Spin-glass systems, concentrated

In this section we deal with ESR in metallic systems where the ESR-active probe is substituted at relatively high concentrations (i.e. more than 10%) or where the ESR probe is a constituent of the alloy itself, i.e. Gd metal, GdAgIn or GdA. In many of these alloys or compounds one can observe magnetic ordering. In such cases we consider only investigations of the resonance absorption above the Curie or Neel temperature, i.e. we do not include measurements on ferro-, ferri- or antiferromagnetic compounds. The demarcation line to resonance experiments in spin-glass systems in many cases is rather arbitrary. 

Recent zero-field studies of the free in longitudinal fields have revealed new and unique information on spin glass and other systems but here we concentrate on muonium and muonium-like states in zero magnetic field. In this case, precession is not observed in the classical sense of the word but rather a modulation of the muon polarization with time. This can be most easily understood in the case of muonium itself in terms of the isotropic Hamiltonian of Equation 30. As noted above, muonium is formed via the "capture" of an electron from the stopping medium. Since the fi is longitudinally polarizedl"3 (a ) but the captured e" is not (Qg or Pq), muonium forms initially in two spin states, defined by A> = lV°e> = I 1 1> and B> = I o /3e> = 1//2 10> +... 

Fig. 24. Longitudinal field spectra for a Gaussian (left) and a Lorentzian (right) field distribution. The Gaussian case refers to spin freezing around 8.5 K in CePtSn, a concentrated spin system (Kalvius et al. 1995a) the Lorentzian case to a dilute Cu(Mn) spin glass below its glass transition temperature of 10.8K. The values of the longitudinal fields are (from top to bottom) 640, 320, 160, 80, 40 and OG (Uemuia et al. 1981). In both cases the set of spectra unambiguously proves that the spin systems are static.

The dilute spin glasses are a special topic within p,SR because they generate distinctive muon spin relaxation via the Lorentzian field distribution. This was discussed as a possible field distribution (eq. 33) for p,SR in sect. 3.2.2 (the static ZF relaxation fimction, eq. (34), is shown in fig. 20). Whereas the Gaussian field distribution is expected (and often observed) in dense-moment systems, Walstedt and Walker (1974) predicted that the Lorentzian distribution applies in the dilute-moment limit (magnetic concentration goes to zero) of spin glasses, and Uemura and collaborators (Uemura et al. 1985, and references cited therein) observed it with xSR in the frozen state of dilute Cu(Mn) and Au(Fe). 

The physical properties of concentrated VF and HF systems and of compounds are much less understood than those of dilute systems. In the case of transition metals in a non-magnetic host one has already for rather small concentrations (of the order of one percent) magnetic order or spin-glass structure due to direct... 

Fig. 3. Temperature-dopant concentration T-x) phase diagram delineating the regions of superconductivity and antiferromagnetic ordering of the Cu ions for the hole-doped La2, SrjjCu04 and electron-doped Nd2 jCe,Cu04 systems. AFM, antiferromagnetic phase SG, spin-glass phase SC, superconducting phase.

A careful analysis of the paramagnetic susceptibility by Morgownik and Myd-osh (1983) confirms both complications being present in 3d systems, e.g. in Cu, Au and Pt host-metals with even low ( 8 atom%) Mn and Fe impurity concentrations. Nevertheless, there exist competing interactions which are responsible for the appearance of the spin-glass state in such systems. 

Here we will concentrate on lanthanide systems which built up well-localized moments. In the class of dilute lanthanide metals and intermetallic compounds for which spin-glass behavior has been reported, one topic involves 4f-impurities in superconducting hosts. Already a small amount of paramagnetic impurities, as... 

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