Showing papers on "Magnetic anisotropy published in 1995"

Determination of spin- and orbital-moment anisotropies in transition metals by angle-dependent x-ray magnetic circular dichroism.

Abstract: Angle-dependent x-ray absorption spectroscopy in strong magnetic fields is shown to yield precise site specific information about spin and orbital moments in highly anisotropic $3d$ transition metal systems. A new angle averaging spin sum rule allows for the separate determination of spin and magnetic dipole moments in lower than cubic site symmetry. The microscopic origin of the magnetocrystalline anisotropy can be probed directly through the angular dependence of the orbital moment.

High anisotropy L1/sub 0/ thin films for longitudinal recording

Abstract: Bulk L1/sub 0/ phase materials, such as CoPt and FePt, are known for their high magnetocrystalline anisotropy and magnetic moment, properties that are also desirable for high density magnetic recording media. This work reports the preparation, microstructural characterization, magnetic properties, and magnetic recording performance of L1/sub 0/ phase thin (25 /spl Aring/ to 200 /spl Aring/) films. The samples are prepared by sputter deposition and ex-situ anneal. The microstructural characterization is by transmission electron microscopy and X-ray diffraction. High magnetocrystalline anisotropy is apparently maintained for even the thinnest films, where coercivity, H/sub c/, >10,000 Oe is observed for a 25 /spl Aring/ sample. The H/sub c/ of films prepared for magnetic recording measurements is considerably less to ensure that test patterns could be written. Samples for recording measurements were obtained by a reduction in anneal temperature or the codeposition of an oxide with the L1/sub 0/ phase material to limit grain growth during anneal.

Circularly magnetized non-contact torque sensor and method for measuring torque using same

Abstract: A torque sensor (2) comprises a magnetoelastically active element (4), and a magnetic sensor (6) responsive to the field of the magnetoelastically active portion. The magnetoelastically active portion comprises a ring of material (4) endowed with an effective uniaxial magnetic anisotropy such that the circumferential direction is the easy axis, and magnetically polarized in a substantially circumferential direction. The ring (4) is attached to the torqued member (8), such as a rotating shaft, so that application of a torque to the shaft is transmitted to the ring. A magnetic field vector sensor (6) is mounted in a fixed position relative to the ring (4) and oriented so that it responds maximally only to the field due to the axial magnetization components of the ring (4). The output of the sensor (6) is thus proportional to the change in orientation of the magnetization resulting from torque applied to the shaft (8) and transmitted to the ring (4). The magnetoelastically active portion (4) may include two or more axially distinct, magnetically contiguous, oppositely polarized circumferential regions (104, 105).

Microscopic origin of magnetic anisotropy in Au/Co/Au probed with x-ray magnetic circular dichroism.

Abstract: High-field, angle-dependent x-ray magnetic circular dichroism measurements on a Au/Co-staircase/Au structure reveal an anisotropy in the dichroism intensities parallel and perpendicular to the film plane. The size of this effect is related to the anisotropies of the spin density within the Wigner-Seitz cell and of the orbital magnetic moment, both increasing with decreasing Co thickness. The orbital moment anisotropy is shown to be the microscopic origin of the magnetocrystalline energy anisotropy.

Magnetic switching and in‐plane uniaxial anisotropy in ultrathin Ag/Fe/Ag(100) epitaxial films

Abstract: We have studied the process by which the in‐plane magnetization of an ultrathin (4–11 ML) epitaxial iron film reverses under the action of an external magnetic field. Kerr effect measurements reveal a small in‐plane uniaxial anisotropy superimposed on the cubic magnetocrystalline anisotropy which greatly influences the reversal. In addition, we find that depending upon the field orientation, reversal can proceed either via a ‘‘1‐jump’’ mechanism, by the sweeping of 180° domain walls and which gives a classic square hysteresis loop, or by a ‘‘2‐jump’’ mechanism, by the sweeping of 90° domain walls at two distinct applied field strengths—this gives a more unusual hysteresis loop with two irreversible transitions. We have developed a simple phenomenological energy model which explains how so small a uniaxial anisotropy can play so significant a role in the reversal process. The model explains the two reversal mechanisms and predicts with good experimental agreement which should be observed for different appl...

Longitudinal and transverse magneto‐impedance in amorphous Fe73.5Cu1Nb3Si13.5B9 films

Abstract: Magneto‐impedance effects have been observed in thin films of amorphous Fe73.5Cu1Nb3Si13.5B9. Large magneto‐impedance values have been obtained in field‐annealed samples, but not in as‐deposited samples. Transverse magneto‐impedance effects with magnitudes comparable to the longitudinal case have been observed. These results are discussed in terms of appropriate magnetic permeability.

Origin of orbital magnetization and magnetocrystalline anisotropy in TX ordered alloys (where T=Fe,Co and X=Pd,Pt).

Abstract: We present a method to calculate the magnetic anisotropy parameters which is based on a perturbative treatment of the spin-orbit interaction and a Green's function technique in real space. It allows us to interpret the magnetocrystalline anisotropy energy (MAE) in terms of interatomic interactions in the crystal. The method is applied to analyze orbital magnetism and MAE in TX ordered alloys (T=Fe,Co and X=Pd,Pt). The convergence of the orbital moments and MAE in real space and its relation to the problems of Brillouin-zone integration and of oscillatory behavior of MAE as a function of band filling are discussed. A comparison with results obtained by other methods is also given.

Total-energy calculation of the magnetocrystalline anisotropy energy in the ferromagnetic 3d metals

Abstract: We demonstrate that total energy calculations based upon only the local density approximation of density functional theory in combination with an orbital polarization correction can be used to derive the magnetocrystalline anisotropy energy (MAE) for the ferromagnetic metals, bcc Fe, hcp Co, fcc Co, and fcc Ni. In the case of bcc Fe, hcp Co, and fcc Co the calculations reproduce the experimental easy axis as well as the size of the MAE. However, for fcc Ni we obtain the wrong easy axis.

Magnetic anisotropy of iron multilayers on Au(001): First-principles calculations in terms of the fully relativistic spin-polarized screened KKR method.

Abstract: In order to treat the orientation of the magnetic field at surfaces properly, the spin-polarized fully relativistic version of the screened Korringa-Kohn-Rostoker method for semi-infinite systems is presented. Magnetic anisotropy energies up to six iron layers on Au(001) are calculated by using the force theorem, predicting a change from a perpendicular to a parallel magnetization for a layer thickness between three and four layers of Fe, in very good agreement with experimental observations. In particular, the magnetic anisotropy energy is discussed in relation to the orbital magnetic moment and to the orientation of the magnetic field when changed continuously.

Role of magnetic anisotropy in the magnetoimpedance effect in amorphous alloys

Abstract: The magnetoimpedance (MI) effect of amorphous Co70.4Fe4.6Si15B10 ribbons, subjected to different annealing conditions with and without an external field, has been measured up to a frequency of 100 kHz. We show that the large MI effect is the result of transverse magnetic anisotropy, which is present in the as‐cast and the suitably field‐annealed samples.

Artificial fabrication of an L10‐type ordered FeAu alloy by alternate monatomic deposition

Abstract: An FeAu ordered alloy has been fabricated artificially by depositing alternately monatomic layers of Fe and Au. Neither intermediate phase nor intermetallic compound is known for the Fe–Au system in the equilibrium state. Nevertheless, the x‐ray diffraction for prepared FeAu films shows definite superlattice lines corresponding to the L10 ordered structure, and it is metastable at room temperature. The obtained FeAu ordered alloy is ferromagnetic, and it has the Fe moment of 2.5±0.3 μB, a considerably high Curie temperature over room temperature, and a large uniaxial magnetic anisotropy perpendicular to Fe and Au atomic planes.

Shape anisotropy versus magnetic interactions of magnetite grains: Experiments and application to AMS in granitic rocks

Abstract: The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains. Measurement of the low-field magnetic anisotropy of single multi-domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio. Interactions between two elongated grains were studied experimentally using two types of grain arrangement: an “aligned” configuration and a “side-by-side” configuration. For a distance between the grain centers equal to approximately twice the average grain size, the magnetic susceptibility and its anisotropy are enhanced in both configurations, and the direction of kmax, the easiest magnetization axis, is stable in the “aligned” configuration, whereas it rotates toward an orthogonal direction in the “side-by-side” configuration. Depending on the distribution of the interacting magnetite grains, magnetic interactions may therefore either increase the whole-rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.

Anisotropy and orientational dependence of magnetization reversal processes in epitaxial ferromagnetic thin films

Abstract: We have undertaken a detailed study of the macroscopic and microscopic magnetization reversal processes in epitaxial ferromagnetic thin films with varying cubic and uniaxial magnetocrystalline anisotropy strengths. The macroscopic magnetization reversal processes were observed with in-plane magneto-optic Kerr effect (MOKE) vector magnetometry as a function of the relative anisotropy strengths and on the orientation of the applied field with respect to the anisotropy directions. Measurements of the component of magnetization in the plane of the sample and perpendicular to the applied field allow a precise determination of the relative orientation of the hard and easy in-plane anisotropy axes. This can be used to accurately determine the ratio of uniaxial to cubic anisotropy constants, when the ratio is less than one. The ratios obtained from MOKE agree well with those obtained by Brillouin light scattering (BLS). MOKE vector magnetometry reveals loop features that can be associated with either one or two irreversible jumps in the direction of the magnetization, depending sensitively on the anisotropy ratio and the orientation of the applied field. Minimum-energy calculations predict that the reversal process should proceed by a continuous rotation of the magnetization vector with either one or two irreversible jumps between single-domain states, depending on the applied field orientation and the nature of the anisotropy of the film. The calculations provide a good qualitative description of the observed reversal process, although the magnetic microstructure influences the exact values of the switching fields.

Control of an Easy-Axis on Nematic-Polymer Interface by Light Action to Nematic Bulk

Abstract: An appearance of an easy orientation axis of a nematic liquid crystal on the isotropic aligning surface under the light absorption by in-bulk azo dye molecules is observed. It is shown that orientation of LC molecules in the irradiated region is close to the direction of the exciting beam polarization. Accompanying to aligning, the surface driven reorientation effect has been studied. A threshold as well as a nonthreshold reorientation of the director toward the light-induced easy axis were observed. The value of the light-induced anchoring energy depends on the exposure time and achieves 2.5×10-3 erg/cm2. The experimental data are explained within the framework of adsorption of phototransformed molecules from the LC bulk onto the aligning surface.

Low‐field magnetic sensors based on the planar Hall effect

Abstract: Sensitive magnetic field detection devices have been fabricated based on the planar Hall effect. The active material consists of permalloy ultrathin films (6 nm thick) epitaxially grown by molecular beam epitaxy. Uniaxial magnetic anisotropy is induced in the film through ferromagnetic coupling with a Fe/Pd bilayer epitaxially grown on MgO(001). The active layer shows a magnetoresistive ratio ΔR/R=2%. The device gives a sensitivity of 100 V/TA and a minimum detectable field below 10 nT. The detector response is linear over at least four decades. The transverse resistivity is sensitive only to the anisotropic resistivity, and not to the isotropic resistivity term which is highly temperature sensitive. Consequently, the thermal noise at 1 Hz is reduced by four orders of magnitude compared to a similar longitudinal magnetoresistive detector.

Magnetic anisotropies of sputtered fe films on mgo substrates

Abstract: Ferromagnetic resonance (FMR) and superconducting quantum interference device (SQUID) measurements have been used to study the magnetic properties of rf sputtered Fe films on MgO(001) substrates. The dependences of the FMR spectra parameters on the direction of the dc magnetic field turning in the plane of the films were measured in a wide temperature range (20--400 K) for films with thickness L in the range 25--500 \AA{}. The analysis of the angular dependence of the resonance field ${\mathit{H}}_{0}$ allowed us to determine the fourfold cubic anisotropy constant ${\mathit{K}}_{1}$ and the effective magnetization value ${\mathit{M}}_{\mathrm{eff}}$. It was found that both values decrease with decreasing L and approach a constant value below a certain thickness. A theory of FMR is outlined demonstrating that for the case of the dc magnetic field lying in a film plane, the anisotropy constant can be interpreted as a combination of a volume anisotropy contribution and a 1/L-dependent contribution from the surface anisotropy up to the thickness L\ensuremath{\le}${10}^{3}$ \AA{}. This means that for the experimentally studied thickness range the films may be considered as ``dynamically thin films'' with respect to surface perturbations. Then the peculiar thickness dependence of the ${\mathit{K}}_{1}$ value can be explained assuming that the relaxation of the strain due to the mismatch between film and substrate extends to distances as far as 45 \AA{} from the film-substrate interface. Since our SQUID measurements show that the saturation moment does not depend on the thickness, it is concluded that the thickness dependence of the effective magnetization ${\mathit{M}}_{\mathrm{eff}}$ is caused by a second-order uniaxial anisotropy arising mainly from the broken symmetry of the crystal field at surfaces and near the edges of interfacial dislocations.

Magneto-impedance in sputtered amorphous films for micro magnetic sensor

Abstract: The magneto-impedance (MI) effect in RF sputtered (CoFe)/sub 80/B/sub 20/ zero-magnetostrictive amorphous films are presented. The relation between annealing conditions and MI characteristics was investigated. Impedance Z monotonously decreased with increasing applied field Hex when a high frequency sinusoidal current was applied to the sample annealed in a rotational field. In case of the samples having transverse anisotropy, Z sharply increases with Hex for the region Hex

Perpendicular magnetic anisotropy and strong magneto‐optic properties of SrRuO3 epitaxial films

Abstract: Epitaxial films of the ferromagnetic perovskite SrRuO3 were measured with a bulk magnetometer and with a local magneto‐optic Sagnac interferometer in transmission and in reflection. We find a magnetic easy axis perpendicular to the films, and for saturated remanent magnetization along this direction the Faraday rotation and the Kerr rotation at λ=840 nm are about 0.75×105 deg/cm and 0.85°, respectively. The temperature dependence of the remanent magnetization in the low temperature limit is dominated by spin‐wave excitations, yielding a notable decrease with T3/2. Using Sagnac–Kerr scanning and transmission electron microscopy imaging we correlate the coercivity with the grain size.

Electrochemical growth of superparamagnetic cobalt clusters

Abstract: The magnetization of stabilized cobalt colloids in tetrahydrofuran has been studied by a superconducting quantum interference device (SQUID) and magnetic balance measurements in dependence of applied magnetic field and temperature. The colloids are generated by a newly developed electrochemical method which allows one to generate clusters containing about 1000 atoms with a narrow size distribution. The final size distribution of the clusters is examined by high resolution transmission electron microscopy and small angle x‐ray scattering. The magnetization curves have been determined with special emphasis on changes at the freezing point of the solution. The curves of the liquid phase can be reasonably described by the Langevin function and the magnetic moments of isolated cobalt clusters that have been recently measured by Stern–Gerlach experiments. Deviations that appear at the freezing point can be understood in terms of magnetic anisotropy effects. It is shown that the cluster sizes and the susceptibil...

Epitaxial strain and magnetic anisotropy in ultrathin Co films on W(110).

Abstract: The role of epitaxial strain for the anisotropies of ultrathin films is studied experimentally for the case of Co(0001) films on W(110), based on the measurement of anisotropies using torsion oscillation magnetometry combined with measurement of the strain by high-angular-resolution low-energy electron diffraction. Up to a thickness of t=2 nm, the films grow in a state of constant strain, governed by pseudomorphism in the direction ([11\ifmmode\bar\else\textasciimacron\fi{}00]Co\ensuremath{\parallel}[11\ifmmode\bar\else\textasciimacron\fi{}0]W), which results in a true volume-type strain anisotropy. Above 2 nm, a relaxation of strain is observed which scales roughly with 1/t and therefore results in an apparently surface-type contribution to strain anisotropy, superimposed on a reduced volume contribution. In-plane and out-of-plane volume and surface-strain anisotropies are calculated from the observed strain. For the four anisotropy constants, their calculated differences between the two regimes above and below 2 nm agree with their differences as determined by magnetometry.

Magnetoresistance effect device, and magnetoresistance effect type head, memory device, and amplifying device using the same

Abstract: A magnetoresistance effect device of the invention includes: a substrate; and a multilayer structure formed on the substrate. The multilayer structure includes a hard magnetic film, a soft magnetic film, and a non-magnetic metal film for separating the hard magnetic film from the soft magnetic film. The magnetization curve of the hard magnetic film has a good square feature, and the direction of a magnetization easy axis of the hard magnetic film substantially agrees to the direction of a magnetic field to be detected.

High density magnetic recording media design and identification: susceptibility to thermal decay

Abstract: Systematic simulation studies of magnetic and recording properties of thin film media at ultrahigh density have been performed with thermal effects taken into account In the bulk magnetic property study, the focus is on coercivity A formula for the ratio of the writing coercivity to that from the vibrating sample magnetometer (VSM) is given The writing of di-bits and their subsequent thermal decay are simulated for a wide range of medium parameter values From these simulations, the medium performance is mapped out on an anisotropy-constant-saturation-magnetization plane and the range of proper medium parameters suitable for stable storage determined Ways to predict the thermal decay of written information by bulk measurements are discussed

Epitaxial ferromagnetic MnAs thin films grown by molecular beam epitaxy on Si (001) substrates

Abstract: We have grown ferromagnetic MnAs thin films on Si(001) substrates by molecular beam epitaxy. Epitaxial monocrystalline MnAs films with the growth plane of (1101) were obtained when the Si surface was first exposed to an As4 flux and then Mn and As4 fluxes were codeposited. It was found that the very first monolayer of As on Si(001) plays an essential role to obtain epitaxial MnAs thin films. Magnetization measurements indicate that the easy axis of the MnAs thin films is in‐plane, along the [1120] of MnAs and the [110] of Si, normal to the substrate misorientation. The M‐H curve of a 300‐nm‐thick film shows a hysteresis with a saturation magnetization Ms of 694 emu/cm3 and a coercive field Hc of 94 Oe, when the magnetic field is applied along the easy axis.

Increased exchange anisotropy due to disorder at permalloy/CoO interfaces

Abstract: The exchange anisotropy of Ni80Fe20 (permalloy) films deposited onto bulk single crystal CoO substrates using molecular‐beam epitaxy is strongly affected by the interface structure. Contrary to expectations for uncompensated interfaces, the loop shift increases for samples with greater interface disorder. The results are discussed in the light of proposed exchange anisotropy models. The results suggest that antiferromagnet domain dynamics are an important part of the exchange anisotropy mechanism.