Showing papers on "Magnetization published in 1985"

Interfacial charge-spin coupling: Injection and detection of spin magnetization in metals.

Abstract: The strong inequivalence of spin-up and spin-down subbands in a ferromagnet causes a coupling between the charge and spin transport across the interface of a ferromagnetic and a contiguous paramagnetic metal. This allows the use of sensitive electronic measurements to probe spin transport. Application of small static magnetic fields results in a Hanle effect which permits determination of the spin-relaxation time ${\mathrm{T}}_{2}$. The unique features of the method should make it applicable to a wide range of studies.

Perpendicular magnetic anisotropy in Pd/Co thin film layered structures

Abstract: This letter presents the results of magnetic studies of thin film, periodic, Pd/Co layered structures with ultrathin cobalt (4–13 A). We show that films with Co thicknesses less than 8 A are easy to magnetize along a direction normal to the film surface. The best films have a saturation magnetization of about 500 emu/cm3 and a coercivity of 550 Oe, and thus they are candidates for a vertical magnetic recording medium. We attribute the perpendicular magnetic easy axis in these films to surface anisotropy at Pd/Co interfaces and strain in thin Co layers.

Magnetic scattering of x rays (invited)

Abstract: The scattering of x rays is used to determine the electric charge distribution in matter. Since x rays are electromagnetic radiation, we should expect that they will be sensitive not only to the charge distribution, but also to the magnetization density. That this is indeed the case has been pointed out and studied experimentally. In this paper the magnetic scattering is discussed in a way which allows consideration of the effects of electron binding. The cross section, compared with that for neutron scattering from magnetically ordered materials, is reduced by (ℏω/mc2)2 (about 5×10−4). With a synchrotron radiation source, however, this factor can be made up, and magnetic x‐ray Bragg peaks can be collected in the same time as neutron peaks. Special effects of interest include high momentum resolution, polarization phenomena which separate spin and orbital densities, and resonance effects which give a large enhancement of the x‐ray cross section and which may make the study of surface magnetism possible.

Effect of spin fluctuations on the magnetic equation of state of ferromagnetic or nearly ferromagnetic metals

Abstract: The authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation. The model takes account of both longitudinal and transverse fluctuations in terms of four physically transparent parameters which may be determined independently from the equation of state in the T=0 limit and from inelastic neutron scattering, or calculated directly from a semi-empirical band structure model near the Fermi level fitted for example to experimental Fermi surface areas. For parameters of the same order of magnitude as those recently determined in the weakly spin-polarised metal Ni3Al, the model yields approximately a quadratic temperature dependence of the spontaneous magnetisation over a wide range well below the Curie temperature (Tc), a nearly linear inverse susceptibility well above Tc, and nearly linear magnetic isotherms (Arrott plots) at high magnetic fields. These results are qualitatively consistent with the behaviour observed in many magnetic metals near the ferromagnetic instability at low temperatures. For Ni3Al the model yields good quantitative agreement with experiment for the magnitude of the Curie temperature Tc, for the ratio peff/p0 of the high- to low-temperature effective magnetic moments, and for the coefficient of the quadratic (T2) variation of the magnetisation with temperature well below Tc, without the use of any free adjustable parameters. Finally the authors show that the model also provides a good quantitative description of the paramagnetic susceptibility and transition temperature of the more complex magnetic system MnSi, the only other unsaturated magnetic metal for which all of the microscopic parameters are well known.

Diamagnetic susceptibility of superconducting clusters: Spin-glass behavior.

Abstract: We present a theory for the diamagnetic response of weakly linked superconducting clusters. In the model, superconducting grains, each small compared to a penetration depth, are weakly coupled into closed loops. These support screening supercurrents in response to an external magnetic field. In a magnetic field, a large cluster can support many supercurrent-carrying states of nearly equal energy, but energy barriers between these states tend to inhibit hops from one state to another at low temperatures. The picture is similar to that often proposed for spin glasses. An important consequence is predicted to be a large difference between the dc and ac susceptibilities at low temperatures. The former, an equilibrium property, will fall off much more rapidly with field than the latter, which is generally a property of the metastable states. In addition, the magnetization of a cluster varies discontinuously with field; for a sufficiently large cluster, the magnetization is everywhere discontinuous. To check these conjectures, two examples are studied. The first consists of single loops of random areas and orientation, which can be solved analytically at zero temperature. The second involves random two-dimensional clusters of many closed loops, and is studied via careful Monte Carlo simulation at various temperaturesmore » and fields. Both examples display the expected strong differences between ac and dc susceptibilities at low temperatures. Our predictions are found to be quite similar to the experimental results of Bastuscheck et al. (Phys. Rev. B 24, 6707 (1981)) for the fibrous superconductors NbSe/sub 3/ and TaSe/sub 3/.« less

Magnetic properties of rare-earth-iron-boron permanent magnet materials

Abstract: Static magnetic measurements have been carried out on single crystals of Nd2Fe14B, Sm2Fe14B, and Y2Fe14B from 4.2 to 590 K. Values of K1 estimated from high field measurements at room temperature are 4.5, −12, and 1.1 MJ/m3 for Nd2Fe14B, Sm2Fe14B, and Y2Fe14B, respectively. Anisotropic behavior of the magnetization versus magnetic field curves in the basal plane has been observed for Sm2Fe14B, indicating large amplitude of the high order coefficients, K2 and K3. In Nd2Fe14B, the magnetization has been found to tilt from the c axis and simultaneously increase in magnitude. Average Fe moment is estimated to be 2.23 μB/Fe at 4.2 K from the saturation magnetization of Y2Fe14B.

Magnetic properties of the Nd2(Fe1−xCox)14B system

Abstract: We have investigated the magnetic properties of the Nd2(Fe1−xCox)14B system to improve the thermal properties of the Nd‐Fe‐B magnets. Nd2(Fe1−xCox)14B exists in the tetragonal form in the entire range of 0≤x≤1. In this system, the replacement of Fe by Co significantly increases the Curie temperature. The room‐temperature magnetization of Nd2(Fe1−xCox)14B has its maximum value at x=0.1. However, because of the decrease in the anisotropy energy and the saturation magnetization by further substitution of Co for Fe, a reasonable substitution range of Co is suggested to be x<0.2 in the sintered Nd‐Fe‐B magnet. In this range of Co, we have succeeded in improving the reversible temperature coefficient of the remanence for the Nd‐Fe‐B magnets.

Density of states and de Haas-van Alphen effect in two-dimensional electron systems.

Abstract: The density of states of two-dimensional electron systems in GaAs/AlGaAs single-layer and multilayer heterostructures has been determined through measurements of the high-field magnetization. Our results reveal a substantial density of states between Landau levels, even in high-mobility single quantum wells. There is no existing theoretical explanation for this anomaly.

Cytoplasmic motions, rheology, and structure probed by a novel magnetic particle method.

Abstract: The motions of magnetic particles contained within organelles of living cells were followed by measuring magnetic fields generated by the particles. The alignment of particles was sensed magnetometrically and was manipulated by external fields, allowing non-invasive detection of particle motion as well as examination of cytoplasmic viscoelasticity. Motility and rheology data are presented for pulmonary macrophages isolated from lungs of hamsters 1 d after the animals had breathed airborne gamma-Fe2O3 particles. The magnetic directions of particles within phagosomes and secondary lysosomes were aligned, and the weak magnetic field produced by the particles was recorded. For dead cells, this remanent field was constant, but for viable macrophages, the remanent field decreased rapidly so that only 42% of its initial magnitude remained 5 min after alignment. A twisting field was applied perpendicular to the direction of alignment and the rate at which particles reoriented to this new direction was followed. The same twisting was repeated for particles suspended in a series of viscosity standards. Based on this approach, the low-shear apparent intracellular viscosity was estimated to be 1.2-2.7 X 10(3) Pa.s (1.2-2.7 X 10(4) poise). Time-lapse video microscopy confirmed the alignment of ingested particles upon magnetization and showed persistent cellular motility during randomization of alignment. Cytochalasin D and low temperature both reduced cytoplasmic activity and remanent-field decay, but affected rheology differently. Magnetic particles were observed in association with the microtubule organizing center by immunofluorescence microscopy; magnetization did not affect microtubule distribution. However, both vimentin intermediate filaments and f-actin reorganized after magnetization. These data demonstrate that magnetometry of isolated phagocytic cells can probe organelle movements, rheology, and physical properties of the cytoskeleton in living cells.

Magnetic transducer head utilizing magnetoresistance effect

Abstract: In the MR magnetic head of the present invention, its sensing element (2) comprises a plurality of superposed magnetic layers (4,5) having magnetoresistance effect in at least one of them and a nonmagnetic intermediate layer (3) sandwiched therebetween, and a sensing current (i) is fed to flow in the sensing element (2) in the same direction as a signal magnetic field applied to the element. Each of the magnetic layers (4,5) is so formed as to have an easy axis of magnetization substantially perpendicular to the signal magnetic field or to have an isotropic magnetic characteristic in the magneticfilm plane, thereby avoiding generation of Barkhausen noise with certainty.

Polarized neutron study of the compounds Y2Fe14B and Nd2Fe14B

Abstract: Outstanding permanent magnet properties were recently observed in a Nd‐Fe‐B compound which was shown to crystallize in a new phase, R2Fe14B. Polarized neutron measurements are reported on Y2Fe14B and Nd2Fe14B single crystals. The 3d Fe moment on different sites in Y2Fe14B at 4.2 K is closely related to local environment. Its value is maximum for atoms in σ‐like layers at the center of an Fe antiprism. On the contrary, it is reduced to 1.95 μB by 4d‐3d electron transfer and hybridization for Fe atoms which have the largest coordinance number of Y atoms, 4. The measurements at 250 K reveal a larger thermal decrease of the 3d moment for Fe atoms which exhibit shortest Fe‐Fe interatomic distances. This property reveals a reduction of 3d magnetic interactions for short distances as was previously observed in R2Fe17 compounds. In Nd2Fe14B, the low values obtained at 4.2 K for Nd magnetic moments suggest that the magnetic structure, determined by a competition between 3d‐4f exchange interactions and crystal‐fiel...

Magnetic properties of R2Fe14B single crystals

Abstract: A whole new class of high‐performance permanent magnet materials is based on the ternary tetragonal structure R2Fe14B, where R is one of the rare‐earth elements. We have successfully grown single crystals of this structure with R=Y, Nd, and Tb. Y is a nonmagnetic rare‐earth substitute, while Nd and Tb couple ferro‐ and ferrimagnetically, respectively, relative to the iron moment. All three of the compounds have [001] easy axes at room temperature, although the Nd compound exhibits a spin reorientation away from the [001] below about 150 K. Nd2Fe14B has a saturation induction at room temperature of 16.2 kG, which places an upper limit of approximately 65.6 MGOe on the energy product obtainable by magnets based on that material. While Tb2Fe14B exhibits a smaller magnetization because of ferrimagnetic coupling of the rare earths and the iron, it also has an extremely large magnetic anisotropy which is nearly temperature independent between 4.2 and 300 K.

Magnetic properties of amorphous metals

Abstract: Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe x Ni 1− x ) 75 P 16 B 5 Al 3 and Fe x B 1− x . The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mossbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.

Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles

Abstract: Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles were investigated. Saturation magnetization of these particles decreased with decreasing particle size. This result can be explained with the existence of a nonmagnetic thin layer, several angstroms in thickness, on the surface of the particles. Particle size dependence of coercivity for these particles, below 1000 A in size, follows the superparamagnetism theory. For the decrease in coercivity above 2000 A, multidomain nucleation is inferred as one of the possible mechanisms. Particle shape dependence of coercivity is well explained with the coherent rotation model, considering shape anisotropy.

rf sputtering of highly Bi‐substituted garnet films on glass substrates for magneto‐optic memory

Abstract: Using glass substrates, we have prepared highly Bi‐substituted garnet [(Bi,Y,Fe,Al)8O12 and (Bi,Gd,Fe,Al)8O12] films by rf diode sputtering which was followed by annealing for crystallization. The films were polycrystalline garnet with no preferred orientation and no impurity crystalline phase detectable by x‐ray analysis, which contained 1.1–1.4 Bi ions per formula unit. The films crystallized directly during the sputter deposition when the substrate temperature Ts>440 °C. Film quality was best when Ts was just below 440 °C. The film had magnetic anisotropy perpendicular to the film plane, which is stress induced. In the (Bi,Gd,Fe,Al)8O12 film, the magnetization normal to the film plane exhibited strong coercivity, Hc=600 Oe, and a large remanence‐to‐saturation ratio, Mr/Ms=1, while in the (Bi,Y,Fe,Al)8O12 film, weak Hc (200 Oe) and small Mr/Ms (0.47). These films had large Faraday rotation, typically 1.5×104 deg/cm at λ=633 nm, owing to the high Bi substitution.

CeCu2: a new Kondo lattice showing magnetic order

Abstract: The authors report measurements on the lattice parameter, electrical resistivity, thermal conductivity, thermoelectric power, susceptibility, magnetisation and specific heat as well as preliminary neutron diffraction results for the orthorhombic compound CeCu2. The Ce valence is close to three and the overall crystal-field splitting seems to be approximately=200K. CeCu2 behaves as a Kondo lattice system with a 'Kondo temperature' in the crystal-field ground state being of the order of 10-20K. Below 3.5K CeCu2 shows magnetic order of an anisotropic antiferromagnetic type.

Ba–ferrite thin‐film disk for perpendicular magnetic recording

Abstract: The authors have prepared Ba–ferrite film disks and evaluated their magnetic read/write characteristics. The c‐axis‐oriented Ba–ferrite thin films were formed epitaxially on c‐axis‐oriented ZnO films by means of a targets‐facing‐type sputtering system. The direction of easy magnetization in the film with perpendicular crystalline anisotropy is almost perpendicular to the film plane. Thus, the perpendicular remanence is more than 60 times as large as the in‐plane one. The simple Ba–ferrite films, however, have a coercive force, Hc⊥ above 2 kOe which is too large to write on by means of a conventional head. The substitution of In3+ in the Ba–ferrite films makes the Hc⊥ values decrease to about 1 kOe without increasing the critical temperature, Tcritical, of c‐axis orientation. The read/write characteristics of the films were measured with a MnZn–ferrite ring head. The reproduced signal exhibits clear dipulsive waveforms as the dipulse ratio exceeds 0.7. Furthermore, the overwrite ratio decreases below −28 dB for the In3+‐substituted Ba‐ferrite film disk. The Ba–ferrite film disks have promising potential as a high‐density perpendicular magnetic recording medium.

Origin of the Field-Induced SDW in the Bechgaard Salts

Abstract: The energetic gain giving rise to the magnetic-field-induced SDW state in the Bechgaard salts is clarified to originate in the effect of the field to the electronic orbital motion. A variational total energy of the SDW state under the field is obtained on the basis of the anisotropic two-dimensional Hubbard model. Its semi-classical and fully quantum evaluations for the case of the most basic set of the SDW wave vector and the magnetization clearly show a gain of the total energy under the field, which is in fair agreement with the observed increase of the perpendicular magnetic susceptibility. The total energy proves to be lowered by quantization of the closed orbits, since the zero-field state density in the energy region of closed-orbits is a decreasing function of energy. The nature of the successive SDW phase transitions under the field is briefly discussed.

Magnetic field structures in active compact radio sources

Abstract: The analysis of simultaneous multifrequency linear polarimetry data between 1.4 GHz and 90 GHz for about 20 active, compact radio sources at six epochs from 1977 December 10 1980 July is presented. In addition, monthly 8 Ghz polarization data on the same sources were examined. The general polarization characteristics of these sources can be well described in terms of magnetic fields which are largely turbulent and slightly anisotropic. The magnetic field symmetry axes are generally aligned with the source structural axes on the milli-arcsecond scale (OJ 287 is a notable exception.) Monte Carlo calculations indicate that observed polarization variations and in particular rotator polarization events can be produced in this model as a consequence of random walks generated through evolution of the turbulent magnetic field. 43 references.