Experimental evidence for magnetic two level systems in spin glasses (invited)

Abstract

We report about low temperature measurements of susceptibility, magnetization, and time dependence of the thermoremanent magnetization on amorphous magnetic alloys (PdCuSi) with transition metals Mn, Fe, and Co in a concentration of x=5 to 20 at. %. Alloys with low concentrations show single peak spin-glass behavior. The more concentrated samples show magnetic double transitions, with an upper transition at Tc into a ferromagnetic state, followed by a lower transition at Tg into a state which has a spin- or cluster-glass character. Evidence for this comes from the onset of irreversibility and time dependence of the remanence, features which are absent above Tg. The results can be interpreted by thermal relaxation of cluster moments over anisotropy barriers, or in an equivalent picture by the existence of magnetic two-level systems (MTLS). Direct evidence for the existence of MTLS comes from investigations of the phonon thermal conductivity κPh(T) studied as a function of temperature and magnetic field. Enhancement of κPh(T) in a field of 5 T is found for Fe and Co, yet only in the cluster-glass range T<Tg. We conclude that the phonons, coupled via a 1-s coupling to the cluster moments, scatter inelastically from the MTLS; i.e., the phonons excite the magnetic clusters over their anisotropy barriers. The results are discussed within the framework of a phenomenological model describing the low temperature properties of spin glasses in terms of a distribution of asymmetrical double well potentials.