The spin-autocorrelation function in spin glasses above the freezing temperature

The purpose of this work is to investigate spin glasses above the freezing temperature, where dynamics of magnetic correlations have been observed. In the temperature range well above two times the freezing temperature the Markovian Two Level Jump (MTJP) model was rather successful to describe the experimental findings in HFM measurements of the spin glasses AuFe, where Fe atoms are diluted in a diamagnetic matrix, and in the cubic Laves phase Y(Fe,Al)2, where magnetic spins are placed in a pauli paramagnetic matrix. This MTJP model proposes a two state system, where the 57-Fe probe atom is embedded in a correlated region - the life time of this state is ton - and a state, in which the atoms experiences no correlation. The temperature dependence of ton was found to follow a power law between two times Tf and 10 times Tf. Two different exponents were obtained: -1 for samples with low Fe content and -2 for spin glasses with high Fe content. A major goal of the present work is to test the power law findings in spin glasses with Fe concentrations different to the ones investigated and in other classes of spin glass types for temperatures above Tf with HFM and μSR measurements. For the current investigations additional AuFe and Y(Fe,Al)2 samples in the intermediate concentration range are investigated. To check if the findings are more generally valid the study is extended to spin glasses with competing exchange interactions. In the alloys CrFe and NiFeMn the spin glass phase occurs in the concentration regime between antiferromagnetic and ferromagnetic phase. The successful analysis of all recorded HFM spectra by means of the MTJP model is shown. For temperatures above two times Tf the proposed power law behavior for high Fe and low Fe content is confirmed for all spin glasses investigated. Below this temperature range a sharp increase in the life time of correlated states is observed. The life times derived from μSR experiments show a similar power law behavior in temperature dependence. However, the correlation times are a factor 10 - 100 shorter than those analyzed in the HFM measurements. (author)