Exchange Bias And Training Effect In Magnetic Nanoparticles System

The exchange bias effect in magnetic nanoparticle systems are one of the foreland fields of spintronics. The exchange bias effect can beat the superparamagnetic limit in ultrahigh density media, so the study on magnetism and exchange bias effect in ferromagnetic/antiferromagnetic nanocomposites becomes particularly important, and it will lie the foundations of developing practical applications of spintronic devices. In this thesis, our studies focus on the composites system of ferrite embedded an antiferromagnetic oxide matrix. The microstructure and the exchange bias effect in these composites system are investigated, by changing the internal factors and external factors of the samples to achieve the measurement of the exchange bias effect on the macro-controlling. The main contents of this thesis can be summarized as follows:1. The effect of the microstructure on magnetism and exchange bias in magnetic nanoparticle system have been studied. The results showed that the Cu0.85Fe0.15O nanoparticles by chemical co-precipitation method had the best exchange bias effect in low temperature. The effect of particle size on exchange bias was very obvious in Fe-doped CuO nanoparticles, which were sintered at different temperatures (350℃≤Ts≤650℃). It is found that the system shows magnetic properties transition from paramagnetic to ferromagnetic with increasing grain size, with the increase in the particles size, exchange bias field decreases monotonously.Meanwhile, the coercivity was largest with d=20nm. Different doping concentration Cu1-xFexO nanoparticles were also investigated. The ferromagnetic CuFe2O4 phase was only observed of the microstructure as 0.10≤x≤0.30, the CuFe2O4 and Fe2O3 phase could be found as 0.40≤x≤0.65. The exchange bias field accompanying vertical magnetization shift was observed in the system at low temperatures, and as x=0.20, they were up to maximum. The trend of exchange bias effect with the doping concentration shows that the pining force of AFM phase can play a significant role in pinning the uncompensated interfacial moments. 2. The effect of temperature field, measurement magnetic field and cooling magnetic field on the exchange bias were investigated. For the sample Cu1-xFexO (x=0.15) with d=20nm, both horizontal and vertical magnetization shift decreased with the increasing temperature when the samples were cooled below 50K under field cooling. The observed exchange bias effect can be interpreted considering that the magnetic coupling between ferromagnetic and antiferromagnetic and the exitence of spin-glass-like phase. As the temperature is 10 K, the HEB decreasd with the increasing measurement field,meanwhile the He increased with the increasing measurement field. The exchange bias effect has been observed for Cu1-xFexO with different field cooling, and as HFC=10 kOe, the exchange bias field was the largest.3. Consecutive hysteresis loops showed that the HEB and HC decrease with magnetic field cycling, which is referred to as the training effect, it shows that there are two different reversing mechanism in the ferromagnetic phase. For NiFe2O4/NiO nanoparticle, it is found that training effect strongly depends on the cooling fields and measuring temperatures. After training, the relative change of HEB becomes more pronounced with the increase of cooling fields and measuring temperatures. This behavior is discussed in terms of the evolution of the metastable spin configurations at the interfaces, and fitted by a recent theoretical model.4. The MFe2O4/CuO(M=Co,Ni) composites were synthesized by direct two-phase composite method, and MFe2O4 by sol-gel technique and CuO by coprecipitation method. The exchange bias effect of the magnetic nanoparticle system was investigated, and the relation between different ferromagnetic phase and exchange bias effect was also studied. It is found that the exchange bias effect of CoFe2O4/CuO system is more pronounced, the TB of the system is more high.The strong magnetic property of CoFe2O4 can play a significant role in coupling between the FM/AFM interface.