铜铁稀磁合金中铁磁杂质之间相互作用对低温热电势的影响

本文提出在铜铁稀磁合金中高浓度铁磁杂质之间的相互作用对低温热电势的影响巨大, 基于耦合杂质理论, 得出了高浓度铜铁稀磁合金的热电势在4-100 K的温度范围内随温度变化的理论曲线. 理论曲线与铁杂质浓度含量为0.1%(at) Fe, 0.13%(at) Fe和0.15%(at) Fe原子百分比的铜铁合金热电势实验值符合, 为推动低温铜铁稀磁热电偶的应用提供了理论分析基础.     

Magnetic behaviour of cobalt,iron and manganese dissolved in palladium

This paper is meant to be a report on the experimental work on dilute Pd-based alloys with Co, Fe and Mn. These alloys exhibit the phenomenon of giant moments. The importance of measurements on paramagnetic alloys is emphasized. From these measurements the conclusion can be drawn that Co and Fe dissolved in Pd does not behave like a normal paramagnet, i.e. according to a Brillouin function. This result makes it possible to explain the existing discrepancy in the interpretations of magnetic measurements on one hand and of specific-heat experiments on the other.

The main conclusions of this paper are:

The giant moment should be accounted for by ‘normal’ values of the magnetic quantum number (3/2 for Co, 2 for Fe and 5/2 for Mn) and a large value of geff.

Paramagnetic alloys of Mn in Pd behave according to Brillouin functions, but alloys of Co or Fe in Pd do not. Hence, a number of interpretations of magnetic measurements should be considered as incorrect.

The localized model for ferromagnetism can well account for the magnetic ordering of dilute Pd-based alloys (certainly if c < 1 at.%). A straightforward generalization of the Weiss molecular-field model may be applied.

The transition temperature of Pd-Mn alloys is not proportional to the concentration, but after scaling the behaviour is similar to what has been found for Pd-Co and Pd-Fe alloys. The concentration dependence can be explained from a calculation of the strength of the interaction between two impurity atoms as a function of the distance.

Comparison between alloys with equal concentrations shows that the magnetic ordering in Pd-Mn is not at all exceptional, but analogous to that in Pd-Co and in Pd-Fe. It should be mentioned, however, that Pd-Mn at c > 3 at.% is a so-called spin glass.

Addition of Ag or Rh to Pd alloys with Co, Fe and Mn has important influences on their properties. Unfortunately these effects are not completely understood.     

High field magnetic susceptibility of NiCr and NiV alloys

The variation of the density of states at the Fermi level with impurity concentration is investigated by means of magnetic measurements. This variation is found to be small for NiCr and NiV dilute alloys, in contrast with specific heat results.     

Green's function theory of the Kondo effect in dilute magnetic alloys

The exact solution of thet-matrix integral equation derived from the self-consistent Nagaoka equations in the theory of dilute magnetic alloys is established. It is shown that the unique solution for thet-matrix involving all Kondo type anomalies can be found under quite general assumptions. Using the exact solution we have calculated thermodynamic properties of dilute magnetic alloys. It is found that the excessive specific heat of the system due to the anomalous scattering of conduction electrons from the magnetic impurities is of the order ofBoltzmann's constant per local moment at low temperatures. In the limit of vanishing temperature the specific heat goes to zero asymptotically as (lnT)^?4. Finally the entropy difference of the interacting system as compared to the free system is calculated.     

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.     

Anomalous specific heat in La : Ce alloys

The magnetic contribution to the specific heat in La : Ce alloys shows a broad anomaly at ≈ 1.2°K which is weakly dependent on the concentration. This anomaly, which accounts for a substantial portion of the entropy of a doublet, cannot be attributed to either usual magnetic ordering or to a dilute alloy Kondo effect, but may represent the behavior of a concentrated Kondo system.     

Pressure as a parameter in the study of dilute magnetic alloys

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 important in the latter case. Evidence for a pressure-induced spin glass to Kondo transition in LaCe is also presented. It is suggested that the present results fit into a larger picture that magnetism, when present, tends to be weakened when high pressure is applied.     

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Green's function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.     

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.     

Magnetic field dependence of the impurity specific heat in dilute magnetic alloys

Using a trial sunction scheme at finite temperature, the impurity specific heat has been calculated in terms of the Wolff model with an external magnetic field. When local moments exist, the magnetic field effect is drastic under accessible experimental condition. In the absence of local moments, the specific heat is practically field independent. If accurate specific heat can be measured, it provides a new method to detect local moments in dilute magnetic alloys.     

Resistance minimum and heavy fermions

The phenomenon of the resistance minimum in dilute magnetic alloys is explained in terms of the s-d interaction which takes account of scattering of the conduction electron off the magnetic impurities in metals. Some of the intermetallic compounds which involve rare earth elements or uranium show a very large electronic specific heat and remain non-magnetic even though they show a Curie-like susceptibility at higher temperatures. These phenomena are also explained based on the s-d interaction model.     

Magnetic properties of dilute MgCe alloys

The magnetic susceptibility of monocrystalline dilute MgCe alloys has been investigated. An anisotropic susceptibility was found and explained as the consequence of a crystalline field splitting of the ground state of about 3.2 K.     

Intermediate valence state of Yb in Ag

Results of electrical resistivity measurements at low temperatures of dilute alloys of Yb in Ag reveal weak Kondo-like resistivity minima, in apparent contradiction with the previous magnetic susceptibility measurements of AgYb by other workers who concluded that Yb was in a divalent state and essentially non-magnetic in Ag. The present results together with the weak magnetic behaviour evident in the susceptibility measurements suggest that Yb is in the ‘Intermediate Valence State’ in Ag.     

dHvA observation of spin-dependent scattering by magnetic pairs of Co impurities in Au

We report the first dHvA observations of interaction effects between impurities, via measurements of the spin-dependent scattering of conduction electrons by magnetic pairs of Co impurities in Au. The local moment associated with Co pairs produces the only spin-dependent scattering in these dilute alloys because isolated Co impurities (which are also present) carry no local moment at low temperatures. The results are used to estimate the concentration (or average separation) of magnetic pairs.     

The ferromagnetic transition temperature of dilute Pd-Co,Pd-Fe and Pd-Mn alloys

Results of resistivity measurements on dilute Pd-Mn alloys with concentrations from 0.15 at .% to 0.7 at .% are discussed. A calculation of the transition temperature of Pd-Co, Pd-Fe and Pd-Mn alloys with concentrations between 0.05 at .% and 3 at .% is presented and compared with the available experimental data.     

Temperature and field dependent scattering in noble metal alloys

The scattering formalism of Blaker and Harris which is appropriate to potential scattering in dilute cubic alloys is modified to include the spin-dependent scattering. An expression for the total T-matrix which describes the ordinary and exchange scattering by magnetic impurities is obtained. The new formalism is compared with other work in the area and is used to discuss the dHvA data for Kondo alloys based on Cu and Au.     

High magnetic field behavior of iron impurities in chromium

Mössbauer measurements in ⁵⁷Fe in dilute FeCr alloys in external magnetic fields up to 140 kOe are presented. The magnetic hyperfine interactions are compared with results obtained by Herbert, Clark and Wilson and are discussed in terms of a model due to Housley and Dash.     

Influence of interstitials and non magnetic impurities on the Kondo effect

The influence of interstitials and non magnetic impurities on the anomalous resistivity, thermopower and Kondo temperature of dilute magnetic alloys was investigated generalizing a model proposed by Bohnen and Fischer. Numerical results are given as a function of the distance between the interstitial (or non magnetic impurity) and the magnetic impurity using their scattering phase shifts as parameters. The Kondo anomalies are altered considerably, if the magnetic impurity is very close to the non magnetic scattering potential, e.g. if it is part of an interstitial dumbbell.This work is part of a doctoral thesis of G.Wehr at the Technische Universität München     

Giant magnetic moments in metals as thermodynamic fluctuations

The nucleation of giant magnetic moments in certain dilute alloys is interpreted in terms of a Landau-Ginsburg type fluctuation theory. Beyond a certain threshold value of the coupling energy of the bare impurity spin to the spin density of the host a characteristic fluctuation localized around the impurity spontaneously acquires a non-zero, autonomous value. As observed, the magnitude of the giant moment decreases with increasing impurity concentration, at least at low concentrations.