Non-dipolar magnetic coupling in a strongly interacting superparamagnet: nanogranular Fe26Cu8Ag66

Unambiguous evidence for non-dipolar, as well as dipolar, inter-granular interactions has been found in the strongly interacting superparamagnetic mechanical alloy Fe26Cu8Ag66. This alloy comprised 5-6 nm BCC Fe-Cu grains in a matrix of similarly sized FCC Ag grains, and was of excellent purity and homogeneity. Magnetic relaxation was measured via DC magnetometry, AC susceptibility, muon spin relaxation and Moessbauer effect experiments. A phenomenological model in which magnetic interactions increased the apparent size of the grains, while retaining the Arrhenius law behaviour of non-interacting grains, predicted the observed variation in blocking temperature as a function of measurement time, with an apparent grain size d* approximately three times larger than the actual size. Significant differences were found in comparisons of temperature-dependent hysteresis data and the predictions of a dipolar interactions model, indicating the additional presence of non-dipolar interactions, most likely RKKY, in the alloy. Corroborating evidence was found in a low-temperature spin-glass-like peak in AC susceptibility data, indicating the possible influence of isolated Fe atoms or clusters within the Ag matrix in mediating inter-granular RKKY interactions