Experimental studies of the low-temperature properties of spin glasses

Abstract

The recent experimental advances in the characterization of the low T properties of spin glasses are reviewed. A macroscopic anisotropy with a triadic character is found to characterize the spin glass state at low T, whatever its remanence O , and can be modified at will in RKKY spin glasses by addition of non magnetic impurities. The irreversible properties of spin glasses might be explained by successive jumps in phase space over energy barriers with a flat distribution of heights which are associated with the exchange couplings rather than the anisotropic interactions. Other marked differences with the case of fine magnetic grains,such as a single crossover,form non-ergodic to paramagnetic behavior and the probable occurrence of an equilibrium field cooled state,are stressed. The low-T specific heat and the correlated resistivity data in metallic spin glasses indicate the existence of a large density of magnon-like excitations,which have been simulated numerically. From zero field NMR experiments it can be seen that the local spin correlation functions do not markedly differ throughout the sample. The limited NMR and USR relaxation data available indicate that these local correlation functions have long time tails although it cannot be decided yet whether they correspond at low T to a 1/w or ωv−1 noise spectrum, with V<1.