Evidence for the Existence of Magnetic Two-Level Systems in Amorphous Spin-Glasses

Susceptibility and remanence as well as the phonon thermal conductivity ${\kappa }^{\mathrm{ph}}$ are studied as a function of temperature and magnetic field on amorphous spin-glasses ${\mathrm{}\left(\mathrm{PdCuSi}\right)\mathrm{}}_{90}$T${\mathrm{M}}_{10}$, TM=Mn, Fe, Co. For the Fe and Co alloys ${\kappa }^{\mathrm{ph}}\mathrm{}\left(T\right)\mathrm{}$ is enhanced in a field of 5 T below $T_g$. The results give evidence for thermal activation of cluster moments (magnetic two-level systems) by inelastic scattering of phonons, coupled via spin-orbit interaction to the magnetic two-level systems. Contributions of spin-wave-like excitations to ${\kappa }^{\mathrm{ph}}(T, H)$ cannot be detected.