Exchange-interaction by the induced growth of the soft phase on the extreme hard phase

The significant change is reported in interparticle interaction for nanocomposites of ε-Fe2O3 and FeCo as the grain size of the soft phase (FeCo) increases upon annealing. It undergoes a transition from the dipolar interaction dominant regime into the exchange-interaction dominant regime, starting from the crystalline soft phase size of 9.6 nm, which is coincident with the theoretical critical dimension. The effective anisotropy increases exponentially as the ordering dimension of the soft phase increases with the ratio between hard and soft phases stays constant, causing a delay in the demagnetization process. The emerged exchange-interaction is understood by the induced growth of the soft phase on the extreme hard phase.