Showing papers on "Magnetic anisotropy published in 1990"

Grain size dependence of coercivity and permeability in nanocrystalline ferromagnets

Abstract: Amorphous ribbons of composition Fe/sub 74.5-x/Cu/sub x/Nb/sub 3/Si/sub 13.5/B/sub 9/ (x=0, 1 at.%) have been annealed between about 500 degrees C and 900 degrees C. This produced a series of crystallized samples with grain sizes between about 10 nm and 300 nm and with coercivities H/sub c/ and initial permeabilities mu /sub i/ varying over several orders of magnitude. The best soft magnetic properties (H/sub c/ approximately=0.01 A/cm and mu /sub i/ approximately=80*10/sup 3/) were observed for the smallest grain sized of about 10 nm. With increasing grain size D, coercivity steeply increases following a D/sup 6/-power law (up to D approximately=50 nm). H/sub c/ then runs through a maximum of H/sub c/ approximately=30 A/cm and decreases again for grain sizes above 150 nm according to the well-known 1/D law for polycrystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal. >

First-principles calculation of the magnetocrystalline anisotropy energy of iron, cobalt, and nickel

Abstract: The magnetocrystalline anisotropy energies of the elements iron, cobalt, and nickel have been calculated by means of the linear muffin-tin orbital (LMTO) method in the atomic-sphere approximation (ASA) within the framework of the local-spin-density approximation (LSDA). The so-called ``force theorem'' is used to express the total-energy difference, when spin-orbit coupling is included, as a difference in sums of Kohn-Sham single-particle eigenvalues. The results depend strongly on the location and dispersion of degenerate energy bands near the Fermi surface, and particular attention must be paid to the convergence of the Brillouin-zone integral of the single-particle eigenvalues. The calculated values of the anisotropy energy are too small by comparison with experiment, and we do not predict the correct easy axis for cobalt and nickel. We find that the variation of the anisotropy energy with changes in strain, in the magnitude of the spin-orbit coupling, for different choices of the exchange-correlation potential and for varying numbers of valence electrons are not capable of explaining these incorrect results. By comparing our calculated energy bands with those obtained by a full-potential linear augmented plane wave (FLAPW) method we conclude that the discrepancy is not attributable to terms in the potential that are neglected in the ASA.

Reversible transition between perpendicular and in-plane magnetization in ultrathin films.

Abstract: The magnetization in ultrathin Fe layers (2.5-3.5 atomic layers) on Cu(100) reversibly switches between perpendicular (at low temperature) and in-plane magnetization (at higher temperature). The switching temperature decreases with increasing film thickness. The switching transition is attributed to the temperature dependence of the perpendicular anisotropy. The transition is accompanied by a loss of magnetization over a temperature range of 20-30 K and shows evidence for a canted-spin configuration

Soft magnetic materials

Abstract: The state of art is reviewed with an emphasis on recent research results and a view to identifying areas in which further developments in materials and processing might lead to even better properties and greater application of novel technology. Basis magnetic considerations are discussed, namely, B-H loop shape, core loss, magnetic anisotropy and annealing, and magnetostriction and stress effects. Materials and applications for power frequency devices are examined, covering core loss considerations, silicon steel development, metallic glasses, and high silicon materials. High-frequency and pulse applications, magnetic recording heads, and sensor and transducer applications are also discussed. Basic research questions and future directions with respect to core loss, magnetization, and stability are examined. >

Magnetic anisotropy in low-dimensional ferromagnetic systems: Fe monolayers on Ag(001), Au(001), and Pd(001) substrates.

Abstract: A second-variation full-potential linear augmented-plane-wave total-energy method for thin-film ferromagnetic systems is used to study the spin-orbit-interaction contribution to the magnetic anisotropy. For a free-standing Fe monolayer, the spin magnetization is determined to lie in the plane. Results for Fe monolayers on Au(001), Ag(001), and Pd(001) substrates indicate a preference for the spin direction to be perpendicular to the plane of the film. Computational details for this magnetic anisotropy are also discussed.

Hysteresis properties of ultrathin ferromagnetic films

Abstract: We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.

Magnetic anisotropy in epitaxial Co superlattices.

Abstract: A series of epitaxial Co-Au and Co-Cu superlattices has been grown by molecular-beam-epitaxy techniques, with Co thicknesses varying from 5 to 40 \AA{} and constant Au and Cu thicknesses of 16 and 25 \AA{}, respectively. We compare the magnetic properties of both types of superlattice. Crossover to an easy axis normal to the film plane occurs at a thickness of 19 \AA{} of Co in the Co-Au series and 10 \AA{} of Co in the Co-Cu series. We show that the observed magnetic behavior of both of these systems can be accounted for by contributions from demagnetization, magnetocrystalline anisotropy, and magnetoelastic anisotropy. The magnetoelastic term depends on the strain present in Co layers, which in turn is a function of the Co-layer thickness. In our calculation we employ measured Co-layer strains, determined by x-ray scattering, in combination with the known elastic and magnetostriction constants of bulk Co.

Mössbauer effect in small particles

Abstract: Mossbauer spectroscopy is very sensitive to the special magnetic properties of ultrafine particles. Studies of particles in the superparamagnetic state allow determination of the particle size and the magnetic anisotropy energy constant A strong magnetic interaction between the particles may result in ordering of the magnetic moments of particles which would be superparamagnetic if they were non-interacting. This so-called superferromagnetic state can also be characterized by Mossbauer spectroscopy measurements. Furthermore, because a large fraction of the atoms in very small particles are in the surface layer, Mossbauer spectroscopy can be applied for studies of surface magnetism in small particles.

Ferromagnetic resonance of particulate magnetic recording tapes

Abstract: The room‐temperature ferromagnetic resonance (FMR) spectra of γ‐Fe2O3, CrO2, and barium ferrite particulate magnetic recording tapes have been measured at microwave frequencies of 9.35 and 35 GHz for various orientations of the static and high‐frequency magnetic fields with respect to the tape. For CrO2 tapes, the influence of the width of the angular distribution of the particle orientations on the FMR spectra has been studied from the nearly isotropic case up to the highly oriented case. Hysteretic behavior for a CrO2 tape as well as the effect of tape calendering for a γ‐Fe2O3 tape has been observed by FMR. Experimental results are found to be in reasonable agreement with results of theoretical calculations based on a model of an ellipsoidal single‐domain particle with both shape and magnetocrystalline anisotropy. Magnetostatic interaction inside the magnetic film has been introduced by expressing the total magnetostatic energy as a combination of a part dependent on particle shape and a part dependent...

A mean field Stoner-Wohlfarth hysteresis model

Abstract: A model of magnetic hysteresis is proposed. It is based on the Stoner-Wohlfarth coherent rotation model, but with a macroscopic mean field interaction (similar to Weiss's molecular field theory), added to represent interaction between domains. Rotation of domain magnetization vectors gives both reversible and irreversible changes in bulk magnetization. A qualitative correlation is obtained between hysteresis curves calculated using this theoretical model and experimentally measured hysteresis curves of a ferrite permanent magnet. >

Magnetization of a Dy(Fe11Ti) single crystal.

Abstract: Magnetization curves have been measured in the temperature range from 4.2 to 300 K along the (100), (110), and (001) directions of a Dy(Fe{sub 11}Ti) single crystal in fields up to 7 T. The magnetic moment is along (100) below 58 K and parallel to the {bold c} axis above 200 K. Between the two spin-reorientation transitions (first order at 58 K, second order at 200 K) there is a canted spin structure where the net magnetization lies in a (010) plane and is inclined at an angle to the {bold c} axis. Three first-order magnetization processes are observed as a function of applied field below 150 K. All the data are used to derive a set of five crystal-field coefficients at the single rare-earth site of the ThMn{sub 12} structure: {ital A}{sub 20}={minus}32.3 K{ital a}{sub 0}{sup {minus}2}, {ital A}{sub 40}={minus}12.4 K{ital a}{sub 0}{sup {minus}4}, {ital A}{sub 44}=118 K{ital a}{sub 0}{sup {minus}4}, {ital A}{sub 60}=2.56 K{ital a}{sub 0}{sup {minus}6}, {ital A}{sub 64}=0.64 K{ital a}{sub 0}{sup {minus}6}. Spin reorientation observed in the other members of the {ital R}(Fe{sub 11}Ti) ({ital R}==rare earth) series, except those in Tb(Fe{sub 11}Ti), are explained by the same crystal-field coefficients.

Epitaxial ferromagnetic τ-MnAl films on GaAs

Abstract: We report the growth of epitaxial τ‐MnAl ferromagnetic films on GaAs substrates by molecular beam epitaxy (MBE). Reflection high‐energy electron diffraction and x‐ray diffraction show that the τ‐phase films grow with the c axis of the tetragonal unit cell normal to the {100}GaAs substrate surface. In the bulk, τ‐MnAl is a metastable ferromagnetic phase with uniaxial magnetocrystalline anisotropy. The large hysteresis observed in the Hall resistance versus applied magnetic field suggests that the easy magnetization direction is indeed parallel to the c axis in the MBE‐grown films. The growth of these ferromagnetic films with perpendicular magnetization on compound semiconductor substrates creates the possibility of novel devices that combine magnetic memory and magneto‐optic functions with semiconductor electronics and photonics.

Magnetic properties of (rare earth)Al2 intermetallic compounds

Abstract: In this paper the most important magnetic properties of non-S-state ferromagnetic single crystalline REAl2 (RE ≡ rare earch) intermetallic compounds with well localized 4f electrons are reviewed. Particular emphasis is placed on the static magnetization and on the magnetic excitations. The review demonstrates that for a single REAl2 compound a cubic crystalline field (CF) provides an excellent basis for a quantitative description of the magnetization, including the magnetocrystalline anisotropy by using molecular-field theory. This approach is also appropriate in many cases for the magnetic part of the molar specific heat in a temperature range where contributions from spin waves can be neglected. Taking into account the magnetic interactions of different ions within the random-phase approximation, it is also possible in most cases to describe the low-temperature behaviour of the magnetic excitations. In general a consistent description of various other important magnetic properties can also be g...

Magnetic anisotropies in Fe(110) films on W(110)

Abstract: Magnetic anisotropies in epitaxial Fe(110) films on W(110) were analyzed using torsion oscillation magnetometry (TOM) in situ in UHV. Films with clean surfaces and films coated by Cu, Ag, and Au were analyzed. Out-of-plane anisotropies were determined by standard TOM methods, in-plane anisotropies from hard-axis magnetization loops. Since all anisotropies showed a clear linear dependence on reciprocal film-thickness, a straightforward separation of volume and surface type anisotropies could be performed. Volume anisotropies can be explained as a superposition of standard fourth-order magnetocrystalline and strain anisotropies, caused by residual strain of the Fe coincidence lattice on W. Surface anisotropies, both in-plane and out-of-plane, are related by Neel's model to the bulk magnetoelastic constants, to a surprisingly good approximation.

Resolution of ferrimagnetic and paramagnetic anisotropies in rocks, using combined low-field and high-field measurements

Abstract: SUMMARY A method of separation of ferrimagnetic and paramagnetic components of magnetic anisotropy from the total anisotropy is developed. From the anisotropy measurement of a specimen in a low field (susceptibility anisotropy) and from measurement in two high fields stronger than the saturation field of the ferrimagnetic fraction present, the following anisotropy components can be calculated: susceptibility anisotropy (field-independent) of the paramagnetic fraction, low-field (susceptibility) anisotropy of the ferrimagnetic fraction and high-field anisotropy of the ferrimagnetic fraction. Errors possibly arising from imperfect saturation of the ferrimagnetic fraction in the field practically available are analysed.

Magnetisation processes, hysteresis and finite-size effects in model multilayer systems of cubic or uniaxial anisotropy with antiferromagnetic coupling between adjacent ferromagnetic layers

Abstract: Magnetisation processes have been investigated for model multilayer systems where antiferromagnetic interactions couple adjacent ferromagnetic layers. In this first study, only coherent rotations of the magnetisation of each layer are taken into account. Depending on the direction of the applied magnetic field, the initial moment configuration and the magnetocrystalline anisotropy, various first- or second-order magnetic transitions may be observed. The cases of cubic and uniaxial anisotropy bilayer systems are treated in detail. Spin-flop and spin-flip transitions are calculated to occur for both symmetries when starting from antiferromagnetic configurations that are parallel to the field axis. In the cubic case, various other transitions have been found. In particular, first-order transitions between symmetric and nonsymmetric states have been calculated, involving asymmetric behaviour of the magnetisation vectors of adjacent layers. Such transitions give rise to a transverse magnetisation. All the critical transition fields have been calculated as a function of the anisotropy and are reported in the various phase diagrams. Hysteresis loops have also been calculated. They generally consist of an upper and a lower part shifted symmetrically about the origin as in the case of bulk antiferromagnets. The influence of the number of layers has been investigated in some particular cases. It is shown that in most instances when the number of layers n becomes very large (i.e. when boundary effects disappear), the multilayer behaves like a bilayer but with different transition-field values. For small numbers of layers, whether or not the magnetism is compensated (n even or odd) has a very strong effect on the magnetisation processes.

Soft magnetism of crystalline Fe based alloy sputtered films

Abstract: The physical origin of soft magnetic properties in Fe-based alloy films with large magnetocrystalline anisotropy and magnetostriction is systematically examined. The relations between the microstructure of the film and soft magnetic properties are discussed for various kinds of Fe-based alloy films such as Fe-N, Fe-Si-Al, and Ni-Fe-Nb fabricated by DC magnetron sputtering. Magnetic anisotropy dispersion is analyzed based on the micromagnetic ripple theory proposed by H. Hoffmann (1969). Analysis of dynamic differential susceptibility suggests that initial permeability is strongly related to the structure constant S, which must be reduced in order to realize high initial permeability. It has been demonstrated that the reduction of grain size and induced lattice deformation are very effective in decreasing the value of S with respect to controlling the microstructure of the film. Several factors which must be taken into account for achieving high initial permeability in Fe-based alloy films are examined. >

Complex magnetic properties of the rare-earth copper oxides, R2CuO4, observed via measurements of the dc and ac magnetization, EPR, microwave magnetoabsorption, and specific heat

Abstract: We report the results of an extensive investigation of the magnetic properties of a large series of undoped ${R}_{2}$${\mathrm{CuO}}_{4}$ single crystals with R==Pr, Nd, Sm, Eu, and Gd (which are the host compounds for the newly discovered series of electron cuprate superconductors) and mixture versions of the form ${A}_{2\mathrm{\ensuremath{-}}x}$${B}_{x}$${\mathrm{CuO}}_{4}$, with A==Pr, Nd, Sm, Eu, or Gd, and B==Gd, Tb, or Dy. We have measured dc and ac magnetization, microwave magnetoabsorption, EPR, and specific heat. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low-field dc magnetization, an unusual anisotropy in the EPR resonance field for R=Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temeperature at \ensuremath{\sim}270 K is associated with antiferromagnetic ordering of the Cu moments which are strongly coupled within the ${\mathrm{CuO}}_{2}$ layers. The lower, at \ensuremath{\le}20 K, cannot be attributed to antiferromagnetic ordering of the R moments and is tentatively attributed to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of high-temperature superconductivity.

Magnetization hysteresis and flux pinning in twinned and untwinned YBa2Cu3O7−δ single crystals

Abstract: Measurements of the magnetization hysteresis of the same crystal in the twinned and untwinned state reveal that pinning due to twinning planes may contribute a substantial, anisotropic fraction of the observed critical currents. A linear scaling between magnetization hysteresis and sample dimension shows that the twinned and untwinned crystal does not exhibit any subgranular effects.

First-passage-time approach to overbarrier relaxation of magnetization

Abstract: We consider the irreversible dynamics of the magnetization vectorM in a single-domain particle. The dynamics is given by a stochastic phenomenological equation due to Gilbert. It contains a damping field proportional toM and a corresponding white noise field component. The probability distribution function satisfies a Fokker-Planck equation derived by Brown. We give the overbarrier decay rateκ out of a metastable minimum. First we rederive the well-known expression forκ for an axially symmetric model. We argue that this result is unphysical. For systems of general point symmetry of the magnetic anisotropy energy we giveκ in both the low-damping and intermediate- to high-damping limits.

Two‐dimensional magnetic phase transition of ultrathin iron films on Pd(100)

Abstract: Epitaxial Fe films grown on Pd(100) are used to study monolayer magnetism, critical behavior, and surface magnetic anisotropy, by means of in situ surface magneto‐optical Kerr‐effect measurements. Auxiliary LEED‐Auger observations in 10−11 Torr vacuum are used to characterize the (1×1) epitaxy and the layer‐by‐layer film‐growth mode. Ferromagnetic hysteresis loops were detected for all Fe thicknesses from 0.6–4 monolayers (ML) with the TC monotonically increasing with thickness, independent of the easy‐axis orientation. The easy axis is perpendicular to the film plane below a critical thickness of 2.5 ML for 100‐K film growth, and reorients in‐plane above this thickness, and for all thicknesses for films grown at 300 K. The temperature dependence of the magnetization was obtained from the height of the Kerr loops in the remanent state and used to extract an effective magnetization exponent β for different film thicknesses and spin orientations. A value of β=0.127±0.004 is reported for a 1.2‐ML Fe film wit...

Experimental probe for thin-film magnetism in p(1×1) Pd and V on Ag(100)

Abstract: We report high-sensitivity experiments utilizing the magneto-optic Kerr effect which probe for ferromagnetism in ultrathin epitaxial films of Pd and V on Ag(100) substrates. Our experiments fail to detect unambiguous manifestations of ferromagnetism in these two thin-film systems.

The influence of stress on ferromagnetic hysteresis

Abstract: It is argued that attempts to describe the stress magnetization behavior of ferromagnetic materials have, in general, relied upon unsubstantiated assumptions regarding the distribution of domains and the relative motion of domain walls under the combined influences of applied field and mechanical stress. An empirical description of stress-induced magnetization is presented as a practical extension of the Jiles-Atherton model of ferromagnetic hysteresis. It permits a semiquantitative description of the stress magnetization characteristics of ferromagnetic materials and highlights the profound influence of previous magnetic history. It is concluded that measuring the B/H characteristics of a material as a function of stress and invoking the Jiles-Atherton theory of ferromagnetism could provide a computational basis for predicting the magnetic behavior of complex structures under mechanical stress. >

Magnetization and anisotropy of Co/Pd multilayer thin films

Abstract: Multilayered Co/Pd thin films were prepared by sequential electron‐beam evaporation of Co and Pd onto Si substrates at room temperature. The thicknesses of the Co sublayer and of the Pd sublayer were varied between 2.0–10.3 and 4.5–22.3 A, respectively. As the Pd sublayer thickness was varied at constant Co thickness, broad maxima in the saturation magnetization Ms and intrinsic perpendicular anisotropy energy Ku were observed at a Pd thickness of about 10 A. At this maximum, Ms per Co volume is larger than the saturation magnetization of bulk Co. This is believed to be caused by the polarization of the Pd atoms within about 10 A of the Co layer. Ku and Ms per Co volume both decrease with increasing Co layer thickness.