Showing papers on "Magnetization published in 1989"

Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange

Abstract: The electrical resistivity of Fe-Cr-Fe layers with antiferromagnetic interlayer exchange increases when the magnetizations of the Fe layers are aligned antiparallel. The effect is much stronger than the usual anisotropic magnetoresistance and further increases in structures with more than two Fe layers. It can be explained in terms of spin-flip scattering of conduction electrons caused by the antiparallel alignment of the magnetization.

Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo

Abstract: In this study the exchange between 1H magnetization in "free" water (1Hf) and that in a pool with restricted motion (1Hr) was observed in tissues in vivo using NMR saturation transfer methods Exchange between these two pools was demonstrated by a decrease in the steady-state magnetization and relaxation times of 1Hf with radiofrequency irradiation of 1Hr The pseudo-first-order rate constant for the movement of magnetization from 1Hf to 1Hr was approximately 1 s-1 in kidney and approximately 3 s-1 in skeletal muscle in vivo Proton NMR imaging demonstrated that this exchange was tissue specific and generated a novel form of NMR image contrast The extent of exchange between 1Hf and 1Hr as well as the topological correlation of the exchange with relaxation weighted images suggests that this pathway is a major determinant of the observed relaxation properties of water 1H in vivo

MR imaging of motion with spatial modulation of magnetization.

Abstract: A novel magnetic resonance imaging technique provides direct imaging of motion by spatially modulating the degree of magnetization prior to imaging. The preimaging pulse sequence consists of a radio-frequency (RF) pulse to produce transverse magnetization, a magnetic field gradient to "wrap" the phase along the direction of the gradient, and a second RF pulse to mix the modulated transverse magnetization with the longitudinal magnetization. The resulting images show periodic stripes due to the modulation. Motion between the time of striping and image formation is directly demonstrated as a corresponding displacement of the stripes. This technique can be used to study heart wall motion, to distinguish slowly moving blood from thrombus, and to study the flow of blood and cerebrospinal fluid.

Tight-binding approach to the orbital magnetic moment and magnetocrystalline anisotropy of transition-metal monolayers

Abstract: A perturbative theory of magnetocrystalline anisotropy and orbital moment in itinerant ferromagnets is presented that clearly outlines the close connection between these two quantities. The theory is used to study the magnetocrystalline anisotropy in transition-metal monolayers. The importance of the crystal-field energy and of the filling of the valence band is emphasized. For the first time the orbital contribution to the magnetization in monolayers is estimated; it is shown that it may produce an anisotropy in the magnetization of the order of 0.1\ensuremath{\mu}B per atom.

Switching mechanisms in cobalt-phosphorus thin films

Abstract: Studies have shown that although the interactions between grains in magnetic thin-film recording media can enhance the squareness, these interactions are also responsible for media noise. The authors made measurements on Co-P thin films that give a simple and practical way of investigating the role of interactions in the magnetic processes that occur in these films. All magnetic measurements were made on a PAR 155 vibrating sample magnetometer with a resolution of 10/sup -5/ EMU. The field measurements were made with a Bell-Gauss Hall probe calibrated with a proton magnetometer to give an accuracy of >

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

Abstract: We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 05 Pb 05 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV

Kim model for magnetization of type‐II superconductors

Abstract: We have calculated the initial magnetization curves and complete hysteresis loops for hard type‐II superconductors. The critical‐current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical‐state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two‐dimensional problem to a one‐dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the di...

Perpendicular magnetic anisotropy and magneto-optical Kerr effect of vapor-deposited Co/Pt-layered structures

Abstract: We prepared by vapor deposition at room temperature thin (500 A) Co/Pt multilayers or layered structures directly onto glass or Si substrates. They show a preferential magnetization perpendicular to the film plane for Co thicknesses below 12 A and a 100% perpendicular remanence for Co thicknesses below 4.5 A. The magnetic anisotropy can be explained by an interface contribution to the anisotropy. We also investigated the magneto‐optical (MO) polar Kerr effect of these multilayers. Because of their excellent magnetic properties and their potentially high oxidation and corrosion resistance, these Co/Pt‐layered structures are very promising candidates for MO recording. The Kerr rotation θk at λ=820 nm for a 35×(4.0 A Co+12.7 A Pt)‐layered structure, which has 100% magnetic remanence, is modest (−0.12°), but the reflectivity R is high (70%), which results in a respectable figure of merit Rθ2k. Furthermore, the Kerr effect increases towards shorter wavelengths and thus favors future higher‐density recording.

Magnetic and magneto‐optical properties of rare‐earth transition‐metal alloys containing Dy, Ho, Fe, Co

Abstract: Amorphous rare‐earth transition‐metal alloys of composition REl−xTMx with RE=Dy, Ho; TM=Fe,Co and 0

Predicted time dependence of the switching field for magnetic materials.

Abstract: A scaling relationship between switching fields, of which remanent coercivity is a prominent example, and measurement time is derived. The energy barrier to thermal fluctuations is found to exhibit a (3/2-power dependence on the difference between the applied field and the nonthermally assisted switching field. This (3/2-power dependence contrasts with the 2-power dependence which has been widely assumed in the literature. Implications for magnetic viscosity and the orientational dependence of the time-dependent switching field in certain small, isolated particles are also discussed.

Direct Solution of the Landau-Lifshitz-Gilbert Equation for Micromagnetics

Abstract: A mathematical framework is presented for solving the Landau-Lifshitz-Gilbert equation expressed in Cartesian components of magnetization according to the backward difference method without conflicting with the constraint of constant magnetization. Test calculation shows that the method allows the use of a large time step almost independent of spatial mesh size and damping constant. The derived program is used to calculate the magnetization structure of a crosstie wall in a Permalloy film yielding calculated structures which closely resemble the electron-holography image of an actual cross-tie wall. It is also used to investigate magnetization reversal mechanisms in fine ferromagnetic particles by pursuing time dependent changes in magnetization structures. The paper gives detailed descriptions of the reversal mechanisms which differ depending on the size of the particle.

Optical second-harmonic generation from magnetized surfaces.

Abstract: We propose optical second-harmonic generation as a means to probe surface magnetization. It is shown that surface magnetization can induce a number of nonlinear susceptibility elements that would vanish otherwise. They are presented for the (001), (110), and (111) surfaces of a fcc centrosymmetric crystal. An order-of-magnitude estimate, using the microscopic expression of the nonlinear susceptibility, suggests that these induced elements are detectable by optical second-harmonic generation with appropriate polarization combinations. The second-harmonic signals from magnetized and nonmagnetized surfaces should exhibit characteristically different rotational anisotropy.

Perpendicular magnetic anisotropy and magnetostriction of sputtered Co/Pd and Co/Pt multilayered films

Abstract: Magnetic properties of Co/Pd and Co/Pt multilayers were studied as a function of sputtering gas pressure. It was found that the magnetic properties of the films depended upon the sputtering gas pressure, and large coercivity (several kOe) and a perfect squareness of the perpendicular hysteresis loop were attained by the deposition at the high gas pressure. These multilayers are suitable for perpendicular magnetic recording media. In the Co/Pd, the origin of the perpendicular magnetic anisotropy is mainly an interfacial anisotropy, although there exists a stress‐induced anisotropy for the films with the relatively short periodicity. The perpendicular magnetic anisotropy of the Co/Pt can be concerned with the Co‐Pt mixture formed at the interface of the multilayer structure. The magnetostriction of the Co/Pd films with the relatively short periodicity is extraordinarily large with a negative sign, since Co atoms adjacent to Pd atoms at the interface could induce larger magnetostriction than that of pure Co....

Intrinsic magnetic properties of the iron-rich ThMn12-structure alloys R(Fe11Ti); R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu

Abstract: Magnetic properties of the series of ThMn12-structure intermetallic compounds R(Fe11Ti) have been determined for rare earths from Nd to Lu plus Y. The highest Curie temperature (607 K) is for R=Gd, and R-Fe exchange interactions are much stronger for light rare earths than for heavy ones. The temperature dependence of the iron sublattice magnetisation and anisotropy are determined for the Y and Lu compounds. Spin reorientation transitions are found as a function of temperature for the rare earths with a negative second-order Stevens coefficient alpha J(Nd, Tb, Dy), and a set of crystal-field parameters is derived to account for the transitions in a consistent way. A sharp increase in magnetisation observed for Sm(Fe11Ti) below 130 K in a field of about 10 T applied perpendicular to the easy direction indicates that J-mixing may be important for Sm3+. Compared with R2Fe14B, the iron anisotropy in R(Fe11Ti) is greater, and the rare-earth anisotropy is much weaker at low temperature, with the opposite sign for the rare-earth crystal-field coefficient A20. The average iron moment is 1.7 mu B in R(Fe11Ti) at 4.2 K; Mossbauer spectra are analysed to yield the average moments on each site. Limits set by the intrinsic magnetic properties on the performance of magnets made from these families of alloys are discussed.

Critical-state model for harmonic generation in high-temperature superconductors.

Abstract: High-temperature superconductors exhibit harmonic generation when immersed in an ac magnetic field. To explain this phenomenon, we propose a macroscopic critical-state model as an alternative to the loop model used by Jeffries et al. While the original Bean model of the critical state only predicts odd harmonics, our extended model also predicts even harmonics by taking into account the dependence of the critical current upon magnetic field. The results of our measurements of harmonic signals as a function of ac magnetic field, dc magnetic field, temperature, and harmonic number are consistent with the proposed model. In particular, we find that, as the magnetic field is increased, the critical current crosses over from the Bean regime, where ${J}_{c}$ is independent of field, to the Anderson-Kim regime, where ${J}_{c}$ is approximately inversely proportional to the field.

Direct measurement of the temperature-dependent magnetic penetration depth in Y-Ba-Cu-O crystals.

Abstract: The temperature dependence of the magnetic penetration depth lambda is determined directly from low-field dc magnetization measurements on single crystal Y-Ba-Cu-O (T/sub c/ = 89.7 K, ..delta..T/sub c/ = 0.2 K). The results are consistent with the behavior expected from a BCS (s wave) superconductor.

Magnetic resonance determination of the antiferromagnetic coupling of Fe layers through Cr.

Abstract: Variable frequency ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes in single-crystal Fe/Cr/Fe(001) sandwiches grown by molecular-beam epitaxy. Magnetization M and magnetoresistance measurements also were carried out on these samples, which exhibited antiferromagnetic (AF) layer alignment for 12 A\r{}lt(Cr)25 A\r{}. The FMR data reveal two resonance modes with complex frequency dependences for the AF-aligned samples. The detailed FMR and M vs H behavior can be quantitatively explained by an AF coupling parameter J which has a thickness dependence peaked about t(Cr)=16 A\r{}.

Critical currents near 106 A cm−2 at 77 K in neutron-irradiated single-crystal YBa2Cu3O7

Abstract: THE defects in as-grown bulk YBa2Cu3O7 have been shown to act as weak flux pinning sites, leading to a reversible magnetization1, magnetization decay2 and non-zero resistivity3 in moderate magnetic fields at temperatures near the transition temperature, Tc. These effects limit the technological utility of the as-grown material3. This is not entirely surprising, as both single crystals and polycrystalline materials are grown under conditions expected to minimize the defect density, but the weak pinning in these materials has nevertheless raised the issue of a possible intrinsic limitation to the critical current density. Here we report the attainment of a critical current density, Jc, of ∼ 6 x 105 A cm-2 at 77 K and 9 kOe, in a single crystal of Yba2Cu3O7 irradiated with fast neutrons to introduce defects. (In both irradiated and unirradi-ated crystals, the critical current density was inferred by measuring the magnetic moment resulting from an induced persistent current.) This hundredfold enhancement of Jc relative to unirradiated crystals demonstrates that artificially induced defects can act as strong flux pinning sites, and analysis of the defects should increase our understanding of flux pinning in high- Tc superconductors.

Spiral phase of a doped quantum antiferromagnet.

Abstract: A low density of vacancies in a 2D, spin-1/2, Heisenberg antiferromagnet leads (for a range of effective couplings) to a metallic phase with incommensurate antiferromagnetic order, i.e., with the staggered magnetization rotating in a plane with the wave number proportional to the density. This structure originates from the polarization of the antiferromagnetic dipole moments of the vacancies. The excitation spectrum of this spiral state includes an interesting low-lying mode. Implications for neutron scattering and normal-state resistivity are discussed.

Sherrington-Kirkpatrick model in a transverse field: Absence of replica symmetry breaking due to quantum fluctuations.

Abstract: The Sherrington-Kirkpatrick model under a transverse field is studied here employing the Suzuki-Trotter formula to map the model to an equivalent classical one. The effective Thouless-Anderson-Palmer free energy is used to study the stability of the system, and Monte Carlo computer simulations of the effective classical model are performed to obtain the phase diagram and the magnetization overlap distribution. Our results indicate a trivial overlap distribution due to quantum fluctuations. The phase diagram shows a slight initial increase in the glass transition temperature ${T}_{g}$ as the transverse field is switched on, confirming that obtained by Yokota.

Microwave ion source

Abstract: A microwave ion source suitable for an apparatus which requires ions of an element of high reactivity such as oxygen, fluorine, etc., the microwave ion source being arranged to transmit microwaves between outer and inner conductors of a coaxial line. An ion extraction electrode is formed at least partly of a low magnetic permeability material while an acceleration electrode is formed of a high magnetic permeability material. The acceleration electrode is formed so as to have a structure in which a low magnetic permeability material of a certain thickness is stacked on the high magnetic permeability material at a plasma chamber side and openings of ion exit holes are formed in the portion of the low magnetic permeability material. A permanent magnet constituting a magnetic field generating means is provided to surround the microwave lead-in coaxial line. The direction of magnetization of the permanent magnet is made to coincide with the axial direction of the coaxial line. The end surface of the permanent magnet at the microwave lead-in side is coupled with the periphery of the high magnetic permeability material of the acceleration electrode through another high magnetic permeability material to form a magnetic path. The plasma chamber is formed of a dielectric insulator which transmits microwaves well. It is possible to realize an ion source in which ions can be extracted with a high electric field, and in which a high current ion beam can be extracted for a long time.

Particulate magnetic recording media: a review

Abstract: After describing briefly the principles of magnetic recording and the manufacture of media, the author reviews the history, characteristics, and development trends of the most important particulate magnetic recording materials. These are acicular iron oxides, chromium dioxide, cobalt-modified iron oxides, acicular metal particles, and barium ferrite. A trend common to most of the materials is particle size reduction. >

Temperature dependence of lower critical fields in Y-Ba-Cu-O crystals

Abstract: The temperature dependence of the anisotropic lower critical fields ${H}_{c1}$ in single-crystal Y-Ba-Cu-O is determined from the onset of flux penetration in zero-field-cooled dc magnetization versus temperature from ${T}_{c}$ down to $0.5{T}_{c}$. An extended Bean-critical-state model is developed to define this onset consistently. The ${H}_{c1}'\mathrm{s}$ are linear in temperature near ${T}_{c}$ as in Ginzburg-Landau theory and consistent with a clean local-limit BCS form with extrapolated ${H}_{c1}(T=0)=180$ Oe for $\mathrm{H}\ensuremath{\perp}\stackrel{^}{c}$ and 530 Oe for $\mathrm{H}\ensuremath{\parallel}\stackrel{^}{c}$.

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Abstract: Electronic band-structure calculations are presented for the ferromagnetic compounds CrTe, Cr3Te4 and Cr2Te4 and Cr2Te3. In these compounds the Cr3d-Te5p covalency and the Cr3d(z2)-Cr3d(z2)-overlap along the c axis are the most important interactions. The magnetic polarisation of Te is parallel to the Cr local moment in CrTe, antiparallel to it in Cr2Te3 and about zero in Cr2Te4. Measurements of electronic transport properties (resistivity, Hall effect and thermo-electric power) and magnetic properties of Cr1- delta Te( delta =0.1) and Cr3+xTe4(x=0.2) indicate that these chromium tellurides are p-type metals, with strong interaction between the holes in the Te5p band and the Cr magnetic moments. In the literature the variation of the magnetic properties of Cr3.2Te4 near Ts approximately=100 K has been attributed to a change from a canted antiferromagnetic to a collinear ferromagnetic structure. However, our Hall-effect measurements indicate that the spin structure is not collinear ferromagnetic above Ts.

Large magnetic hysteresis in a melt‐textured Y‐Ba‐Cu‐O superconductor

Abstract: Melt‐textured YBa2Cu3O7−δsuperconductor with a long and well‐aligned grain structure is shown to exhibit magnetic hysteresis at 77 K which is the largest ever reported for bulk polycrystalline Y‐Ba‐Cu‐O. The large ΔM resulted in a magnet‐like behavior as well as a strong suspension phenomenon. It is shown, contrary to the previous reports, that the suspension behavior observed in Y‐Ba‐Cu‐O is a generic consequence of large grain size, and not due to the presence of Ag oxide or Ag particles. There appears to be no substantial enhancement in flux pinning and J c by these particles. Comparisons of magnetization behavior in various YBa2Cu3O7−δ samples (polycrystals, silver‐oxide doped, melt‐textured, and single crystals) indicated that for applied fields substantially larger than H c 1 the current flow that gives rise to the observed magnetization is intragranular. The pinning force and hence the critical current is roughly the same [J c (magn)∼104 A/cm2] within a factor of ∼2 regardless of grain size, grain boundary configuration, presence of second phase particles, or increased dislocation and twin densities.

Spin-wave results for the triangular Heisenberg antiferromagnet

Abstract: We perform the standard spin-wave analysis of the triangluar Heisenberg quantum antiferromagnet. Contrary to the variational calculation of Anderson, we diagonalize exactly the quadratic part of the spin-wave Hamiltonian and obtain results in a 1/S expansion. We compute the ground-state staggered magnetization per spin and the ground-state energy. For spin (1/2, the agreement with the variational values of Huse and Elser suggests that the trangular lattice has long-range magnetic order.