Exchange Bias Effect And Training Effect In NiFe₂O₄/NiO Nanocomposites System

The study on exchange bias effect in ferromagnetic/antiferromagneticnanocomposites is one of the foreland fields of spintronics. The exchange bias effect canovercome the super-paramagnetic behavior arising from the decreased size offerromagnetic nanoparticles and provide potential solutions to increase the density ofstorage. In this thesis, our studies focus on the composites system of ferromagnet(NiFe2O4) embedded in antiferromagnet matrix (NiO). The microstructure, exchange biaseffect and training effect in these composites system are investigated, by changing theinternal factors and external factors of the samples to achieve the measurement of theexchange bias effect and training effect on the macro-controlling. The main contents ofthis thesis can be summarized as follows:1. A brief introduction on phenomena and laws of magnetic nanomaterials and thebackground of the exchange bias effect which focus on the conception, basic theory andadvancement. Based on the introduction, the basis of subject selection and studysignificance was put forward. A nanogranular system of multiferroic NiFe2O4nanoparticles embedded in an antiferromagnetic NiO matrix had been synthesized throughthe chemical co-precipitation method on the basis of a high-temperature phase segregationprinciple.2. The effect of the microstructure on exchange bias in magnetic nanoparticle systemhad been studied. A nanogranular system of Ni1-xFexO (x=0.09) was prepared by chemicalco-precipitation method. The effect of particle size ranging from~3nm to~55nm onexchange bias was very obvious in Ni-doped NiO nanoparticles, which were sintered atdifferent temperatures (550°C≤TS≤1000°C). With the increase of the particles size,exchange bias field increases in the beginning and then decreases. Both exchange biasfield (HEB) and vertical magnetization shift (MShift) can be exhibited below250K after fieldcooling procedure. The HEBand MShiftdecrease monotonically with crystalline size, andtheir behavior strongly depend on the crystalline size of NiFe2O4nanoparticles. Linearrelationship between HEBand MShiftis observed for systems with smaller sizes (DNFO≤8nm), reveals a straightforward correlation between them. This phenomenon isascribed to the interfacial exchange coupling between FiM NiFe2O4clusters andspin-glass-like (SGL) phase. As DNFOis above12nm, the dependence of HEBand MShiftdeviates from the linear relationship, which is discussed in terms of the superimposedcontribution from the exchange coupling between FiM NiFe2O4core with the SGL phase,and the exchange coupling between FiM NiFe2O4core and AFM NiO phases at theinterfaces.3. Consecutive hysteresis loop on the exchange coupled NiFe2O4/NiO nanogranularsystems showed that the HEBdecrease with magnetic field cycling, which is referred to asthe training effects. It suggests that two distinct forms of training mechanism are existed.One is related to an athermal contribution leading to the abrupt single cycle training, whilethe other is the conventional thermal activation mechanism responsible for the gradualreduction of HEBduring the subsequent cycles. With the increase of particle size, therelative change of HEBand enhanced coercivity (△HC) after training display anonmonotonic size-scaling behavior and reaches the maxima for S5with DNFO~22nm. Inthis system, this largest reduction reveals the weakest dynamic stability of the interfacialspin configuration during training procedure. Moreover, different decay rate of HEBand△HCwith field cycles are observed supporting the dual behavior of the interfacialuncompensated spins. The interfacial frozen spins are suggested to account for theappearance of HEB, the rotatable spins are linked to the△HC.