Exchange Bias Effect In CoCr₂O₄/Cr₂O₃Nanocomposites System

The study on exchange bias effect in ferromagnetic/antiferromagneticnanocomposites is one of the foreland fields of spintronics. Because, the exchange biaseffect can beat the superparamagnetic limit in ultrahigh density media. In this thesis, thestudies focus on the composites system of ferromagnet embedded in antiferromagnetmatrix. The microstructure and the exchange bias effect in these composites system areinvestigated, by changing the internal factors and external factors of the samples toachieve the measurement of the exchange bias effect on the macro-controlling. Thecontents of thesis are summarized as follows:1. A brief introduction about the exchange bias effect was presented, mainly focusingon the basic theory, conception and advancement of the exchange bias effect. Then, thestudy significance and subject selection was put forward. A nanogranular system ofmultiferroic CoCr₂O₄nanoparticles embedded in an antiferromagnetic Cr2O3matrix hadbeen synthesized by the high-temperature phase segregation route from a Co-doped Cr2O3matrix.2. The effect of the microstructure on exchange bias in magnetic nanoparticle systemhad been studied. A nanogranular system of Cr_{2（1-x）}Co_2xO₃（x=0.1） was prepared bychemical co-precipitation method. The effect of particle size on exchange bias was veryobvious in Co-doped Cr2O3nanoparticles, which were sintered at different temperatures（600°C≤TS≤1100°C）. With the increase of the particles size, exchange bias field increasesin the beginning and then decreases. Different doping concentration Cr_{2（1-x）}Co_2xO₃nanoparticles system was also investigated. Different from the traditional FM/AFMcomposites with T_C>T_N, the Cr_{2（1-x）}Co_2xO₃nanoparticles system has a T_CTC>TNT_N.3. The effects of measurement temperature and cooling magnetic field on theexchange bias were investigated. For the sample Cr_{2（1-x）}Co_2xO₃（x=0.1） measurement bydifferent temperature, both horizontal and vertical magnetization shift were observed when the samples were cooled below70K under field cooling. The exchange bias effecthad been observed for Cr_{2（1-x）}Co_2xO₃with different field cooling. With the increase of thecooling field, exchange bias field increases quickly and then reach the saturation value.The similar behave of H_EBand Mshiftcan be interpreted considering that the magneticcoupling between ferromagnetic and antiferromagnetic and the exitence of spin-glass-likephase.