Size-dependent training effect in exchange coupled NiFe2O4/NiO nanogranular systems

The particle size dependent training effect has been investigated on the exchange coupled NiFe2O4/NiO nanogranular systems, with average particle size (DNFO) of NiFe2O4 ranging from ∼3 nm to ∼55 nm. For all samples, analysis of the field cycles (n) dependence on exchange bias fields (HEB) suggests the existence of two distinct forms of training mechanism during training procedure. One is related to an athermal contribution leading to the abrupt single cycle training, the other is the conventional thermal activation mechanism responsible for the gradual reduction of HEB during the subsequent cycles. With the increase of particle size, the relative change of HEB and enhanced coercivity (△HC) after training display a nonmonotonic size-scaling behavior and reaches the maxima for DNFO ∼ 22 nm. In this system, this largest reduction reveals the weakest dynamic stability of the interfacial exchange coupling energy during field cycle process. Moreover, different decay rate of HEB and ΔHC with field cycles are observed supporting the dual behavior of the interfacial uncompensated spins. The interfacial frozen spins are suggested to account for the appearance of HEB, while the rotatable spins are linked to the ΔHC.