Study On The Regulation Of Magnetism And Magnetic Exchange Effect In Spinel Of Co₂MO₄ （M Denotes Other Metal）

The Co-based oxides with spinel structure,with nominal component Co₂MO₄（M denotes other metal）,contain two sets of magnetic sublattices i.e.,A and B,which refers to oxygen tetrahedron and octahedron as the basic structural unit,repectivily.Due to the different magnetic exchange interactions between magnetic ions sited in both inter-and intra-sublattices of A and B,the Co-based spinels show rich magnetic characteristics.In this dissertation,based on the non-magnetic and/or magnetic ion doping at Sn site in Co₂SnO₄ system,the effects on the magnetic perporty,low-temperature spin-glass state behavior and the resulting magnetic exchange bias effect have been systematically studied and analyzed.Furthermore,it has been experimentally studied and analyzed for the doping effect for Y at Mn site on magnetism and the resulting exchange spring effect in Co₂Mn O₄ system.The main work and results are as follows:（1）For single-phase spinel Co₂SnO₄ samples,it have been experimental studied that the magnetic susceptibilityχ_FC（T）andχ_ZFC（T）,which is measured respectively in field cooling（FC）and zero field cooling（ZFC）processes under different magnetic fields,and the magnetization hysteresis loops carried out in different given temperature.The results ofχ_FC（T）andχ_ZFC（T）measurements showed that the sample undergoes the transition from high-temperature paramagnetic state to low-temperature ferrimagnetic state near T_C～42 K.As the temperature decreases below a certain temperature T_B,a“spin-glass state”emerged in the sample with the freezing behavior.The temperatures corresponding to the peaks of ac susceptibility moved to the higher temperature with the increasing in the frequency,which provides a strong experimental evidence for the formation of spin-glass phase below T_B.With the formation of spin-glass phase,a series of anomalies appear in the low-temperature magnetism of Co₂SnO₄ system,such as negative ZFC susceptibility at low temperature,ZFC susceptibility changes from negative to positive with the increase of magnetic field,and the magnetic loop changes from the hysteresis loop,at higher temperature lower than T_C,to a straight line with zero loop area at low temperature.The quantitative analysis based on Néel molecular field theory showd that the exchange interactions between magnetic ions inter-and intra-sublattices of A and B are antiferromagnetic in nature.It is believed that the complex magnetic properities at low temperature in Co₂SnO₄ system is related to the complex magnetic exchange and the formation of spin-glass state at low temperature.（2）Based on the single-phase Co₂SnO₄ sample,it has been discussed that the doping effect of the trivalent non-magnetic metal ion T³⁺（where T=Al,Sc and Y）at Sn site on the magnetic properties.The results showed that the doping of trivalent non-magnetic ions contributes to the improvement of the para-ferrimagnetic transition temperature T_C,the coercive field H_C and the residual magnetization M_Rbelow T_C,thus enhancing the overall magnetism at low temperature.We explain that this improvement attributed to the doping of trivalent non-magnetic metal ions makes the part of Co²⁺ions in B lattice turned into Co³⁺ions.This change in the valence state of cobalt ions not only increases the inherent magnetic moment of ions,but also enhanced the exchange interactions within B-B sublattices and between B-A sublattice.Based on the measurement and analysis of both dc and ac susceptibility,the dynamic behavior of spin-glass state at low temperature has been discussed.The results showed that,on the one hand,the T³⁺doping increased the freezing temperature of spin-glass state,and on the other hand,was benefit to grow the volume of spin-glass phase.The aforementioned growth tends to conducive to increasing the effective contact interface between ferrimagnetic state and spin-glass state.Using the pinning effect of the surface spins in the spin-glass phase on the spins of the ferrimagnetic phase at phase boundary,the obvious magnetic exchange bias effect at low temperature（T<20 K）is realized in the doped samples.Owing to the magnetic exchange bias effect,the system greatly increases the magnetism at low temperature.For instance,the coercivity field H_C at 10 K increased from ablout 0for x=0 sample to～2.1,1.3 and 2.9 k Oe for Al-,Sc-and Y-doped samples,respectively.（3）Based on the single-phase Co₂SnO₄ sample,it has been discussed that the doping effect of Fe at Sn sites on the magnetic propertie.The results ofχ_FC（T）andχ_ZFC（T）measurements showed that,in the doping ranges of x≤0.3,Fe doping induced the obvious increase of the magnetic transition temperature and the ferromagnetism,however,the dependence of magnetic behavior on temperature shows a behavior similar to that of undoped samples.The analysis of Curie-Weiss fitting,based on high temperature susceptibility,showed that with the increase of Fe doping,the dominated magnetic exchange in the system changes from antiferromagnetic exchange,x<0.2,to ferromagnetic exchange,x>0.2,and this change leading to the hysteresis loop characteristics of low Fe-doped samples are obviously different from those of Fe doped samples with higher doping content.The ac susceptibility measurements showed that the spin-glass state can be observed in low Fe-doped content samples ranges in x≤0.3 at low temperature,but for high Fe-doped content samples where x=0.4,the spin glass state basically disappears at low temperature.Based on the pinning effect of the frozen spins in the spin-glass phase on the ferrimagnetic phase on the surface of the phase boundary,a significantly enhanced exchange bias effect can be observed in the Fe-doped samples with low doping amount at low temperature,where the strongest exchange bias effect appears in the x=0.2 sample.It is considered that this strongest exchange bias effect could be related to the concurrence of the strong antiferro-and ferromagnetic exchange interactions in the sample.In addition,taking the x=0.2sample as an example,the experimental results are qualitatively and quantitatively analyzed on the effects of cooling field and exercise effect on exchange bias effect are also experimentally studied.（4）Based on the single-phase Co₂SnO₄ sample,it has been discussed that the doping effect of Cr at Sn sites on the magnetic propertie.The results of magnetic susceptibility showed that with the Cr doping at Sn site,the macro magnetism of the sample can be enhanced.The analysis of ferrimagnetic Curie-Weiss fitting showed that the dominant interaction in the sample gradually changes from the inter-sublattice interaction between A and B,for undoped sample,to the interaction within B sublattices in the sample.The ac susceptibility experiments showed that all the Cr-doped samples being characterised by spin-glass freezing behavior at low temperature,and with the increase of Cr doping,there is a trend of transition to the cluster spin-glass in the samples.Particularly,a spontaneous exchange bias effect can be observed in Cr-doped samples,which becomes gradually significant with the increase of Cr doping.Based on the experimental study and theoretical analysis of the hysteresis phenomenon of the initial magnetization curves,this spontaneous exchange bias effect can be attributed to the field-induced super-ferromagnetic phase in Cr doped samples.（5）Considering the coexistence of"soft/hard"ferromagnetic phase at low temperature in the Y-dopped Co₂SnO₄ system,the magnetic Mn ion is used to replace the non-magnetic Sn ion,and consequencely it has been carried out that the Y doping effect on the magnetic properities at Mn site in Co₂Mn O₄.Magnetic susceptibility measurement and analysis showed that in addition to the para-ferrimagnetic phase transition near the temperature T_C,there is another phase transition near a certain temperature T^*below T_C of Y-doped samples.Fuerthermore,the magnetic entropy change analysis also confirmed the magnetic phase transition near the temperature of T^*.With the phase transition near T^*,Y-doped samples show magnetic anomalies at low temperature.Based on the analysis of magnetic entropy change and the magnetic hysteresis loops,it is considered that Y-doped samples show the coexistence of"soft"and"hard"ferromagnetic phases at low temperature.Using the coupling of the"soft/hard"ferromagnetic phase boundary,an obvious exchange spring effect can be observed at low temperature.Due to the exchange spring effect,the magnetic energy product（BH）_max up to 3.1 MJ·m^-3 can be obtained at 2 K.