Showing papers on "Magnetic anisotropy published in 1999"

Modern Magnetic Materials: Principles and Applications

Abstract: Introduction and Overview. Magnetostatics. Classical and Quantum Phenomenology of Magnetism. Quantum Mechanics, Magnetism, and Exchange in Atoms and Oxides. Quantum Mechanics, Magnetism, and Bonding in Metals. Magnetic Anisotropy. Magnetoelastic Effects. Magnetic Domain Walls and Domains. Magnetization Process. Soft Magnetic Materials. Amorphous Materials: Magnetism and Disorder. Magnetism in Small Structures: Exchange Coupling and Nanocrystals. Hard Magnetic Materials. Magnetic Annealing and Directional Order. Electronic Transport in Magnetic Materials. Surface and Thin-Film Magnetism. Magnetic Recording. Appendices. Index.

Quantum Phase Interference and Parity Effects in Magnetic Molecular Clusters

Abstract: An experimental method based on the Landau-Zener model was developed to measure very small tunnel splittings in molecular clusters of eight iron atoms, which at low temperature behave like a nanomagnet with a spin ground state of S = 10. The observed oscillations of the tunnel splittings as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum interference of two tunnel paths of opposite windings. Transitions between quantum numbers M = -S and (S - n), with n even or odd, revealed a parity effect that is analogous to the suppression of tunneling predicted for half-integer spins. This observation is direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system.

The correlation between mechanical stress and magnetic anisotropy in ultrathin films

Abstract: The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

Progress and outlook for MRAM technology

Abstract: We summarize the features of existing semiconductor memories and compare them to Magnetoresistive Random Access Memory (MRAM),a semiconductor memory with magnetic bits for nonvolatile storage. MRAM architectures based on Giant Magnetoresistance (GMR) and Magnetic Tunnel Junction (MTJ) cells are described. This paper will discuss our progress on improving the material structures, memory bits, thermal stability of the bits, and competitive architectures for GMR and MTJ based MRAM memories as well as the potential of these memories in the commercial memory market.

Model for exchange bias in polycrystalline ferromagnet-antiferromagnet bilayers

Abstract: This paper describes a model for polycrystalline ferromagnet-antiferromagnet bilayers. Independent antiferromagnetic grains are coupled to a ferromagnetic film both by direct coupling to the net moments at the interfaces of the grains and by spin-flop coupling. Rotation of the ferromagnetic magnetization applies a torque to the antiferromagnetic spins at the interface of each grain which winds up partial domain walls in the antiferromagnet. The model explains both the unidirectional anisotropy that gives rise to the well-known shifted hysteresis loops, and the hysteretic effects observed in rotational torque and ferromagnetic resonance experiments. The unidirectional anisotropy comes from grains in which the antiferromagnetic order is stable as the magnetization is rotated. The hysteretic effects come from grains in which the antiferromagnetic order irreversibly switches as the domain wall is wound up past a postulated critical angle. For all of the models considered here, spin-flop coupling does not contribute to the unidirectional anisotropy.

Single-Molecule Magnet Behavior of a Tetranuclear Iron(III) Complex. The Origin of Slow Magnetic Relaxation in Iron(III) Clusters

Abstract: The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear iron(III) methoxo-bridged cluster of formula Fe4(OCH3)6(dpm)6 (where Hdpm = dipivaloylmethane) is reported. The cluster has a ground spin state of S = 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D = −0.2 cm-1 of the S = 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To investigate the origin of the magnetic anisotropy we have evaluated the projection of the single-ion and dipolar contributions to the zfs of the ground state. The zfs tensors of the three structurally independent iron(III) centers have been calculated from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm)3 have been recorded...

Inductive measurement of ultrafast magnetization dynamics in thin-film Permalloy

Abstract: An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni81Fe19). Data are presented for impulse- and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are clearly observed. The oscillation frequency increases as the dc bias field parallel to the anisotropy axis increases as predicted by classical gyromagnetic theory. The data are fitted to the Landau–Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. Damping for step excitations is characterized by an anomalous transient damping which rapidly increases at low dc bias field. Transformation of the data to the frequency domain reveals a higher order precessional mode which is also preferentially excited at low dc bias fields. A possible source for both phenomena is precessional mode saturation for large peak rotations. The technique has the potential for 20 ps resolution, although only 120 ps resolution is demonstrated due to the limited bandwidth of the waveguides used.

Propagation of a magnetic domain wall in a submicrometer magnetic wire

Abstract: The motion of a magnetic domain wall in a submicrometer magnetic wire was detected by use of the giant magnetoresistance effect. Magnetization reversal in a submicrometer magnetic wire takes place by the propagation of a magnetic domain wall, which can be treated as a “particle.” The propagation velocity of the magnetic domain wall was determined as a function of the applied magnetic field.

Superparamagnetic Relaxation and Magnetic Anisotropy Energy Distribution in CoFe2O4 Spinel Ferrite Nanocrystallites

Abstract: Superparamagnetism is a unique feature of magnetic nanoparticles. Spinel ferrite nanoparticles provide great opportunities for studying the mechanism of superparamagnetic properties. CoFe2O4 nanocrystallites have been synthesized with a microemulsion method. The neutron diffraction studies and the temperature-dependent decay of magnetization show the superparamagnetic relaxation occurring in these nanoparticles. The neutron diffraction shows a high degree of inversion with the 78% tetrahedral sites occupied by Fe3+ cations. The nanoparticles with a 12 nm diameter have a blocking temperature around 320 K. The field-cooled and zero-field-cooled magnetization measurements display a divergence below the blocking temperature. The energy barrier distribution of magnetic anisotropy is derived from the temperature-dependent decay of magnetization. The magnetic anisotropy is clearly the origin of the divergence in the field-cooled and zero-field-cooled magnetization measurements. The energy barrier distribution fu...

Magnetic anisotropy barrier for spin tunneling in Mn 12 O 12 molecules

Abstract: Electronic structure calculations on the nature of electronic states and the magnetic coupling in Mn-acetate $[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}(R\mathrm{COO}{)}_{16}({\mathrm{H}}_{2}\mathrm{O}{)}_{4}]$ molecules have been been carried out within the generalized gradient approximation to the density functional formalism. Our studies on this 100-atom molecule illustrate the role of the nonmagnetic carboxyl host in stabilizing the ferrimagnetic ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}$ core and provide estimates of the local magnetic moment at the various sites. We provide a first density-functional-based prediction of the second-order magnetic anisotropy energy of this system. Results are in excellent agreement with experiment. To perform these calculations we introduce a simplified exact method for spin-orbit coupling and magnetic anisotropy energies in multicenter systems. This method is free of shape approximations and has other advantages as well. First, it is valid for periodic boundary conditions or finite systems and is independent of basis set choice. Second, the method does not require the calculation of electric field. Third, for applications to systems with a finite energy gap between occupied and unoccupied electronic states, a perturbative expansion allows for a simple determination of the magnetic anisotropy energy.

M-hexaferrites with planar magnetic anisotropy and their application to high-frequency microwave absorbers

Abstract: Magnetic and microwave absorbing properties have been investigated in the M-type barium ferrites (BaFe/sub 12-2x/A/sub x/Co/sub x/O/sub 19/) with planar magnetic anisotropy. For the tetravalent A ions, Ti/sup 4+/ and Ru/sup 4+/ are chosen and the samples are prepared by a conventional ceramic processing technique. At the substitution ratio with in-plane anisotropy which is estimated from the minimum coercivity, the saturation magnetization of the Ru-Co substituted specimens were about twice as large as those of Ti-Co. This leads to higher magnetic resonance frequency, and thus a large absorption loss (in sintered specimens) and small reflection loss (in rubber composites) are predicted at the higher frequencies in the Ru-Co substituted specimens. High-frequency noise suppressors or anti-reflection absorbers can be proposed for use with M-hexaferrites with planar magnetic anisotropy.

Magnetization Directions of Individual Nanoparticles

Abstract: The magnetization directions of individual monodomain nanoparticles as small as 5 nanometers in diameter are determined using the Foucault method of Lorentz microscopy. A model is developed to explain the images and diffraction patterns of samarium cobalt nanoparticles as a function of the aperture shift direction. Thermally induced changes in the magnetization direction of superparamagnetic magnetite nanoparticles were observed but with a much slower rate than expected, due to surface anisotropy. When the time scale for magnetization reversal is much shorter than the data acquisition time, as in carbon-coated iron cobalt alloy nanoparticles, the images show an average of such thermally induced changes.

Magnetic properties of nanostructured CuFe2O4

Abstract: The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mossbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mossbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1) × 105 erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.

Ferromagnetic shape memory in the NiMnGa system

Abstract: Strain versus field measurements for a ferromagnetic shape memory alloy in the NiMnGa system demonstrate the largest magnetostrictive strains to date of nearly 1.3%. These strains are achieved in the martensitic state through field-induced variant rearrangement. An experimental apparatus is described that provides biaxial magnetic fields and uniaxial compressive prestress with temperature control while recording microstructural changes with optical microscopy. The magnetostrictive response is found to be sensitive to the initial state induced by stress-biasing the martensitic variant structure, and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity. Experimental results are interpreted by using a theory based on minimization of a micromagnetic energy functional that includes applied field, stress, and demagnetization energies. It is found that the theory provides a good qualitative description of material behavior, but significantly overpredicts the amount of strain produced. Issues concerning the martensitic magnetic anisotropy and variant nucleation are discussed with regard to this discrepancy.

Temperature dependence of exchange bias in polycrystalline ferromagnet-antiferromagnet bilayers

Abstract: We describe a simple model for the temperature dependence of the exchange bias and related effects that result from coupling a ferromagnetic thin film to a polycrystalline antiferromagnetic film. In this model, an important source of temperature dependence comes from thermal instabilities of the antiferromagnetic state in the antiferromagnetic grains, much as occurs in superparamagnetic grains. At low enough temperatures, the antiferromagnetic state in each grain is stable as the ferromagnetic magnetization is rotated and the model predicts the unidirectional anisotropy that gives rise to the observed exchange-bias loop shift. At higher temperatures, the antiferromagnetic state remains stable on short time scales, but on longer time scales, becomes unstable due to thermal excitations over energy barriers. For these temperatures, the model predicts the high field rotational hysteresis found in rotational torque experiments and the isotropic field shift found in ferromagnetic resonance measurements.

Antiferroquadrupolar ordering and magnetic properties of the tetragonal DyB2C2 compound

Abstract: Magnetic properties were investigated on a single-crystalline DyB 2 C 2 compound with the tetragonal structure. Spontaneous magnetizations appear along the a- and [1 1 0]-directions below T C =15.3 K. A very small shoulder is observed only along the c -axis at T Q =24.7 K, although large λ-type anomalous specific heats are observed at T C and T Q . There exists a large magnetic anisotropy in the tetragonal basal plane which is observed in magnetization processes below T C . Neutron powder diffraction experiments reveal that magnetic reflections are observed only below T C . In the magnetically ordered phase, the Dy moments are arranged to be perpendicular to neighbors along the c -axis. The results are well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one. DyB 2 C 2 is a novel compound with a high antiferroquadrupolar ordering temperature of 24.7 K.

Magnetic properties of submicron co islands and their use as artificial pinning centers

Abstract: We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a $2\ifmmode\times\else\texttimes\fi{}2$-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.

Microscopic tips having stable magnetic moments and disposed on cantilevers for sensing magnetic characteristics of adjacent structures

Abstract: A magnetic force microscope (MFM) needle has a magnetic material with a magnetic moment that is pinned in a preferred direction. The magnetic moment can be of lower than conventional magnitude without risking an undesirable change in the direction of magnetization. The magnetic needle can have a ferromagnetic layer (or layers) that is stabilized by an antiferromagnetic layer (or layers). The needle can be employed as a magnetoresistance sensitivity microscope (MSM) to map the sensitivity of a magnetic sensor, such as a magnetoresistive (MR) or giant magnetoresistive (GMR) sensor. Alternatively, the needle can be employed in measuring magnetic fields, such as with a high frequency magnetic force microscope (HFMFM).