Exchange Bias Effect In Ferrimagnetic/Antiferromagnetic Oxide System

With rapid development of spintronic devices,the storage density of magnetic memory devices is greatly improved,which further makes the magnetoelectric devices miniaturization.Meanwhile,with reduction of magnetic device scale,particle size of magnetic materials is also reduced.When ferromagnetic particle size exceeds the critical value,magnetic materials will show the superparamagnetic effect,which can result in the losing of magnetic storage function.Exchange bias effect has attracted wide atttention because it can overcome the superparamagnetic behavior in nanoscale materials.In addition,exchange bias effect is a unique physical phenomenon,which also possesses very important scientific significance.At present,although more and more materials present the exchange bias effect,the research systems still have some limitations.The study of exchange bias effect in ferrimagnetic chromium oxides is more scarce.At the same time,the complex physical mechanisms of exchange bias effect in ferrimagnetic chromium oxides also need to be explored.Based on the above problems,this paper is focused on the exchange bias effect of ferrimagnetic chromium oxides.The main research results and innovations are as follows:(1)NiCr2O4/Cr2O3 sample with fine interfacial contact and homogeneous distribution is successfully synthesized by chemical coprecipitation and high temperature annealing processes based on the phase segregation thought.Experimental results show that the sample exhibits significant exchange bias effect at low temperature.Moreover,the exchange bias effect strongly depends on the measurement temperature and the applied cooling field.With the given cooling field,exchange bias effect is weakening with the increase of measurement temperature and finally disappears near 70 K.Meanwhile,the exchange bias effect is greatly enhanced with the increase of cooling field when the measurement temperature is stable.What is more,exchange bias effect tends to be saturated when the cooling field is more than 10 kOe.On the basis of experiment analysis,it is considered that the observed exchange bias effect is due to the pinning effect of frozen antiferromagnetic uncompensated spins on the ferrimagnetic spins at interfaces.At the same time,through measurement and analysis of training effect,it is confirmed that the emergence of training effect is attributed to the athermal and thermal activation mechanisms.(2)CoCr2O4/NiO nanoparticle system with smaller CoCr2O4 particles and larger NiO particles is directly formed as homogeneous mixing through high-energy ball milling.The results suggestion that CoCr2O4/NiO nanoparticle system exhibits an observable exchange bias effect at low temperature.At 10 K,the exchange bias field HEB and longitudinal magnetization shift Mshift are respectively 872 Oe and 0.074 emu/g.As temperature increases,the exchange bias effect gradually weakens and finally disappears near 90 K.(3)NiCr2O4/NiO system with well interfacial contact and homogeneous distribution is successfully prepared by chemical coprecipitation and high temperature annealing treatment based on phase segregation idea.Experimental results exhibit that the exchange bias effect is obviously presented at low temperature.With measurement temperature of 10 K,HEB and Mshift are respectively 11.86 kOe and 0.7 emu/g,which are much higher than those values of NiCr2O4/Cr2O3 under the same conditions.In addition,the influence of measurement temperature,cooling field and cycle number on exchange bias effect is distinctly different from NiCr2O4/Cr2O3.Moreover,the existence of spin-glass-like phase is proved by different methods.Based on the pinning interaction between frozen spins and reversible spins in spin-glass-like phase,the influence of temperature,cooling field and cycle number on exchange bias effect is analyzed and explained detailedly.These unusual exchange bias observations in NiCr2O4/NiO can be attributed to the formation of spin-glass-like phase at interfaces in the sample,which increases the area of spin frustration.What is more,the pinning force between frozen spins and reversible spins is stronger,which results in the significantly enhanced exchange bias effect in NiCr2O4/NiO.(4)Magnetic behavior,exchange bias effect,and the existence of spin-glass-like phase in NiCr1.9Mn0.1O4 single phase sample are respectively measured and analyzed systematically.The results show that,NiCr1.9Mn0.1O4 displays a similar exchange bias effect to that of NiCr2O4/NiO.The A-T law,high field relaxation,influence tendency of temperature and cooling field on HEB and Mshift,and the impact of training effect on HEB and Mshift are respectively researched.Based on the above measurement and analysis,it is considered that exchange bias effect in single phase NiCr1.9Mn0.1O4 sample comes from the introduction of Mn in Cr sites,which changes the original coupling between ferromagnetic and antiferromagnetic interaction of NiCr2O4.The variation of coupling interaction leads to the spin frustration and the spin-glass-like phase,which finally gives rise to the similar exchange bias effect to that of NiCr2O4lNiO.