Showing papers on "Magnetic anisotropy published in 2000"

Property variation with shape in magnetic nanoelements

Abstract: Nanometre scale magnetic particles (`nanoelements' or `nanomagnets') form a rich and rapidly growing new area in condensed matter physics, with many potential applications in data storage technology and magnetic field sensing. This paper reviews an extensive study into the influence of shape on the properties of nanomagnets in the size range 35-500 nm. Elliptical, triangular, square, pentagonal and circular geometries have all been considered. It is shown that the size, thickness and geometric shape of nanomagnets all play a vital role in determining the magnetic properties. The shape, size and thickness of a nanomagnet are shown to be linked to its magnetic properties by two distinct phenomena. The first is called configurational anisotropy and describes the role played by small deviations from uniformity in the magnetization field within the nanostructures, which allow unexpected higher-order anisotropy terms to appear. These anisotropies can often dominate the magnetic properties. The second is the competition which exists between exchange energy and magnetostatic energy. This competition determines whether the nanomagnets exhibit single domain or incoherent magnetization and also controls the non-uniformities in magnetization which lead to configurational anisotropy. Understanding the influence of shape opens the way to designing new nanostructured magnetic materials where the magnetic properties can be tailored to a particular application with a very high degree of precision.

Chemical Control of Superparamagnetic Properties of Magnesium and Cobalt Spinel Ferrite Nanoparticles through Atomic Level Magnetic Couplings

Abstract: A correlation between the electron spin−orbital angular momentum coupling and the superparamagnetic properties has been established in MgFe2O4 and CoFe2O4 spinel ferrite nanoparticles The contribution to the magnetic anisotropy from the Fe3+ lattice sites is almost the same in both nanocrystallites as neutron diffraction studies have shown a similar cation distribution in these two types of spinel ferrite nanoparticles Due to the strong magnetic couplings from Co2+ lattice sites, the blocking temperature of CoFe2O4 nanoparticles is at least 150 deg higher than the same sized MgFe2O4 nanoparticles Mossbauer spectroscopy studies demonstrate that the magnetic anisotropy of CoFe2O4 nanoparticles is higher than that of the same size MgFe2O4 nanoparticles These studies indicate that the superparamagnetic properties of nanoparticles can be controlled through chemically adjusting the magnetic anisotropy energy

Micromagnetic modelling—the current state of the art

Abstract: The increasing information density in magnetic recording, the miniaturization in magnetic sensor technology, the trend towards nanocrystalline magnetic materials and the improved availability of large-scale computer power are the main reasons why micromagnetic modelling has been developing extremely rapidly. Computational micromagnetism leads to a deeper understanding of hysteresis effects by visualization of the magnetization reversal process. Recent advances in numerical simulation techniques are reviewed. Higher order finite elements and adaptive meshing have been introduced, in order to reduce the discretization error. The use of a hybrid boundary/finite element method enables accurate stray field computation for arbitrary shaped particles and takes into account the granular microstructure of the material. A dynamic micromagnetic code based on the Gilbert equation of motion to study the time evolution of the magnetization has been developed. Finite element models for different materials and magnet shapes are obtained from a Voronoi construction and subsequent meshing of the polyhedral regions. Adaptive refinement and coarsening of the finite element mesh guarantees accurate solutions near magnetic inhomogeneities or domain walls, while keeping the number of elements small. The polycrystalline microstructure and assumed random magnetocrystalline anisotropy of elongated Co elements decreases the coercive field and the switching time compared to zero anisotropy elements, in which vortices form and move only after a certain waiting time after the application of a reversed field close to the coercive field. NiFe elements with flat, rounded and slanted ends show different hysteresis properties and switching dynamics. Micromagnetic simulations show that the magnetic properties of intergranular regions in nucleation-controlled Nd-Fe-B hard magnetic materials control the coercive field. Exchange interactions between neighbouring soft and hard grains lead to remanence enhancement of isotropically oriented grains in nanocrystalline composite magnets. Upper limits of the coercive field of pinning-controlled Sm-Co magnets for high-temperature applications are predicted from the micromagnetic calculations. Incorporating thermally activated magnetization reversal and micromagnetics we found complex magnetization reversal mechanisms for small spherical magnetic particles. The magnetocrystalline anisotropy and the external field strength determine the switching mechanism. Three different regimes have been identified. For fields, which are smaller than the anisotropy field, magnetization by coherent switching has been observed. Single droplet nucleation occurs, if the external field is comparable to the anisotropy field, and multi-droplet nucleation is the driving reversal process for higher fields.

Strain-dependent magnetic phase diagram of epitaxial La0.67Sr0.33MnO3 thin films

Abstract: Effects of lattice strain on magnetic behavior of epitaxial La0.67Sr0.33MnO3 thin films grown by 90° off-axis sputtering have been studied. The size of epitaxial strain was varied using four different substrates, i.e., (001) LaAlO3, (001) SrTiO3, (001) La0.3Sr0.7Al0.65Ta0.35O9, and (110) NdGaO3. The observed magnetism of coherent epitaxial films grown on these substrates, particularly anisotropy and Curie temperature, exhibit strong correlations with lattice strains. Spin reorientation transitions have been observed. The dependence of Curie temperature on the bulk and Jahn–Teller strains has been determined.

Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)

Abstract: Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression includin...

Magnetic properties of self-assembled Co nanowires of varying length and diameter

Abstract: Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal.

Rare-Earth-Containing Magnetic Liquid Crystals

Abstract: Rare-earth-containing metallomesogens with 4-alkoxy-N-alkyl-2-hydroxybenzaldimine ligands are reported. The stoichiometry of the complexes is [Ln(LH)(3)(NO3)(3)], where Ln is the trivalent rare-earth ion;(Y, La, and Pr to Lu, except Pm) and LH is the Schiff base. The Schiff base ligands are in the zwitterionic form and coordinate through the phenolic oxygen only. The three nitrate groups coordinate in a bidentate fashion. The X-ray single-crystal structures of the nonmesogenic homologous complexes [Ln(LH)(3)(NO3)(3)]where Ln = Nd(III), Tb(III), and Dy(III) and LH = CH3OC6H3(2-OH)CH=NC4H9, are described. Although the Schiff base Ligands do not exhibit a mesophase, the metal complexes do (SmA phase). The mesogenic rare-earth complexes were studied by NMR, IR, EPR, magnetic susceptibility measurements, X-ray diffraction,and molecular modeling. The metal complexes in the mesophase have a very large magnetic anisotropy, so that these magnetic liquid crystals can easily be aligned by an external magnetic field.

Correlation between antiferromagnetic interface coupling and positive exchange bias

Abstract: The induced moment in antiferromagnetic (AFM)--ferromagnetic (FM) $({\mathrm{FeF}}_{2}\ensuremath{-}\mathrm{Fe}$ and ${\mathrm{MnF}}_{2}\ensuremath{-}\mathrm{Fe})$ bilayers has been studied from the shift along the magnetization axis of the exchange-biased hysteresis loops. The magnetization shift depends strongly on the cooling field and microstructure of the AFM layer. The shift for small cooling fields can be opposite to the cooling field, indicating that, in some cases, the presence of the FM layer induces an antiferromagnetic coupling at the interface. Samples with negative magnetization shifts (antiferromagnetic coupling) exhibit large changes in exchange bias ${H}_{E}$ as a function of cooling field and positive exchange bias. Samples with positive magnetization shifts (ferromagnetic coupling) show almost no change in ${H}_{E}$ with cooling field and the exchange bias field remains always negative. These results confirm the theoretical assumption that an antiferromagnetic interface coupling is necessary to observe positive exchange bias.

Calculations of the susceptibility of interacting superparamagnetic particles

Abstract: A model of the magnetic properties of a dispersion of interacting superparamagnetic particles in a solid matrix is presented. The model uses Monte Carlo techniques and is capable of predicting the time and temperature dependence of the magnetic properties. The model is applied to the study of the magnetic behavior of a cobalt granular system, particularly the low-field susceptibility. It is shown that strongly interacting systems at high density exhibit non-Langevin behavior and give a strongly nonlinear variation of susceptibility with packing density. The temperature dependence of the initial susceptibility shows the characteristic peak observed experimentally, with the peak temperature increasing with packing density. The field cooled (FC) and zero field cooled (ZFC) magnetization are also studied. The field dependence of the FC magnetization is shown to depend on the interparticle interactions and also on the orientational easy axis distribution. The FC magnetization is found to exhibit a peak resulting from the interactions. This behavior is finally related to the energy barrier distribution of the system (and its dependence on the interactions) using the temperature decay of remanence. It is also shown that the remanence calculated from the complete hysteresis loop at each temperature differs from the values obtained by increasing the temperature of a system initially at saturation remanence. The evolution of magnetic properties as a function of the magnetic state and history points to the importance of collective phenomena. Calculations of a spin-spin correlation function show the existence of a state with short-ranged order at low temperatures.

Nanostructured FePt:B2O3 thin films with perpendicular magnetic anisotropy

Abstract: FePt/B2O3 multilayers were deposited by magnetron sputtering onto 7059 glass substrates. By annealing the as-deposited films at 550 °C, nanostructured FePt:B2O3 films consisting of FePt grains with L10 structure, embedded in a glassy B2O3 matrix, were obtained. The c axes of the FePt grains can be made to align with the film normal direction, which results in a perpendicular anisotropy constant of 3.5×107 erg/cc. The films remain layered structures after annealing when the B2O3 layer thickness exceeds 16 A. The nanostructure of the films was investigated by transmission electron microscopy. The coercivities and the average grain sizes of the films are dependent on the B2O3 concentrations, with coercivities varying from 4 to 12 kOe, while average grain sizes vary from 4 to 17 nm. Strong perpendicular anisotropy, adjustable coercivity, and fine grain size suggest this nanocomposite system might have significant potential as recording media at extremely high areal density.

Magnetic dynamics of weakly and strongly interacting hematite nanoparticles

Abstract: The magnetic dynamics of two differently treated samples of hematite nanoparticles from the same batch with a particle size of about 20 nm have been studied by Mossbauer spectroscopy. The dynamics of the first sample, in which the particles are coated and dispersed in water, is in accordance with the Neel expression for the superparamagnetic relaxation time of noninteracting particles. From a simultaneous analysis of a series of Mossbauer spectra, measured as a function of temperature, we obtain the median energy barrier KBuVm /k 55706100 K and the preexponential factor t 051.3 20.8 11.9 310 210 s for a rotation of the sublattice magnetization directions in the rhombohedral ~111! plane. The corresponding median superparamagnetic blocking tempera- ture is about 150 K. The dynamics of the second, dry sample, in which the particles are uncoated and thus allowed to aggregate, is slowed down by interparticle interactions and a magnetically split spectrum is retained at room temperature. The temperature variation of the magnetic hyperfine field, corresponding to different quantiles in the hyperfine field distribution, can be consistently described by a mean field model for ''super- ferromagnetism'' in which the magnetic anisotropy is included. The coupling between the particles is due to exchange interactions and the interaction strength can be accounted for by just a few exchange bridges between surface atoms in neighboring crystallites.

First-principles calculations of magnetic interactions in correlated systems

Abstract: We present a method to calculate the effective exchange interaction parameters based on the realistic electronic structure of correlated magnetic crystals in local approach with the frequency dependent self-energy. The analog of ``local force theorem'' in the density-functional theory is proven for highly correlated systems. The expressions for effective exchange parameters, Dzialoshinskii-Moriya interaction, and magnetic anisotropy are derived. The first-principles calculations of magnetic excitation spectrum for ferromagnetic iron, with the local correlation effects from the numerically exact QMC scheme, are presented.

Magnetic properties of monodispersed Co/CoO clusters

Abstract: Monodispersed Co/CoO cluster assemblies with the mean cluster sizes of 6 and 13 nm have been prepared by a plasma-gas condensation type cluster beam deposition apparatus. We measured the effects of the oxygen gas flow rate during deposition, temperature, and cluster size on the coercivity and hysteresis loop shift induced by field cooling. The large exchange bias field (10.2 kOe) and coercivity (5 kOe) were observed at 5 K for the monodispersed Co/CoO cluster assembly with $d=6\mathrm{nm}.$ The correlations between unidirectional anisotropy and uniaxial anisotropy, training effect and magnetic relaxation can be interpreted by the hypothesis of a spin disorder in the interfacial layer between the antiferromagnetic CoO shell and the ferromagnetic Co core.

Large field induced strain in single crystalline Ni–Mn–Ga ferromagnetic shape memory alloy

Abstract: A room temperature free shear strain of 5.7% is reported in a single crystal of Ni–Mn–Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm×2 mm×25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5° bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from √ to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}bct and were observed to lie almost normal to the long axis of the sample tested.

Epitaxial growth and magnetic properties of single-crystal Co2MnGe Heusler alloy films on GaAs (001)

Abstract: Single-crystal Co2MnGe Heusler alloy films were epitaxially grown on GaAs (001) substrates by molecular beam epitaxy. In situ reflection high-energy electron diffraction patterns and Auger spectroscopy confirmed the high-quality growth and stoichiometry. At 5 K, a saturation magnetization of 1000 emu/cm3 was measured. In-plane ferromagnetic resonance shows narrow linewidths and four-fold plus uniaxial anisotropy. A room-temperature resistivity of 115 μΩ cm has also been determined. The temperature dependence of the resistivity shows metallic behavior down to low temperatures.

High temperature 2:17 magnets: relationship of magnetic properties to microstructure and processing

Abstract: A comprehensive and systematic study on Sm(Co/sub bal/Fe/sub v/Cu/sub y/Zr/sub x/)/sub z/ magnets is made to completely understand the effects of composition and processing on their magnetic properties. These studies include the compositions required for the formation of cellular/lamellar microstructure, the effects of compositions (x, y, v, z) and processing (aging temperature and time) on microstructure and magnetic properties, the evolution of microstructure and magnetic properties and the magnetic hardening. All of these effects and results may be summed up as changing microstructure and microchemistry, which leads to a variation of the distribution and amount of Cu at the 1:5 cell boundaries. This becomes the most predominant factor controlling the pinning strength and therefore the coercivity and its temperature dependence.

Structural tuning of the magnetic behavior in spinel-structure ferrite thin films

Abstract: Epitaxial cobalt ferrite thin films provide a model system for understanding the magnetic properties of spinel structure ferrite thin films We demonstrate that the anomalous magnetic behavior, observed in our antiphase-boundary-free films, can be explained in terms of cation distribution and lattice distortions The magnetic anisotropy is a delicate balance between strain anisotropy, due to internal lattice strain or equivalent external strain effects, and magnetocrystalline anisotropy, due to cation distribution By annealing the cobalt ferrite thin films and hence varying the cobalt cation distribution among the tetrahedral and octahedral sites and the strain state of the films, we are able to tune the symmetry and the magnitude of the magnetic anisotropy

CoPt and FePt Thin Films for High Density Recording Media

Abstract: Granular CoPt/C and FePtJC films, consisting of nanoparticles of the highly anisotropic fct CoPt (FePt) phase embedded in a carbon matrix, were made by co-sputtering from pure Co50Pt50 (Fe50Pt50) and C targets using a tandem deposition mode. The as-made films showed a disordered face centered cubic (fcc) structure, which was magnetically soft and had low coercivity. Magnetic hardening occurred after heat treatment at elevated temperatures, which led to increase in coercivity with values up to 15 kOe. The hardening originated from the transformation of the fcc phase to a highly anisotropic face centered tetragonal phase (fct) with anisotropy K > 107 erg/cm3. Transmission electron microscopy studies showed FePt particles embedded in C matrix with a particle size increasing from below 5 nm in the as-made state to 15 nm in the fully annealed state. These results are very promising and make these materials potential candidates for high-density magnetic recording.

Chemical order induced by ion irradiation in FePt (001) films

Abstract: We demonstrate that the long-range order parameter S of sputtered FePt (001) films may be improved by using postgrowth He ion irradiation. This was demonstrated both on disordered (S∼0) and partially ordered (S∼0.4) films in which S was increased up to 0.3 and 0.6, respectively. X-ray diffraction analysis showed that these changes are due to irradiation-induced chemical ordering. The changes in the magnetic hysteresis loops correlate with the expected perpendicular magnetic anisotropy increase. This method may find applications in ultrahigh-density magnetic recording.

Magnetic pinning in superconductor-ferromagnet multilayers

Abstract: We argue that superconductor/ferromagnet multilayers of nanoscale period should exhibit strong pinning of vortices by the magnetic domain structure in magnetic fields below the coercive field when ferromagnetic layers exhibit strong perpendicular magnetic anisotropy. The estimated maximum magnetic pinning energy for single vortex in such a system is about 100 times larger than the pinning energy by columnar defects. This pinning energy may provide critical currents as high as 106−107 A/cm2 at high temperatures (but not very close to Tc) at least in magnetic fields below 0.1 T.

Size dependence of the properties of hematite nanoparticles

Abstract: Samples of nanoparticles of hematite (α-Fe2O3) with average size between 6 and 27 nm have been studied by use of Mossbauer spectroscopy. The superparamagnetic relaxation was analysed on the basis of the Neel-Brown expression for the relaxation time, τ = τ0 exp[KV/kT]. It was found that the value of τ0 increases with increasing particle volume, V, whereas the magnetic anisotropy energy constant, K, decreases. The electric quadrupole interaction, the isomer shift and the magnetic hyperfine field extrapolated to 0 K were found to be essentially independent of particle size.

Crystal-field interaction in the pyrochlore magnet Ho2Ti2O7

Abstract: Neutron time-of-flight spectroscopy has been employed to study the crystal-field interaction in the pyrochlore titanate Ho2Ti2O7. The crystal-field parameters and corresponding energy-level scheme have been determined from a profile fit to the observed neutron spectra. The ground state is a well separated Eg doublet with a strong Ising-like anisotropy, which can give rise to frustration in the pyrochlore lattice. Using the crystal-field parameters determined for the Ho compound as an estimate of the crystal-field potential in other pyrochlore magnets, we also find the Ising type behavior for Dy. In contrast, the almost planar anisotropy found for Er and Yb prevents frustration, because of the continuous range of possible spin orientations in this case.

Spin tunneling and phonon-assisted relaxation in Mn 12 -acetate

Abstract: We present a comprehensive theory of the magnetization relaxation in a Mn-12-acetate crystal in the high-temperature regime (T greater than or similar to 1 K), which is based on phonon-assisted spin tunneling induced by quartic magnetic anisotropy and weak transverse magnetic fields. The overall relaxation rate as function of the longitudinal magnetic field is calculated and shown to agree well with experimental data including all resonance peaks measured so far. The Lorentzian shape of the resonances, which we obtain via a generalized master equation that includes spin tunneling, is also in good agreement with recent data. We derive a general formula for the tunnel splitting energy of these resonances. We show that fourth-order diagonal terms in the Hamiltonian lead to satellite peaks. A derivation of the effective linewidth of a resonance peak is given and shown to agree well with experimental data. In addition, previously unknown spin-phonon coupling constants are calculated explicitly. The values obtained for these constants and for the sound velocity are also in good agreement with recent data. We show that the spin relaxation in Mn-12-acetate takes place via several transition paths of comparable weight. These transition paths are expressed in terms of intermediate relaxation times, which are calculated and which can be tested experimentally.

Light ion irradiation of Co/Pt systems: Structural origin of the decrease in magnetic anisotropy

Abstract: We study the structural properties of Pt/Co/Pt systems submitted to ${\mathrm{He}}^{+}$ ion irradiation, in order to understand why the magnetic anisotropy can be decreased in a controlled way. It is shown by grazing x-ray reflectometry that the irradiation-induced Pt and Co atom displacements can be largely accounted for by a simple ballistic recoil mechanism model. Our results indicate that even in these nm-thick films, irradiation may affect the upper and lower interfaces differently. Specifically, the upper Co interface undergoes short-range mixing, resulting in roughness, whereas the lower Co interface mostly evolves by longer-range mixing, leading to alloy formation. Irradiation also releases strain in these Co-Pt systems, but has no chemical ordering effect. Together with slow asymmetric interface roughening, the cobalt tensile strain relaxation at low fluences accounts for the magnetic anisotropy decrease. The type of analysis we propose could be useful to understand why other magnetic properties, such as interlayer exchange coupling, can be controlled by light ion irradiation.

Soft high saturation magnetization (Fe/sub 0.7/Co/sub 0.3/)/sub 1-x/N/sub x/ thin films for inductive write heads

Abstract: (Fe/sub 07/Co/sub 03/)/sub 1-x/N/sub x/ (or FeCoN) alloy single layers and FeCoN film sandwiched between two very thin (5 nm) permalloy layers have been synthesized by RF diode sputtering The saturation magnetization of the as-deposited FeCoN single layers was found to be around 245 kG, the same as the pure Fe/sub 07/Co/sub 03/ alloy; and the minimum hard-axis coercivity was 5 Oe In contrast, the sandwiched FeCoN films have a hard axis coercivity of 06 Oe, an excellent in-plane uniaxial anisotropy with an anisotropy field of 20 Oe The optimized FeCoN films exhibit a BCC structure with a strong {110} fiber texture and the resistivity is 55 /spl mu//spl Omega//spl middot/cm The combination of high saturation and low coercivity, makes the FeCoN films a very promising candidate for the write head materials for future magnetic recording

High-frequency EPR spectra of

Abstract: A detailed multifrequency high-field-high-frequency EPR (95-285 GHz) study has been performed on the single-molecule magnet of formula [Fe8O2(OH)12(tacn)6]Br8 x 9H2O, in which tacn = 1,4,7-triazacyclononane. Polycrystalline powder spectra have allowed the estimation of the zero-field splitting parameters up to fourth order terms. The single-crystal spectra have provided the principal directions of the magnetic anisotropy of the cluster. These results have been compared with an evaluation of the intra-cluster dipolar contribution to the magnetic anisotropy; this suggests that single-ion anisotropy is the main contributor to the magnetic anisotropy. The role of the transverse magnetic anisotropy in determining the height of the barrier for the reversal of the magnetization is also discussed.

Magnetic anisotropy and strain states of (001) and (110) colossal magnetoresistance thin films

Abstract: The magnetic anisotropy of epitaxial colossal magnetoresistance films on SrTiO3 substrates is dominated by the strain or magnetocrystalline anisotropy depending on the orientation of the film. While the magnetic anisotropy of (001) films is determined by magnetocrystalline anisotropy, that of (110) films is determined by magnetoelastic effects. Furthermore while the microstructure in the thin films has a significant effect on the coercive field, it has a correspondingly negligible effect on the magnetic anisotropy. We find a roughening surface morphology with increasing thickness that is more pronounced in (110) oriented films and can be attributed to the growth kinetics.

Magnetic properties of YVO3 single crystals

Abstract: The magnetic properties of ${\mathrm{YVO}}_{3}$ single crystals have been studied in the temperature range from 350 to 4.2 K and in magnetic fields up to 7 T. It is found that in an applied field less than 4 kOe remarkable magnetization reversals occur at two distinct temperatures: an abrupt switch at ${T}_{s}=77$ K associated with a first-order structure phase transition and a gradual reversal at ${T}^{*}\ensuremath{\approx}95$ K without a structural anomaly. Most interestingly, the magnetization always switches to the opposite direction if the crystal is cooled or warmed through ${T}_{s}$ and ${T}^{*}$ in a field less than \ensuremath{\sim}500 Oe. In higher magnetic fields the magnetization does not change sign but has a minimum at ${T}^{*}$ and a sudden change at ${T}_{s}.$ A possible mechanism for the observed peculiar magnetic behavior is discussed, related to the competition of the single-ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya interaction accompanied by a change of orbital ordering.

FePt:SiO2 granular thin film for high density magnetic recording

Abstract: Nanocomposite FePt:SiO2 thin films consisting of high anisotropy FePt particles embedded in a SiO2 matrix have been successfully fabricated by annealing the as-deposited FePt/SiO2 multilayers. By adjusting the annealing temperatures and compositions, films were obtained with coercivity of 3.8 kOe and grain size of 10 nm, which are suitable for high-density magnetic recording. Magnetic activation volumes were measured and thermal stability is discussed.