Showing papers on "Magnetization published in 1999"
TL;DR: A review of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors, is given in this paper.
Abstract: Mixed-valence manganese oxides (R1-χAχ)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, ...
1,757 citations
Book•
26 Nov 1999
TL;DR: The classical and quantum properties of magnetism have been discussed in this article, including magnetization, magnetization in small structures, exchange couplings and nanocrystals, and magnetic recording.
Abstract: Introduction and Overview. Magnetostatics. Classical and Quantum Phenomenology of Magnetism. Quantum Mechanics, Magnetism, and Exchange in Atoms and Oxides. Quantum Mechanics, Magnetism, and Bonding in Metals. Magnetic Anisotropy. Magnetoelastic Effects. Magnetic Domain Walls and Domains. Magnetization Process. Soft Magnetic Materials. Amorphous Materials: Magnetism and Disorder. Magnetism in Small Structures: Exchange Coupling and Nanocrystals. Hard Magnetic Materials. Magnetic Annealing and Directional Order. Electronic Transport in Magnetic Materials. Surface and Thin-Film Magnetism. Magnetic Recording. Appendices. Index.
1,425 citations
TL;DR: In this article, an approach to calculate the magnetocaloric effect from the combined heat capacity and magnetization data is proposed, based on the assumption that heat capacity is magnetic-field independent.
Abstract: Accurate values for the magnetocaloric effect can be obtained from both magnetization and heat-capacity data. A reliable estimate of the experimental errors in the calculated magnetocaloric effect can be made from the known experimental errors of the measured physical properties. Attempts in the past to simplify the basic thermodynamic relation to allow the calculation of the adiabatic temperature change from the heat capacity at constant field and the magnetic entropy change calculated from the magnetization data fail because the assumption that heat capacity is magnetic-field independent is erroneous. A suitable approach to carry out these calculations from the combined heat capacity and magnetization data is suggested.
724 citations
TL;DR: A general solution of the exchange problem in the high-nuclearity spin clusters (HNSC) containing arbitrary number of exchange-coupled centers and topology is developed and the general expression for the INS cross-section of spin clusters interacting by all kinds of exchange interactions is derived for the first time.
Abstract: A general solution of the exchange problem in the high-nuclearity spin clusters (HNSC) containing arbitrary number of exchange-coupled centers and topology is developed. All constituent magnetic centers are supposed to possess well-isolated orbitally non-degenerate ground states so that the isotropic Heisenberg-Dirac-Van Vleck (HDVV) term is the leading part of the exchange spin Hamiltonian. Along with the HDVV term, we consider higher-order isotropic exchange terms (biquadratic exchange), as well as the anisotropic terms (anisotropic and antisymmetric exchange interactions and local single-ion anisotropies). All these terms are expressed as irreducible tensor operators (ITO). This allows us to take full advantage of the spin symmetry of the system. At the same time, we have also benefitted by taking into account the point group symmetry of the cluster, which allows us to work with symmetrized spin functions. This results in an additional reduction of the matrices to diagonalize. The approach developed here is accompanied by an efficient computational procedure that allows us to calculate the bulk magnetic properties (magnetic susceptibility, magnetization, and magnetic specific heat) as well as the spectroscopic properties of HNSC. Special attention is paid to calculate the magnetic excitations observed by inelastic neutron scattering (INS), their intensities, and their Q and temperature dependencies. This spectroscopic technique provides direct access to the energies and wave functions of the different spin states of the cluster; thus, it can be applied to spin clusters in order to obtain deep and detailed information on the nature of the magnetic exchange phenomenon. The general expression for the INS cross-section of spin clusters interacting by all kinds of exchange interactions, including also the single-ion zero-field splitting term, is derived for the first time. A closed-form expression is also derived for the particular case in which only the isotropic exchange interactions are involved. Finally this approach has been used to model the magnetic properties as well as the INS spectra of the polyoxometalate anion [Ni(9)(OH)(3)(H(2)O)(6)(HPO(4))(2)(PW(9)O(34))(3)](16)(-), which contains a central magnetic cluster formed by nine exchange-coupled Ni(II) ions surrounded by diamagnetic phosphotungstate ligands (PW(9)O(34))(9)(-).
554 citations
TL;DR: In this paper, the authors developed a theory of the extrinsic contributions to the ferromagnetic resonance linewidth and frequency shift of ultrathin films by two magnon scattering by defects at surfaces and interfaces.
Abstract: We develop a theory of the extrinsic contributions to the ferromagnetic resonance linewidth and frequency shift of ultrathin films. The basic mechanism is two magnon scattering by defects at surfaces and interfaces. In the presence of dipolar couplings between spins in the film, one realizes short wavelength spin waves degenerate with the ferromagnetic resonance (FMR) mode, provided the magnetization is parallel to the film surfaces. Defects on the surface or interface thus scatter the FMR mode into such short wavelength spin waves, producing a dephasing contribution to the linewidth, and a frequency shift of the resonance field. The mechanism described here is inoperative when the magnetization is perpendicular to the film.
478 citations
TL;DR: In this article, the transverse and longitudinal polarizations of recoiling protons from the elastic scattering of polarized electrons with unpolarized protons were measured over a range of four momentum transfer squared between 0.5 and 3.5 GeV-squared.
Abstract: The ratio of the proton's elastic electromagnetic form factors was obtained by measuring the transverse and longitudinal polarizations of recoiling protons from the elastic scattering of polarized electrons with unpolarized protons. The ratio of the electric to magnetic form factor is proportional to the ratio of the transverse to longitudinal recoil polarizations. The ratio was measured over a range of four-momentum transfer squared between 0.5 and 3.5 GeV-squared. Simultaneous measurement of transverse and longitudinal polarizations in a polarimeter provides good control of the systematic uncertainty. The results for the ratio of the proton's electric to magnetic form factors show a systematic decrease with increasing four momentum squared, indicating for the first time a marked difference in the spatial distribution of charge and magnetization currents in the proton.
477 citations
TL;DR: In this paper, the volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random face-centred cubic iron-nickel alloy, in which they allow for non-collinear spin alignments, that is, spins that may be canted with respect to the average magnetization direction.
Abstract: In 1897 Guillaume1 discovered that face-centred cubic alloys of iron and nickel with a nickel concentration of around 35 atomic per cent exhibit anomalously low (almost zero) thermal expansion over a wide temperature range. This effect, known as the Invar effect, has since been found in various ordered and random alloys and even in amorphous materials2. Other physical properties of Invar systems, such as atomic volume, elastic modulus, heat capacity, magnetization and Curie (or Neel) temperature, also show anomalous behaviour. Invar alloys are used in instrumentation, for example as hair springs in watches. It has long been realized that the effect is related to magnetism2,3; but a full understanding is still lacking. Here we present ab initio calculations of the volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random face-centred cubic iron–nickel alloy, in which we allow for non-collinear spin alignments—that is, spins that may be canted with respect to the average magnetization direction. We find that the magnetic structure is characterized, even at zero temperature, by a continuous transition from the ferromagnetic state at high volumes to a disordered non-collinear configuration at low volumes. There is an additional, comparable contribution to the net magnetization from the changes in the amplitudes of the local magnetic moments. The non-collinearity gives rise to an anomalous volume dependence of the binding energy, and explains other peculiarities of Invar systems.
449 citations
TL;DR: In this article, the synthesis, crystal structure, and magnetic characterization of a novel tetranuclear iron(III) methoxo-bridged cluster of formula Fe4(OCH3)6(dpm)6 (where Hdpm = dipivaloylmethane) is reported.
Abstract: The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear iron(III) methoxo-bridged cluster of formula Fe4(OCH3)6(dpm)6 (where Hdpm = dipivaloylmethane) is reported. The cluster has a ground spin state of S = 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D = −0.2 cm-1 of the S = 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To investigate the origin of the magnetic anisotropy we have evaluated the projection of the single-ion and dipolar contributions to the zfs of the ground state. The zfs tensors of the three structurally independent iron(III) centers have been calculated from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm)3 have been recorded...
400 citations
TL;DR: In this article, the spin-split density of states of a high mobility two-dimensional electron gas (2DEG) was observed at a ferromagnet-semiconductor interface.
Abstract: Spin injection at a ferromagnet-semiconductor interface is observed by projecting the spin-polarized current in the ferromagnet onto the spin-split density of states of a high mobility two-dimensional electron gas (2DEG). For a given polarization of carriers in the 2DEG, reversing the magnetization orientation of the ferromagnet modulates the interface resistance. Equivalently, reversing the polarization of the 2DEG carriers by reversing the bias polarity gives the same resistance modulation. Interface resistance changes of order 1% at room temperature indicate interfacial current polarizations of order 20%.
397 citations
TL;DR: In this article, the Curie relaxation was used to explain the low-field part of the NMRD profiles, which can only explain the crystal's internal anisotropy energy, a concept which clarifies the important difference between superpara-and paramagnetic compounds.
Abstract: Evaluating and understanding the performances of magnetic colloids as contrast agents for MRI requires a theory describing their magnetic interactions with water protons. The field dependence of the proton longitudinal relaxation rate (nuclear magnetic relaxation dispersion profiles) in aqueous colloidal suspensions of superparamagnetic particles is based on the so-called Curie relaxation, which essentially accounts for the high field part of the NMRD profiles (B0>0.02 T). The low-field part of the NMRD profiles can only be explained by the crystal’s internal anisotropy energy, a concept which clarifies the important difference between superpara- and paramagnetic compounds: the anisotropy energy modifies both the electronic precession frequencies and the thermodynamic probability of occupation of the crystal magnetic states. Our theory clearly explains why a low-field dispersion exists for suspensions of small size crystals, and why it does not for large crystals’ suspensions. This important effect is due...
395 citations
TL;DR: In this article, an overview of exchange-spring coupled magnetic films and multilayers, including fabrication methods, and the characterization and modeling of the magnetization reversal processes is provided.
TL;DR: In this article, an inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described using 50 nm films of Permalloy (Ni81Fe19) and data are presented for impulse and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis.
Abstract: An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni81Fe19). Data are presented for impulse- and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are clearly observed. The oscillation frequency increases as the dc bias field parallel to the anisotropy axis increases as predicted by classical gyromagnetic theory. The data are fitted to the Landau–Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. Damping for step excitations is characterized by an anomalous transient damping which rapidly increases at low dc bias field. Transformation of the data to the frequency domain reveals a higher order precessional mode which is also preferentially excited at low dc bias fields. A possible source for both phenomena is precessional mode saturation for large peak rotations. The technique has the potential for 20 ps resolution, although only 120 ps resolution is demonstrated due to the limited bandwidth of the waveguides used.
TL;DR: In this article, colloidal assemblies are used to synthesize FCC cobalt nanoparticles and the magnetic properties are compared when they are dispersed in a solvent and organized in 2D superlattices.
Abstract: Colloidal assemblies are used to synthesize FCC cobalt nanoparticles. The particles are coated, extracted from micelles, and characterized by transmission electron microscopy, small angle X-ray scattering, and electron and X-ray diffraction spectroscopy. These cobalt metal particles are stable in air, have a narrow size distribution, and on deposition on a graphite support, spontaneously form a 2D hexagonal network. The magnetic properties are compared when they are dispersed in a solvent and organized in 2D superlattices. Changes in the hysteresis loop and in the blocking temperature are observed and attributed to collective flip of the magnetization of adjacent particles.
TL;DR: In this paper, the electrochemical synthesis of nanoparticles of γ-Fe2O3 was performed in an organic medium, and the size distributions of the particles were narrow, with the average sizes varying from 3 to 8 nm.
Abstract: The electrochemical synthesis of nanoparticles of γ-Fe2O3 was performed in an organic medium. The size was directly controlled by the imposed current density, and the resulting particles were stabilized as a colloidal suspension by the use of cationic surfactants. The size distributions of the particles were narrow, with the average sizes varying from 3 to 8 nm. The amorphous character of the nanoparticles was clearly established by X-ray powder diffraction and TEM analysis. The microstructure of this phase could nevertheless be spectroscopically related to maghemite, γ-Fe2O3. 57Fe Mossbauer spectroscopy and magnetization measurements indicated that the dry powders exhibit superparamagnetic behavior at room temperature.
TL;DR: In this article, the authors highlight the increasing interest of polyoxometalates in molecular magnetism, providing at the same time a perspective of the state-of-the-art in this area.
TL;DR: In this paper, the superconducting spin switch was proposed and theoretically justified, which is based on a four-layer antiferromagnet/ferromagnetic/superconductor/fermanet spin-valve-like structure.
Abstract: The principle of a novel device, which is called a superconducting spin switch or a spin valve for supercurrent, is proposed and theoretically justified. It is based on a four-layer antiferromagnet/ferromagnet/superconductor/ferromagnet spin-valve-like structure. Calculations show that this structure has either zero value or lower superconducting transition temperature for the parallel alignment of magnetizations in the ferromagnetic layers as compared with an antiparallel alignment of magnetizations. Thus, the supercurrent flowing through the superconducting layer can be switched by rotating the magnetization of the top free ferromagnetic layer by a weak external magnetic field.
TL;DR: In this article, the effect of pulsed currents on magnetization reversal was studied on single ferromagnetic nanowires of diameter about 80 nm and 6000 nm length, and the injected current triggered the magnetisation reversal at a value of the applied field distant from the switching field by as much as 20%.
Abstract: The effect of pulsed currents on magnetization reversal were studied on single ferromagnetic nanowires of diameter about 80 nm and 6000 nm length. The magnetization reversal in these wires occurs with a jump of the magnetization at the switching field Hsw, which corresponds to unstable states of the magnetization. A pulsed current of about 107 A/cm2 was injected at different values of the applied field close to Hsw. The injected current triggered the magnetization reversal at a value of the applied field distant from the switching field by as much as 20%. This effect of current-induced magnetization reversal is interpreted in terms of the action of the spin-polarized conduction electrons on the magnetization.
TL;DR: In this article, a temperature-dependent decay of magnetization is used to study superparamagnetism in magnetic nanoparticles, and the magnetic anisotropy is clearly the origin of the divergence in the field-cooled and zero-fieldcooled magnetization measurements.
Abstract: Superparamagnetism is a unique feature of magnetic nanoparticles. Spinel ferrite nanoparticles provide great opportunities for studying the mechanism of superparamagnetic properties. CoFe2O4 nanocrystallites have been synthesized with a microemulsion method. The neutron diffraction studies and the temperature-dependent decay of magnetization show the superparamagnetic relaxation occurring in these nanoparticles. The neutron diffraction shows a high degree of inversion with the 78% tetrahedral sites occupied by Fe3+ cations. The nanoparticles with a 12 nm diameter have a blocking temperature around 320 K. The field-cooled and zero-field-cooled magnetization measurements display a divergence below the blocking temperature. The energy barrier distribution of magnetic anisotropy is derived from the temperature-dependent decay of magnetization. The magnetic anisotropy is clearly the origin of the divergence in the field-cooled and zero-field-cooled magnetization measurements. The energy barrier distribution fu...
Book•
01 Jan 1999
TL;DR: Takei et al. as mentioned in this paper presented a list of World's major Ferrite Suppliers and their application in magnetic material processing and applied them to a variety of applications such as DC and low frequency applications.
Abstract: Foreword Takeshi Takei. Preface. Acknowledgements. 1. Applications and Functions of Ferromagnetic Material. 2. Basics of Magnetism - Source of Magnetic Effect. 3. The Magnetization in Domains and Bulk Materials. 4. AC Properties of Magnetic Materials. 5. Materials for Permanent Magnet Applications. 6. DC and Low Frequency Applications. 7. Soft Cobalt-Iron Alloys. 8. Metallic Materials for Magnetic Shielding Applications. 9. High Permeability-High Frequency Metal Strip. 10. Metal Powder Cores for Telecommunications. 11. Crystal Structure of Ferrites. 12. Chemical Aspects of Ferrites. 13. Microstructural Aspects of Ferrites. 14. Ferrite Processing. 15. Ferrite Inductors and Transformers for Low Power. 16. Soft Magnetic Materials for EMI Suppression. 17. Ferrites for Entertainment Applications. 18. Ferrite Transformers and Inductors at High Power. 19. Materials for Magnetic Recording. 20. Ferrites for Microwave Applications. 21. Miscellaneous Magnetic Material Applications. 22. Physical-Thermal Aspects of Magnetic Materials. 23. Magnetic Measurements-Materials and Components. Bibliography. Appendix 1: Abbreviations and Symbols. Appendix 2: List of World's Major Ferrite Suppliers. Appendix 3: Units Conversion from CGS to MKS(SI) System. Index.
TL;DR: In this article, Monte Carlo simulation studies are performed to examine the implications of octahedral cation (Fe, Mo) site disorder for magnetization in the double-perovskite Sr2FeMoO6.
Abstract: Monte Carlo simulation studies are performed to examine the implications of octahedral cation (Fe, Mo) site disorder for magnetization in the double-perovskite Sr2FeMoO6. Correlations between the near-neighbor cation distributions and the spin distributions are identified to gain insight into the spin arrangement within, and on the periphery of a given transition element cation cluster. It is shown that the drop in the magnetic moment is nearly linear with the increase in the mis-site defect concentration for the case of randomly created defects. Implications of the concomitant presence of mis-site defects and oxygen vacancies are also analyzed.
TL;DR: Ultrafast magnetic field pulses as short as 2 picoseconds are able to reverse the magnetization in thin, in-plane, magnetized cobalt films, triggered by fields as small as 184 kiloamperes per meter.
Abstract: Ultrafast magnetic field pulses as short as 2 picoseconds are able to reverse the magnetization in thin, in-plane, magnetized cobalt films. The field pulses are applied in the plane of the film, and their direction encompasses all angles with the magnetization. At a right angle to the magnetization, maximum torque is exerted on the spins. In this geometry, a precessional magnetization reversal can be triggered by fields as small as 184 kiloamperes per meter. Applications in future ultrafast magnetic recording schemes can be foreseen.
Patent•
23 Nov 1999
TL;DR: In this article, a spin valve for magnetoresistive head is described. Butler et al. proposed a method and a system for providing a synthetic pinned layer, a non-magnetic spacer layer, and a free layer.
Abstract: A method and system for providing a spin valve for use in a magnetoresistive head is disclosed. The method and system include providing a synthetic pinned layer, a nonmagnetic spacer layer, and a free layer. The free layer has a first magnetization canted from a first direction by a first angle. The nonmagnetic spacer layer is disposed between the free layer and the synthetic pinned layer. The synthetic pinned layer has a second magnetization in a second direction. The second direction is canted from a third direction that is transverse to the first direction by a second angle. The second magnetization is substantially orthogonal to the first magnetization.
TL;DR: In this paper, a theory based on an effective staggered field, orthogonal to the applied uniform field, resulting from a staggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading to a sine-Gordon quantum field theory was developed.
Abstract: Recent experiments on the $S=\frac{1}{2}$ antiferromagnetic chain compound, Cu benzoate, discovered an unexpected gap scaling as approximately the $\frac{2}{3}$ power of an applied magnetic field. A theory of this gap, based on an effective staggered field, orthogonal to the applied uniform field, resulting from a staggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading to a sine-Gordon quantum field theory, has been developed. Here we discuss many aspects of this subject in considerable detail, including a review of the $S=\frac{1}{2}$ chain in a uniform field, a spin-wave theory analysis of the uniform plus staggered field problem, exact amplitudes for the scaling of gap, staggered susceptibility and staggered magnetization with field or temperature, intensities of soliton and breather peaks in the structure function, and field and temperature dependence of the total susceptibility.
TL;DR: In this article, the Kramers-Kronig relations for a dusty medium are generalized to include the possibility of magnetic grains, and the magnetic permeability as a function of frequency is discussed for several candidate grain materials.
Abstract: Thermal fluctuations in the magnetization of interstellar grains will produce magnetic dipole emission at ν100 GHz. We show how to calculate absorption and emission from small particles composed of material with magnetic, as well as dielectric, properties. The Kramers-Kronig relations for a dusty medium are generalized to include the possibility of magnetic grains. The magnetic permeability as a function of frequency is discussed for several candidate grain materials. Iron grains, or grains containing iron inclusions, are likely to have the magnetic analog of a Frohlich resonance in the vicinity of ~50-100 GHz, which results in a large magnetic dipole absorption cross section. We calculate the emission spectra for various interstellar grain candidates. Although "ordinary" paramagnetic grains or even magnetite grains cannot account for the observed "anomalous" emission from dust in the 14-90 GHz range, stronger magnetic dipole emission will result if a fraction of the grain material is ferromagnetic, as could be the case given the high Fe content of interstellar dust. The observed emission from dust near 90 GHz implies that not more than ~5% of interstellar Fe is in the form of metallic iron grains or inclusions (e.g., in "GEMS"). However, we show that if most interstellar Fe is in a moderately ferromagnetic material, with the magnetic properties suitably adjusted, it could contribute a substantial fraction of the observed 14-90 GHz emission, perhaps comparable to the contribution from spinning ultrasmall dust grains. The two emission mechanisms can be distinguished by measuring the emission from dark clouds. If ferromagnetic grains consist of a single magnetic domain and are aligned, the magnetic dipole emission will be linearly polarized, with the polarization depending strongly on frequency.
18 May 1999
TL;DR: In this paper, highfrequency noise suppressors or anti-reflection absorbers were proposed for use with M-hexaferrites with planar magnetic anisotropy, which leads to higher magnetic resonance frequency, and thus a large absorption loss in sintered specimens and small reflection loss (in rubber composites) are predicted at the higher frequencies in the substituted specimens.
Abstract: Magnetic and microwave absorbing properties have been investigated in the M-type barium ferrites (BaFe/sub 12-2x/A/sub x/Co/sub x/O/sub 19/) with planar magnetic anisotropy. For the tetravalent A ions, Ti/sup 4+/ and Ru/sup 4+/ are chosen and the samples are prepared by a conventional ceramic processing technique. At the substitution ratio with in-plane anisotropy which is estimated from the minimum coercivity, the saturation magnetization of the Ru-Co substituted specimens were about twice as large as those of Ti-Co. This leads to higher magnetic resonance frequency, and thus a large absorption loss (in sintered specimens) and small reflection loss (in rubber composites) are predicted at the higher frequencies in the Ru-Co substituted specimens. High-frequency noise suppressors or anti-reflection absorbers can be proposed for use with M-hexaferrites with planar magnetic anisotropy.
TL;DR: Thermally induced changes in the magnetization direction of superparamagnetic magnetite nanoparticles were observed but with a much slower rate than expected, due to surface anisotropy.
Abstract: The magnetization directions of individual monodomain nanoparticles as small as 5 nanometers in diameter are determined using the Foucault method of Lorentz microscopy. A model is developed to explain the images and diffraction patterns of samarium cobalt nanoparticles as a function of the aperture shift direction. Thermally induced changes in the magnetization direction of superparamagnetic magnetite nanoparticles were observed but with a much slower rate than expected, due to surface anisotropy. When the time scale for magnetization reversal is much shorter than the data acquisition time, as in carbon-coated iron cobalt alloy nanoparticles, the images show an average of such thermally induced changes.
TL;DR: In this article, the structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements.
Abstract: The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mossbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1) × 105 erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.
TL;DR: The states emerging in the scaling limit are compared to those of a homogeneous system where the same magnetization current is driven by a bulk field, and it is found that the expectation values of various quantities agree in the two systems.
Abstract: We study the connection between magnetization transport and magnetization profiles in zero-temperature XX chains. The time evolution of the transverse magnetization m(x,t) is calculated using an inhomogeneous initial state that is the ground state at fixed magnetization but with m reversed from -m(0) for x 0. In the long-time limit, the magnetization evolves into a scaling form m(x,t)=Phi(x/t) and the profile develops a flat part (m=Phi=0) in the (x/t) 1/2 while it expands with the maximum velocity c(0)=1 for m(0)-->0. The states emerging in the scaling limit are compared to those of a homogeneous system where the same magnetization current is driven by a bulk field, and we find that the expectation values of various quantities (energy, occupation number in the fermionic representation) agree in the two systems.
TL;DR: In this paper, the authors show that perovskite manganites show a ferromagnetic ground state when the holes are optimally doped (usually 0.2, x, 0.5) and anisotropic antiferromagnetic (AFM) phases for x.
Abstract: Manganese oxides with the general composition R12xAxMnO3 (where R and A are rare- and alkaline-earth ions, respectively) have attracted considerable attention because of their unusual magnetic and electronic properties. In some of these materials, metal-insulator transitions can be observed where both conductivity and magnetization change markedly. The x 0 and x 1 end members of the R12xAxMnO3 family are insulating and antiferromagnetic (AF) with the Mn ion in the Mn 31 and Mn 41 state, respectively. For intermediate x, the average Mn valence is noninteger and the material is generally metallic or semiconducting. Most of the perovskite manganites show a ferromagnetic (FM) ground state when the holes are optimally doped (usually 0.2 , x , 0.5) and anisotropic antiferromagnetic (AFM) phases for x . 0.5. The half-doped manganites, with x 1 , are very particular. Magnetically these systems form FM
TL;DR: In this paper, the role of transverse anisotropy in the dynamics of the magnetization is elucidated through the comparison of the tunneling rate of two Fe8 cluster compounds.