
performed M¨ossbauer measurements. The T2 NMR spin–spin relaxation time of the
nanoparticles has also been measured as a function of temperature and found to be two orders
of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in
the longitudinal magnetization of the nanoparticles in the low temperature limit may account
for the shortening and the temperature dependence of the T2 relaxation time. Thus, the low
temperature NMR results are in accord with the mechanism of collective magnetic excitations,
due to the precession of the magnetization around the easy direction of the magnetization at an
energy minimum, a mechanism originally proposed to interpret M¨ossbauer experiments in
magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is
also discussed.
Journal of Physics Condensed Matter 03/2012; 24:156001. · 2.55 Impact Factor
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ABSTRACT: The structural and magnetic properties and spin dynamics of dextran coated and uncoated γ-Fe(2)O(3) (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), (57)Fe nuclear magnetic resonance (NMR), Mössbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined Mössbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed Mössbauer measurements. The T(2) NMR spin-spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T(2) relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret Mössbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.
Journal of Physics Condensed Matter 03/2012; 24(15):156001. · 2.55 Impact Factor
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ABSTRACT: Magnetic particles are currently applied to special biomedical and environmental applications owing to their unique magnetic, morphological and substance-carrying capabilities. Very recently we introduced Magnetically Assisted Hemodialysis (MAHD), an innovative therapeutic application of Ferromagnetic Nanoparticles (FNs) for the treatment of End-Stage Renal Disease (ESRD). MAHD can be employed for the selective and efficient removal of toxins that, although of high biological importance, they cannot be handled by current Hemodialysis strategies. This work is focused on evaluating the biocompatibility of Fe3O4 FNs with cells of donated human blood, namely red blood cells (RBCs), white blood cells (WBCs) and platelets (Plts). To that end, optical microscopy and atomic force microscopy were employed for the morphological examination of blood cells that were maturated under the presence of Fe3O4 FNs by means of mild incubation up to 120 min at T=20 degrees C. As a conclusion we have not detected noticeable interference between RBCs, WBCs and Pits with FNs for the maturation conditions and the extreme FNs concentrations examined in this work.
Journal of Nanoscience and Nanotechnology 09/2010; 10(9):6110-5. · 1.56 Impact Factor
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ABSTRACT: Iron oxide Ferromagnetic Nanoparticles (FNs) such as magnetite (Fe3O4) and maghemite (γ-Fe2O3) are currently employed in biomedical applications owing to their relatively high biocompatibility. Recently, we have introduced a novel application of Fe3O4 FNs in the so-called Magnetically Assisted Haemodialysis (MAHD), a promising concept that can be employed for the treatment of End-Stage Renal Disease. The key characteristic of MAHD is the selective removal of toxins that cannot be removed by current low- and high-flux dialysers that are extendedly used during conventional Haemodialysis (HD). In addition, MAHD could enable the more efficient removal of all toxins when compared to conventional HD so that the duration of dialysis session could be decreased. This is an important benefit that could significantly improve the quality of life of patient. The present work focuses on the in vitro evaluation of the biocompatibility of both bare Fe3O4 FNs and Fe3O4-Bovine Serum Albumin Conjugates (Fe3O4-BSA Cs) with blood cells, namely Red Blood Cells (RBCs), White Blood Cells (WBCs) and Platelets (Plts). Their solubility in whole human blood medium is also carefully evaluated. Both issues are fundamental for the MAHD application since the latter is based on the intravenous injection of FN Cs into the bloodstream of the patient. Atomic force microscopy and optical microscopy were employed for the investigation of both surface characteristics and overall morphology of blood cells, respectively. Samples of donated blood, where bare Fe3O4 FNs or Fe3O4-BSA Cs were added, were maturated under mild incubation for durations up to 120 min. We investigated two representative temperatures, T=20 oC owing to easy experimental realization, and T=37 oC trying to simulate human body conditions. We did not observe noticeable interference of either bare Fe3O4 FNs or Fe3O4-BSA Cs with RBCs, WBCs and Plts. More importantly we did not observe any degradation of the surface of RBCs and WBCs that were maturated under the presence of bare FNs or Cs in concentrations that strongly exceed the ones used for the treatment of iron-deficiency anaemia. Incidents where either bare FNs or Cs were bound onto the surface of RBCs or internalised by WBCs were very rare. Our observations suggest high biocompatibility of both bare Fe3O4 FNs and Fe3O4-BSA Cs with blood cells, while the solubility depends on the BSA content of the Fe3O4-BSA Cs.
Current Nanoscience 04/2009; 5(2):177-181. · 1.78 Impact Factor
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ABSTRACT: CoPt and FePt compound films with L10 ordered structure have been intensively studied, due to their extremely high uniaxial magnetocrystalline anisotropy which makes them suitable for application in ultra high density magnetic recording media. A basic requirement in these type of media is the development of strong perpendicular magnetic anisotropy. The role of Ag underlayers in promoting strong (001) crystallographic texture and perpendicular magnetic anisotropy in post-annealed Ag/CoPt and Ag/FePt bilayers (BLs) has already been reported, along with a possible correlation between L10 formation and development of (001) crystallographic texture. In this work we present new data, which provide further evidence that there is indeed such a correlation during the annealing process of Ag/CoPt BLs. The most obvious manifestation of this correlation is the fact that the X-ray intensity ratios I001/I002 (used as a measure of the degree of L10-ordering) and I002/I111 (used as a measure of the crystallographic texture) and the coercivity Hc and relative remnant magnetization mr (for field H normal to the surface of the films) exhibit the same kind of dependence from the thickness of the Ag underlayer. Comparison with respective crystallographic data from post-annealed Ag/CoPt nanocomposites (NCs) shows that in the case of NCs the (001) texture starts to degrade for lower total film thickness, compared to the case of BLs. This difference can be attributed to the structural incoherence in the growth of the CoPt grains imposed by the presence of Ag inside the Ag/CoPt NCs, while in BLs Ag is only used as an underlayer. Based on the above data and on detailed Heavy Ion Elastic Rutherford Back Scattering (HIRBS) measurements, performed on post-annealed Ag/CoPt NCs, we propose a possible mechanism for the interpretation of the observed correlation. The proposed mechanism is based on the reduction of total strain (residual strain of as-deposited film and transformation strain due to deformation of the unit cell as L10-CoPt is formed) throughout the annealing process.
Journal of Physics Conference Series 03/2009; 153(1):012060.
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ABSTRACT: We report on the influence of stray fields for both low and high magnetic fields applied parallel to trilayers consisting of a low-T Nb interlayer and two outer Ni Fe layers having in-plane anisotropy. At low magnetic fields these trilayers exhibit a pronounced magnetoresistance effect. Its dynamic transport behavior is presented through detailed I-V characteristics. More importantly, the detailed evolution of the longitudinal and transverse magnetic components of the trilayers is presented from close to well below T . These data clearly show that below T and for low magnetic fields the transport properties of the Nb interlayer are influenced by transverse stray-fields that motivate subsequent transverse magnetic coupling of the outer Ni Fe layers. By generalizing this experimental finding we propose that the generic prerequisite for the occurrence of intense magnetoresistance effects in relative trilayers is that the coercive fields of the outer ferromagnetic layers should almost coincide since the simultaneous occurrence of magnetic domains all over their surface will promote a transverse magnetic coupling mediated by the accompanying transverse stray fields. Finally, the trilayer's upper-critical field exhibits a pronounced suppression for low magnetic fields indicative of a behavior, while for high values the conventional behavior is recovered. A similar process is observed in both Nb-Ni Fe bilayers and Nb single layers. However, significant qualitative and quantitative differences exist between these samples. Based on a mechanism that is motivated by longitudinal stray-fields existing exclusively in the high-field regime we propose a possible interpretation for this experimental finding.
03/2009;
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ABSTRACT: Magnetically assisted hemodialysis is a development of conventional hemodialysis and is based on the circulation of ferromagnetic nanoparticle-targeted binding substance conjugates (FN-TBS Cs) in the bloodstream of the patient and their eventual removal by means of a 'magnetic dialyzer'. Presented here is an in vitro investigation on the biocompatibility of bare Fe(3)O(4) FNs and Fe(3)O(4)-bovine serum albumin Cs with blood cells, namely red blood cells (RBCs), white blood cells (WBCs) and platelets (Plts). Atomic force microscopy (AFM) and optical microscopy (OM) enabled the examination of blood cells at the nanometer and micrometer level, respectively. The observations made on FN- and C-maturated blood samples are contrasted to those obtained on FN- and C-free reference blood samples subjected to exactly the same maturation procedure. Qualitatively, both AFM and OM revealed no changes in the overall shape of RBCs, WBCs and Plts. Incidents where bare FNs or Cs were bound onto the surface of RBCs or internalized by WBCs were very rare. Detailed examination by means of OM proved that impaired coagulation of Plts is not initiated/promoted either by FNs or Cs. Quantitatively, the statistical analysis of the obtained AFM images from RBC surfaces clearly revealed that the mean surface roughness of RBCs maturated with bare FNs or Cs was identical to the one of reference RBCs.
Nanotechnology 12/2008; 19(50):505101. · 3.98 Impact Factor
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ABSTRACT: By using x-ray diffraction, magnetization and Mössbauer spectroscopy techniques we have studied the magnetoelectric Al2−xFexO3 (x = 0.8,0.9 and 1.0) compound. Ac-susceptibility and magnetization measurements revealed magnetic transitions at TN = 180,210 and 260 K for x = 0.8, 0.9 and 1.0 respectively, that can be attributed to the Néel temperatures of ferrimagnetic to paramagnetic phase transition for all samples. Mössbauer spectra for the three samples were recorded between 4.2 and 295 K. Above the Néel temperature the paramagnetic spectra can be analyzed by three quadrupole doublets associated with the octahedral Fe1, Fe2 and Fe4 sites. The values of the hyperfine parameters show that iron ions are in the high spin Fe3+ state. The spectrum area of the doublet with larger quadrupole splitting increases with x, and in combination with x-ray diffraction results it can be attributed to the iron which occupies the Fe4 site. Below TN(x) the Mössbauer spectra are magnetically split and at T = 4.2 K consist of six broad lines, indicating either a hyperfine magnetic field distribution (P(Hhyp)) or that the three octahedral sites give three unresolved sextets. The most probable value of Hhyp (the maximum value of P(Hhyp)) follows a power law indicative of a second order transition, in agreement with ac-susceptibility and magnetization measurements. The width of P(Hhyp) increases drastically toward low hyperfine magnetic fields as temperature increases. In addition, an appreciable percentage of the iron nuclei sense a hyperfine field with values in the interval [0,Hmax]. This behavior can be explained by assuming that several magnetic sites with different superexchange parameters exist.
Journal of Physics Condensed Matter 09/2008; 20(41):415222. · 2.55 Impact Factor
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ABSTRACT: The in vitro utilization of biocompatible ferromagnetic nanoparticles (BFNs) in hemodialysis (HD), routinely used today for the treatment of end stage renal disease (ESRD), is introduced in this work. The proposed strategy is termed magnetically assisted hemodialysis (MAHD) and it aims to become a more efficient development of conventional HD. The method is based on the production of biocompatible ferromagnetic nanoparticles-targeted binding substances conjugates (BFNs-TBSs Cs) constructed of BFNs and specifically designed TBSs that should have high affinity and binding capacity for target toxic substances (TTSs) which must be removed from the ESRD patient subjected to HD. Antibodies or even specific proteins could serve as the TBS of the desired BFNs-TBSs Cs. The BFNs-TBSs Cs should be administered to the patient timely prior to the MAHD session so as to bind with the desired TTSs during their free circulation in the vascular network. Eventually, the complete BFNs-TBSs-TTSs structure can be selectively removed during the MAHD session by means of an external inhomogeneous magnetic field that is applied either at the dialyzer or at other collection point(s) along the blood circulation line of the dialysis machine. The advantages of MAHD over conventional HD regarding the patient's comfort and overall health status are discussed in detail among practical issues. To examine this proposition we employed Fe(3)O(4) and bovine serum albumin (BSA) as the BFN and the TBS constituents respectively, since they are both highly biocompatible. By means of x-ray diffraction, atomic force microscopy, circular dichroism spectropolarimetry, UV-vis spectrophotometry, SQUID magnetometry, and nuclear magnetic resonance we evaluated (i) the structural/morphological characteristics, (ii) the magnetic retraction efficiency, and most importantly (iii) the toxin binding affinity and capacity of both bare Fe(3)O(4) BFNs and Fe(3)O(4)-BSA Cs by performing in vitro experiments on specific TTSs. Homocysteine and p-cresol were chosen as representative TTSs and were investigated in great detail. The results obtained prove the in vitro applicability of the proposed MAHD method. Corrections were made to this article on 6 November 2007 (see figure 10 caption and lines 10 and 11 of page 11). The corrected electronic version is identical to the print version.
Nanotechnology 12/2007; 18(49):495102. · 3.98 Impact Factor
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ABSTRACT: It is generally believed that superconductivity and magnetism are two antagonistic long-range phenomena. However, as it was preliminarily highlighted in D. Stamopoulos et al. [Phys. Rev. B 75, 014501 (2007)] and extensively studied in this work under specific circumstances these phenomena instead of being detrimental to each other may even become cooperative so that their synergy may promote the superconducting properties of a hybrid structure. Here, we have studied systematically the magnetic and transport behavior of such exchange biased hybrids that are comprised of ferromagnetic (FM) Ni80Fe20 and low-Tc superconducting (SC) Nb for the case where the magnetic field is applied parallel to the specimens. Two structures have been studied: FM-SC-FM trilayers (TLs) and FM-SC bilayers (BLs). Detailed magnetization data on the longitudinal and transverse magnetic components are presented for both the normal and superconducting states. These data are compared to systematic transport measurements including I-V characteristics. The comparison of the exchange biased BLs and TLs that are studied here with the plain ones studied in D. Stamopoulos et al. [Phys. Rev. B 75, 184504 (2007)] enable us to reveal an underlying parameter that may falsify the interpretation of the transport properties of relevant FM-SC-FM TLs and FM-SC BLs investigated in the recent literature: the underlying mechanism motivating the extreme magnetoresistance peaks in the TLs relates to the suppression of superconductivity mainly due to the magnetic coupling of the two FM layers as the out-of-plane rotation of their magnetizations takes place across the coercive field where stray fields emerge in their whole surface owing to the multidomain magnetic state that they acquire. Comment: 20 pages, 20 figures
10/2007;
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ABSTRACT: We report on the transport and magnetic properties of hybrid trilayers and bilayers that consist of low spin-polarized Ni80Fe20 exhibiting in-plane but no uniaxial anisotropy and low-Tc Nb. We reveal a magnetoresistance effect that is pronounced. In our trilayers the magnetoresistance exhibits an increase of two orders of magnitude when the superconducting state is reached: from the conventional normal-state values 0.6 % it goes up to 1000 % for temperatures below Tc. In contrast, in the bilayers the effect is only minor since from 3% in the normal state increases only to 70 % for temperatures below Tc. Magnetization data of both the longitudinal and transverse magnetic components are presented. Most importantly, we present data not only for the normal state of Nb but also in its superconducting state. Strikingly, these data show that below its Tc SC the Nb interlayer under the influence of the outer Ni80Fe20 layers attains a magnetization component transverse to the external field. By comparing the transport and magnetization data we propose a candidate mechanism that could motivate the pronounced magnetoresistance effect observed in the trilayers. Adequate magnetostatic coupling of the outer Ni80Fe20 layers is motivated by stray fields that emerge naturally in their whole surface due to the multidomain magnetic structure that they attain near coercivity. Atomic force microscopy is employed in order to examine the possibility that such magnetostatic coupling could be promoted by interface roughness. Referring to the bilayers, although out-of-plane rotation of the magnetization of the single Ni80Fe20 layer is still observed, in these structures magnetostatic coupling does not occur due to the absence of a second Ni80Fe20 one so that the observed magnetoresistance peaks are only modest.
06/2007;
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ABSTRACT: In this work we study the transport properties of hybrids that consist of exchange biased ferromagnets (FMs) combined with a low-Tc superconductor (SC). Not only different FMs but also various structural topologies have been investigated: results for multilayers of La(1-x)CaxMnO3 combined with Nb in the form of [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3]15/Nb, and for more simple Ni80Fe20/Nb/Ni80Fe20 trilayers and Ni80Fe20/Nb bilayers are presented. The results obtained in all hybrid structures studied in this work clearly uncover that the exchange bias mechanism promotes superconductivity. Our findings assist the understanding of the contradictory results that have been reported in the recent literature regarding the transport properties of relative FM/SC/FM spin valves.
02/2007;
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D. Stamopoulos
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ABSTRACT: In this work we study the transport properties of hybrids that consist of exchange biased ferromagnets (FMs) combined with a low-Tc superconductor (SC). Not only different FMs but also various structural topologies have been investigated: results for multilayers of La1−xCaxMnO3 combined with Nb in the form of [La0.33Ca0.67MnO3∕La0.60Ca0.40MnO3]15∕Nb, and for more simple Ni80Fe20∕Nb∕Ni80Fe20 trilayers and Ni80Fe20∕Nb bilayers are presented. The results obtained in all hybrid structures studied in this work clearly uncover that the exchange bias mechanism promotes superconductivity. Our findings assist the understanding of the contradictory results that have been reported in the recent literature regarding the transport properties of relative FM/SC/FM spin valves.
Phys. Rev. B. 01/2007; 75(1).
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ABSTRACT: In the present work we study experimentally the influence that the domain structure of a fer- romagnet (FM) has on the properties of a superconductor (SC) in bilayers and multilayers of La0.60Ca0.40MnO3/Nb and FePt/Nb proximity hybrids. Specific experimental protocols that were employed in the performed magnetization measurements enabled us to directly uncover a generic property of FM/SC hybrids: in the absence of an external magnetic field, the multidomain struc- ture of the FM promotes the nucleation of superconductivity, while its monodomain state strongly suppresses it. Our experimental findings support recent theoretical studies proposing that when an inhomogeneous exchange field is offered by the FM to the SC the superconducting pairs are not susceptible to pair-breaking.
05/2006;
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D. Stamopoulos
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ABSTRACT: In the present work we study experimentally the influence that the domain structure of a ferromagnet (FM) has on the properties of a superconductor (SC) in bilayers and multilayers of La0.60Ca0.40MnO3∕Nb and FePt∕Nb proximity hybrids. Specific experimental protocols that were employed in the performed magnetization measurements enabled us to directly uncover a generic property of FM/SC hybrids: In the absence of an external magnetic field, the multidomain structure of the FM promotes the nucleation of superconductivity, while its monodomain state strongly suppresses it. Our experimental findings support recent theoretical studies [ A. I. Buzdin and A. S. Mel’nikov Phys. Rev. B 67 020503 (2003) T. Champel and M. Eschrig Phys. Rev. B 71 220506 (2005)] that suggest the formation of the so-called domain-wall superconductivity and propose that when an inhomogeneous exchange field is offered by the FM to the SC the superconducting pairs are not susceptible to pairbreaking. In contrast, when magnetic homogeneity is restored in the FM the SC’s properties are strongly suppressed.
Phys. Rev. B. 04/2006; 73(13).
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G Papavassiliou,
M Pissas,
G Diamantopoulos,
M Belesi,
M Fardis, D Stamopoulos,
A G Kontos,
M Hennion,
J Dolinsek,
J-Ph Ansermet,
C Dimitropoulos
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ABSTRACT: By using nuclear magnetic resonance techniques we show that for T<30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.
Physical Review Letters 03/2006; 96(9):097201. · 7.37 Impact Factor
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ABSTRACT: In this work we present magnetization data on hybrids consisting of multilayers (MLs) of man- ganites [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3]15 in contact with a low-Tc Nb superconductor (SC). Although a pure SC should behave diamagnetically in respect to the external magnetic field in our ML-SC hybrids we observed that the magnetization of the SC follows that of the ML. Our intriguing experimental results show that the SC below its TSC c becomes ferromagnetically coupled to the ML. As a result in the regime where diamagnetic behaviour of the SC was expected its bulk magne- tization switches only whenever the coercive field of the ML is exceeded. By employing specific experiments where the ML was selectively biased or not we demonstrate that the ML inflicts its magnetic properties on the whole hybrid. Possible explanations are discussed in connection to recent theoretical proposals and experimental findings that were obtained in relative hybrids. Comment: To appear in Phys. Rev. B
11/2005;
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ABSTRACT: We studied by M\"{o}ssbauer spectroscopy the Na$_{0.82}$CoO$_2$ compound using 1% $^{57}$Fe as a local probe which substitutes for the Co ions. M\"{o}ssbauer spectra at T=300 K revealed two sites which correspond to Fe$^{3+}$ and Fe$^{4+}$. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co$^{+3}$, Co$^{+4}$ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe$^{4+}$ and all of Fe$^{3+}$, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.
04/2005;
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ABSTRACT: We studied the crystal and magnetic structure of the La1−xCaxMnO3 compound for (0.11⩽x⩽0.175) using stoichiometric samples. For x<0.13 the system’s ground state is insulating canted antiferromagnetic. For 0.13⩽x⩽0.175 below the Jahn-Teller transition temperature (TJT) the crystal structure undergoes a monoclinic distortion. The crystal structure can be described with P21∕c space group which permits two Mn sites. The unit-cell strain parameter s=2(a−c)∕(a+c) increases for T<TJT, taking the maximum value at the Curie point, and then decreases. Below TM∕M∕∕≈60 K s abruptly changes slope and finally approaches T=0 K with nearly zero slope. The change of s at TM∕M∕∕ is connected to a characteristic feature in the magnetic measurements. As x increases towards the ferromagnetic metallic boundary, although s is reduced appreciably, the monoclinic structure is preserved. The monoclinic structure is discussed with relation to the orbital ordering, which can produce the ferromagnetic insulating ground state. We also studied samples that were prepared in air atmosphere. This category of samples shows ferromagnetic insulating behavior without following the particular variation of the s parameter. The crystal structure of these samples is related to the so-called O* (c>a>b∕√2) structure.
Phys. Rev. B. 03/2005; 72(6).
