Dynamics of spin-glass freezing

The unique properties of STf(tv) described above are a challenge to get more insight into the dynamics of spin-glass freezing. At present, however, no theory is available to be compared with. Nevertheless the data already demonstrate the... 

Particularly valuable information on the dynamics of spin-glass freezing has recently been gained by experiments where both the real and imaginary component (x x") of the complex susceptibility x(T, co) have been measured accurate-... 

The tSR data are compatible with either dense spin glass freezing or with the sudden condensation of a coherent Kondo state (for a discussion see, for example, Grewe and Steglich 1991) with slow, but dynamic spin correlations at 2.5K. The latter interpretation ties in with the specific heat data, especially in applied fields. The former could be explained in terms of the magnetic polaron model formulated by Kasuya et al. (1993a,b). 

Thus, 8 Kb NMR studies on Rb0.4s(ND4)o.55D2P04 reveal that the deuteron "pseudo-spin-glass" transition is just not a simple kinetic slowing down process, but due to a gradual condensation of randomly polarized clusters. The results provide novel information on the temperature dependence and distribution of local RFs and their dynamics. Further, the NMR data suggest that the "pseudospin O-D-O deuteron intra-bond motion determines freezing dynamics. 

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... 

In the paramagnetic regime (T > Tm), the spectra in a weak LF (needed to suppress the depolarization by Cu nuclear dipoles) for x > 0.08 were most easily fitted to a power exponential (exp[—(At) ]) relaxation. Hence the summary label relaxation rate in fig. 113 (left) refers to the static width Aeff (see eq. 74) for T dynamic rate A for r > Tu- The variation of power p was studied in some detail for the 10% sample. A decrease fromp w 1 at high temperatures top w 0.6 close to Tm was found. This is another indication that a disordered spin-glass-like state is approached and 7m might best be considered a spin freezing temperature. This spin-glass-like state, however,... 

The authors of this experiment (Murani 1981, Mezei 1983) interpret their results as a clear evidence that the freezing in spin glasses is a purely dynamical gradual process, without any static phase transition. 

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