Magnetic Correlations in Spin Glasses above the Freezing Temperature investigated by High Field Mössbauer Spectroscopy

Abstract

The strong dynamics of magnetic correlations are observed by high field 57Fe Mössbauer investigations for the spin glasses AuFe and Y(Fe, Al)2 in the temperature interval between the freezing temperature Tf and approximately 7 to 10Tf. All recorded spectra can be analysed with a model which takes into account these dynamics by allowing the formation of correlated states after a stochastically varying time τoff and their decay after a time τon, restricting the number of subspectra to the different number of Fe atoms in the first neighbour shell. Whereas for the different Fe environments, in addition to centre shift and quadrupole splitting, different hyperfine fields of the two stochastically formed states can always be resolved, the same set of τon and τoff values was obtained for all subspectra above approximately 4Tf. The external field hinders the decay of the correlated regions. The temperature dependence of ¿ on scales with Tf and exhibits a power law with exponents of –2 for the Fe-rich and –1 for the Fe-poor compounds.

Spin correlation times in above the freezing temperature

“…The shape of the HFM spectra of Cr 0.825 Fe 0.175 above T f is characteristic for the influence of dynamics-pronounced minimum at low and broad shoulders at high absolute velocities (figure 5). The similarity of the spectra with those found for AuFe and Y(Fe,Al) 2 in the spin glass region suggests the applicability of the same analysis (Pösinger et al 1993, Bogner et al 1998b, which takes into account dynamics based on a Blume formalism (Blume 1968). The model process can be described by a jump in time between two discrete hyperfine field levels,…”

“…This means on the other hand that the influence of the dynamics can be resolved in the used field range up to temperatures of approximately 10 T f . For comparable Fe nn environments B cl is 10-20% smaller than the one found for the AuFe system (Bogner et al 1998b) but exhibits a similar temperature dependence. In the whole temperature range investigated B cl is oriented antiparallel to the applied field, since for each subspectrum the intensity of the m = 0 transitions vanishes.…”

“…Dynamics of magnetic correlations have been observed in spin glasses well above the freezing temperature (T f ). Detailed investigations were performed especially for AuFe by µSR (Uemura et al 1985, Keren et al 1996, neutron spin echo (Mezei 1983) and highfield Mössbauer (HFM) measurements (Pösinger et al 1993, Bogner et al 1998b. For the analysis of the Mössbauer spectra in an applied magnetic field a model was proposed based on a two level jump process, between a state in which the probe atom is embedded in a correlated region-the lifetime of this state is τ on -and a state in which the atom experiences no correlation.…”

“…Au 0.85 Fe 0.15 , Y(Fe 0.75 Al 0.25 ) 2 ) and −1 for samples with low Fe content ((e.g. Au 0.95 Fe 0.05 , Y(Fe 0.40 Al 0.60 ) 2 ) (Pösinger et al 1993, Bogner et al 1998b.…”

Mössbauer investigation of Fe0.5In1.5S3

“…The spectra could only be analysed by a superposition of several subspectra (the chosen number is arbitrary), representing a hyperfine field distribution and strongly supporting the presence of a spin glass order as observed in the magnetic measurements. The appearance of hyperfine splitting (Figure 3) above the magnetically determined freezing temperature can be attributed to the different time windows of the two experiments and is typical for spin glasses [12]. The spin glass ground state is caused by the high degree of disorder present in this spinel structure, which does not allow that all nearest neighbour antiferromagnetic exchange interactions (indicated by the high Curie-Weiss temperature of −50 K [11]) are satisfied.…”