摘要:

A vector moving Preisach model has been developed that describes partially oriented particulate media by pseudoparticles, consisting of a distribution of several basic particles with various orientations of easy axes. The model uses switching modes calculated by a micromagnetic simulation of physical particles. In order to test this model, the angular dependence of the magnetic properties was measured by a vibrating sample magnetometer for several samples of commercial recording media that had different degrees of particle orientation. The experimental results are in good agreement with the orientation dependence of the squareness and coercivity, as predicted by the model.

ISSN:0021-8979    

DOI:10.1063/1.348338

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

It has been reported1that the spin canting observed in &ggr;‐Fe2O3is confined to a thin surface layer. We have examined by Mo¨ssbauer spectroscopy acicular &ggr;‐Fe2O3particles, ∼250 A˚ in diameter, before and after coating them with57Fe, which provided a surface layer 1.3 A˚ thick, if uniform. Measurements were made in applied fieldsHato 60 kOe collinear with the &ggr;‐ray direction at temperaturesTfrom 4.2 to 296 K. For the coated particles, 50% of the signal arose from the adsorbed57Fe. For a givenHaandT, the relative heights of the &Dgr;m=0 transitions were the same for coated and untreated particles. This indicates that the spin canting, which is visible at 4.2 K even at 60 kOe, is a bulk phenomenon and is not confined to a thin surface layer. The spectra were fit with a distributionp(H) in hyperfine fieldsH. A tailing to low‐Hvalues from the main (core) peaks was observed for both sets of particles, but was more extensive for the treated particles. Thus the tailing presumably is due to surface Fe with weaker exchange coupling. Fitting2the magnetic moment vsHacurves of the untreated sample, we obtained a paramagnetic high‐field susceptibility term which increased nearly linearly with decreasingT, as did the Mo¨ssbauer canting. Potential implications of these results for magnetization reversal in these particles will be discussed.<ks>

ISSN:0021-8979    

DOI:10.1063/1.348339

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The Mo¨ssbauer spectrum of a small particle (250 A˚ diameter), partially reduced &ggr;‐Fe2O3sample coated sequentially with57Fe and Co was obtained in a 1.8‐kOe field transverse to the &ggr;‐ray propagation direction at room temperature. The adsorbed57Fe, less than a monolayer thick, provided half the Mo¨ssbauer signal. The data were fit with a distribution in hyperfine fields. The relative areas of the Mo¨ssbauer lines of each subspectrum are in the ratio 3:2p:1:1:2p:3, withpthe common polarization; largepimplies smallHK.1For several possibilities ofB‐site toA‐site population ratios and variations in core isomer shifts, the resultant spectrum least‐squares fit values ofpwere found to be about 1.4. Theoretical calculations1of composite Mo¨ssbauer line intensities based on a model2of a rigid random‐spin shell and a core with itspdetermined by the anisotropy field obtained1from magnetization vs applied field measurements yield a compositep≊1.2. Since this value is substantially smaller than experimentally observed, the outer shell spins are not rigidly pinned even in this weak applied field. A simple theoretical free‐energy minimization calculation of a linear chain of ferrimagnetically coupled atoms in an external field applied along the hard axis shows that a high anisotropy ‘‘shell’’ will cause a parabolic dependence of sublattice spin angle vs position along the chain, with the maximum deviation from collinearity of a few degrees.<ks>

ISSN:0021-8979    

DOI:10.1063/1.348340

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

As part of our effort to evaluate substituted barium ferrites for particulate magnetic recording,1we have obtained room‐temperature Mo¨ssbauer spectra for BaFe12−2xCoxTixO19(CoTi), withx=0, 0.15, 0.30, 0.45, 0.60, 0.75, 0.80, 0.85, and 0.90. With increasingx, resolution steadily worsens as the 12‐K lines broaden and change shape, but monotonic trends are evident for all spectral features, suggesting a gradual change in site occupation and local atomic environment. Following our earlier work on the well‐resolved CoMo spectra (up tox=2.0),2we have successfully fit a five‐site Lorentzian model forx<−0.45, however with the 12‐K peak width varying independently of the other sites. A three‐site model fits the main features of all spectra with a nearly linear decrease withxof hyperfine field (ionic moment) of ∼13%, 11%, and 9%, respectively, for the 4fvi, 4fiv, and 12‐K sites, while the 12‐K spectral area and hence site occupation increases; this is in contrast to CoMo, where the fields are independent ofxand the 12‐K area decreases withx. The correct fitting model is of course more complex than either of these, but is greatly clarified by the use of magnetically oriented absorbers with the gamma rays passing parallel to the magnetization direction. This procedure removes lines 2 and 5 from the sextet components and allows the full shape of 12‐K lines 1 and 6 to be observed. We will discuss the results obtained in this way and relate them to trends in the bulk magnetic properties of CoTi‐substituted barium ferrite.

ISSN:0021-8979    

DOI:10.1063/1.348341

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The influence of impurity potentials on vacancies in thet‐Jmodel is studied through the exact diagonalization of systems with 8 and 16 sites. For a wide range of model parameters, the ground state transforms according to the two‐dimensional vector representation ofC4vand hasS=1/2. The inclusion of symmetry breaking effects present for an isolated impurity in an infinite system yields an anisotropic ground state which may couple to local lattice distortions. Implications for magnetic order, spectroscopy, and the metal‐insulator transition in the CuO2planes of the cuprate superconductors are discussed. The relationship to the behavior of free vacancies and the role of finite size effects are considered. A mean‐field Hubbard model analysis is proposed as a supplementary approach.

ISSN:0021-8979    

DOI:10.1063/1.348342

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A good knowledge of the magnetic properties of cuprates, from their insulating AF state to their metallic state, is required prior to a clarification of the origin of high temperature superconductivity. NMR techniques have proven to be quite successful in studying the magnetic properties on a local scale. The89Y NMR, which can be detected even in the AF and paramagnetic phases of YBa2Cu3O6+x, allows one to study the influence of oxygen content or impurity substitutions on the 3D AF ordering. The associated changes of spin correlation length and interlayer exchange couplings will be discussed. In the metallic state of YBa2Cu3O6+x, NMR shift data taken on the various nuclear sites,89Y,17O, and63Cu have allowed one to conclude that the covalency between Cu(3dx2−y2) and O(2p&sgr;) holes result in a spin system which behaves as a single spin fluid. The large decrease of the susceptibility of the CuO2planes at lowTfor oxygen depleted samples, as well as the differentTdependencies of the89Y,17O, and63Cu relaxation rates, is attributed to the existence of AF correlations at the wave vectorQof the AF structure of YBa2Cu3O6. However, the89Y and17O relaxation rates, which filter theQcomponent of the dynamic susceptibility, exhibit a behavior similar to that of a Fermi liquid, for large enough hole doping. TheTvariations of the spin susceptibility and of the AF correlations are discussed.

ISSN:0021-8979    

DOI:10.1063/1.348343

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The results of recent inelastic neutron scattering investigations of the magnetic excitations in La2CuO4are reviewed. Experiments were performed using a wide range of incident neutron energies. Our study of the paramagnetic phase is consistent with a picture for the magnetic dynamics which shows conventional spin waves at higher energies and an overdamped response due to spin‐wave interactions at lower energies. The evolution of the magnetic dynamics with doping in the La2−x(Ba,Sr)xCuO4system is discussed. 

ISSN:0021-8979    

DOI:10.1063/1.348344

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Exchange‐coupled double layers consisting of two ferrimagnetic layers of different composition of the general system GdTb‐FeCo were experimentally studied by magnetic reversal experiments using the Faraday effect. A novel type of magnetic reversal experiment was developed in which one of the layers is in a partly demagnetized state when the other layer is reversed. In order to interpret the resulting complicated Faraday hysteresis curves, phenomenological domain formation models are developed, including the following processes which are not possible in single layers: creation/annihilation of interface wall; reversal of the magnetization against (antiparallel to) the external field; vertical motion of the interface wall; competing reversal processes in the two layers. A method is described to estimate the distribution of compensation temperatures in a layer from reversal curves of double layers.

ISSN:0021-8979    

DOI:10.1063/1.348346

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The magnetic properties of molecular‐beam‐epitaxy iron films grown on (001) GaAs substrates were studied using a SQUID magnetometer. In uncapped films, where there is an oxidized iron layer on top of the film, theM‐Hloops shift from being symmetric about the origin when the film is cooled in applied fields to temperatures below 100 K. The observed behavior is attributed to a unidirectional exchange anisotropy generated by the antiferromagnetic ordering of the oxidized iron surface. By comparing the magnitude of the exchange coupling observed in these films with that observed in the Co‐CoO system, the oxide that forms on the iron surface is most likely FeO. In addition, the magnetic training effect or the effect of cycling through the hysteresis loops at fixed temperatures has been studied. It is noted that the temperature dependence of the measured exchange coupling is different from that which is usually reported. The reason for this difference is unknown.

ISSN:0021-8979    

DOI:10.1063/1.348347

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

This work describes a detailed investigation into the magnetic and structural properties of permalloy‐tantalum multilayered thin films produced by vacuum evaporation. Their microstructure was investigated using high‐resolution TEM and the magnetic properties were measured with vibrating‐sample and vibrating‐reed magnetometers. The results show a reduction in coercivity for the multilayer films which is independent of the number of layers but depends strongly on the magnetic layer thickness. The tantalum layer is shown to be continuous and microcrystalline down to 25‐A˚ thickness, but the interface between the layers is irregular and may give rise to additional magnetostatic coupling as the tantalum layer thickness is reduced.

ISSN:0021-8979    

DOI:10.1063/1.348348

出版商:AIP    

年代:1991

数据来源: AIP