Dynamical susceptibility of weakly interacting ferromagnetic nanoclusters

The ac-susceptibility of weakly interacting ferromagnetic nanoclusters located at the vertices of a periodic lattice in a static applied field is calculated using a perturbation theory. The samples are nonspherical which gives rise to predominant shape effects due to dipolar interactions. The effects observed on the maximum of the two components of the dynamical susceptibility when increasing the strength of the dipolar interactions are opposite for prolate and oblate samples. It is also shown that a static applied field can invert these effects. The peak position of the out-of-phase component of the ac-susceptibility shifts towards lower temperatures with increasing the strength of the dipolar interactions in most cases in agreement with previous numerical studies on isotropic systems. It is shown that the opposite behaviour can be observed in the high damping case because of a sample shape effect.

Influence of the magnetic state on the chemical order-disorder transition temperature in Fe-Ni permalloy

In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.