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A nucleation-based analytical methodology helps enhance the magnetic strength of aligned two-phase magnets by exchange hardening in nanoscale combinations of a soft-phase and an oriented hard-phase. Breakdown of this high energy magnet occurs in the limit of isotropic magnets having enhanced remanence. The high cost of production of such giant-energy magnets limits their range of application.

The study of the reaction between nitrogen gas and an intermetallic compound with particular reference to Sm2Fe17, treating the Sm2Fe17N(sub y) system as a gas solid solution, shows that there is a large uniaxial strain near the surface of nonuniformly nitrided particles and even in the absence of nitrogen there is core expansion. The reaction of nitrogen atoms with the metal lattice in terms of the net reaction energy U(sub 0) is described by a simple lattice gas models. The equilibrium nitrogen concentration as a function of nitrogenation temperature and gas pressure is calculated.

The gas phase interstitial modification of rare-earth intermetallics is studied. Net reaction energies for nitrogen in Sm2Fe17 and Nd(Fe11Ti) are U0 = - 57 kJ/mole and U0 = - 51 kJ/mole, respectively. The equilibrium nitrogen concentration is calculated as function of temperature and gas pressure using a simple lattice gas model. For nitrogen in Sm2Fe17, refined diffusion parameters D0(N) = 1.02 mm2/s and Ea(N) = 133 kJ/mole, determined by thermopiezic analysis of the initial stage of nitrogen absorption, are used to calculate nitrogen profiles and the time dependence of the mean nitrogen content during nitrogenation. Similiar values are obtained for nitrogen in Nd(Fe11Ti), whereas the activation energies for hydrogen in Sm2Fe17 and Nd(Fe11Ti) are 31 kJ/mole and 45 kJ/mole, respectively. The elastic stress and strain profiles during nitrogenation are calculated. Important results are a large uniaxial strain near the surface of non-uniformly nitrided particles, and core expansion even in the absence of any nitrogen there. Curie temperature and K1 profiles are calculated and suggestions are made regarding the influence of stress on coercivity and disproportionation of the material.

It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition- metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independentelectron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rareearth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy. [ABSTRACT FROM AUTHOR]

Nanoscale magnets with characteristic dimensions in the range of 1-100 nm are important in several areas of nanoscience and technology. First, this length scale spans the typical important dimensions of exchange lengths and domain-wall widths, which means that significant control of magnetic properties can be obtained by varying grain or particle dimensions. Second, the nonequilibrium synthetic processes used for clusters, particles, and films, often lead to new real-space crystal structures with completely novel spin structures and magnetic properties. Third, a basic-science challenge in this class of matter involves the spin-polarized quantum mechanics of many-electron systems containing 10-10 000 atoms. Finally, the materials under study may have important future applications in high-density data storage, ultra-small spintronic devices, or high-energy magnetic materials. In this article, we discuss our recent work on novel Fe-Au nanoclusters, MnAu-Mn coreshell structures, and complex high-anisotropy Co-rich intermetallic compound clusters. We also present new results on Fe-based alloys including the magnetic properties of semiconducting FeSi2 nanoclusters and spin correlations in FeGe nanocluster films. [ABSTRACT FROM AUTHOR]

Single-phase noncubic Mn-Ga films with a thickness of about 200 nm were fabricated by an in situ annealing of [Mn(x)/Ga(y)/Mn(x)]5 multilayers deposited by e-beam evaporation. Mn-Ga alloys prepared in three different compositions Mn2Ga5 and Mn2Ga were found to crystallize in the tetragonal tP14 and tP2 structures, respectively. Mn3Ga crystallizes in the hexagonal hp8 or tetragonal tI8 structures. All three alloys show substantial magnetocrystalline anisotropy between 7 and 10 Mergs/cm3. The samples show hard magnetic properties including coercivities of Mn2Ga5 and Mn2Ga about 12.0 kOe and of Mn3Ga about 13.4 kOe. The saturation magnetization and Curie temperature of Mn2Ga5, Mn2Ga, and Mn3Ga are 183 emu/cm3 and 435 K, 342 emu/cm3 and 697 K, and 151 emu/cm3 and 798 K, respectively. The samples show metallic electron transport up to room temperature. [ABSTRACT FROM AUTHOR]

First-principle calculations are used to explore the use of magnetic degrees of freedom in catalysis. We use the Vienna Ab-Initio Simulation Package to investigate both L10-ordered FePt and CoPt bulk materials and perform supercell calculations for FePt nanoclusters containing 43 atoms. As the catalytic activity of transition-metal elements and alloys involves individual d levels, magnetic alloying strongly affects the catalytic performance, because it leads to shifts in the local densities of states and to additional peaks due to magnetic-moment formation. The peak shift persists in nanoparticles but is surface-site specific and therefore depends on cluster size. Our research indicates that small modifications in stoichiometry and cluster size are a useful tool in the search for new catalysts. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of substituted [L1.sub.0] compounds are studied. It is shown that ordering has a strong impact on the magnetism of the alloys, particularly on the anisotropy.

Magnon-plasmon interactions are modeled by considering the spin-dependent dielectric response of atom placed in crystalline environment. Magnetic excitations of intra-atomic origin are analyzed which are found to be related to ordinary magnons but involve longitudinal magnetic degrees of freedom and involve much higher energies.

The temperature dependence of the magnetic anisotropy of ferromagnetic materials is analyzed. The findings conclude explain that magnetocrystalline anisotropy is, essentially, a single-ion property by embedding the atom in a metallic or nonmetallic crystalline environment.

Intergranular interactions in nanostructured FePt:Ag thin films and their effect on magnetic hysteresis are investigated. The films, produced by muitilayer deposition plus rapid thermal annealing, consist of FePt nanoparticles embedded in a silver matrix. They are investigated by magnetization measurements and magnetic force microscopy. Analytical model calculations, supported by micromagnetic simulations, are used to elucidate the relation between coercivity, hysteresis-loop slope, and spatial correlations during magnetization reversal. The analytical calculations yield simple expressions for the loop slope and the coercivity as a function of the intergranular exchange. Small intergranular exchange enhances the coercivity, but for strong exchange there is a self-energy cutoff, associated with the onset of cooperative reversal and preventing alpha from becoming negative. Index Terms--Anisotropy, magnetic recording, magnetization reversal.

High-coercivity CoPt magnetic force microscope tips have been modified by focused ion beam milling to improve the resolution of magnetic domain images. The magnetic materials around the apex have been removed, leaving a 30-nm diameter magnetic particle at the tip end. Due to the smaller amount of magnetic material, the stray field from this new tip is significantly reduced, and the spatial resolution of the magnetic domain images is improved. The tip is used to obtain high-resolution domain images of a CoCrPt-Si[O.sub.2]/Ru perpendicular recording medium with linear recording densities from 800 to 1000 kfci. Magnetic patterns of 900 kfci, corresponding to a bit size of 28 nm, are well resolved. From the analysis of the power spectrum of the track profiles for these images, a spatial resolution as good as 11 nm under ambient conditions with a commercial magnetic force microscope is achieved. Index Terms--Domain images, focused ion beam technology, magnetic force microscopy, magnetic recording.

Electron microscopy, x-ray diffraction and superconducting quantum interference device magnetometry are used for examining the FePt and Fe3O4 nanotubes produced by hydrogen reduction in nanochannels of porous alumina templates. Fe3O4 tubes are formed by a thermally decomposed Fe nitrate solution at 250 degrees C that is reduced in hydrogen for 2.5 h at the same temperature.

The magnetic properties and switching behavior of anisotropic magnetic nanodots are examined. A strong interdot interaction is observed in dot separations of less than 50 nm resulting in a probable controlled switching of neighboring dots.

Linear muffin-tin orbital calculations were used to examine the electronic band structure of ordered equiatomic compounds of 3d transition elements (Mn, Fe, Co, Ni) with nonmagnetic 4d and 5d elements (Rh, Pd, Pt, Ir). Mn spins in a given plane prefer to align antiferromagnetically as indicated by the pronounced negative exchange interactions between four Mn-Mn first nearest pairs, J(sub 11) observed in all Mn compounds.

The temperature dependence of the magnetocrystalline anisotropy of PtCo and its atomic origin are investigated by first-principle and model calculations. The Pt spin moment necessary to realize the leading 5d anisotropy contribution is due to neighboring Co atoms. At finite temperatures, Co spin disorder strongly reduces the Pt moment and the anisotropy. This is in contrast to the situation encountered in 3d and 3d-4f magnets, where the atomic magnetic moments remain largely conserved, even above the Curie temperature. A consequence of the L[1.sub.0] mechanism is that the [K.sub.1] (T) curve of exhibits a negative curvature, in contrast to the unfavorable positive curvature for rare-earth transition-metal magnets. Index Terms--Anisotropy, finite-temperature magnetism, inteatomic exchange, L[1.sub.0] magnets, permanent magnets, PtCo.

The Curie temperature of rare-earth Co intermetallics is investigated by self-consistent spin-polarized electronic-structure calculations on Y--Co compounds. The total exchange interaction of a given site with all other sites ([J.sub.o) is calculated by the linear-muffin-tin-orbital method using the local force theorem and employed to obtain mean-field Curie-temperature estimates. The theoretical predictions for Y[Co.sub.3], Y[Co.sub.5], and [Y.sub.2][Co.sub.17] are in fair agreement with the experimental Curie temperatures. For Y[Co.sub.5], the variation of exchange interactions ([J.sub.ij]) with distance is analyzed, and it is discussed how the presence of nonequivalent cobalt sites affects the Curie temperature of the compounds. Index Terms--Curie temperature, exchange interactions, magnetic moment, permanent magnets, rare-earth cobalt intermetallics, Y[Co.sub.5].

Research into cooperative and noncooperative magnetization processes in magnetic nanostructures is presented. It is found that the Preisach model overestimates the coercivity. In general, cooperative effects are strongest for narrow switching-field distribution.

Research into the magnetism of antimony overlayers on a ferromagnetic substrate is presented. This approach produces a layer-specific analysis of the spin polarization. The relatively long range of the Sb spin polarization is attributable to the semimetallic nature of the Sb overlayer.

The net Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between well-separated spin distributions can be mapped onto a RKKY interaction between two point-like effective moments. In 4f metals and alloys, the RKKY interaction gives rise to a magnetoelectric two-ion anisotropy.

A study was conducted to analyze the effects of the shape of the energy landscape on the magnetic viscosity. The evolution of a quantum-mechanical system was characterized by the time-dependent Schrodinger equation. Experimental results indicated that the activation energy related to thermally activated hopping changes the magnetic viscosity by minimizing the attempt-frequency prefactor.

A study was conducted to analyze magnetic hardening in SmCo(sub x)-Co multilayers and heat treated nanocomposite films. The films were prepared by dc and rf sputtering. Magnetization loops were determined by an alternating gradient force magnetometer and a superconducting quantum interference device magnetometer. Experimental results indicated that a thick chromium underlayer is needed for magnetic hardening.

The anisotropy of fee (111) Fe/Ni and Fe/Ni/Fe layers is investigated. The films exhibit an oscillating behavior of the preferential magnetization direction depending on whether the surface layer is Ni or Fe. Anisotropy fields are obtained from polar and perpendicular Kerr hysteresis loops and yield a comparatively small perpendicular Fe/Ni interface anisotropy of about 0.15 mJ/[m.sup.2]. The real-space origin of the interface anisotropy is the interlayer hybridization of the yz, zx, and [z.sup.2] orbitals. Index terms - interface anisotropy, ultrathin films, Kerr hysteresis

The diffusion of nitrogen in R(sub2)Fe(sub17) intermetallics by a method that reconciles the solid-solution and immobilization theories of nitrogen diffusion, has been studied. Two-sublattice diffusions may lead to sharp concentration profiles, although the phase structure of the nitride remains unchanged. There is a global relaxation time, that controls the smoothing of concentration gradients.

The effect of interstitial nitrogen on the phase transition and magnetocaloric behavior of MnAs1-xSixNδ (x=0.03, 0.06, and 0.09) is investigated. The interstitial nitrogen atoms cause the step-scanned x-ray diffraction peaks to shift toward lower angles and lower the Curie temperature, whereas silicon addition increases the Curie temperature to near room temperature. The thermal hysteresis is reduced to nearly 0 in 1-xSixNδ, which is beneficial to practical applications. For a field change of 5 T, the largest magnetic entropy change and refrigerant capacity are 14.6 J Kg-1 K-1 at 247 K and 360 J K-1, which is slightly higher than the entropy change in the parent alloy. Finally, we briefly discuss the occurrence and origin of the "virgin effect" in MnAs. [ABSTRACT FROM AUTHOR]

We report on optical, structural, and magnetic properties of two substantially different cobalt nanostructure thin films deposited at an oblique angle of incidence of 85° away from the substrate normal. Comparison is made between an achiral columnar thin film grown without substrate rotation and a chiral nanocoil sculptured thin film by glancing angle deposition with substrate rotation. Generalized spectroscopic ellipsometry is employed to determine geometrical structure properties and the anisotropic optical constants of the films in the spectral range from 400 to 1000 nm. The magnetic properties are analyzed with a superconducting quantum interference device magnetometer. Both nanostructure thin films show highly anisotropic optical properties such as strong form birefringence and large dichroism. In particular, Co slanted columnar thin films are found to possess monoclinic optical properties. Magnetic measurements at room temperature show hysteresis anisotropy with respect to a magnetic field either parallel or perpendicular to the nanostructures’ long axis. We find extremely large coercive fields of approximately 3 kOe for our achiral columnar nanostructures. [ABSTRACT FROM AUTHOR]

The crystal-field parameters [A.sub.20] and [A.sub.40] were determined from the magnetocrystalline anisotropy constants [K.sub.1] and [K.sub.2] in a linear approximation for [Sm.sub.2][Fe.sub.17][Z.sub.x] (Z = N, C) compounds. For the materials investigated, the values of [K.sub.1] and [K.sub.2] in a temperature range from 4.2 - 533 K have been obtained by a fit of demagnetization curves. The parameters [A.sub.n0] are calculated in two ways: i) from the [K.sub.i] at zero temperature and ii) from the temperature dependence of the [K.sub.i]. Both methods give equivalent results. The value [Mathematical Expression Omitted] has been obtained which corresponds to an inverse THOMAS-FERMI screening length q [approximately equal to] 2.3 [[Angstrom].sup.-1] calculated within the framework of the screened charge model.

The expansion of magnetic domains in thin films with perpendicular anisotropy is investigated. To determine the domain-wall-velocity as a function of the applied magnetic field a selfconsistent magnetic-viscosity approach is used. The main predictions of the theory are a linear behaviour in the limit of moderately strong fields, a quasi-exponential behaviour for fields close to the propagation field, and a negative velocity for very small and negative fields. The predictions of the theory are compatible with domain-wall investigations on Au/Co/Au sandwiches.

Elastic and magnetic properties of ultrathin iron films on tungsten(110) are investigated. In situ film stress measurements during growth show stress values of 25 GPa per deposited monolayer. Our experiments indicate that the tremendous film stress triggers the formation of a misfit dislocation network at a coverage of 1.5 monolayers. The effect of the film stress and its spatial variation on the magnetic behavior are discussed. We find an high coercivity of order 0.3 T for 1.5 monolayer thick films. A model is presented that explains the high coercivity in terms of strong domain wall pinning.

The magnetism of sesquilayer bcc iron films on W(110) is investigated. Coercivity data and theoretical calculations indicate that the freezing of the magnetization at low temperatures reflects exchange-dominated domain-wall pinning rather than superparamagnetic spin-glass behaviour.

FePt nanoparticles with a size of 3 nm and thermally stable room-temperature ferromagnetism are investigated. The monodisperse nanoparticles were prepared by chemical synthesis and a salt-matrix annealing technique. Structural and magnetic characterizations confirmed the phase transition from the disordered face-centered cubic structure to the L10 structure with the chemical ordering parameter of 0.62±0.05. Analysis in blocking temperature and fitting of temperature dependence of switching field reveals that the transformed 3 nm nanoparticles have a magnetic anisotropy constant of (2.8±0.2)×106 J/m3, smaller than those for the bigger particles and the fully ordered L10 bulk phase. [ABSTRACT FROM AUTHOR]

Structural and magnetic properties of laminated (Nd,Dy)(Fe,Co,Nb,B)5.5 /Fe nanocomposites are investigated. Normally, the addition of the soft phase to the hard phase enhances the remanence but deteriorates the permanent-magnet performance of the material by reducing the coercivity. In the present system, the coercivity increases to 1608 kA/m (20.2 kOe) in thermally processed Nd-Dy-Fe-Co-Nb-B(15 nm)/Fe(4 nm) multilayered nanocomposites, which is higher than that of the single-layer hard-magnetic film. The abnormally high coercivity is achieved by annealing at relatively high temperature, which breaks the laminated structure of the as-deposited multilayer. A likely physical explanation of the enhanced coercivity is the introduction of the domain-wall pinning sites that counteract the inevitable decrease of the nucleation field. [ABSTRACT FROM AUTHOR]