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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.

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.

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].

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 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.

Fine cobalt and silver powders have been ball-milled to yield a granular solid of composition [Co.sub.30][Ag.sub.70] in the form of coarse powder. The material is an intimate mixture of grains of fcc Co in a silver matrix. The cobalt is in the single-domain size range, and the samples exhibit coercivity (0.07 T at 296 K) and unusual thermomagnetic effects, including field-induced unidirectional anisotropy. There is a large, isotropic negative magnetoresistance, which exceeds 10% at 4.2 K.

Size effects in two-phase Boolean magnets composed of an aligned hard phase and a soft phase with high magnetization are investigated. In 'overcritical' magnets, where the size of the soft regions is larger than the Blochwall width [[Delta].sub.h] of the hard phase, the energy product is reduced compared to the corresponding 'undercritical' magnets with small soft regions, where theoretical energy products as high as 1 MJ/[m.sup.3] have been predicted. Nevertheless, the theoretical energy product only gradually drops with increasing size of the soft regions, and it may still exceed the theoretical energy product 516 kJ/[m.sup.3] of [Nd.sub.2][Fe.sub.14]B.

Non-equilibrium nitrogenation of transition-metal rich rare-earth intermetallics can be used to increase the nitrogen content beyond its equilibrium concentration. For example, nitrogenation of [Sm.sub.2][Fe.sub.17] using ammonia yields volume expansions up to 8.3 vol %, which correspond to a composition [Sm.sub.2][Fe.sub.17][N.sub.x] with x [approximately equal to] 4. This effect is related to catalytic action at the metal surface where ammonia is decomposed into nitrogen and hydrogen, with nitrogen absorption on a site where it is weakly bound, as an intermediate step. For overloaded [Sm.sub.2][Fe.sub.17][N.sub.x] with x > 3 the net reaction energy [U.sub.o] = 5 [+ or -] 10 kJ/mole indicates a nearly temperature-independent occupation of the non-9e sites.

Exchange hardning of nanostructured two-phase systems composed of an aligned hard phase and a soft phase with high magnetization is investigated using a micromagnetic approach which accounts for interactions between the soft regions. For Sm2Fe17N3 (2.5 nm)/Fe65Co35(9nm) multilayers an energy product as high as 1 MJ/m cube (120 MGOe) is predicted, with a rare-earth content of only 5wt%. Giant energy products may also be achieved in suitable cellular and disordered structures.

Continuum and layer-resolved calculations are used to investigate the spin structure in the vicinity of grain boundaries. Reduced exchange in the grain-boundary region gives rise to a quasidiscontinuity of the magnetization and yields a perturbation which decays exponentially inside the grains. An effective intergranular exchange is obtained as a micromagnetically well-defined function of the grain-boundary exchange, and it is discussed how grain boundaries affect the hysteresis loops of nanostructures. Index Terms--Anisotropy, grain boundaries, intergranular exchange, thin films.

The high-temperature coercivity of mechanically alloyed and subsequently annealed R[Co.sub.5] (R = Sm and Y) is studied. The annealed materials have the hexagonal Ca[Cu.sub.5] structure with 2:17 (or 1:7) regions as a minor phase. High-temperature magnetic measurements show that the coercivities of materials decrease with increasing temperature from room-temperature to 873 K, but that the temperature coefficient of the coercivity of Y[Co.sub.5] is much smaller than that of Sm[Co.sub.5]. This behavior is explained in terms of the intrinsic temperature variation of the magnetocrystallline anisotropy. Index Terms--Anisotropy, coercivity, finite-temperature magnetism, permanent magnets.

Magnetic properties of Co-C nanodot arrays produced by direct interferometric laser annealing are investigated by magnetic force microscopy (MFM) and magnetization measurements. The dots are formed by locally annealing sputtered amorphous Co-C films in regions where the laser intensity is highest. As-sputtered Co-C films do not exhibit ferromagnetic order at room temperature, but MFM shows that the dots become magnetic upon annealing, possibly due to the agglomeration or phase separation of Co-rich clusters. The dots are embedded in either a paramagnetic or weakly magnetic matrix. The magnetic properties of the generated pattern can be changed by varying the laser power. The present results show that direct interferometric lithography may become a useful tool for fabricating future patterned magnetic nanostructures. Index Terms--Artificially structured materials--small particles and patterned films, magnetic domains.

Several systems of nanocomposite thin films have been prepared by plasma sputtering and heat treatment. By choosing suitable multilayer structures of the as-deposited films and subsequent heat-treatment processes, the nanostructures of the films have been tailored. The thermal processing is the key to control the morphology. Appropriate heat-treatment processes have been found for the systems studied, which result in the desired nanostructures. For the first time a nearly ideal nanostructure with the soft-phase grains embedded homogeneously in the hard phase grains has been obtained for the FePt-[Fe.sub.1-x][Pt.sub.x](X~0.3) system. Effective intergrain exchange coupling has been realized. As a consequence, high energy products up to 50 MGOe have been achieved. Index Terms--exchange coupling, heat treatment, permanent magnets, thin films.

First-principle supercell calculations are used to determine how 3d elemental additions, especially Fe additions, modify the magnetization, exchange and anisotropy of L10-ordered ferromagnets. Calculations are performed using the VASP code and partially involve configurational averaging over site disorder. Three isostructural systems are investigated: Fe-Co-Pt, Mn-Al-Fe, and transition metal-doped Fe-Ni. In all three systems the iron strongly influences the magnetic properties of these compounds, but the specific effect depends on the host. In CoPt(Fe) iron enhances the magnetization, with subtle changes in the magnetic moments that depend on the distribution of the Fe and Co atoms. The addition of Fe to MnAl is detrimental to the magnetization, because it creates antiferromagnetic exchange interactions, but it enhances the magnetic anisotropy. The replacement of 50% of Mn by Fe in MnFeAl2 enhances the anisotropy from 1.77 to 2.5 MJ/m^3. Further, the substitution of light 3d elements such as Ti, V, Cr into L10-ordered FeNi is shown to substantially reduce the magnetization. [ABSTRACT FROM AUTHOR]

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr18\-xHfxCo82 ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr2Co11, orthorhombic Zr2Co11, hcp Co, and cubic Zr6Co23 phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr2Co11 (rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (Hc) and the saturation magnetization (Ms) are found to linearly increases with x (x\leq 2), then Hc slightly increases and Ms slightly decreases with increasing x. The maximum energy product (BH)\max at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m^3 and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials. [ABSTRACT FROM AUTHOR]

Nanostructured Mn3 - x Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D019 and cubic L21 Heusler crystal structures based on the concentration of Mn in Mn3 - xGa. Upon vacuum-annealing the samples at 450 ^\circC for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D022 structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn1.9 Ga and Mn2.1Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn2.6 Ga and Mn3.0Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn3 - x Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn2.6 Ga and Mn3.0 Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn3 - xGa. The maximum magnetization of 57 emu/g (300 emu/cm^3) and the coercivity of 6.5 kOe were measured in the Mn1.9 Ga and Mn3.0Ga ribbons, respectively. [ABSTRACT FROM AUTHOR]

This study presents the structural and magnetic properties of melt-spun HfCo7, HfCo7-xFe_x (0.25\leqx\leq 1), and HfCo7Six (0.2\leqx\leq 1.2) alloys. Appreciable permanent-magnet properties with a magnetocrystalline anisotropy of about 9.6–16.5 Mergs/cm^3, a magnetic polarization Js\approx 7.2–10.6 kG, and coercivities Hc=0.5–3.0 kOe were obtained by varying the composition of these alloys. Structural analysis reveals that the positions of x-ray diffraction peaks of HfCo7 show good agreement with those corresponding to an orthorhombic structure having lattice parameters of about a=4.719\ \\rm\AA, b=4.278\ \\rm\AA, and c=8.070\ \\rm\AA. Based on these results, a model crystal structure for HfCo7 is developed and used to estimate the magnetic properties of HfCo7 using density-functional calculations, which agree with the experimental results. [ABSTRACT FROM AUTHOR]

The effect of Zr addition on nanostructure and magnetic properties in nanocrystalline Sm1 - xZrxCo5 (x = 0-0.6) has been investigated. (Sm, Zr)Co5 with the CaCu5 structure was synthesized by melt spinning. The lattice parameters a and b decrease with x, whereas c increases. Thus, the unit cell volume of (Sm, Zr)Co5 shrinks because the smaller Zr atoms occupy the sites of the larger Sm atoms. Zr addition decreases the grain size and induces the formation of planar defects. The coercivity decreases with x, due to weakening of magnetocrystalline anisotropy energy and effective intergrain exchange coupling. A very high coercivity of 39 kOe and energy product of 13.9 MGOe are obtained for x = 0. The remanence of (Sm, Zr)Co5 increases with x. For x \leq 0.4, the energy product slightly decreases with x. The results show that 40% of the Sm can be replaced by the less expensive Zr, with an energy-product reduction of only 10%. In addition, the planar defects are responsible for the change of coercivity mechanism from the nucleation-type of reverse domain for the x = 0 to the pinning-type of domain wall for the x = 0.4. [ABSTRACT FROM AUTHOR]

The effect of an applied electric field on the magnetic properties of L 10-ordered CoPd thin films is investigated by first-principle calculations. Both the magnetic moment and the magnetocrystalline anisotropy of the surface atoms are changed by the electric field, but the net effect depends on the surface termination. The magnetocrystalline anisotropy switches from in-plane to perpendicular in the presence of external electric field. Typical magnetic-moment changes are 0.1 \muB per eV/\AA. The main mechanism is the shift of the Fermi level, but the anisotropy change also reflects a crystal-field change due to incomplete screening. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the high-temperature coercivity of mechanically alloyed and subsequently annealed samarium-cobalt and ytrrium-cobalt permanent magnets. Sample preparation and structure; Magnetic properties of the samples; Discussion.