Showing papers on "Magnetization published in 1998"
TL;DR: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K as mentioned in this paper.
Abstract: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K. The Curie temperature determined from inflection points of the experimental magnetic susceptibility and heat capacity is in excellent agreement with those obtained from the magnetocaloric effect and Arrot plots. Above 2 T the temperature of this transition increases almost linearly with the magnetic field at a rate of $\ensuremath{\sim}6\mathrm{K}/\mathrm{T}$ in fields up to 7.5 T. The spin reorientation transition, which occurs at 227(2) K in the absence of a magnetic field, has been confirmed by susceptibility, magnetization, and heat-capacity measurements. Magnetic fields higher than 2--2.5 T apparently quench the spin reorientation transition and Gd retains its simple ferromagnetic structure from the ${T}_{C}(H)$ down to $\ensuremath{\sim}4\mathrm{K}.$ The nature of anomaly at $T\ensuremath{\cong}132\mathrm{K},$ which is apparent from ac susceptibility measurements along the $c$ axis, is discussed. The presence of large amounts of interstitial impurities lowers the second-order $\mathrm{paramagnetic}\ensuremath{\leftrightarrow}\mathrm{ferromagnetic}$ transition temperature, and can cause some erroneous results in the magnetocaloric effect determined in pulsed magnetic fields. The magnetocaloric effect was studied utilizing the same samples by three experimental techniques: direct measurements of the adiabatic temperature rise, magnetization, and heat capacity. All three techniques, with one exception, yield the same results within the limits of experimental error.
862 citations
TL;DR: In this paper, the contribution of the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behavior of ultrathin single films is reviewed.
Abstract: The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.
780 citations
TL;DR: In this paper, a simple phenomenological model for the magnetization process and field-induced strain by twin-boundary and phaseboundary motion is proposed for both the strong and weak anisotropy cases.
Abstract: The large magnetic-field-induced strains observed in martensitic phases based on Ni2MnGa and in other magnetic shape memory alloys are believed to arise from a process of twin-boundary motion rather than magnetostriction. The dependence of strain on magnetization, e(M), generally shows a large component that is linear (rather than quadratic) in M below saturation (quadratic dependence being typical of magnetostrictive strain). A simple phenomenological model for the magnetization process and field-induced strain by twin-boundary and phase-boundary motion is proposed for both the strong and weak anisotropy cases. The model is shown to account for the nearly linear dependence of strain on magnetization in the martensitic phases of these materials. It shows the field dependence of the magnetization and strain to be functions of an effective stiffness constant, C, the transformation strain, e0, and the magnetic anisotropy of the martensitic phase, Ku, through two reduced field parameters, he=MsH/Ce02 and ha=M...
632 citations
TL;DR: In this article, two colloids (Coll-I and Coll-II) have been obtained by changing the organometallic concentration in the polymer and the particle size deduced from analyses of the magnetic susceptibilities and magnetization curves are consistent with those measured by high-resolution transmission electronic microscopy (HRTEM).
Abstract: Monodispersed nanoparticles of cobalt have been prepared by an original method using the decomposition under hydrogen of an organometallic precursor in the presence of a stabilizing polymer. Two colloids (Coll-I and Coll-II) have been obtained by changing the organometallic concentration in the polymer. Observation by high-resolution transmission electronic microscopy (HRTEM) showed Co particles well isolated and regularly dispersed in the polymer with a very narrow size distribution centered around 1.5 nm (Coll-I) and 2 nm (Coll-II) diameter. These particles are superparamagnetic above the blocking temperature 9 K (Coll-I) and 13.5 K (Coll-II). The particle size deduced from the analyses of the magnetic susceptibilities and magnetization curves are consistent with those measured by HRTEM. Magnetization at 5 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom for both samples, where $〈{\ensuremath{\mu}}_{\mathrm{Co}}〉=1.94\ifmmode\pm\else\textpm\fi{}0.05$ ${\ensuremath{\mu}}_{B}$ for the smallest particles. High-field magnetization measurements, up to 35 T, increases nearly linearly with the applied field. This is equivalent to an increase of the mean magnetic moment with $〈{\ensuremath{\mu}}_{\mathrm{Co}}〉=2.1\ifmmode\pm\else\textpm\fi{}0.1$ ${\ensuremath{\mu}}_{B}$ at 35 T for the smallest particles. The effective magnetic anisotropies are found to be larger than that of the bulk materials and decrease with increasing particle size. This set of data allows us to conclude that the enhanced magnetization, its increase with applied magnetic field, and the enhanced effective magnetic anisotropy are associated with the large influence of the surface atoms and are more significant with decreasing size.
496 citations
TL;DR: In this article, the authors measured the equilibrium length scale of dynamic heterogeneities close to the glass transition by means of a new multidimensional NMR experiment, which was obtained from a proton spin diffusion experiment combined with two 2D exchange sequences via appropriate back and forth transfer of magnetization between the two spins.
Abstract: We directly measure the equilibrium length scale of dynamic heterogeneities close to the glass transition by means of a new multidimensional NMR experiment. The spatial information is gained from a proton spin diffusion experiment combined with two 2D ${}^{13}\mathrm{C}$ exchange sequences via appropriate back and forth transfer of magnetization between ${}^{13}\mathrm{C}$ and ${}^{1}\mathrm{H}$ spins. For poly(vinyl acetate) at 10 K above the glass transition we detected a length scale of $3\ifmmode\pm\else\textpm\fi{}1\mathrm{nm}$.
490 citations
Book•
01 Jan 1998
TL;DR: The Magnetization Process: Coherent Rotation and Domain Wall Motion as discussed by the authors The Magnetic Hysteresis in Preisach Systems: Collections of Bistable Units and Vector Relations.
Abstract: Introduction: Magnetic Hysteresis. Types of Hysteresis. Maxwells Equations and Thermodynamics: Maxwells Equations in Magnetic Media. Magnetic Work and Thermodynamics. Magnetic Free Energy: Exchange and Anisotropy. Micromagnetics. Magnetic Domains and Domain Walls. The Magnetization Process: Coherent Rotation. Domain Wall Motion. Magnetization Curves. Coercivity Mechanisms. Eddy Currents. Preisach Systems: Collections of Bistable Units. Hysteresis in Preisach Systems. Appendixes: Systems of Units. Vector Relations. Reciprocity Theorems. Micromagnetic Parameters. Stochastic Processes. Bibliography. Index.
484 citations
TL;DR: In this article, the magnetic properties of a new class of nanocrystalline magnets are described, which are based on the α- and α′-FeCo phases, and offer large magnetic inductions to elevated temperatures.
Abstract: The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the ...
425 citations
TL;DR: Off-axis electron holography in the transmission electron microscope was used to correlate the physical and magnetic microstructure of magnetite nanocrystals in magnetotactic bacteria to find shape anisotropy was found to dominate magnetocrystalline an isotropy in elongated crystals.
Abstract: Off-axis electron holography in the transmission electron microscope was used to correlate the physical and magnetic microstructure of magnetite nanocrystals in magnetotactic bacteria. The magnetite crystals were all single magnetic domains, and the magnetization directions of small superparamagnetic crystals were constrained by magnetic interactions with larger crystals in the chains. Shape anisotropy was found to dominate magnetocrystalline anisotropy in elongated crystals. A coercive field between 300 and 450 oersted was determined for one chain.
404 citations
TL;DR: In this paper, the magnetization of a conducting ferromagnet in the presence of a spin-polarized current is derived and the current effects enter in the form of a topological term in the Landau-Lifshitz equation.
Abstract: We derive a continuum equation for the magnetization of a conducting ferromagnet in the presence of a spin-polarized current. Current effects enter in the form of a topological term in the Landau-Lifshitz equation. In the stationary situation the problem maps onto the motion of a classical charged particle in the field of a magnetic monopole. The spatial dependence of the magnetization is calculated for a one-dimensional geometry and suggestions for experimental observation are made. We also consider time-dependent solutions and predict a spin-wave instability for large currents.
390 citations
TL;DR: In this paper, the temperature dependence of spin-polarized tunneling is investigated between 77 and 420 K for various ferromagnetic tunnel junctions, and the dominant one is elastic, spinpolarised tunneling between the two spin electrodes, each with an electron polarization $P$ that decreases with increasing temperature.
Abstract: The temperature dependence of spin-polarized tunneling is investigated between 77 and 420 K for various ferromagnetic tunnel junctions. Both the junction resistance and the magnetoresistance decrease with increasing temperature $T.$ The experimental results are successfully described by a model that includes two current contributions. The dominant one is elastic, spin-polarized tunneling between the two ferromagnetic electrodes, each with an electron polarization $P$ that decreases with $T$ due to thermally excited spin waves according to $P\ensuremath{\propto}(1\ensuremath{-}\ensuremath{\alpha}{T}^{3/2}),$ i.e., in the same way as the surface magnetization. A smaller second conductance is due to assisted, spin-independent tunneling which we find to be proportional to ${T}^{1.35\ifmmode\pm\else\textpm\fi{}0.15}.$
360 citations
TL;DR: In this article, the magnetoresistance behavior of Fe3O4 in polycrystalline thin film, powder compact, and single-crystal form is compared and it is shown that negative magnetoreduction with peaks at the coercive field, observed in thin films and powder compacts but not in the single crystal, is due to fieldinduced alignment of the magnetization of contiguous grains.
Abstract: The magnetoresistance behavior of Fe3O4 in polycrystalline thin film, powder compact, and single-crystal form are compared. Negative magnetoresistance with peaks at the coercive field, observed in thin films and powder compacts but not in the single crystal, is due to field-induced alignment of the magnetization of contiguous grains. The effect is associated with intergranular transport of spin-polarized electrons.
TL;DR: In this paper, the amorphous nature of CoFe2O4 particles was confirmed by various techniques, such as scanning and transmission electron microscopy (SEM and TEM), electron microdiffraction, and X-ray diffractograms.
Abstract: Nanostructured CoFe2O4 particles were prepared by a sonochemical approach, first by preparation of the amorphous precursor powders, followed by heat treatment at relatively very low temperatures. The precursor was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)5 and Co(NO)(CO)3, in Decalin at 273 K, under an oxygen pressure of 100−150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as scanning and transmission electron microscopy (SEM and TEM), electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mossbauer, and electron paramagnetic resonance (EPR) spectral studies indicated that the as-prepared amorphous particles were superparamagnetic. The Mossbauer parameters and the significantly low (45 emu/g) observed saturation of magnetization of the annealed sample, compared to that of the bulk sample (72 emu/g), reflected its nanocrystalline nature.
TL;DR: In this paper, the electrochemical preparation of a CoNiFe film with a very high value of Bs (2.0 −2.1 T) was described, which can find applications in miniaturization of electromechanical devices and in high-density magnetic data storage.
Abstract: Magnetic materials are classed as ‘soft’ if they have a low coercivity (the critical field strength Hc required to flip the direction of magnetization). Soft magnetic materials are a central component of electromagnetic devices such as step motors, magnetic sensors, transformers and magnetic recording heads. Miniaturization of these devices requires materials that can develop higher saturation flux density, Bs, so that the necessary flux densities can be preserved on reducing device dimensions, while simultaneously achieving a low coercivity. Common high-Bs soft magnetic films currently in use are electroplated CoFe-based alloys1,2,3,4 electroplated CoNiFe alloys5,6,7 and sputtered Fe-based nanocrystalline8,9,10,11 and FeN films12,13,14. Sputtering is not suitable, however, for fabricating the thick films needed in some applications, for which electrochemical methods are preferred. Here we report the electrochemical preparation of a CoNiFe film with a very high value of Bs (2.0–2.1 T) and a low coercivity. The favourable properties are achieved by avoiding the need for organic additives in the deposition process, which are typically used to reduce internal stresses. Our films also undergo very small magnetostriction, which is essential to ensure that they are not stressed when an external magnetic field is applied (or conversely, that external stresses do not disrupt the magnetic properties). Our material should find applications in miniaturization of electromechanical devices and in high-density magnetic data storage.
TL;DR: In this article, the crystal structures and magnetic properties of a series of magnetic compounds, MII{N(CN)2}2, where M=Cu (1), Ni (2), Co (3) and Fe (4), were presented.
Abstract: We present the crystal structures and magnetic properties of a series of magnetic compounds, MII{N(CN)2}2, where M=Cu (1), Ni (2), Co (3) and Fe (4), and [Mn{N(CN)2}2(C2H5OH)2]Z·(CH3)2CO (5). In the isostructural compounds 1–4, the dicyanamide anion is triply coordinating through its three nitrogen atoms. It bridges the metal ions to form infinite 3D metal-organic frameworks with a rutile-type structure. The framework contains doubly bridged M(–Nâ–·C–N–Câ–·N–)2 ribbons that link approximately orthogonally through the amide nitrogen atoms. The Jahn–Teller distortion in 1 has a strong influence on the packing arrangement (M–N bond lengths: 1.98 and 2.47 A for 1 and 2.10 and 2.15 A for 3). On lowering the temperature the bond distances in 1 remain unchanged except for a decrease of the M–Namide length to 2.45 A. Magnetic data for 1 obey the Curie–Weiss law (Θ=-2.1 K). 2 and 3 are ferromagnets with Curie temperatures (TC) of 9 and 21 K and are characterized by hysteresis loops of 710 and 7975 Oe at 2 K, remnant magnetization, magnetization approaching the expected saturation (gS) of 2 and 3 µB in high field, absorptive component (χ″) in the AC magnetization and λ peak in the heat capacity data. 4 is similarly characterized and shows behaviour that is characteristic of a canted antiferromagnet: the Weiss constant is temperature dependent (+3 K in the range 200–300 K), there is a sharper peak than for 1 or 2 in the AC magnetization and the isothermal magnetization at 3 K increases monotonically to ≈1.3 µB (expected to be 4 µB for ferromagnetic alignment of the spins) in a field of 8 T. Its coercive field (17800 Oe) is the largest observed for any metal-organic compound and exceeds those of alloys of SmCo5 and Nd2Fe14B. The maximum energy product (B · H) is the highest for 3 and is comparable to alloys of Sm–Co. We attribute the large coercive field to a combination of single ion and particle shape anisotropies. 5 is paramagnetic at high temperature with Θ=-3 K. Below 16 K it behaves as a canted antiferromagnet with a very weak resultant spontaneous magnetization.
TL;DR: In this article, the superparamagnetic properties of the MgFe2O4 spinel ferrite nanoparticles with the particle size from about 6 to 18 nm were studied.
Abstract: Superparamagnetism is a unique and important aspect of magnetism in nanoparticles. The superparamagnetic properties of the MgFe2O4 spinel ferrite nanoparticles with the particle size from about 6 to 18 nm are studied. The blocking temperature is a function of the particle size and increases with increasing particle size. The coercivity of MgFe2O4 nanoparticles also is a function of the particle size below the blocking temperature. When the temperature rises above the blocking temperature, the nanoparticles show nonhysteresis magnetization behaviors. With these interesting superparamagnetic properties, MgFe2O4 nanoparticles have potentials for applications such as ferrofluids, magnetocaloric refrigeration, and the contrast agents for magnetic resonance imaging.
TL;DR: In this paper, the effect of exchange coupling on the coercive field was determined within the framework of the nucleation model from the temperature dependence of the coercive fields, and reversibility measurements of the demagnetization curves in the second quadrant gave important information about the magnetization processes in exchange coupled magnets.
TL;DR: The magnetic domain structure and magnetization curves of chemically ordered epitaxial FePt (001) films with perpendicular magnetic anisotropy are discussed in this article, where films were dc magnetron sputtered from a Fe50Pt50 alloy target onto Pt seeded MgO (001), at substrate temperatures of 550 °C.
Abstract: The magnetic domain structure and magnetization curves of chemically ordered epitaxial FePt (001) films with perpendicular magnetic anisotropy are discussed. Films were dc magnetron sputtered from a Fe50Pt50 alloy target onto Pt seeded MgO (001) at substrate temperatures of 550 °C. The thickness of the FePt layers was varied between 18 and 170 nm. Specular and grazing incidence x-ray diffraction measurements confirm the presence of the anisotropic, face centered tetragonal (L10) crystal structure. Long range chemical order parameters of up to 0.95 and small mosaic spread, similar to results reported for FePt (001) films grown by molecular beam epitaxy. For film thicknesses ⩾50 nm in-plane and out-of-plane hysteresis measurements indicate large perpendicular magnetic anisotropies and at the same time low (about 10%) perpendicular remanence. Magnetic force microscopy reveals highly interconnected perpendicular stripe domain patterns. From their characteristic widths, which are strongly dependent on the film...
TL;DR: In this paper, the authors investigated the magnetic-field-induced phase transitions of the charge carriers and found that the destruction of the real-space ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect.
Abstract: We have investigated the magnetic-field-induced phase transitions of ${R}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ($R=\mathrm{Pr}$ and Nd, $x=0.50,$ 0.45 and 0.50, 0.45, 0.40) by measurements of magnetization, magnetoresistance, and magnetostriction utilizing a nondestructive long-pulse magnet (generating up to 40 T). We observed processes where magnetic fields destroy the real-space ordering of the charge carriers and cause insulator-to-metal phase transitions over the whole temperature region below about 250 K. We found that the destruction of the charge ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect. The different profiles of the temperature vs transition field curve depending on the carrier concentration $x$ may be ascribed to the difference in the entropy between the commensurate and the discommensurate charge-ordered state. It turned out that the stability of the charge-ordered state is strongly correlated with the colinear antiferromagnetic ordering of the localized Mn moments.
TL;DR: In this paper, a dc magnetization study of the critical phenomena in a high-quality single crystal of the double-exchange ferromagnet was performed and it was shown that the magnetization-field-temperature behavior below and above the critical temperature obeys scaling.
Abstract: We report a dc magnetization study of the critical phenomena in a high-quality single crystal of the double-exchange ferromagnet ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$. We have determined the critical temperature $({T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}354.0\ifmmode\pm\else\textpm\fi{}0.2\mathrm{K})$ and the critical exponents, $\ensuremath{\beta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.37\ifmmode\pm\else\textpm\fi{}0.04$ and $\ensuremath{\gamma}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.22\ifmmode\pm\else\textpm\fi{}0.03$. Here $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ are the critical exponents for the temperature dependence of the spontaneous magnetization just below ${T}_{C}$ and initial susceptibility just above ${T}_{C}$. Using these values of $\ensuremath{\beta}$, $\ensuremath{\gamma}$, and ${T}_{C}$, we find that the magnetization-field-temperature $(M\ensuremath{-}H\ensuremath{-}T)$ behavior below and above ${T}_{C}$ obeys scaling, following a single equation of state in which $M/({1\ensuremath{-}T/T}_{C}{)}^{\ensuremath{\beta}}$ is uniquely related to $H/({1\ensuremath{-}T/T}_{C}{)}^{\ensuremath{\beta}+\ensuremath{\gamma}}$.
TL;DR: In this paper, the authors demonstrate how elastic strain can alter the magnetic and electrical properties of single-domain epitaxial thin films (1000 A thick) on vicinal (001) SrTiO3 substrates.
Abstract: By lifting an epitaxial thin film off its growth substrate, we directly and quantitatively demonstrate how elastic strain can alter the magnetic and electrical properties of single-domain epitaxial SrRuO3 thin films (1000 A thick) on vicinal (001) SrTiO3 substrates. Free-standing films were then obtained by selective chemical etching of the SrTiO3. X-ray diffraction analysis shows that the free-standing films are strain free, whereas the original as-grown films on SrTiO3 substrates are strained due to the lattice mismatch at the growth interface. Relaxation of the lattice strain resulted in a 10 K increase in the Curie temperature to 160 K, and a 20% increase in the saturation magnetic moment to 1.45 μB/Ru atom. Both values for the free-standing films are the same as that of the bulk single crystals. Our results provide direct evidence of the crucial role of the strain effect in determining the properties of the technologically important perovskite epitaxial thin films.
TL;DR: In this paper, the current density in type-II superconductor circular disks of arbitrary thickness, or cylinders of finite length, in an axial magnetic field is calculated from first principles by treating the super-conductor as a conductor with nonlinear resistivity or with linear complex resistivity, both caused by thermally activated depinning of Abrikosov vortices.
Abstract: The current density in type-II superconductor circular disks of arbitrary thickness, or cylinders of finite length, in an axial magnetic field is calculated from first principles by treating the superconductor as a conductor with nonlinear resistivity or with linear complex resistivity, both caused by thermally activated depinning of Abrikosov vortices. From these currents follows the magnetic field inside and outside the specimen and the magnetic moment, which in its turn determines the nonlinear and linear ac susceptibilities. The magnetization loops and nonlinear ac susceptibilities are obtained directly by time integration of an integral equation for the current density, which does not require any cutoff or approximation of the magnetic field outside the cylinder. With increasing thickness the results go over from the recently obtained solutions for thin disks in a perpendicular field to the classical behavior of long cylinders in a parallel field. Here this direct method is applied to homogeneous disks with constant thickness, but it applies to any axially symmetric superconductor with arbitrary cross section and inhomogeneity.
TL;DR: In this paper, a plane-wave pseudopotential scheme for noncollinear magnetic structures was developed, based on a generalized local spin-density theory in which the direction of the magnetization is a continuous variable of position.
Abstract: We develop a plane-wave pseudopotential scheme for noncollinear magnetic structures, based on a generalized local spin-density theory in which the direction of the magnetization is a continuous variable of position. We allow the atomic and magnetic structures to relax simultaneously and self-consistently. Application to small Fe clusters yields noncollinear magnetic structures for Fe-3 and Fe-5. The components of the magnetization density vary smoothly with position. The spin direction undergoes sizable changes only in the regions of small charge and spin density between the atoms and is generally uniform in the magnetic regions of the atoms.
TL;DR: In this paper, an Fe2O3−SiO2 composite was prepared by a gelation method that adopts tetraethoxysilane and iron(III) nitrate as starting materials.
Abstract: An Fe2O3−SiO2 composite was prepared by a gelation method that adopts tetraethoxysilane and iron(III) nitrate as starting materials. The dried gel was treated at increasing temperatures, and the samples were characterized by XRD, TEM, magnetic susceptibility measurements, and EPR and Mossbauer spectroscopies. Nanometer size (3−4 nm) X-ray-amorphous iron(III) oxide particles are observed in the samples treated at low temperature. These particles display superparamagnetic behavior in the Mossbauer spectra and susceptibility measurements, and their magnetic moments indicate antiferromagnetic clustering. The occurrence of two sites for iron ions, one in the bulk and one on the surface of nanoparticles, is suggested by EPR and Mossbauer spectroscopies. Heating of the samples to higher temperatures (T > 700 °C) gives rise to a small increase of the particle size. Simultaneously XRD and TEM exhibit the formation of γ-Fe2O3 crystalline particles, Mossbauer spectra reveal a large change in the magnetization, magne...
TL;DR: In this paper, a two-step silica-coating process, sol−gel followed by dense liquid coatings, is described, which produces magnetic nanocomposites of the highest protection against acid attack at the lowest level of silica coatings with maximized magnetization.
Abstract: A novel two-step silica-coating process, sol−gel followed by dense liquid coatings, is described. The maghemite surfaces coated with silica, using the two-step process, are characterized by X-ray photoelectron spectroscopy, electrokinetics, leaching test, and magnetization measurement. Compared with the single-step sol−gel or dense-liquid-coating process, the two-step process produces magnetic nanocomposites of the highest protection against acid attack at the lowest level of silica coatings with maximized magnetization.
TL;DR: In this paper, a cubic-type magnetocrystalline anisotropy was found in CoFe 2 O 4 fine powders with particle sizes between 43 A (4.3 ) and 465 A (46.5 ).
TL;DR: In this paper, high-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a singlemolecule magnet.
Abstract: The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜz2) with D = −0.53 cm-1 and a quartic zero-field splitting (B40O40)with B40 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by...
TL;DR: In this paper, the authors reported the observation of multiple temperature-induced magnetization reversals in YVO3 and proposed a possible mechanism involving a change in orbital ordering which may be generic to a broad class of transition metal oxides.
Abstract: The total energy of a magnet in a magnetic field is lowest when the magnetic moment is aligned parallel to the magnetic field. Once aligned, the magnetic moment can be reversed by applying a sufficiently large field in the opposite direction. These properties form the basis of most magnetic recording and storage devices. But the phenomenon of magnetization reversal in response to a change in temperature (in a small magnetic field) is rarer. This effect occurs in some ferrimagnetic materials consisting of two or more types of antiferromagnetically ordered magnetic ions1, and forms the operational basis of ferrimagnetic insulators. Here we report the observation of multiple temperature-induced magnetization reversals in YVO3. The net magnetic moment is caused by a tilting of the antiferromagnetically aligned moments of (crystallographically identical) V3+ ions, due to orthorhombic distortion in the crystal structure. We observe an abrupt switching at 77 K associated with a first-order structural phase transition, and a gradual reversal at ∼95 K without an accompanying structural change. The magnetization always reverses if the crystal is cooled or warmed through these two temperatures in modest fields. We propose a possible mechanism involving a change in orbital ordering which may be generic to a broad class of transition metal oxides.
TL;DR: In this article, the intrinsic resistivity of high-quality epitaxial films in the paramagnetic phase has a thermally activated form, whereas the extrinsic resistivity found in polycrystalline films follows a variable range hopping law.
Abstract: The high-temperature resistivity of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{0.7}{\mathrm{Ba}}_{0.3}{\mathrm{MnO}}_{3}$ thin films on various substrates ${\mathrm{LaAlO}}_{3},{\mathrm{SrTiO}}_{3},$ and Si was investigated. We show that the intrinsic resistivity of high-quality epitaxial films in the paramagnetic phase has a thermally activated form, whereas the extrinsic resistivity found in polycrystalline films follows a variable range hopping law. The small activation energies measured are not compatible with the formation of a band gap; instead they indicate polaron hopping. The polaron mobility is drastically enhanced by the ferromagnetic ordering. We show that the reduction of the polaron activation energy below the Curie temperature is linear in the magnetization. The shift of the metal-insulator transition temperature depends sublinearly on the applied field.
IBM1
TL;DR: In this paper, the authors measured and simulated the dynamics of magnetization reversal in 5 nm by 0.8 by 1.6 µm thin films with spin-polarized tunnel junction and showed that the magnetization direction of the film can be probed by measuring the tunneling resistance of the junction.
Abstract: We have measured and simulated the dynamics of magnetization reversal in 5 nm by 0.8 by 1.6 $\ensuremath{\mu}\mathrm{m}$ ${\mathrm{Ni}}_{60}{\mathrm{Fe}}_{40}$ thin films. The films measured form the upper electrode of a spin-polarized tunnel junction so that the magnetization direction of the film can be probed by measuring the tunneling resistance of the junction. When a magnetic field pulse is applied, the time to switch the film magnetization changes from greater than 10 ns to less than 500 ps as the pulse amplitude is increased from the coercive field to 10 mT and beyond. We have simulated these transitions using micromagnetic modeling of the exact experimental conditions. The simulations agree well with the experimental measurements.
TL;DR: In this paper, the magnetic and structural properties of Co ferrite thin films were investigated with an x-ray diffractometer, a vibrating sample magnetometer and atomic force microscopy.
Abstract: Thin films with cobalt ferrite layers on thermally oxidized silicon wafers were fabricated by a sol-gel method. Magnetic and structural properties of the films were investigated with an x-ray diffractometer, a vibrating sample magnetometer and atomic force microscopy. The crystallization temperature for Co ferrite thin films was determined by using Mossbauer spectroscopy. Co ferrite films annealed at and above 450 °C have only a single phase spinel structure without any preferred crystallite orientation. Their rms surface roughness is less than 3 nm and the size of grains is about 30 nm for annealing temperatures greater than 650 °C. Films fired at and above 550 °C have moderate saturation magnetization and there is no significant difference of their magnetic properties for external fields applied parallel and perpendicular to their planes. The coercivity shows a strong dependence on the annealing temperature.