Spin-glass systems, dilute

To summarize, the analysis of dynamical properties have shown that the H0B22C2N system is not a simple superparamagnet, nor a typical 3D spin glass, but a new 2 dimensional spin glass system (Mori and Mamiya, 2003). In fact, a dilute triangular lattice magnetic system. 

After the pioneering work of Owen et al. (1956, 1957) on diluted localized moments in metals (Cu Mn), twenty years later a large amount of experimental ESR work has been devoted to classical spin-glass systems. The experiments can be divided in two groups one in the SG state at T -c Tg and one at T > Tg, where Tg is the glass transition temperature which depends on the measuring frequency. 

The canonical spin glass consists of a noble metal (Au, Ag, Cu, or Pt) diluted with a transition metal ion, such as Fe or Mn. The magnetic interaction in such systems is mediated by the conduction electrons, leading to an indirect exchange interaction—the RKKY (Ruderman and Kittel [70], Kasuya [71], and Yosida [72]) interaction, whose coupling constant J R) oscillates strongly with distance r between the spins as... 

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). 

ZF and LF-ftSR has now been reported by Dunsiger et al. (2000). The ZF relaxation function is root-exponential at all temperatures down to 0.025 K, indicative of a dilute spin system with substantial dynamics. This supports the idea of isolated islands nucleated around defects, but indicates only slowed fluctuations, not full freezing. The apparent spin fluctuation rate drops starting near 1 K (where bulk probes see effects they attribute to short-range magnetic order, Schiffer et al. 1994), but does not extrapolate to zero, and shows no effect around 0.14K. Thus p,SR sees no spin-glass transition. All of this is generally consistent with the neutron diffraction results. In LF at 0.1 K, the relaxation... 

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... 

Metallic crystals Metallic systems, like Fe impurities in Au, are often cited as classical spin glasses, referring to the pioneering experiment by Cannella and Mydosh (1972) when a cusp in the ac susceptibility, a (T ), in low magnetic fields is observed in these dilute alloys. In such systems, however, the interactions are rather complicated and not precisely known ... 

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... 

To discuss this new type of ordered phase in spin glasses, one would like to have a microscopic model where the actual interactions and anisotropies are considered and the average over a realistic description of the site dilution disorder is performed. Clearly this is a difficult task and up to now no realistic model of a spin glass has been solved analytically. In addition, there exists another difficulty because a proper treatment of systems with quenched disorder like spin glasses involves averaging the free energy F rather than the partition function Z... 

In this section characteristic properties of spin glasses at temperatures well below the freezing temperature Tf are discussed, being determined, e.g. by the maximum of the low-field, low-frequency ac-susceptibility. A number of fascinating phenomena are known to occur in spin glasses at low temperatures and they had been well documented in dilute alloys such as CwMn and AuFe (Owen et al. 1957, Schmitt and Jacobs 1957, Kouvel 1961, Tournier and Ishikawa 1964) long before these systems became objects of such intense and systematic studies. 

This insulating dilution system has been studied in great detail in the spin-glass regime, 0.13 x < 0.51, as described above. Early ac susceptibility measurements (Maletta and Felsch 1980d) displayed in fig. 86 can be taken as a first hint at a RSG behavior of the compounds with Eu concentrations 0.51 x 0.65. Note that the different plateau values in fig. 86 are only due to different sample geometries in the various measurements, which is checked experimentally. 

Hiraoka et al. (1986) have found the onset of spin-glass order at the dilution concentration (1 —x) =0.11 in similar compounds of the diluted antiferromagnetic system, Eu Ybj Se, by measurements of ac-x Eu-NMR. 

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