Yu. P. Irkhin

CHARGE SCREENING AND MAGNETIC ANISOTROPY IN METALLIC RARE-EARTH SYSTEMS

The calculation of magnetic anisotropy constants is performed beyond the point charge model for a continuous charge density distribution of screening conduction electrons. An important role of the non-uniform electron density, in particular, of the Friedel oscillations, in the formation of crystal field is demonstrated. Such effects can modify strongly the effective ion (impurity) charge and even change its sign. This enables one to justify the anion model, which is often used at discussing experimental data on hydrogen-containing systems. Possible applications to the pure rare-earth metals and RCo5 compounds are discussed. The deformation of magnetic structure near the interstitial positive muon owing to the strong local anisotropy, and the corresponding contribution to the dipole field at the muon are considered.

Spin fluctuations with strong DOS energy dependence

Abstract It is shown that a sizeabel spin fluctuation contribution to temperature dependence of magnetic susceptibility X ( T ) in transition metals exists only in the case of strong energy dependence of electron density of states (DOS) g (ϵ). Calculation of X ( T ) for Ni within the frame of the Hertz-Klenin theory and on the ground of the g (ϵ) linear model leads to the relatively small fluctuation contribution. The effective Curie-Weiss behaviour of X ( T ) also arises mainly due to singularities of g (ϵ).

Spin dependence of impurity scattering in ferromagnetic metals

Abstract Modern results of band structure calculations are applied to calculate the spin dependence of impurity resistivity within the two-current model of Fert and Campbell. Non-magnetic and transition-metal impurities in Ni and Fe hosts are considered and a qualitatively satisfactory description of the experimental situation is obtained. The adequacy of the Anderson and s-d exchange models is discussed. Effective s-d exchange integrals for impurities with well-defined magnetic moments are estimated. The important role of conduction electron spin polarization in explaining the experimental data is demonstrated. On this basis, the role of d-electrons in conductivity is discussed.

Determination of magnetic anisotropy of complex rare-earth compounds from Mössbauer and NMR spectra

Experimental data and theoretical papers on the magnetic anisotropy (MA) of rare-earth-transition metal intermetallic compounds are reviewed. Discrepancies between the experimental data obtained by different authors, as well as between these data and the theoretical calculations of the MA constants, are indicated. A technique is proposed for determining the crystal-field parameters and the effective charges Qi* of ions in intermetallic compounds. Using experimental Mössbauer and NMR spectroscopic data, possible values of Qi* are determined for R2T17−x and R2T17−xTix compounds, which allow one to find the MA constants of these systems with different R and T in a unified way. The problem of the sign of the contribution from the rare-earth metal sublattice to the MA is discussed. The heavy x dependence of this contribution in the R2T17−xTix system is explained to be due to the contribution to the crystal field from Ti ions in the dumbbells.

The anion and cation effects in the magnetic anisotropy of rare-earth compounds: Charge screening by conduction electrons

The formation of magnetic anisotropy (MA) in rare-earth compounds with transition metals has been analyzed. The screening of the charges creating the crystal field by conduction electrons has been shown to play an important role. The calculations took into account the Friedel charge-density oscillations. The model used for RCo5 is the point-charge crystal field including nonuniform screening by conduction electrons with an anisotropic Fermi surface. The mechanisms of strong MA due to light-element impurities (hydrogen and nitrogen) are considered. The effective charge of an impurity can heavily depend on its ionic radius and the characteristics of the Fermi surface (in particular, on the Fermi momentum kF) of the screening electrons. The screening of the cation and anion charge in hydrides and nitrides based on the R2Fe17 and RFe11Ti intermetallic compounds is discussed.

Barnes-Mössbauer rule for compounds formed by rare-earth and transition metals

The question of applicability of the technique proposed in [1–3] to calculate the gain S for the R2Fe14B compound, whose physical properties are of practical interest, is discussed.

Magnetic anisotropy, first-order phase transition, and Brown’s paradox in rare-earth metals

The anomalous behavior of magnetization curves (first-order magnetization process, FOMP) of highly anisotropic rare-earth metal compounds was studied. It is pointed out that FOMP assignment to a large contribution from higher magnetic anisotropy constants Ki comes into conflict with the point-ion approximation of crystal field theory. An alternative interpretation based on the multisublattice model is proposed; this model makes use only of first-order sublattice constants kj derived from independent experimental NMR data. Direct calculations provide a satisfactory explanation of the FOMP in Tb2Fe17. A possible connection of the FOMP with Brown’s paradox in the theory of domain structure and coercive force is proposed.

Magnetic anisotropy, first-order magnetization process and the Brown paradox in rare-earth compounds

Abstract Anomalous behavior of magnetization curves in rare-earth compounds is considered. It is shown that the explanation of this effect due to the large values of magnetic anisotropy constants K i of higher order contradicts the point charge approximation of the crystal field theory. An alternative approach within the framework of sublattice model and first-order magnetic anisotropy constants k 1 only is proposed. The sublattice values of k 1 are taken from independent nuclear magnetic resonance data. The direct calculation leads to satisfactory explanation of experiment in Tb 2 Fe 17 crystal. In conclusion, a possible relation of first-order magnetization process and Brown paradox in coercivity and domain structure is discussed.

The effect of spatial quantization of the electronic spectrum on the exchange interaction in multilayers

A strong influence of the quantization of the electronic spectrum of nonmagnetic spacers in multilayers is discovered on the dependence of the exchange interaction on the thickness of the spacer. It is shown that the antiferromagnetic dip is observed experimentally in Fe/Cr multilayers, as small thicknesses can be interpreted in terms of the simple RKKY approximation.

Hall effect in microscopically inhomogeneous magnetic alloys

A theory of the generalized conductivity for the normal component of the Hall effect is developed. It is shown that the normal Hall effect coefficient R0 of microscopically inhomogeneous magnetic alloys GdZnxCu1−x, which at low temperatures consist of ferro-, antiferro-, and paramagnetic phases, can be described satisfactorily on the basis of an effective-medium theory. The experimentally observed relationship between the coefficient R0(x) and the resistivity ρ(x) is obtained.