Showing papers on "Magnetic anisotropy published in 1991"

A mechanism of magnetic hysteresis in heterogeneous alloys

Abstract: It is suggested that in many ferromagnetic materials there may occur particles distinct in magnetic character from the general matrix, and below the critical size, depending on shape, for which domain boundary formation is energetically possible. For such single-domain particles, change of magnetization can take place only by rotation of the magnetization vector. As the field changes continuously, the resolved magnetization may change discontinuously at critical values of the field. The character of the magnetization curves depends on the degree of magnetic anisotropy of the particle and on the orientation of easy axes with respect to the field. The magnetic anisotropy may arise from the shape of the particle, from magnetocrystalline effects, and from strain. A detailed quantitative treatment is given of the effect of shape anisotropy when the particles have the form of ellipsoids of revolution, along with a less detailed treatment for the general ellipsoidal form.

Magnetoresistive sensor based on the spin valve effect

Abstract: A magnetoresistive (MR) sensor comprising a first and a second thin film layer of a magnetic material separated by a thin film layer of a non-magnetic metallic material. The first ferromagnetic layer is magnetically soft. The magnetization direction of the first layer of magnetic material is set substantially perpendicular to the magnetization of the second layer of magnetic material at zero applied field, and the magnetization direction of the second layer of magnetic material is fixed. A current flow is produced through the MR sensor, and the variations in voltage across the MR sensor are sensed due to changes in resistance of the MR sensor produced by rotation of the magnetization in the first layer of magnetic material as a function of the magnetic field being sensed. The variation of the resistance with the angle between the magnetizations of the first and second layers of magnetic material has been defined as the spin valve (SV) effect. It is also shown that, by a suitable direction of the current with respect to the fixed magnetization, the (SV) magnetoresistance can be added constructively to the usual anisotropic magnetoresistance.

Interface magnetic anisotropy in epitaxial superlattices

Abstract: We have studied the uniaxial magnetic anisotropy of Co/Pd superlattices grown under identical conditions by molecular-beam epitaxy along the three crystal axes: [001], [110], and [111]. Our measurements unambiguously demonstrate that the large systematic variations of the anisotropy energy with crystal orientation result solely from differences in the volume contribution to the anisotropy. We find the perpendicular interface anisotropy to be independent of the epitaxial orientation (0.63\ifmmode\pm\else\textpm\fi{}0.05 erg/${\mathrm{cm}}^{2}$), and hence to be an intrinsic property of the Co/Pd interface.

Magnetic anisotropies and exchange coupling in ultrathin fcc Co(001) structures.

Abstract: Metastable fcc structures using ultrathin layers of metastable fcc Co(001) and fcc Cu(001) were grown by molecular-beam epitaxy. The growth was studied using reflection high-energy electron-diffraction (RHEED) patterns and RHEED intensity oscillations. The magnetic properties were investigated by employing ferromagnetic-resonance (FMR) and surface magneto-optic Kerr effect (SMOKE) techniques. A detailed discussion of the underlying physical mechanisms in FMR and SMOKE studies is presented. The temperature and thickness dependences of the perpendicular uniaxial and fourfold in-plane anisotropies are given and discussed. The role of lattice strains in magnetic anisotropies is demonstrated. FMR measurements revealed that the magnetic properties of single Co layers are different from those in Co/Cu/Co trilayers becuase of a lattice-strain relaxation in the multilayer samples. The exchange coupling between fcc Co(001) layers separated by a fcc Cu(001) interlayer was studied for several structures. The exchange coupling in Co structures was found to be anisotropic. Hysteresis loops were measured by means of SMOKE. Magnetic-trilayer hysteresis loops are complex. Micromagnetic calculations were carried out to explain their main features.

Science and Technology of Nanostructured Magnetic Materials

Abstract: Thin Films, Surfaces and Interfaces -- Electronic Structure and Magnetism of Metal Surfaces, Overlayers and Interfaces -- Growth and Magnetic Properties of Metastable Structures -- Spin-Resolved Photoemission -- Correlation of Crystalline and Electronic Structure in Epitaxial FCC-Cobalt Monolayers on Cu(100) -- Hybrid Ferromagnetic/Semiconductor Structures -- Mossbauer Studies of Ultrathin Magnetic Films of Fe/Ag(100) -- Spin-Dependence of Absorbed and Reflected Current on Fe(110) -- MBE Growth of Metal/Semiconductor Interfaces -- Surface and Interface Magnetism -- Ferromagnetic Resonance Studies of BCC Epitaxial Ultrathin Fe(001)/Cu(001) Bilayers and Fe(001)/Cu(001)/Fe(001) Trilayers -- Laser Ablation Deposition of Metallic Thin Films -- Exchange Coupled Films for Magneto-Optic Applications -- Temperature Dependence of Micromagnetic Domain Structure in Cobalt Films -- Hyperfine Interaction Techniques Applied to the Study of Surfaces and Interfaces -- Surface Magnetostriction -- Multilayers -- Magnetic Rare Earth Artificial Metallic Superlattices -- X-Ray Characterization of Magnetic Multilayers and Superlattices -- The Characterization of Interface Roughness and Other Defects in Multilayers by X-Ray Scattering -- Magnetism of Nanostructured Rare Earth Multilayers -- FMR Studies of Metallic Magnetic Thin Films in Layered Structures -- Compositionally Modulated Magnetic Multilayers: Temperature- and Modulation-Dependent Properties -- Structural and Magnetic Properties of Epitaxial Co/Pd Superlattices -- First-Principles Calculation of the Magnetocrystalline Anisotropy Energy of ConPdm Multilayers -- Structural and Magnetic Studies in Co-Pt Multilayers -- Magnetic Properties of Hexagonal Fe/Ru Superlattices With Short Periodicity -- Magnetic Studies of Fe-Si Compositionally Modulated Thin Films -- Mossbauer Spectroscopy of the Fe/Ni Interface -- Analysis of Amorphous Dysprosium-Transition Metal Nanoscale Magnetic Multilayers -- Transport Properties of Thin Metallic Films and Multilayers -- Domain Walls, Magnetic Domains And Techniques For Their Observation -- Micromagnetics of Longitudinal Recording Media -- MO-Recording: The Switching Process and Its Relation to the Magnetic Properties of Thin Films -- Micromagnetic Computations of Magnetization Configurations -- Domain Walls and Wall Structure -- Domain Wall Multiplication in Amorphous Ferromagnetic Alloys -- Electron Microscope Methods for Imaging Internal Magnetic Fields at High Spatial Resolution -- Scanning Tunneling Microscopy and Force Microscopy Applied to Magnetic Materials -- Special Session on Spin-Polarized Vacuum Tunneling -- Magnetic Imaging Via Scanning Electron Microscopy with Polarization Analysis -- Atomic Scale Probe into High-Tc Superconductors Using Scanning Tunnelling Microscopy -- Magnetic Anisotropy and Random Magnets -- Magnetic Anisotropy -- Random Anisotropy in Magnetic Materials -- Perpendicular and In-Plane Anisotropy in Amorphous Tb-Fe -- Magnetostriction in Amorphous Ferromagnets -- Anderson Localization in 3-Dimensional Amorphous Alloys: Evolution with the Content of Magnetic Ions -- On the Law of Approach to Saturation in the Series of Amorphous Alloys a-Dyxd1?xNi -- Magnetoresistance of Amorphous U1?xSbx Films -- Absence of Temperature-Driven First-Order Phase Transitions in Systems with Random Bonds -- Magnetic Semiconductors and Intermetallic Compounds -- Magnetic Behavior of Diluted Magnetic Semiconductors -- Intermetallic Compounds and Crystal Field Interactions -- Crystal-Field and Exchange Interactions in Hard Magnetic Materials -- First Order Magnetization Processes -- Structure and Properties of Novel Ternary Fe-Rich Rare-Earth Carbides -- Fine Particles -- Granular Solids -- Ultrafine Magnetic Particles -- Magnetic Nanometer Systems and Mossbauer Spectroscopy -- Some Topics in Fine Particle Magnetism -- Mossbauer Studies of Fine Fe-Based Particles -- Mossbauer Studies of Fine Particles of Fe-Cr-B -- Chemical Preparation of Amorphous Fe-Cr-B Particles -- Composition and Structure of Fe-Ni-B Alloy Particles Prepared by Chemical Reduction with NaBH4 -- Quantum Effects in Ultrafine Nd-Fe-B Particles -- Magnetization Reversal in Clusters of Magnetic Particles -- Electric and Magnetic Properties of Small Systems -- Existence of Frequency Cut-Off in the Spin Wave Spectrum of Small Magnetic Particles -- Magnetic Hysteresis and Permanent Magnets -- Mechanically Alloyed Permanent Magnets -- Melt-Spun Magnets -- Solid NdFeB Magnets Made by Gas Atomization and Extrusion -- The Role of Microstructure in Permanent Magnets -- Lorentz Microscopy Studies in Permanent Magnets -- Coercivity in Hard Magnetic Materials -- Micromagnetism and Magnetization Processes in Modern Magnetic Materials -- Micromagnetic Approach to Magnetic Hysteresis -- Magnetic Hysteresis in Disordered Magnets -- Coercivity of Nanostructured Materials -- Magnetic Hysteresis of CoPt Films -- Technology and Applications of Permanent Magnets -- Author Index.

Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single‐crystal films (invited)

Abstract: Single‐crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X‐ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic mom...

Magnetocrystalline anisotropy and orbital moments in transition-metal compounds.

Abstract: The magnetocrystalline anisotropy energy and anisotropy of the orbital angular momentum have been calculated from first prinicples for Co and for a variety of intermetallic compounds including ${\mathrm{YCo}}_{5}$. For all compounds the predicted easy axes are in agreement with experiment. A strong correlation between the anisotropy of the orbital angular momentum and the energy is found for the compounds that do not contain Pt. For those that do contain Pt, Pt is shown to contribute significantly to the anisotropy energy.

Anisotropic Nd–Fe–B thin‐film magnets for milli‐size motor

Abstract: Efforts were made to obtain anisotropic thin‐film magnets at low substrate temperature. This is an important criterion for practical applications such as to build motors. The influence of substrate materials as well as film thickness on the c‐axis orientation were studied. It has been shown that thin‐film magnets with the easy axis of magnetization normal to the film plane could be deposited at a substrate temperature of around 450 °C by choosing the composition near the line from Nd13Fe76B11 to Nd13Fe70B17 in the ternary phase diagram. It was found that the anisotropic film magnets could be also deposited on the metallic substrate. The c‐axis orientation tended to be isotropic with an increase in film thickness. The obtained results were used to fabricate a milli‐size motor by depositing 20‐μm‐thick Nd–Fe–B films on a silicon steel disk substrate of 5‐mm diam. The milli‐size motor exhibited a torque of 0.8 g mm at a rotational speed of 3000 rpm.

Cation distribution and intrinsic magnetic properties of Co‐Ti‐doped M‐type barium ferrite

Abstract: The structural and magnetic properties of BaFe12−2xCoxTixO19 (0≤x≤1.0) M‐type barium ferrite have been investigated by means of neutron powder diffraction and high‐field magnetization measurements. The cationic distribution and magnetic moments of the five different metallic sublattices are determined and compared to the experimental saturation magnetization. It is found that about 50% of Co ions occupy tetrahedral sites, thus being ineffective in the reduction of the uniaxial magnetic anisotropy, while Ti ions prefer the 4fVI octahedral sites. All in all, a clear hierarchy of sublattice preferences is defined. Although the collinearity of the magnetic structure is progressively broken, mainly above x≊0.7, the overall behavior remains ferrimagnetic. The strong local spin canting is tentatively attributed to the localization of diamagnetic Ti ions in the 4fVI octahedral sites.

Magnetic and crystallographic properties of novel Fe‐rich rare‐earth nitrides of the type RTiFe11N1−δ (invited)

Abstract: We succeed in inserting a number of nitrogen atoms into the RTiFe11 intermetallics. The nitrides retain the ThMn12‐type structure, but with an increase in the unit cell volume. The crystallographic sites located by nitrogen atoms are determined by using neutron diffraction techniques. The nitrogen atoms are found to have an effect of increasing Curie temperature and saturation magnetization. Moreover, an essential change in magnetocrystalline anisotropy is observed upon nitrogenation. By all of these effects, the NdTiFe11N1−δ compounds have excellent intrinsic magnetic properties favorable for permanent magnet applications.

Magnetic properties of a new family of ternary rare‐earth iron nitrides R2Fe17N3−δ (invited)

Abstract: A series of interstitial ternary nitrides R2Fe17N3−δ has been prepared via a gas‐solid reaction for R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y. These nitrides have structures related to the Th2Zn17 or Th2Ni17 structures of the R2Fe17 parent compounds, but the unit cell volumes are 6%–7% greater, and the Curie temperatures are approximately 400 K higher. The Fe‐Fe exchange interactions are increased by a factor of 2.8 by nitrogenation, whereas the R‐Fe exchange interactions are little changed. All compounds exhibit easy‐plane anisotropy at room temperature, except for Sm2Fe17N3−δ, which shows strong uniaxial anisotropy and may be used to make permanent magnets. The Er and Tm compounds exhibit spin reorientations below room temperature. The anisotropy due to the iron sublattices is easy plane (K1 = −1.3 MJ/m3 at 4.2 K for Y2Fe17N3), but it changes sign to easy axis with cobalt substitution (K1 ≊ 1.0 MJ/m3 at 4.2 K for Y2(Fe1−xCox)17N3−δ when x≥0.2).

Commensurability effect of magnetoresistance anisotropy in the quasi-one-dimensional conductor tetramethyltetraselenafulvalenium perchlorate, (TMTSF)2ClO4.

Abstract: We have observed a clear series of fine structures superposed on the angular dependence of magnetoresistance in the metallic and magnetic-field-induced spin-density-wave phases in the low-dimensional organic superconductor (TMTSF){sub 2}ClO{sub 4}. They appear at the field angles whose tangent satisfies a fractional condition reflecting the crystal geometry. This new oscillatory phenomenon is considered to result from a resonance effect, i.e., commensurate versus incommensurate, between two periodicities in the electron motion along an open orbit under tilted magnetic fields.

Spin-wave theory of two-dimensional ferromagnets in the presence of dipolar interactions and magnetocrystalline anisotropy

Abstract: A spin-wave theory of two-dimensional ferromagnets taking into account both the long-range dipolar interactions and a uniaxial magnetocrystalline anisotropy is presented. We show that in the case of the perpendicular easy axis the stabilization of long-range ferromagnetic order at finite temperature is due mainly to the anisotropy-induced gap at the bottom of the spin-wave spectrum, whereas the dipolar interactions play a negligible role; on the other hand, in the case of easy-plane anisotropy, the spin-wave spectrum remains gapless, so that the stabilization of long-range ferromagnetic order is due in turn to the long-range character of the dipolar interactions. The temperature dependence of the magnetization is calculated in both cases. The results are discussed in connection with experimental data reported for fcc (001) cobalt monolayers epitaxially grown onto (001) copper single crystals.

Magnetization reversal in (100) Fe thin films.

Abstract: The longitudinal and transverse magneto-optical Kerr effects have been used for understanding the magnetization processes in single crystal Fe/GaAs (100) thin films. By using both of these Kerr effects it is possible to concurrently detect two orthgonal in-plane magnetization components. Presented here are Kerr hysteresis curves for magnetic fields directed along the in-plane 〈100〉 and 〈110〉 directions of the Fe films. For the 〈100〉 direction the magnetization curves are square, while for the 〈110〉 unusual overshoots are present in the Kerr hysteresis curves. In order to understand the origin of these curves for the 〈110〉 direction, simulations were done using the Fresnel reflection coefficients for the in-plane Kerr effects and a coherent-rotation model for the magnetization process. There is good agreement between the simulated Kerr hysteresis curves and the experimental data. The overshoots along with the general analyzer dependence of the hysteresis curves are reproduced in the simulations. However, the magnitude of the reversal and saturation fields of the modeled loops cannot be brought into agreement with the data. For applied fields near the 〈110〉 directions, the analysis suggests that the reversal occurs through the nucleation and/or unpinning of 90\ifmmode^\circ\else\textdegree\fi{} domains at two distinct transition fields followed by coherent rotation.

Magnetic properties of ultrafine nickel particles

Abstract: Ultrafine nickel particles have intriguing physical and chemical properties, which are interesting both in fundamental and applied research. The size of the particle was controlled by gas pressure. X‐ray diffraction studies showed that fine Ni particles have fcc crystal structure and are coated with thin Ni oxide on the surface. Electron micrographs showed a spherical particle shape, forming a long chain. Size dependence of magnetic properties were studied. The specific magnetic moment drastically decreases when reducing the diameter d of particles <15 nm. The coercivity Hc also approaches zero when d is about 15 nm. Therefore, we can suppose that the critical size for superparamagnetism at room temperature is about 15 nm. According to the superparamagnetic formula KV=25 kT, the value of the magnetic anisotropy constant can be determined, K = −5.8 × 105 (erg/cc). It is found to be larger than bulk Ni [K=−3.4–5.1 × 104 (erg/cc)], the same as Fe particles. The maximum of coercivity at room temperature is ab...

Surface magnetism; magnetization and anisotropy at a surface

Abstract: A review is given of surface magnetism. In particular the interplay of magnetization and anisotropy at a surface is discussed, after a survey of experimental and theoretical results for magnetic moment and anisotropy at surfaces or interfaces of semi-infinite magnets and thin films. Finally, the importance for surface (or interface) analysis of the random single-ion anisotropy model is stressed.

Anisotropy compensation and magnetostriction in TbxDy1−x(Fe1−yTy)1.9 (T=Co,Mn)

Abstract: From magnetization (M) and magnetostriction (λ) measurements as a function of magnetic field and stress, the temperatures of anisotropy compensation, Tm, for technologically important TbxDy1−x(Fe1−yTy)1.9 [T=Co,Mn (0.3≤x≤0.5) (0≤y≤0.3)] were determined. Measurements of M and λ encompassing Tm were made under compressive stresses from 8.8 to 36 MPa and for temperatures from −196 to +130 °C. In agreement with earlier measurements, Tm decreases with increasing Tb. Substitution of Mn for Fe for fixed x also decreases Tm. In contrast with these observations is the increase of the anisotropy compensation temperature with the replacement of Fe by small amounts of Co. In the cases of both (1) increasing Tb content and (2) increasing Co content, the Curie temperature TC increases, yielding, in general, a higher magnetic moment and saturation magnetostriction of these alloys. Thus, compensation at a given temperature may be obtained in an improved class of Laves phase compounds, R(1)xR(2)1−x(Fe1−yCoy)2, where rare ...

Magnetic anisotropies in single and multilayered structures (invited)

Abstract: Ultrathin magnetic metallic structures provide a variety of systems in which unique magnetic properties can be engineered. The investigation of magnetic anisotropies in ultrathin structures has brought exciting results to the basic studies of magnetism in systems with reduced dimensionality and to the engineering of new magnetic materials. Molecular‐beam epitaxy (MBE) techniques turned out to be particularly useful in the formation of new stable and metastable structures whose sharply defined interfaces, lattice relaxations, and lattice reconstructions have resulted in a wide range of interesting magnetic properties. In this presentation the present authors’ recent studies of single and multilayered structures composed of bcc Fe(001), bcc Cu(001), lattice‐expanded Pd(001), fcc Co(001), and fcc Cu(001) will be summarized. Besides presenting their interesting magnetic properties the following techniques will be highlighted: reflection high‐energy electron diffraction (RHEED) and ferromagnetic resonance (FMR). RHEED patterns and RHEED intensity oscillations were used to monitor the MBE growths. It will be shown that RHEED studies provide an effective way to characterize the quality of growth which can then be correlated with the observed magnetic properties. It will be shown that the FMR technique is extremely useful in determining all of the basic magnetic parameters. The role of interfaces, lattice relaxations, and proximity effects on the in‐plane fourfold and perpendicular uniaxial anisotropies will be discussed.

Commensurate fine structure in angular-dependent studies of (TMTSF)2ClO4.

Abstract: Clear fine-structure oscillations in magnetoresistance and torque magnetization are observed at low temperatures in the organic superconductor (TMTSF) 2 Cl 4 as a function of sample orientation in a magnetic field. These fine structures appear as resistance minima and torque maxima at specific field orientations, with one-to-one correspondence. The angular positions of the features are independent of magnetic field, temperature, and cooling rate (disorder), while their magnitudes depend strongly on all three

Exchange interactions and giant magnetoresistance in magnetic multilayers

Abstract: Magnetic layer structures consisting of alternating magnetic and non-magnetic layers exhibit new magnetic phenomena with great potential for applications in magnetic storage technology. It is found experimentally that the magnetic moments of neighbouring magnetic layers are spontaneously aligned ferromagnetically or antiferromagnetically depending on the thickness of the intervening nonmagnetic layers. This implies an oscillatory magnetic coupling between the layers mediated by the nonmagnetic spacer layers. The resistance of the structure when a current is passed in the direction parallel to the layers is much higher in the antiferromagnetic configuration than in the ferromagnetic one. The magnetic configuration of the layers, and hence the resistance of the structure, is strongly influenced by an applied magnetic field. This effect, called giant magnetoresistance, can be exploited to read information from a magnetic disc. The purpose of this review is to introduce the reader to magnetic multila...

Magnetic properties of R2Fe17N2.7 compounds

Abstract: We have studied the magnetic properties and decomposition behaviour of various ternary nitrides of the type R2Fe17N2.7 with R ≡ Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, LuandY. The magnetic anisotropy of the R2Fe17N2.7 compounds was studied on magnetically aligned powders in field strengths up to 35 T. The rare-earth sublattice anisotropy is much stronger in R2Fe17N2.7 than in R2Fe17, leading to an easy c-axis anisotropy in Sm2Fe17N2.7. For Er2Fe17N2.7 and Tm2Fe17N2.7 spin-reorientation transitions were observed upon cooling from room temperature, starting at about 150 and 225 K, respectively. The intersublattice-coupling strength between the R- and Fe-sublattice magnetization was found to remain approximately unaltered after charging R2Fe17 with nitrogen gas.