Magnetic Properties Of Ion Doped RFeO₃ Single-phase Multiferroic Materials

Rare-earth ferrites have got a lot of attention recently because of their distinctive physical features,such as single-phase multiferroicity,magnetoelectric coupling and the magnetocaloric effect of double perovskite oxides at ambient temperature.Single-phase multiferroic materials based on geometric ferroelectric hexagonal LuFeO₃（h-LuFeO₃）demonstrated a high magnetoelectric coupling effect.However,its magnetic phase transition temperature was still too low for practical application.The double perovskite Sr₂Fe Mo O₆ exhibited a higher magnetic exchange interaction than ordinary perovskite because of the ordering of Fe/Mo ions at B-site,showing rich magnetic functional properties.However,the effect of doping regulation on the ordering degree of double perovskite materials on magnetic functional properties remains to be studied.Therefore,this thesis will take hexagonal LuFeO₃ single-phase multiferroic materials,Sr₂Fe Mo O₆ double perovskite oxides as the research object,carry out research on the magnetoelectric coupling effect,magnetic functional properties.The details of research were as follows:Firstly,the magnetic phase transition temperature of h-LuFeO₃ materials was regulated by ion doping.On the one hand,the effect of In³⁺doping with small ionic radius on the structure and magnetism of hexagonal(Lu_1-xIn_x)_0.5Sc_0.5Fe O₃ series samples was studied.The experimental results demonstrate that all samples have antiferromagnetic transitions near 163K and depict a dielectric anomaly,implying a magnetoelectric coupling effect;The magnetic transformation temperature of the system is not related to the lattice parameter c/a completely,but may be related to the inclination angle of the Fe O₅ by K₃ structural distortion.Above the antiferromagnetic transition temperature T_N,a magnetic anomaly temperature T_A was discovered,and T_A can be adjusted to rise to room temperature with the increase of doping amount.On the other hand,we studied the effect of(Zn²⁺,Sn⁴⁺)co-doping on the structure and magnetic properties of hexagonal Lu_0.5[In_1-x(Zn_0.5Sn_0.5)_x]_0.5Fe O₃（0≤x≤0.5）samples,and found that the K₃ structural distortion and magnetic phase transition temperature decreased with the increase of the co-doping amount.decreases.It was attributed to the fact that with the increase of the doping amount x of the co-doping system,the lattice parameter a increased,the c/a decreased,the interaction between Fe ions weakened,and the T_N decreased gradually.In addition,we studied metamagnetic transition of hexagonal(Lu_1-xIn_x)_0.5Sc_0.5Fe O₃（0.1≤x≤0.3）series samples x=0.1,x=0.2 and x=0.3 samples with magnetic transformation in the range of 40-150 K,60-150 K and 80-150 K.According to the M-T curves,it can be inferred that the magnetic properties of the samples ought to be associated to the magnetic structure at the temperature range T_SR and T_N,which may be derived from the magnetic structure transition from B₂ to A₂.Under different doping concentration,the critical field and temperature of transition are different.The maximum critical field moves towards high temperature with the increase of doping content,which was consistent with T_SR.It can be seen that the magnetic structure of the system above T_SR can be affected by the magnetic field.Finally,due to the fact the rare earth ions play a fundamental role in the structure and magnetic interaction of double perovskites,heavy rare earths Ho³⁺was selected to dope the A-site of double perovskites.It was found that in Sr_2-xHo_xFe Mo O₆,the x=0.1 sample obtained a maximum negative magnetic entropy of 0.4631 J·kg^-1·K^-1 and a maximum relative cooling power of 37.5 J·kg^-1 near the room temperature（313 K）and underneath 2 T magnetic fields.In the range of 0≤x≤0.1,magnetocaloric effect also increase with the increasing Ho doping.Our results suggested that the oxide was a suitable candidate for exploring novel working substance in magnetic refrigeration.