ISSN:0094-243X    

DOI:10.1063/1.30044

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The basic ideas behind magnetic impurities in nonmagnetic metallic hosts are reviewed. The atomic, or insulator, picture is used as a beginning. The alterations in wave function and energy level width upon solution in a metal are explored. The resulting covalent exchange is shown to lead to KondolnT behavior in the resistivity and susceptibility. The Kondo temperature is defined. Discrepancies between perturbation theoretic results and experiment are exhibited below this temperature. The Anderson‐Yuval‐Hamann concept of scaling is presented, and Wilson’s low temperature results are discussed using Noziere’s perturbation model. A pedestrian version of scaling is used to find the stable exchange fixed point. The implications of scaling are discussed.

ISSN:0094-243X    

DOI:10.1063/1.30155

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Much of the theoretical and experimental work on phase transitions in the past decade has been concerned with, stimulated by, or tested on magnetic systems, and much of this work has been reported at sessions of this Conference. A review of the development of this field was given, designed primarily for those workers in magnetism who might have been present at conferences on magnetism and magnetic materials, but who did not attend the sessions on critical phenomena.Recent work has been done against the background of the classical theories of Curie, Weiss, Ornstein, Zernike, and Landau. As a result of Onsager’s 1944 solution of the two‐dimensional Ising model, it was realized that the singular behavior of different thermodynamic functions such as magnetization, susceptibility, and specific heat, was not necessarily as predicted in the classical theories. Tentative numerical work in the fifties led to the consideration of various critical indices, the exponents which characterize the nature of divergences in the thermodynamic quantities near Tc. Experimental measurements on these indices showed that they agree much more closely with results of analysis of series expansions than with classical theories. Theoretical developments of the sixties, which were presented at CMMM sessions, included inequalities and equalities relating the indices‐the scaling relations. Experimental tests of these relations on magnetic system were also reported. The most recent developments have included the theoretical calculation of these indices by the renormalization group theories of Wilson and Wilson and Fisher. These theories have shed considerable light on the nature of the singularities near critical points and have given some indication of the factors which affect the singularities.Attention of these conferences in the last few years has been turned to more complex types of critical points (e.g. tricritical points) which can be found in antiferromagnets in external fields, and we can expect close, experimental and theoretical examination of the phase diagrams of magnetic systems in external fields.It should be emphasized that the relationship between the two fields of study, critical phenomena and magnetism, is symbiotic and not parasitic. There has been and will continue to be much beneficial cross‐fertilization between them, as advances in one stimulate developments (both theoretical and practical) in the other, and vice versa.

ISSN:0094-243X    

DOI:10.1063/1.30005

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

In the configuration‐based approached in 3d or 4f impurities in metals, the bare impurity states are taken as conventional ionic‐type several‐electron states, corresponding to well‐defined configurations 3dnor 4fnwith intraconfigurational splitting into L−S terms, crystal‐field levels, etc. The condition for the impurity to remain ’’magnetic’’ as the interaction with the conduction electrons is switched on is that the stabilization energy of the bare‐impurity ground state, relative to levels belonging to higher configurations, be large compared to the mixing width. Under this condition, the ionic‐type intraconfigurational structure of the impurity also continues to be defined. Although a different historical line of development hindered its recognition, there is good evidence for such ionic‐type structure in 3d as well as 4f impurities. Our conclusion is that a configuration‐based description is the most physical one for a magnetic impurity. This conclusion has interesting implications for the theory of magnetism in concentrated metallic systems.

ISSN:0094-243X    

DOI:10.1063/1.30013

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A theory of the volume striction in ferromagnetic transition metal alloys is developed on the basis of the Alexander‐Anderson‐Moriya theory of the exchange interaction. The volume derivative of the interaction between a pair of atoms is calculated under the assumption that a volume change affects the state density of thesband and 3d atomic levels. In fcc Ni‐Fe the number of 3d minority spin electrons of a Fe atom increases with increasing Fe concentration. The volume derivative of the interaction between two Fe atoms shows an anomalous dependence on the Fe concentration through the change of the number of 3d minority spin electrons which explains the large spontaneous volume striction, lattice constant anomaly and negative thermal expansion at low temperatures in invar alloys. The possibility of finding Fe atoms with magnetic moment antiparallel to bulk magnetization and its relevance to the lattice anomalies are discussed also.

ISSN:0094-243X    

DOI:10.1063/1.30031

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

After a brief discussion on the application of X‐ray photoemission (XPS) to the study of multiplet structures and intensities in 4f‐photoemission, we present XPS spectra of 4f‐electrons in rare earth compounds with intermediate valence state. This technique provides an instantaneous picture of the electronic configuration, and in each case shows the presence of both divalent and trivalent ions, which are identified by their multiplet structure and chemical shifts. These results allow a direct estimate of the Coulomb correlation energy Ueffwithin 4f‐shells and demonstrate that Hund’s rule correlation is well preserved within a ’’nonmagnetic’’, delocalized 4f‐shell. We review recent temperature dependent XPS‐studies on two distinct systems, the Tm monochalcogenides and the ’’chemically collapsed’’ (golden) phases of SmS. We analyze their relation to other physical properties, especially lattice constant. We also discuss the implications of these observations for the understanding of the semiconductor‐to‐metal transition in these compounds and the related peculiar magnetic properties.

ISSN:0094-243X    

DOI:10.1063/1.30064

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

There are several branches of the spin‐wave spectrum in materials where the exchange and crystalline field are comparable. Spin waves out of excited states can play an important role by mixing and interacting with the conventional spin waves out of the ground state. These effects are illustrated by neutron scattering results for TbSb, where several branches were observed, for TbAl2, where an excited‐state spin wave has been directly observed, and for Pr3Tlwhere the mode‐mode interaction has been found to be responsible in part for the surprising lack of temperature dependence through Tcof the spin‐wave peak. A theory of the dynamical susceptibility has been developed that includes, the RPA, all spin waves allowed within the ground multiplet, and is found to be in good agreement with experiment in contrast to the singlet‐singlet or singlet‐triplet models. For Pr3Tlas T→Tcthe frequencies of the singlet‐triplet modes decrease but remain finite. The modes within the triplets do tend to zero frequency at Tc, but do not drive the transition since they are entirely transverse. The soft ’’mode’’ is a zero‐frequency mode that appears in the scattering as a central peak that diverges at the phase transition. The central peak, which arises from localized spin fluctuations, has probably been observed in TbSb.

ISSN:0094-243X    

DOI:10.1063/1.30085

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Magnetic susceptibility and lattice parameter measurements1,2indicate that Sm in Sm1−xRxS (R=Y, La, or Gd) undergoes valence transitions as a function of composition and of temperature, similar to SmS under pressure.3In the Sm1−xYxS system, the samples are black colored for x<.15 and gold colored for x≳.3. The valence of Sm increases from divalent at x=0 to ?2.3+at x=.14, and to &angupr;2.7+ for the entire range .3<x<1. No phase transition as a function of temperature is observed in these two composition ranges for 4.2<T<900 K. The magnetic susceptibility is independent of temperature for T<100 K, similar to collapsed SmS under a pressure larger than 7.5 Kbar.4Note that trivalent Sm with J=5/2 as ground state in general exhibits Curie‐like behavior at low temperature. For .15 <x<.3, samples show remarkable temperature‐induced phase transitions. At x=.19, for example, the lattice parameter of the sample contracts on warming at T=190 K but expands again at T≳600 K. Accompany the lattice variation, the color of the sample changes on warming from black to gold at T=190 K and to black again at T≳600 K. The valence of Sm is estimated to change from ?2.4+ to ?2.7+ at T=190 K and back to become more divalent at T≳600 K. We thus obtain a T vs x phase diagram for this system in which the valence of Sm changes smoothly from divalent at x=O to more trivalent as x increases at both low (T<100 K) and high (T≳600 K) temperatures. At some intermediate temperature, a sharp transition from black phase (more divalent) to gold phase (more trivalent) occurs in the composition range .15<x<.3. Similar effects are observed in the Sm1−xLaxS and the Sm1−xGdxS systems. An interconfiguration fluctuation (ICF) model has been suggested for the intermediate valence state of Sm and other rare‐earth ions and their common nonmagnetic state at low temperature.4According to this model, the Sm ion in its intermediate valence state switches from one integral valence state (2+) with 6 local electrons on the 4f shell to the other state (3+) with 5 local electrons of the 4f shell and the extra electron in the conduction (5d) band. If this fluctuation rate is faster than the relaxation rate of the Zeeman levels in the 4f5configuration, proper Boltzmann distribution will not be achieved and the Curie behavior will be suppressed. By including crystal field effects and multiple energy levels into the ICF model, the temperature‐induced transitions between valence states can be explained.2The change of valence is a result of the relative change of the energy levels of the 4f6configuration (with J=1 as the first excited state about 400 K above the J=0 ground state) and the 4f55d configuration (with J=5/2 as the ground state for 4f5shell and t2gfor the 5d electron). At T≳600 K, we propose that the energy levels of the two configurations are such that the J=5/2 level is between the J=1 and the J=0 levels for Sm in an intermediate valence state. Further, the J=5/2 level splits into &Ggr;8and &Ggr;7states in cubic crystal. At T≳600 K, all these states are populated at one instant or another as the Sm ion switches between the two configurations. As the temperature lowers, the J=1 state becomes depopulated. This will increase the population of the 4f55d configuration at the expense of the 4f6configuration and cause the lattice to contract. In addition, the system may find that at a certain temperature its total energy can be lowered if the energy separation between the &Ggr;7and the &Ggr;8levels is widened and that the &Ggr;7level is more populated than the &Ggr;8level which has higher energy. The separation between the two levels can be increased if the crystal field strength is increased. This in turn can be achieved by contraction of the lattice which is happening now as a result of the depopulation of the J=1 level. Further, the t2gband is lowered as the separation of the egand the t2gbranches widens as a result of the increase of the crystal field strength. These effects will further depopulate the J=1 state and a spotaneous transition may result under favorable conditions. As a consequence of this transition, Sm becomes more trivalent, the number of the 5d electrons increases, and the color of the sample changes from black to gold. At an even lower temperature, however, the &Ggr;7state will start to depopulate and the J=0 level becomes more populated. This will increase the relative population of the 4f6configuration and cause the lattice to expand. This in turn weakens the crystal field strength and decrease the relative energy level of the 4f6configuration. This will further populate the J=0 state and again, under a certain condition, a sharp spontaneous transition may occur which returns the system to a more divalent state, decreases the number of the 5d electrons, and changes the color of the sample back to black.

ISSN:0094-243X    

DOI:10.1063/1.30117

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Trivalent samarium ions been observed via electron spin resonance as dilute ’’impurities’’ in the Val Vleck paramagnetic semiconductor SmS at 12 and 17.4 GHz in the temperature range 1.4 to 20 K. The spectra indicate the Sm3+ions to reside in stable, bulk sites of tetragonal symmetry and to have a &Ggr;7doublet ground state. The concentration of these ions appears to be &angupr;0.5−5% in those crystals in which the signals are observable, roughly one third of those examined. These crystals do not exhibit appreciable conductive losses at low temperatures so the ionized electrons are apparently not mobile. These observations indicate that the trivalent state is not an intrinsic property of semiconducting SmS but is induced by the presence of a lattice defect such as a sulfur vacancy which can trap an electron from one of the neighboring Sm2+ions. No control over the Sm3+concentration has been achieved despite various modifications of the crystal growth conditions. The isotropically averaged experimental g‐value g¯=0.70±0.02 is significantly enhanced with respect ot the value 0.476 expected for the &Ggr;7doublet. This is attributed to exchange coupling of the Sm3+moment with the neighboring singlet Sm2+ions. The magnitude and even the sign of the effect appear anomalous, however, in relation to earlier monochalcogenides. Rapid line broadening above 10K may result from spin relaxation as the trivalent state becomes locally itinerant in the Sm shell nearest to the stabilizing lattice effect.

ISSN:0094-243X    

DOI:10.1063/1.30124

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Sm1−xYxS undergoes a semiconductor‐metal transition as a function of the composition x. We have measured the reflectivity of cleaved single crystals as a function of composition and temperature for photon energies from 0.1 to 6eV. The prominent structure of the reflectivity of semiconducting SmS is assigned to transitions from the 4f6level of Sm2+into the crystal field split 5d states. The observed decrease in the intensity of the excied 4f5multiplet structure with increasing doping is assumed to be indicative of an increased admixture of 4f55d states into the 4f6ground state, the Sm ion being in a state of mixed valence. The dc scattering time of the conduction electrons at 300K shows a much stronger scattering for samples in the collapsed metallic phase, indicating a degeneracy of the hybridized 4f level with the Fermi level. For these samples the plasma edge exhibits an unusually large shift to lower energies upon cooling due to a localization of conduction electrons in the admixed 4f level. The concurrent valence transformation of the admixed 4f level manifests itself in an increase in lattice constant, scattering time and in the oscillator strength of the 4f6−4f55d transitions.

ISSN:0094-243X    

DOI:10.1063/1.30135

出版商:AIP    

年代:1975

数据来源: AIP