Showing papers on "Magnetization published in 1988"

Giant flux creep and irreversibility in an Y-Ba-Cu-O crystal: An alternative to the superconducting-glass model.

Abstract: We report strong, anisotropic magnetic relaxation of the field-cooled and zero-field-cooled magnetization along the principal axes of an Y-Ba-Cu-O single crystal and interpret it with a thermally activated flux-creep model. A simple scaling argument shows that high thermal activation causes magnetic irreversibilities and critical currents to drop below the threshold of detectability at a reduced temperature difference $1\ensuremath{-}t$ proportional to ${H}^{\frac{2}{3}}$, a power frequently observed in experiment and in particular in our crystal.

Perpendicular magnetic anisotropy in Pd/Co and Pt/Co thin‐film layered structures

Abstract: rf sputtered Pd/Co and Pt/Co thin‐film layered structures (LS) have perpendicular magnetic anisotropy, when the Co layer is ultrathin (<8 A in Pd/Co and <14 A in Pt/Co). The Co thickness (T) dependence of the anisotropy energy (Ku∼T) and the effective anisotropy field (HK∼1/T) in Pd/Co LS support an interfacial anisotropy as the source of the perpendicular magnetic easy axis. In contrast, the anisotropy is independent of Co thickness for thin Co layers in Pt/Co LS, and thus the mechanism for the perpendicular easy axis is thought to be different. We describe magnetization, resistivity, and x‐ray diffraction data that consistently support sharper interfaces in Pd/Co LS than in Pt/Co LS.

X-ray resonance exchange scattering.

Abstract: Large resonant magnetization-sensitive x-ray scattering is predicted to occur in the vicinity of the L/sub II/, L/sub III/, and M/sub II/--M/sub V/ absorption edges in the rare-earth and actinide elements, and at the K and L edges in the transition elements. These ''magnetic'' resonances result from electric multipole transitions, with the sensitivity to the magnetization arising from exchange. For some transitions, the magnetic scattering will be comparable to the charge scattering. The general features of the observed L/sub III/ resonance in Ho are discussed.

Granular and superconducting-glass properties of the high-temperature superconductors

Abstract: The microstructure of bulk samples of the copper-oxide high-temperature superconductors commonly is describable in terms of anisotropic grains of stoichiometric material separated by layers of nonstoichiometric interface material. The granularity strongly influences the electromagnetic properties, especially the transport critical-current density and the magnetization. In this paper, a simple theoretical model for the granularity is introduced and then used to discuss a number of electrodynamic properties (hysteretic magnetization versus magnetic field, zero-field-cooled and field-cooled magnetization versus temperature, ac susceptibility, and flux creep with logarithmic time dependence). Special attention is drawn to the importance of distinguishing between intragranular and intergranular effects.

Quantum tunneling of magnetization in small ferromagnetic particles.

Abstract: The probability of tunneling of the magnetization in a single-domain particle through an energy barrier between easy directions is calculated for several forms of magnetic anisotropy. Estimated tunneling rates prove to be large enough for observation of the effect with the use of existing experimental techniques.

Magnetic anisotropy in metallic ultrathin films and related experiments on cobalt films (invited)

Abstract: We present experimental and theoretical investigations on the magnetic anisotropy of Co ultrathin films sandwiched by Au. Ferromagnetic resonance experiments revealed the presence of a large perpendicular surface anisotropy that makes the easy magnetization direction become perpendicular to the film plane for Co thicknesses lower than 11 A, as is observed in magnetization measurements. In order to explain this surface anisotropy, we propose various models, taking into account the imperfections of the films. For thicknesses below 11 A, there is a large increase of the coercive field with decreasing thickness. This effect is tentatively interpreted in a model of propagating Bloch walls, where the interfacial roughness plays an important role.

Micromagnetic studies of thin metallic films (invited)

Abstract: A computer simulation model has been developed to conduct micromagnetic studies of thin magnetic films. Thin‐film media are modeled as a planar hexagonal array of hexagonally shaped grains. Each grain is a single domain particle whose magnetization reverses by coherent rotation. The computation utilizes coupled gyromagnetic dynamic equations with phenomenological Landau–Lifshitz damping. In particular, the effects of particle interactions are investigated. The effect of media microstructure on magnetic hysteresis is examined as well as the effect of intergranular exchange coupling. The difference between planar and completely random orientation of the crystalline anisotropy axes is discussed. Recorded transitions are simulated by allowing a pair of perfect transitions to relax. With no intergranular exchange coupling, the transitions show profound irregularity and zig‐zag structure. Intergranular exchange coupling produces more uniform transitions with increased zig‐zag structure amplitude. For a closely ...

Magnetization processes in ferromagnetic cubes

Abstract: A cubic discretization procedure of the micromagnetic energy functional is used to carry out numerical studies of the magnetization process in ferromagnetic cubes. Equilibrium magnetization configurations and their switching behavior are calculated for particle sizes in the range from 100 to 550 A. In the model calculations the particles are assumed to have uniaxial crystalline anisotropy with an anisotropy constant of 18 500 erg/cm3, a saturation magnetization of 370 emu/cm3, and an exchange constant of 10−6 erg/cm. For particle sizes smaller than 520 A the remanent state has a flowerlike magnetization configuration. Beyond 520 A this state is replaced by a vortex structure about the easy axis. For particles smaller than 450 A switching occurs by approximately uniform rotation of the flower state. The switching fields are larger than the corresponding Stoner–Wohlfarth value. Beyond 450 A the application of an external field leads to the formation of a vortex configuration. The switching of the vortex con...

Coupling of electronic charge and spin at a ferromagnetic-paramagnetic metal interface.

Abstract: Microscopic models are presented to elucidate the concept of interfacial charge-spin coupling. At the interface between a ferromagnet and a paramagnet, the spin subbands are loosely coupled, an interfacial conductance may be defined for each, and a result of their inequivalence is that an electric current flowing from a ferromagnetic metal into a paramagnetic metal will be partially spin polarized, i.e., will have an associated current of magnetization. The inverse is also true; nonequilibrium magnetization present in a paramagnetic metal can be detected as an open circuit voltage across an interface between the paramagnet and a ferromagnet. Using this effect, a new technique to measure conduction electron relaxation times is described.

Single‐domain particles in rocks and magnetic fabric analysis

Abstract: Magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks, and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes. We have recently demonstrated however in laboratory produced samples that, as theoretically expected, susceptibility anisotropy is dependent on particle size. Multidomain particles of magnetite, or similar strongly magnetic particles with shape anisotropy, exhibit a maximum susceptibility parallel to their easy (remanent) magnetization axes whereas uniaxial single-domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes. Hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state. By demonstrating uniaxial single-domain anisotropy characteristics for the first time in a rock specimen, we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro- or ferrimagnetic particles in rocks. In particular, apparently foliated rocks can actually be lineated (and vice-versa). Moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence. Remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment.

Magnetism in cupric oxide

Abstract: Single-crystal neutron diffraction measurements have been used to study the long-range magnetic ordering in cupric oxide, CuO. An incommensurate antiferromagnetic structure forms below the Neel temperature on 230(1) K, with a propagation vector (0.506(1)a*-0.483(1)c*) which remains constant down to a magnetic phase transition at 213(1) K. Below the latter temperature, the structure remains antiferromagnetic with a commensurate propagation vector (1/2 0-1/2), and this structure persists to the lowest temperature reached in the investigation (20 K). The arrangement of the copper moments in both phases is such that the n-glide perpendicular to the b axis of the monoclinic cell, space group C2/c, does not reverse the direction of the spin. The two magnetic sublattices related to the C-face-centring scatter in phase quadrature and the relative directions of the spin on them could not be determined. Good agreement is obtained for the commensurate phase with a multipole model for the copper magnetisation density and spins of 0.65(3) mu B directed parallel to b. The lower sublattice magnetisation in the incommensurate phase precluded a meaningful multipole fit, but a reasonable agreement is obtained with a model in which the spins rotate in the a-c plane following an elliptical envelope with major axis directed 33(2) degrees to c in beta obtuse and a maximum moment of 0.38(2) mu B at 215 K. The paramagnetic scattering at ambient temperature and 550 K was measured to try to find the origin of the peak in the susceptibility. No significant paramagnetic scattering could be obtained from a powdered sample although the sensitivity of detection was some five times that required to observe the scattering from a Cu2+ ion in an ideal paramagnet.

Mobile Vacancies in a Quantum Heisenberg Antiferromagnet

Abstract: The ground state of a quantum vacancy in a 2D antiferromagnet is found to involve a long-range dipolar distortion of the staggered magnetization. An effective Hamiltonian of vacancies interacting with long-wavelength spin waves is derived. The implications for the anitferromagnetic long-range order are discussed.

Structure and properties of some novel ternary Fe-rich rare-earth intermetallics (invited)

Abstract: The structures and the magnetic properties of some novel ternary compounds were studied. These compounds have the approximate composition RFe10T2, where T represents Cr, V, Ti, Mo, W, or Si. The structure of all these compounds can be derived from the tetragonal ThMn12 structure type, but the crystallographic position occupied by the different T atoms is not the same. Most of the compounds have a homogeneity range represented by RFe12−xTx. This homogeneity range is fairly large in the case of T=V and strongly asymmetric for T=Ti and W. A structure determination is given for a representative member of the RFe10Si2 family. This structure determination is compared with results obtained previously for T=Mo and V, in which latter compounds the site occupancy of the T component is completely different from that in RFe10Si2. A survey is given of the magnetic and invar properties of RFe10T2 compounds. Special attention is paid to an analysis of the magnetocrystalline anisotropy in these materials.

Formation and magnetic properties of the compounds R2Fe14C

Abstract: It was shown by X-ray diffraction that rare earth compounds of the type R2Fe17 can dissolve appreciable amounts of carbon up to x = 0.6 in R2Fe17Cx at 900 °C. The increase in the lattice constants is accompanied by a strong increase in Curie temperature. From a structure determination (R = 7%) it was found that the carbon atoms dissolve interstitially and occupy the 9e position in the Th2Zn17 structure type (R3m). This ternary carbide is present as the main phase when compounds of the type R2Fe14C are heated above their decomposition temperatures. The decomposition proceeds by means of a solid state transformation and the corresponding transformation temperatures Tt vary strongly with the R component. In several of the light rare earth systems Tt becomes rather low and makes an annealing treatment ineffective with regard to the formation of R2Fe14C phases. The magnetic properties of the R2Fe14C phases are compared with those of R2Fe14B. It is shown that the former follow the magnetic behaviour of the latter almost completely.

Magnetostriction ‘‘jumps’’ in twinned Tb0.3Dy0.7Fe1.9

Abstract: Large ‘‘jumps’’ in the magnetostriction have been observed in twinned single crystals of Tb03Dy07Fe19 (Terfenol‐D) for magnetic fields parallel to the crystalline [112] direction The interpretation of these large magnetostriction discontinuities is based upon a model of twinned dendritic Terfenol‐D in which the magnetization of one twin jumps between two [111] directions while the magnetization of the remaining twin undergoes a continuous rotation of the magnetization The field dependence of the magnetization and magnetostriction of cubic single crystals with λ111≫λ100 was calculated using an expression which included the anisotropy constants K1 and K2 and compressive loads along [112] With K1=−06 J/m3 and K2=−20 J/m3 (values appropriate for Terfenol‐D near room temperature), magnetization ‘‘jumping’’ is predicted For the twinned crystal, the jump in the magnetostriction was calculated to be greater than 1000 ppm Because of this large magnetostriction, it is possible to configure a device to p

Noncollinear magnetic structure of CoFe2O4 small particles

Abstract: The magnetic structure of small CoFe2O4 particles has been investigated as a function of the particle size. Samples (in the 10–100 nm size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic applied field, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect of the crystallites that make up a particle is the origin of this phenomenon. A model is proposed in which the CoFe2O4 crystallites that make up a particle consist of a core with the usual spin arrangement and a boundary surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependency of this structure is also examined.

New critical-state model for critical currents in ceramic high- T c superconductors

Abstract: Critical-current and magnetization measurements are performed on a large number of ceramic superconductors of the YBa/sub 2/Cu/sub 3/O/sub 7/ family. The results show that critical currents in present ceramics depend strongly on sample geometry and are, in fact, completely determined by self-field limitation. The strong magnetic-field dependence leads to a general relation between critical current and magnetization which is experimentally found to be satisfied. A quantitative critical-state model based on an extension of Bean's theory is outlined which is able to predict transport and magnetic properties using microscopic current vs magnetic field relations. The underlying microscopic parameters are extracted from a comparison of the model calculation with measured transport and magnetization data.

Magnetic properties of amorphous ferromagnets (invited)

Abstract: Some magnetic properties of amorphous ferromagnets are well described within the random‐anisotropy real‐space model. This model assumes that the neighboring spins are ferromagnetically coupled with each other, and that there is a local magnetic anisotropy whose axes are correlated over a small length Ra due to short‐range structural order. The system is characterized by a small parameter λ∼R2aK/A which depends on temperature and on the concentration of magnetic atoms via the local anisotropy K and exchange constant A. In zero magnetic field the local magnetization smoothly rotates over the solid with a characteristic length Rf =Ra/λ2. The zero‐field susceptibility is very sensitive to the exchange, the anisotropy, and the amorphous structure: χ∝A3K−4R−6a. The magnetization law in approaching saturation (M→M0) is universal (M0−M)∝1/(H)1/2 for H<2A/M0R2a. These and other predictions of the model seem to be in a good agreement with many recent experimental results.

Hysteretic levitation forces in superconducting ceramics

Abstract: Magnetization forces between small permanent magnets and bulk high Tc superconducting ceramic materials have been measured and show marked hysteretic behavior as a function of the distance between the magnet and superconductor. Magnetic forces of up to 2500 dyn, both normal and tangential to the superconductor surface, have been made. Near‐reversible forces are obtained for small motions of the magnet, however, which are believed to be related to flux pinning.

High‐resolution magnetic imaging of domains in TbFe by force microscopy

Abstract: High‐resolution images of domains written in a magnetic thin film have been obtained for the first time using force microscopy. The sample consisted of 500‐A‐thick Tb19Fe81 with magnetization of 109 emu/cm3. Micron‐sized magnetic domains were thermomagnetically written in the sample using a focused laser beam. Domain images were obtained by observing the magnetic interaction of the sample with a small vibrating magnetized iron tip. Typical observed force gradients were in the range 0.8×10−4–6×10−4 N/m and the forces were in the range 10−12–10−11 N. The spatial resolution of the technique was on the order of 1000 A. This was sufficient resolution to see irregularities in those laser written marks which were recorded using low bias field.

Observation of enhanced properties in samples of silver oxide doped YBa2Cu3Ox

Abstract: Samples of YBa2Cu3O(x) doped with silver oxide (Tc is about 93 K) have exhibited attractive forces in gradient magnetic fields, both normal and tangential to the surfaces, which are more than twice the sample weight. This allows sample suspension below a rare earth magnet. Critical current density was increased by about 100 at 77 K. Persistent fields, flux pinning, magnetization, and modeling are described.

Experimental evidence for a transverse magnetization of the Abrikosov lattice in anisotropic superconductors.

Abstract: The torque on a superconductor in a magnetic field H has been thought to be dominated by trapped flux or sample shape effects, but it has recently been suggested that an anisotropic type-II material should experience an intrinsic torque for H(c1) much less than H, which in turn is less than H(c2). The predicted phenomenon results from transverse magnetization of the Abrikosov lattice. Measurements are presented on copper-oxide superconductors which delineate the experimental regime in which extrinsic effects are negligible and confirm the existence of the predicted intrinsic torque.

Exponential H and T decay of the critical current density in YBa2Cu3O7- delta single crystals.

Abstract: We report magnetic measurements on single crystals of YBa/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/. The magnetic critical current density in the Cu-O basal planes (1.5 x 10/sup 6/ Acm/sup 2/ at 4.2 K) decreases exponentially with temperature as well as with field for Tapprox. >50 K. This is ascribed to current tunneling through micro- Josephson-junctions. The behavior is radically different from that associated with macrojunctions typical of ''granular'' samples. It is argued that the anisotropy and the T-H anomalous behavior of J/sub c/ are connected with the T dependence and the anisotropy of both the coherence length and the electron mean free path.

Physical origin of limits in the performance of thin-film longitudinal recording media

Abstract: The application of thin-film media for Winchester-type rigid disk recording is considered The inherent close contact between the magnetic particles in the film introduces strong exchange coupling between them in addition to the magnetostatic interaction generally seen in the particulate-binder-type media The coupling causes the formulation of an intrinsic magnetic cluster that has a dimension much larger than the individual magnetic crystallites The exchange-coupling-induced magnetic cluster becomes the basic unit of magnetization reversal in the film under recording, and hence it is believed to cause noise and increase bit shift in the media It is shown that if a proper interparticle separation is created in the film, the exchange-coupling effect can be reduced or eliminated, so the resulting film will have the low media noise and bit shift required for high-performance-drive applications >

Superconductivity in the nd-sr-ce-cu-o system

Abstract: A new high-Tc superconductor Nd-Sr-Ce-Cu-O system has been found by the resistivity and magnetization measurements. The highest onset transition temperature is about 28 K. The superconducting phase may be described as (Nd1-xSrx-yCey)2CuO4-δ.

Force microscopy of magnetization patterns in longitudinal recording media

Abstract: A force microscope with a magnetic tip has been used to examine magnetization patterns in a thin‐film cobalt‐alloy sample similar to that used in magnetic disk recording. Longitudinal magnetic bits were written on discrete tracks with a recording head flown over the surface of the sample. After minimal sample preparation, images were obtained showing strong magnetic contrast. Model calculations for the expected image contrast were found to be in excellent qualitative agreement with experimental results. By using a constant height imaging mode, enhanced contrast for fine detail was obtained.

Wetting and layering in the nearest-neighbor simple-cubic Ising lattice: A Monte Carlo investigation.

Abstract: Critical, tricritical, and first-order wetting transitions are studied in a simple-cubic nearest-neighbor Ising model, with exchange J in the bulk and exchange ${J}_{s}$ in the surface planes, by applying suitable bulk and surface fields H and ${H}_{1}$. Monte Carlo calculations are presented for systems of size L\ifmmode\times\else\texttimes\fi{}L\ifmmode\times\else\texttimes\fi{}D, in a thin film geometry with D=40 layers and two free L\ifmmode\times\else\texttimes\fi{}L surfaces, with L ranging from L=10 to L=50. In addition, evidence for prewetting transitions and for layering transitions (the latter occur for temperatures T less than the roughening temperature ${T}_{R}$) is presented. We study the magnetization ${m}_{1}$ in the surface layer, susceptibilities ${\ensuremath{\chi}}_{11}$=\ensuremath{\partial}${m}_{1}$/\ensuremath{\partial}${H}_{1}$ and ${\ensuremath{\chi}}_{1}$=\ensuremath{\partial}${m}_{1}$/\ensuremath{\partial}H, surface excess magnetization ${m}_{s}$ and energy ${U}_{s}$, as well as magnetization and energy profiles [m(z) and U(z)] as a function of the distance z from the surface. We compare our results to various theoretical predictions; in particular, our results for critical wetting are consistent with simple mean-field behavior, the different singular behavior predicted by various renormalization-group treatments is not observed. We also analyze the qualitative structure of the full surface phase diagram of the model.

Magnetic properties of Fe‐rich rare‐earth intermetallic compounds with a ThMn12 structure

Abstract: Sm(Fe1−xMx)12 ternary compounds based on the tetragonal ThMn12 structure where M is Ti, Si, V, Cr, and Mo were investigated. M atoms have a preference for site occupation. Ti atoms occupy the 8i or 8j site and Cr atoms occupy the 8i site. Curie temperatures on Sm(M,Fe)12 compounds are around 590 K except for the SmMo2Fe10 compound (Tc=483 K). The SmTiFe11 and SmSi2Fe10 compounds have a high saturation magnetization and magnetic anisotropy.