Non-linear ac susceptibility behaviour of collective dynamics in heterogeneous nanomagnetic systems

Abstract

The existence of magnetic systems displaying heterogeneous magnetic structures is common nowadays. The spin structure is heterogeneous at the nanoscopic level, allowing the interplay of different magnetic interactions which can include exchange, double-exchange, RKKY or dipolar interactions. The high degree of spin disorder and/or the presence of magnetically connected clusters or well-formed grains gives rise to magnetic relaxation and (or pseudo) collective dynamics. The relaxation behaviour resembles those observed in archetypal examples such as freezing in canonical (or cluster) spin glasses, and blocking processes, the latter in a superparamagnetism framework. The AC-susceptibility is the only in-situ technique covering several (usually 4) frequency decades, providing a full photograph of the relaxation phenomena. In addition, the AC technique can simultaneously record the non-linear components which, although of very small magnitude, are of great sensitivity when detecting subtle magnetic changes, such as phase transitions or the presence of minuscule magnetic phases. Results of non-linear susceptibility in diverse examples including reentrant ferromagnets (amorphous FeZr and FeZrCuB), a CMR oxide (a crystalline LaPb(Mn0.8Fe0.2)O3 perovskite) and a GMR FeCuAg alloy (ferromagnetic grains in a diamagnetic matrix) are reviewed.