Structure And Magnetic Properties Of Rare-earth Chromite GdCrO₃

Rare earth chromium oxides(RCrO₃)haves been one of research hotspots due to their unique perovskite structure and various physical properties.Also,much addition has been paid to their magnetic properties.If the rare-earth ion R³⁺in RCrO₃ is no-magnetic,the magnetism of the system mainly comes from the magnetic moment of Cr³⁺.If R³⁺is magnetic,the system will show various magnetic phenomena at low temperatures,such as negative magnetization,exchange bias and magnetocaloric effect.These magnetic phenomena are resulted from the exchange interactions between Cr³⁺and Cr³⁺,R³⁺and R³⁺or Cr³⁺and R³⁺,which maybe also affected by the crystal structure of RCrO₃.Most of the studies on rare-earth chromium oxides focus on tuning the magnetic properties through changing the preparation method or elements doping.However,the relationship between the crystal structure and the magnetic properties of rare earth oxides is rarely discussed.Hence,the mechanism of how the structural changes influence the magnetic properties is still unclear.In order to study the influence of crystal structure on the magnetic properties of rare earth chromium oxide,we take GdCrO₃ system as an example.GdCrO₃ crystal structure is altered by doping non-magnetic rare-earth ions Lu³⁺with smaller ion radius as well as changing the oxygen content during sintering process.Then the relationship between the structure and magnetic properties of rare-earth chromium oxide was studied.The main contents are as follows:Firstly,the relationship between the structure and magnetic properties of Lu_xGd_1-xCrO₃(x=0,0.1,0.3,0.5)samples was studied.All the samples were prepared by sol-gel method.The refined fitting results of XRD data show that they were well crystallized and no impurity phase could be detected.With the increase of the doping ratio of Lu³⁺,the lattice shrinks,and the bond angle of Cr³⁺-O^2--Cr³⁺decreases greatly while the bond length of Cr³⁺-O^2-increases slightly.These structural changes have a significant impact on the magnetic properties of the system:the shrinkages of lattice and Cr³⁺-O^2--Cr³⁺bond angle weakens Cr³⁺-O^2--Cr³⁺superexchange interaction resulting in the decrease of Neel temperature(T_N),meanwhile,it also resulting in an enhancement of ferromagnetism arised from canted Cr³⁺spins below T_N,as shown by the increase of Cr³⁺moments(M_Cr).The contraction of Cr³⁺-O^2--Cr³⁺bond angle also strengthens the internal field H_I exerted by M_Cr on Gd³⁺magnetic moment M_Gd,which is opposite to the external field,therefore the value of negative magnetization of the system in the low-temperature region increases with the decrease of Cr³⁺-O^2--Cr³⁺bond angle.Furthermore,these structural changes also have influence on the variation of remanence(M_r)and coercivity(H_C)below the compensation temperature T_comp.M_r and the temperature corresponding to the maximum and minimum value of H_C decreases with the shrinkage of lattice and the decrease of Cr³⁺-O^2--Cr³⁺bond angle,while the maximum value of H_C increases gradually.Secondly,the relationship between the structure and magnetic properties of GdCrO₃samples was studied.All the samples were prepared by the sol-gel method under different sintering atmospheres(0.1 MPa N₂,Air,0.1 MPaO₂,0.3 MPaO₂).The refined fitting results of XRD data show that all the samples were well-crystallized,and the samples sintered under higher oxygen atmosphere have smaller lattice volume and bond angle of Cr³⁺-O^2--Cr³⁺,which could be attributed to the reduction of oxygen vacancy.Though the shrinkages of lattice and Cr³⁺-O^2--Cr³⁺bond angle can weaken the superexchange of Cr³⁺-O^2--Cr³⁺,however,the decrease of oxygen vacancy also leads to the increase of Cr³⁺-O^2--Cr³⁺networks in the system and enhances the superexchange of Cr³⁺-O^2--Cr³⁺.As a result of the competition between bond angle and oxygen vacancy,T_N fluctuates slightly.The M-T curve fitting results further prove that M_Cr and H_I increase with the decrease of Cr³⁺-O^2--Cr³⁺bond angle.The analysis of the initial magnetization curves of the samples at different temperatures shows that the decrease of the Cr³⁺-O^2--Cr³⁺bond angle enhances M_Cr,which makes the magnetization at low temperature(5 K)increase.The calculation results show that the magnetic entropy change(?S_M)and the cooling capacity(RCP)of the samples sintered in high oxygen atmosphere will obviously increase under low temperature and strong magnetic field,which indicates that the decrease of oxygen vacancy will effectively enhance the magnetocaloric effect of GdCrO₃ system.