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Neutron powder diffraction (NPD) and x-ray magnetic circular dichroism (XMCD) spectroscopy are employed to investigate the magnetism and spin structure in single-phase B20 Co1.043Si0.957. The magnetic contributions to the NPD data measured in zero fields are consistent with the helical order among the allowed spin structures derived from group theory. The magnitude of the magnetic moment is (0.3 ± 0.1)μB/Co according to NPD, while the surface magnetization probed by XMCD at 3 kOe is (0.18–0.31)μB/Co. Both values are substantially larger than the bulk magnetization of 0.11μB/Co determined from magnetometry at 70 kOe and 2 K. These experimental data indicate the formation of a helical spin phase and the associated conical states in high magnetic fields. [ABSTRACT FROM AUTHOR]

Investigation of the structural, electronic and magnetic properties of full-Heusler Co2VIn as well as half-Heusler CoVIn Cobalt based Heusler compounds using density functional theory (DFT) leads to the general conclusion that Co2VIn and CoVIn are half-metallic materials with a gap at the Fermi level in the minority states and majority states respectively. A Hubbard-like Coulomb correlation term U has been included in the DFT (DFT+U) for the computation of the electronic and magnetic properties of the compounds. The structural properties have been calculated for the paramagnetic and ferromagnetic phases, and both Co2VIn and CoVIn are found to be stable in the ferromagnetic phase. The calculated magnetic moments are 2 μB and 0.9 μB per formula unit for Co2VIn and CoVIn respectively.

A modulated structure derived from the inverse Heusler phase (the XA-type and the disordered variant L21B-type) has been observed in rapidly quenched Mn2RuSn ribbons. The powder X-ray diffraction pattern of the quenched ribbons can be indexed as an L21B-type structure. Electron diffraction patterns of the new structure mostly resemble those of the XA-type (and the disordered variant L21B-type) structure and additional reflections with denser spacing indicate a long periodicity. Orthogonal domains of the modulated structure were revealed by a selected-area electron diffraction pattern and the corresponding dark-field transmission electron microscopy images. The structure was further studied by the crystallographic analysis of high-resolution transmission electron microscopy images. A model for the modulated structure has been proposed to interpret the experimental results. [ABSTRACT FROM AUTHOR]

A modulated structure derived from the inverse Heusler phase (the XA‐type and the disordered variant L21B‐type) has been observed in rapidly quenched Mn2RuSn ribbons. The powder X‐ray diffraction pattern of the quenched ribbons can be indexed as an L21B‐type structure. Electron diffraction patterns of the new structure mostly resemble those of the XA‐type (and the disordered variant L21B‐type) structure and additional reflections with denser spacing indicate a long periodicity. Orthogonal domains of the modulated structure were revealed by a selected‐area electron diffraction pattern and the corresponding dark‐field transmission electron microscopy images. The structure was further studied by the crystallographic analysis of high‐resolution transmission electron microscopy images. A model for the modulated structure has been proposed to interpret the experimental results.A new modulated structure derived from the XA‐type Mn2RuSn Heusler compound has been observed and characterized. The structural model has been proposed on the basis of the unit cell of the XA phase and the antiphase boundary shift. [ABSTRACT FROM AUTHOR]

FeNi having the tetragonal L10 crystal structure is a promising new rare-earth-free permanent magnet material. Laboratory synthesis is challenging, however, tetragonal L10 FeNi--the mineral "tetrataenite"--has been characterized using specimens found in nickel-iron meteorites. Most notably, the meteorite NWA 6259 recovered from Northwest Africa is 95 vol.% tetrataenite with a composition of 43 at.% Ni. Hysteresis loops were measured as a function of sample orientation on a specimen cut from NWA 6259 in order to rigorously deduce the intrinsic hard magnetic properties of its L10 phase. Electron backscatter diffraction showed that NWA 6259 is strongly textured, containing L10 grains oriented along any one of the three equivalent cubic directions of the parent fcc structure. The magnetic structure was modeled as a superposition of the three orthonormal uniaxial variants. By simultaneously fitting first-quadrant magnetization data for 13 different orientations of the sample with respect to the applied field direction, the intrinsic magnetic properties were estimated to be saturation magnetization 4πMs = 14.7 kG and anisotropy field Ha = 14.4 kOe. The anisotropy constant K = 0.84 MJ/m3 is somewhat smaller than the value K = 1.3 MJ/m3 obtained by earlier researchers from nominally equiatomic FeNi prepared by neutron irradiation accompanied by annealing in a magnetic field, suggesting that higher Ni content (fewer Fe antisite defects) may improve the anisotropy. The fit also indicated that NWA 6259 contains one dominant variant (62% by volume), the remainder of the sample being a second variant, and the third variant being absent altogether. [ABSTRACT FROM AUTHOR]

The role of dipole-dipole interactions in the self-assembly of dipolar organic molecules on surfaces is investigated. As a model system, strongly dipolar model molecules, p-benzoquinonemonoimine zwitterions (ZI) of type C6H2(...NHR)2(...O)2 on crystalline coinage metal surfaces were investigated with scanning tunneling microscopy and first principles calculations. Depending on the substrate, the molecules assemble into small clusters, nano gratings, and stripes, as well as in two-dimensional islands. The alignment of the molecular dipoles in those assemblies only rarely assumes the lowest electrostatic energy configuration. Based on calculations of the electrostatic energy for various experimentally observed molecular arrangements and under consideration of computed dipole moments of adsorbed molecules, the electrostatic energy minimization is ruled out as the driving force in the self-assembly. The structures observed are mainly the result of a competition between chemical interactions and substrate effects. The substrate's role in the self-assembly is to (i) reduce and realign the molecular dipole through charge donation and back donation involving both the molecular HOMO and LUMO, (ii) dictate the epitaxial orientation of the adsorbates, specifically so on Cu(111), and (iii) inhibit attractive forces between neighboring chains in the system ZI/Cu(111), which results in regularly spaced molecular gratings. [ABSTRACT FROM AUTHOR]

Demagnetizing factors have been calculated for hierarchical mixtures and dispersions of magnetic particles, such as columns of plate-like particles in a matrix and clusters of spherical particles. The theory involves a number of shape and density parameters describing particles, aggregates, and matrix. It approximates distant particles by a homogeneous medium and yields closed and easy-to-use expressions for the effective demagnetizing factor. Index Terms--Biomagnetism, demagnetizing fields, magnetic particles.

The effect of surface anisotropy on the magnetization reversal in small magnetic particles is investigated. The model considers particles of cubic shape cut from a tetragonal crystal with cube faces in the (001) and equivalent planes. In particles having diameters of less than about 10 nm, the coercivity approaches the Stoner-Wohlfarth limit, but the anisotropy field differs from that of the bulk of the particles. With increasing particle size, the nucleation modes acquire the character of magnetic surface or bulk modes that reduce the coercivity. Index Terms--Coercivity, magnetic particles, nucleation, surface anisotropy.

We propose and theoretically investigate a new class of nanostructured magnetic recording films, cluster-pinned recording media. The films consist of magnetic clusters exchange coupled to a continuous hard layer with perpendicular anisotropy and low coercivity. Our calculations yield the coercivity and the cross-track correlation length as a function of film thickness and pinning density and strength. The mechanism is very similar to the Gaunt-Friedel pinning in bulk magnets, which differs from ordinary strong pinning by the self-consistent dependence of wall curvature and coercivity on defect concentration. The main difference is the exponent for the coercivity as a function of the pinning strength, which is equal to 2 in the bulk but equal to 3/2 in thin films. The pinning strength is estimated for various regimes, and it is shown that the diminished domain-wall curvature reduces jitter. Index Terms--Coercivity, domain-wall pinning, magnetic recording.

The coercivity of cellular Sm-Co based permanent magnets is investigated by model calculations. The grain boundaries responsible for the pinning coercivity are modeled as planar inhomogenities with arbitrary cross-sectional geometry. The calculation yields a physically transparent integral equation for the pinning energy, whose derivative is the pinning force. The theory rationalizes experimental data on a semiquantitative level, but without adjustable parameters, and bridges the gap between smooth concentration gradients and abrupt interfaces. Explicit results are obtained for sinoidal profiles, for very thin grain boundaries, and for profiles intermediate between attractive and repulsive pinning. The corrections predicted by the present model elucidate the occurrence of coercivity when the main and grain-boundary phases have the same wall energy. Index Terms--Coercive force, magnetic anisotropy, magnetic films, permanent magnets, samarium alloys.

Highly coercive granular SmCo/CuTi films were produced by thermal processing of sputtered Sm[Co.sub.5]/(CuTi) multilayers and investigated experimentally and theoretically. Electron microscopy shows that the processed films consist of spherical grains that have diameters of 5-10 nm and are embedded in a matrix. Both the grains and the matrix phase exhibit the Ca[Cu.sub.5] structure, but the matrix is probably Cu-rich. The films have high coercivities of up to 50.4 kOe at 300 K. Analytical calculations and micromagnetic simulations yield a transition from a nucleation-type weak-interaction regime to a discrete-pinning-type strong-interaction regime. The transition occurs at packing fractions similar to those encountered in the investigated films and is accompanied by a coereivity maximum. Index Terms--Coercivity, discrete domain-wail pinning, granular films, micromagnetic simulations, samarium-cobalt films.

Crystal structure and magnetic properties of Heusler alloys MnXNiSn (X = Mn, Fe, Co) are investigated using density functional theory and compared with experimental results. The parent alloy Mn2NiSn, which crystallizes in the inverse Heusler structure, is found to be ferrimagnetic, in agreement with previous experimental and theoretical work. The Fe and Co substitutions cause the alloys to assume a fairly well-ordered Y structure and enhance the magnetization substantially. We find that the strong nearest neighbour Mn-X exchange changes from antiferromagnetic (X = Mn) to ferromagnetic (X = Fe, Co), which explains and actually overestimates the experimental changes. A striking feature of the system is that Fe and Co have opposite effects on the Curie temperature Tc: they reduce and enhance Tc, respectively. We qualitatively explain this behaviour in terms of two-sublattice model based on the nearest-neighbour exchange. [ABSTRACT FROM AUTHOR]

The investigation of the magnetocrystalline anisotropy of 5f transition-metal atoms is discussed. The cubic and uniaxial exchange-controlled anisotropies have similar magnitude and exhibit similar temperature dependences.

Permanent-magnet materials are one cornerstone of today's technology, abundant in disk drives, motors, medical equipment, wind generators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of the compound and enhances the magnetocrystalline anisotropy, thereby improving the coercivity of the compound. The energy product reaches 114 kJ/m3 (14.3 MGOe) at room temperature and 86 kJ/m3 (10.8 MGOe) at 200 °C; this value is similar to that of the Dy-free Nd2Fe14B and exceeds that of other rare-earth-free permanent-magnet bulk alloys, as encountered in automotive applications. [ABSTRACT FROM AUTHOR]

A nonequilibrium rapid-quenching method has been used to fabricate NiMnIn and NiMnGa alloys that are chemically and morphologically similar but crystallographically and physically very different. NiMnGa crystallizes in a Ni2In-type hexagonal structure, whereas NiMnIn is a cubic Heusler alloy. Both alloys yield a topological Hall effect contribution corresponding to bubble-type skyrmion spin structures, but it occurs in much lower magnetic fields in NiMnIn as compared to NiMnGa. The effect is unrelated to net Dzyaloshinskii-Moriya interactions, which are absent in both alloys due to their inversion-symmetric crystal structures. Based on magnetic-force microscopy, we explain the difference between the two alloys by magnetocrystalline anisotropy and uniaxial and cubic anisotropies yielding full-fledged and reduced topological Hall effects, respectively. Since NiMnIn involves small magnetic fields (0.02–0.3 kOe) at and above room temperature, it is of potential interest in spin electronics. [ABSTRACT FROM AUTHOR]

The thermal stability of the information stored in magnetic recording media is determined by a complex hierarchy. The leading consideration is the static or zero-temperature magnetization reversal complemented by the intrinsic temperature dependence of the micromagnetic parameters. Thermally activated Arrhenius (or Néel-Brown) processes modify the reversal by realizing paths close to static reversal, whereas “giant fluctuations” corresponding to reversal fields much higher than the nucleation field can safely be excluded. Thermally activated reversal in very thin elongated nanoparticles limits the thermal stability of magnetic recording media but degenerates into coherent rotation as the temperature is lowered, thereby reconciling micromagnetism and thermodynamics. A particularly complicated situation is encountered in alloys, where sublattices containing heavy transition-metal atoms act like earthquakes that modify the energy landscape. [ABSTRACT FROM AUTHOR]

A combination of the x-ray diffraction and transmission electron microscopy techniques is employed to explain the fabrication and the properties of the Ti:TiO core-shell nanoparticles. An increase in the thickness of the TiO shell is shown to lead to an increase in the magnetic moment of the system.

Several studies are conducted to explain the structural, magnetic, as well as the magneto-transport properties of the various different Pt-alloyed MnBi thin films. An increase in the concentration of Pt in the films is shown to lead to a decrease in their saturation magnetization.

The magnetic-force microscopy technique is employed to explain the various magnetic antiphase domains observed in the Co/Ru/Co trilayers. The thickness of the trilayers is shown to exhibit an inverse relationship with the magnetic field.

Nanoclusters of rocksalt TiO, anatase Ti[O.sub.2] and rutile Ti[O.sub.2] are produced by cluster deposition and investigated with transmission-electron microscopy. The analysis of the measurements have shown that the magnetism is located near the surface the clusters and characterized by a nominal value.

Fabricated and characterization of nonepitaxial FePt:X films (X=Cu,Au,CuAu) with tunable magnetic properties are described. The studies have shown that Cu enters the [L1.sub.0] lattice whereas Au segregates at the grain boundaries and has reduced the intergranular exchange coupling.

Cobalt-doped boron carbides produced by simultaneous plasma-enhanced chemical vapor deposition of carborane and cobaltcene are examined. It is shown that the cobalt has strongly hybridized with the molecular orbitals of the icosahedral cage and the states in the region of the band gap have a strong cobalt weight.

The origin of the striking magnetic anisotropy of cobaltous oxide (CoO) is examined by using model calculations. A simple configuration-interaction approach is used to analyze the effect of 3[d.sup.8]-3[d.sup.6] bonds and their interplay with 3[d.sup.7]-3[d.sup.7] configurations.

A mean-field approach is used for calculating the magnetic entropy as a function of temperature, magnetic field, particle size, anisotropy and interaction strength. The studies have shown that the important feature of magnetic nanoparticles is that the entropy has displayed a strong dependence on the magnetic anisotropy.

Analytical and numerical calculations are used for examining the magnetization reversal in very small FePt particles. The nonrectangular hysteresis loops have reflected factors such as grain misalignment, particle-size distribution and different degrees of [L1.sub.0] order in different particles.

Model calculations are used for examining the temperature dependence of the spin polarization of half-metallic ferromagnets. The studies have shown that the gap reduction is a one-electron rather than a correlation effect and adds to the spin mixing.

Magneto-optical measurements are used to examine Pt-Co and Co layers exchange-coupled by a Pt layer. A ferromagnetic or acoustic and an antiferromagnetic or optic mode are identified from the Fourier-transformed excitation spectra and the line positions are used to estimate the interlayer exchange constants.

The impact of magnetic impurities on the behavior of metallic clusters is investigated. A unique feature of the Hubbard-type interaction model is that the Kondo effect is reproduced by an unrestricted Hartree-Fock approximation.

The magnetocrystalline anisotropy of pristine and Co-substituted ε-Fe2O3 is investigated by density functional calculations. The epsilon-iron oxide is the only polymorph of Fe2O3 magnetoelectric in its antiferromagnetic ground states other crystalline forms being α-Fe2O3 (hematite), β-Fe2O3, and γ-Fe2O3 (maghemite). The magnetizations of the four iron sublattices are antiferromagnetically aligned with slightly different magnetic moments resulting in a ferrimagnetic structure. Compared to the naturally occurring hematite and maghemite, bulk ε-Fe2O3 is difficult to prepare, but ε-Fe2O3 nanomaterials of different geometries and feature sizes have been fabricated. A coercivity of 20 kOe [2 T] was reported in nanocomposites of ε-Fe2O3, and an upper bound for the magnetic anisotropy constant K at a low temperature of ε-Fe2O3 is previously measured to be 0.1 MJ/m3. In the Co-substituted oxides, one octahedral or tetrahedral Fe atom per unit cell has been replaced by Co. The cobalt substitution substantially enhances magnetization and anisotropy. [ABSTRACT FROM AUTHOR]

The magnetocrystalline anisotropy of pristine and Co-substituted ε-Fe2O3 is investigated by density functional calculations. The epsilon-iron oxide is the only polymorph of Fe2O3 magnetoelectric in its antiferromagnetic ground states other crystalline forms being α-Fe2O3 (hematite), β-Fe2O3, and γ-Fe2O3 (maghemite). The magnetizations of the four iron sublattices are antiferromagnetically aligned with slightly different magnetic moments resulting in a ferrimagnetic structure. Compared to the naturally occurring hematite and maghemite, bulk ε-Fe2O3 is difficult to prepare, but ε-Fe2O3 nanomaterials of different geometries and feature sizes have been fabricated. A coercivity of 20 kOe [2 T] was reported in nanocomposites of ε-Fe2O3, and an upper bound for the magnetic anisotropy constant K at a low temperature of ε-Fe2O3 is previously measured to be 0.1 MJ/m3. In the Co-substituted oxides, one octahedral or tetrahedral Fe atom per unit cell has been replaced by Co. The cobalt substitution substantially enhances magnetization and anisotropy. [ABSTRACT FROM AUTHOR]

The structural, magnetic and magnetocaloric properties of Fe deficient Pr2-xNdxFe17 (x = 0.5, 0.7) alloys prepared by arc-melting and melt-spinning have been investigated. The room temperature x-ray diffraction patterns show that the samples are nearly single-phase and crystallize in the rhombohedral Th2Zn17-type crystal structure. The Curie temperatures determined from the thermomagnetic curves are 302 K and 307 K for Pr1.5Nd0.5Fe17 and Pr1.3Nd0.7Fe17, respectively. The peak magnetic entropy change and the relative cooling power at field change of 50 kOe are 3.01 J/kgK and 345 J/kg for Pr1.5Nd0.5Fe17, and 4.31 J/kgK and 487 J/kg for Pr1.3Nd0.7Fe17, respectively. The absence of magnetic and thermal hysteresis with relatively high cooling efficiency suggests that the alloys have potential for magnetic refrigeration. [ABSTRACT FROM AUTHOR]

The creation and control of magnetocrystalline anisotropy in bulk and nanostructured magnetic materials remain a continuing challenge, and density functional theory assists the experimental research in developing new high-anisotropy magnetic materials. Co3Si, which crystallizes in the hexagonal CdMg3 structure (space group P63/mmc), is an intriguing magnetic material. For example, Co3Si nanoparticles exhibit high coercivities (17.4 kOe at 10 K and 4.3 kOe at 300 K) in spite of having an easy-plane anisotropy. Here, we used the Vienna ab-initio Simulation Package (VASP) to investigate the electronic structure and magnetic properties of bulk and thin-film Co3Si. We have also studied the thickness dependence of the magnetocrystalline anisotropy of Co3Si (001) thin films. [ABSTRACT FROM AUTHOR]

The realization of the magnetic moment of Mn atoms in ferromagnetic alloys is investigated theoretically. This paper is partly motivated by the recent discovery of high-magnetization Fe–Co–Mn thin films by Snow et al., who reported moments of up to 3.25 $\mu _{\text {B}}$ per atom. Structural and strain effects are discussed with particular emphasis on the distinction between tetragonal martensitic lattice distortions and Bain transitions in body-centered cubic, CsCl, and Heusler alloys. It is outlined how these lattice distortions affect the electronic structure of the alloys, and ab initio calculations are used to determine the net moment of cubic Fe6Co63Mn31. The calculated moment, $2.90~\mu _{\text {B}}$ per atom, is reasonably close to the experimental value. [ABSTRACT FROM AUTHOR]