Magnetoresistance of dilute magnetic alloys

The magnetoresistance of dilute magnetic alloys is calculated. The difficulties appearing in the calculation of scattering times within the density wave model are discussed and overcome semi-empirically.     

Magnetoelastic effects in dilute magnetic alloys

Ultrasonic studies of $\text{Cu}$Mn, $\text{Au}$Mn and $\text{Au}$Fe alloys where the concentrations of the transition metal ranges between 1 and 10% show that the longitudinal sound velocity has a marked field dependence at temperatures below the peak in the magnetic susceptibility and a negligible field dependence above it. This suggests that there may be an ordered magnetic state in these alloys.     

Hall effect in dilute magnetic alloys

We review the theory and the experimental results on the Hall effect in noble metals containing magnetic impurities of transition metals. In order to illustrate the various types of observed effects, we focus succesively on selected systems: $\text{Cu}$Mn, with only enhancement of the ordinary Hall effect due to the existance of different spin-up and spin-down currents; $\text{Au}$Fe and $\text{Au}$Cr, with skew scattering by magnetic impurities; $\text{Cu}$MnT ternary alloys (where T is a non-magnetic impurity), with skew scattering effects due to combined spin—orbit scattering by non-magnetic impurities and spin scattering by Mn impurities. The skew scattering in $\text{Au}$Fe and $\text{Au}$Cr can be ascribed to the orbital character of the impurity moments and accounted for in an orbitally degenerate virtual bound state model. However, the anomalous temperature dependence of the skew scattering in Kondo alloys at low temperature is not well understood. We also present some magnetoresistance data in order to describe the links between the Hall effect and the magnetoresistance in magnetic alloys. In particular, we relate the skew scattering and the magnetoresistance anisotropy observed in $\text{Au}$Cr alloys.     

Spin correlation in dilute magnetic alloys

The correlation of a magnetic impurity spin with the spin density of the conduction electrons in a dilute magnetic alloy is calculated non-perturbationally on the basis of the Nagaoka theory. It is shown that there are anomalies due to the Kondo effect in the long range behaviour of this correlation which contradicts the bound state interpretation of the Kondo effect. The most interesting detail is the appearance of a non-oscillating contribution to the correlation.     

Electronic structure of non-magnetic impurities in dilute alloys of Al

The electronic structure of substitutional non-magnetic impurities Cu, Ag, Cd, Mg, Zn, Ga, In, Ge, Si and Sn in Al is studied using density functional theory. A simple physical model is proposed to calculate the effective charges on impurities in trivalent metal Al. A linear relation is found between the effective charges on impurities and impurity vacancy capture radii. The spherical solid model (SSM) is used to account for discrete nature of the host. The impurity-induced change in charge density, scattering phase shifts, host-impurity potential, residual resistivity and impurity self-energy are calculated. Higher order scattering phase shifts are found significant and the host-impurity potential is found proportional to effective charge on impurity in its vicinity. The self-consistently calculated potential is used to calculate the electric field gradients (EFGs) at the first and second nearest neighbours (1NNs, 2NNs) of impurity. The calculated values are in agreement with the experimental results.     

Unusual magnetic behaviour of Mn spin in magnetic PtFe alloys: Nuclear orientation study

The low temperature nuclear orientation of⁵⁴Mn in Pt₉₉Fe₁, Pt₉₉,₉Fe_0.1 and Pt₉₉,₉₅Fe_0.05 has been studied in the temperature rangé 5–60 mK and in the external magnetic field range 0–9 T. A considerable noncollinearity of Mn magnetic moments with respect to B_ext has been found even at rather high B_ext.     

Transport properties of dilute magnetic alloys

The electrical resistivity, thermopower, and the electronic part of the thermal resistivity of dilute magnetic alloys are calculated in the framework of the Suhl-Nagoaka theory. Using Bloomfield's and Hamann's solution of the Nagaoka equations, we derive expressions for the transport quantities in the limitT? ¯ T_K andT?¯ T_K to order (In ¯T _K /T)^?4 where ¯T _K is the Kondo temperature which may depend on the spin independent scattering. We find that the thermopower and deviations from the Wiedemann-Franz law in this limit decrease as ¦In ¯T_K/T¦^?3 if one neglects a trivial temperature dependence of the thermopower due to the electron-phonon interaction.     

Magnetic susceptibility of dilute magnetic alloys

The magnetic susceptibility of dilute magnetic alloys is calculated using the Nagoaka approximation to the Kondo problem. We use the exact solutions of the Nagaoka equations, or equivalently Suhl's dispersion relations, as obtained recently. Our result is represented by a universal function of a certain temperature parameter. In the case of ferromagnetic coupling no appreciable change of the free spin susceptibility is found over the whole temperature range. In the case of antiferromagnetic coupling we find that the free spin susceptibility is greatly reduced. In fact, for spin 1/2, the result indicates the breakdown of the expansion in terms of the impurity concentration and suggests the onset of impurity ferromagnetism.     

Nearly magnetic antiparallel pairs in dilute alloys

When a dilute alloy exhibits exchange enhancement of the isolated impurity susceptibility, it is shown that pairs of impurities may exhibit a low lying anti-parallel fluctuation mode such that the pair susceptibility in a uniform field is less enhanced than that of the isolated impurity, while the specific heat may be strongly enhanced. N.M.R. properties of such a pair should exhibit a small shift in frequency and a fast nuclear spin lattice relaxation time, leading to possibly large deviations from the Korringa relation.     

On the magnetoresistance in dilute magnetic alloys

Many-particle contributions to extended Suhl equations are considered to derive a magnetoresistance, which saturates for nonzero magnetic fields below the unitatarity limit in agreement with experiments.     

Inelastic collision term in dilute magnetic alloys

The collision term derived by Kohn and Luttinger [1] to describe elastic scattering to third order in the potential is generalized to include inelastic processes for a system of conduction electrons interacting with localized spins. This result is obtained by using the Kadanoff and Baym [6] derivation of the Boltzmann equation.     

Ground state energy of dilute magnetic alloys

The ground state energy of dilute magnetic alloys as described by thes-d-exchange Hamiltonian is calculated within the Nagaoka-Suhl theory. For integral impurity spinS it is shown that for positive coupling constantγ the energy is a holomorphic function ofγ. For negative coupling one obtains an additional singular contribution of the form (?1) ^s?1 e ^1/γ (γ<0). As this changes sign for different integral s-values, the interpretation of the singular part as a binding energy suggested previously does no longer hold. For half integral spin the energy is a singular function for bothγ>0. The singularities are not as elementary as in the case of integral spinS, but are rather related to those of the exponential integral function. In particular, forγ>0 the origin (γ=0) is a branch point. Earlier variational calculations are compared with our new results.