Dilute Magnetic Alloys Research Articles

Transport anomalies and magnetic properties of Sn x Eu1.2?x Mo6S8 alloys

We have measured the resistance R, magnetoresistance, and magnetization of sintered samples of the compounds Sn x Eu1.2−x Mo6S8, where x = 0, 0.12, 0.24, and 0.48. A large resistance anomaly beginning at T ~ 100 K and extending to low temperatures is found in all of the alloys except x = 0.48, which had a high superconducting transition temperature T c ~ 11.3 K. The resistance anomaly, which appears to be correlated with the depression of T c, is also correlated with a large negative magnetoresistance at temperatures T ⩽ 12 K. For all samples except x = 0.48, that part of the magnetoresistance attributed to spin-flip scattering is proportional to the square of the magnetization M 2 and is 30–40% of R (0) for some samples. Magnetization measurements are consistent with a spin-7/2 paramagnetic behavior of Eu2+ ions when allowance is made for a concentration of 20–30% nonmagnetic (J = 0) Eu3+ ions as determined from Mossbauer measurements. The experimental results are inconsistent with predictions of dilute magnetic alloy Kondo models. We speculate that the resistance anomaly and suppression of T c for x ⩽ 0.24 are associated with a semiconductor-like behavior.

On the relationship between transient and CW NMR experiments in dilute magnetic alloys

The enhancement of experimental nuclear magnetic relaxation rates resulting from strong quadrupolar broadening of the resonance is discussed for both transient (spin-echo) and steady-state (CW) techniques. Contrary to recent claims it is shown that the enhancements of the spin-echo decay rate and homogeneous linewidth are identical.

The effect of RKKY interactions on the magnetization of dilute magnetic alloys with hcp hosts

The magnetization due to the presence of magnetic impurities dissolved dilutely in nonmagnetic H.C.P. host metals has been calculated assuming a fine structure splitting by an axially symmetric crystal field and an RKKY interaction between the impurities. To compute the RKKY contribution, the interaction between the impurities is described by a randomly oriented effective internal exchange field H with a probability distribution P(H↘) = Δ/π2(Δ2+H2)2. Comparison of the calculated magnetization with the measured magnetization of a 5 ppm single crystal of MgMn and several polycrystalline ZnMn samples of varying concentration, over the temperature range 2K to 10mK, yields estimates for Δ, the most probable value of the magnitude of the effective exchange field, as well as the dependence of Δ on impurity concentration.

Transport properties of (La, Pr)Sn3 alloys at low temperatures

We report on measurements concerning the transport properties of polycrystalline (La1−x Pr x )Sn3samples with impurity concentrations 0.07 ≤ x ≤ 0.2 in the temperature range 0.3 ≤ T ≤ 12 K. The experimental results of the electrical resistivity, the thermopower, and the thermal conductivity show pronounced anomalies which are referred to crystal-field effects and a Kondo effect of excited levels. A quantitative comparison with existing theories confirms the importance of the aspherical Coulomb scattering in addition to the isotropic spin-exchange scattering of the conduction electrons by the magnetic ions. Taking into account the Kondo effect in a phenomenological ansatz, we are able to fit the magnetic part of the resistivity. Furthermore, we present the first experimental proof of the theoretically predicted influence of crystal-field split impurities on the thermal conductivity in dilute magnetic alloys. The thermopower is qualitatively discussed by a model including both the effect from inelastic scattering on the crystal-field levels and the Kondo effect. Finally the temperature dependence of the Lorenz number is compared with a calculation in the framework of crystal-field theory.

Ion implantation as a tool for studying the Kondo-systemZnMn

In the case ofZnMn alloys it is shown how low temperature ion implantation can be used for the production of dilute magnetic alloys. The Kondo effect in the resistivity ρ(T) was studied between 1.1K and 20 K. The phonon part of the resistivity was subtracted using ρ(T)-curves from Zn-Films with similar disorder produced by self-ion implantation. A logT-dependence with a slope proportional to the Mn-concentration was found below 200 ppm. The influence of disorder on the specific Kondo slope is discussed.

Renormalization-group approach to the Anderson model of dilute magnetic alloys. II. Static properties for the asymmetric case

The temperature-dependent impurity susceptibility for the asymmetric Anderson model is calculated over a broad, physically relevant range of its parameters ${\ensuremath{\epsilon}}_{d}$ (the energy of impurity orbital), $U$ (the Coulomb correlation energy), and $\ensuremath{\Gamma}$ (the impurity-level width). Within the context of renormalization-group theory four fixed points and their associated regimes are identified: (i) the free-orbital regime which is unstable and flows into (ii) or (iv); (ii) the valence-fluctuation regime which is characteristic of the asymmetric Anderson model. Properties are dominated by a temperature-dependent impurity-orbital energy ${E}_{d}(T)\ensuremath{\cong}{\ensuremath{\epsilon}}_{d}+(\frac{\ensuremath{\Gamma}}{\ensuremath{\pi}})\mathrm{ln}(\frac{U}{T})$. If ${E}_{d}(T)$ is negative and large compared to $\ensuremath{\Gamma}$ as $T$ decreases, the system is unstable with respect to (iii), otherwise it flows to (iv); (iii) the local-moment regime is similar to that in the symmetric Anderson model except that it has potential scattering. That is, this regime maps onto the Kondo model with potential scattering, the latter having little effect on the susceptibility; (iv) the frozen-impurity regime, into which all the regimes above flow, is stable, having only irrelevant operators. Furthermore, in the valence-fluctuation regime nonuniversal properties are observed for $\ensuremath{-}{E}_{d}(T)<\ensuremath{\Gamma}$. These conclusions are supported with extensive analytic and numerical calculations, the latter based on the numerical renormalization-group approach. Analytic formulas for the impurity susceptibility and free energy in all four regimes are presented, together with the impurity-specific heat in the frozen-impurity regime.

Renormalization-group approach to the Anderson model of dilute magnetic alloys. I. Static properties for the symmetric case

The temperature-dependent impurity susceptibility for the symmetric Anderson model is calculated for all physically relevant values of its parameters $U$ (the Coulomb correlation energy) and $\ensuremath{\Gamma}$ (the impurity-level width). It is shown that, when $Ug\ensuremath{\pi}\ensuremath{\Gamma}$, for temperatures $Tl\frac{U}{(10{k}_{B})}$ the symmetric Anderson model exhibits a local moment and that its susceptibility maps neatly onto that of the spin-\textonehalf{} Kondo model with an effective coupling given by $\ensuremath{\rho}{J}_{\mathrm{eff}}=\ensuremath{-}\frac{8\ensuremath{\Gamma}}{\ensuremath{\pi}U}$. Furthermore, this mapping is shown for remarkably large values of $|\ensuremath{\rho}{J}_{\mathrm{eff}}|$. At very low temperatures (much smaller than the Kondo temperature) the local moment is frozen out, just as for the Kondo model, leading to a strong-coupling regime of constant susceptibility at zero termperature. The results also depict the formation of a local moment from the free orbital as $T$ drops below $U$, a feature not present in the Kondo model. Finally, when $U\ensuremath{\ll}\ensuremath{\pi}\ensuremath{\Gamma}$ there is a direct transition from free-orbital regime for $T\ensuremath{\gg}\ensuremath{\Gamma}$ to the strong-coupling regime for $T\ensuremath{\ll}\ensuremath{\Gamma}$. The calculations were performed using the numerical renormalization group originally developed by Wilson for the Kondo problem. In addition to the actual numerical calculations, analytic results are presented. In particular, the effective Hamiltonians, i.e., fixed-point Hamiltonian plus relevant and marginal operators, are constructed for the free-orbital, local-moment, and strong-coupling regimes. Analytic formulas for the impurity susceptibility and free energy in all three regimes are developed. The impurity specfic heat in the strong-coupling regime is calculated.

“Crystal fields” for magnetic ions in metals from itinerant electrons

The interaction of conduction electrons with host ions are shown to give rise to “crystal fields”, V C , which act on the magnetic ion in some dilute magnetic alloys. Crystal fields, V N , arising directly from ions neighbouring the magnetic ion have been compared with V C to show that the overall crystal field splitting from all 3d magnetic ions in axial symmetries is enhanced by the p-like character of V C . A 0 4 〈 r 4 〉 from V N is also increased by the d-component of V C . The angular properties of itinerant electron states give rise to the above results. For all rare-earth ions in fcc symmetry the radial and angular components of conduction-electron states with mainly d, but also f character, are shown to give rise to a reversal in sign of A 0 4 〈 r 4 〉 from V N . A 0 6 〈 r 6 〉 is enhanced by the f component of the conduction electrons.

Study of superconductivity in kondo alloys using Josephson tunneling

Information on the resonant spin flip scattering in dilute magnetic alloys can be obtained from superconducting experiments. We report the first measurements of the dc Josephson current in Nb-Nb xO y- Cu Cr/Pb tunnel junctions where Cu Cr is a thin film Kondo alloy in proximity with a superconducting Pb film.

Influence of interference scattering on transport properties of dilute magnetic alloys at finite magnetic field

For the scattering of electrons at magnetic impurities, there is interference between the “normal” and the spin-dependent part of the potential, which at strong magnetic fields can lead to zero net scattering for electrons of one spin direction. Model calculations for magneto-transport properties are presented.

Functional integral approach to the static susceptibility of the Anderson model

We consider the Anderson model for dilute magnetic alloys and use the functional integral approach to calculate the static susceptibility. The Coulomb interaction at the magnetic impurity is represented by the random fields coupled to the charge and the spin fluctuations. On treating the charge fluctuations within the extremal approximation, the free-energy functional is approximated by its potential part plus the harmonic terms in its kinetic part. Within this approximation, the functional integral is evaluated over all the paths by using the transfer matrix technique. The static susceptibility reduces to an expression of a system of independent particles moving in a one-dimensional potential. In the weak-coupling limit (U/πΔ<1,U is the Coulomb interaction at the impurity and Δ is the width of its virtually boundd-state), it behaves like the Pauli susceptibility and coincides with perturbation theory results, up to first order inU/πΔ. In strong-coupling limits (U/πΔ>1), at low temperatures, it shows the tendency to saturation (Kondo effect) and, at high temperatures, behaves like a Curie susceptibility and coincides with the perturbation results, up to first order in πΔ/U.

High pressure study of PdFeMn

Dilute magnetic alloys of PdFeMn have been investigated up to ∼20 kbar, demonstrating that the mechanism for the kneelike sharp rise in the low-field ac susceptibility χ on cooling is different from that for the reversed kneelike sharp drop of χ.

Pressure as a parameter in the study of dilute magnetic alloys

Abstract A large body of data is reviewed, which illustrates how the high pressure technique can be used to gain information about the magnetic and electronic state of dilute magnetic alloys. Values for the pressure dependence of the elementary effective exchange interaction between a magnetic impurity spin and the conduction electron sea are extracted from the data and tabulated for a number of extremely dilute alloys containing both transition metal and rare earth impurities. Results of experiments on host alloy series employing ‘lattice pressure’ are compared to the present ‘external pressure’ results and critically analysed; it is shown how such a comparison can be used to give information about the extent of the electronic screening around an impurity potential. The effect of pressure on impurity-impurity interactions in spin glasses is also examined; a comparison of the pressure studies on extremely dilute alloys to those on spin glasses allows an estimate of the nature of the interaction mechanisms...

Transport properties of a "reverse Kondo alloy" at finite magnetic field: Experiment and theory

Electrical and thermal conductivity and thermoelectric power were theoretically calculated and measured for ($\mathrm{La}$,Gd)${\mathrm{Al}}_{2}$---a dilute magnetic alloy with ferromagnetic coupling of the Gd spins to the host electrons---between 0.3 and 10 K, and up to 8 T. It turned out that the transport properties at finite field deviate markedly from the zero-field behavior, e.g., in the electrical resistivity (where a steplike transition is found between the high-temperature logarithmic increase and a low-temperature plateau). A theoretical result for the Hall coefficient is also given.

Calibration of SQUID magnetometer for dilute alloy studies

A SQUID magnetometer used in the study of dilute magnetic alloys or other weak magnetic systems can be calibrated by using the nuclear paramagnetism of aluminum metal. This material is useful as a calibration standard since magnetic impurities such as Fe and Mn form no local moment. This calibration is compared to others obtained with CMN powder and indium metal.

Moment-conserving decoupling scheme in the Anderson model

The moment-conserving decoupling scheme is applied to the infinite- U Anderson model for dilute magnetic alloys. It is shown that the correct first moment of the spectral density is essential in the occurrence of magnetic solutions.

Influence of oxygen and lattice disorder on the Kondo resistivity of ion implanted InMn alloys

A negative temperature dependence is observed for the resistivity of ion implanted InMn alloys below 5 K. The slope of the curves, where ϱ is proportional to ln T, is strongly influenced by lattice disorder. Oxygen, additionally incorporated into the host lattice, does not significantly change the Kondo resistivity but causes a remarkable variation in the pair breaking behaviour. Both effects are discussed within the Kondo picture for dilute magnetic alloys.

Spin Glasses of Random Dilute Magnetic Alloys

Spin Glasses of Random Dilute Magnetic Alloys

Consequences of the linear part of the specific heat of dilute magnetic alloys

AbstractFor large but finite systems the density of energy states can be obtained uniquely from the specific heat as a function of temperature. In a dilute magnetic alloy with linear dependence of the specific heat on the temperature T (at low T) the density of energy states in the lower energy region is given by a modified Bessel function.

Configuration fluctuations and the dilute-magnetic-alloy problem

A new theoretical treatment of the Anderson model of transition-metal or rare-earth impurities in a simple metal is presented. This study treats the impurity Coulomb correlation exactly and the host impurity coupling to lowest order in the sense of a self-consistent degenerate perturbation theory. The central result of this work is that if an impurity interconfigurational excitation energy (ICEE) is close to the Fermi energy in the atomic limit, a temperature-dependent shift of the ICEE is found for the interacting system. This temperature-dependent shift is shown to give a good description of the experimental observations on dilute magnetic alloys.