Structure,Magnetism And Magnetocaloric Effect Of Perovskite Oxides RFe_1-xMn_xO₃ And RBaCuFeO₅

Perovskite structure oxides are very versatile ceramic materials,and they possess some interesting chemical and physical properties.Perovskite RFe_1-xMn_xO₃(R=rare earth element)materials have won more attention due to the complex magnetic interaction between rare earth ions and Fe/Mnions.In recent years,the research on polycrystalline or single crystal of this series of materials mainly involves the changes of magnetic structure and spin reorientation,ferroelectricity and magnetic dielectric effect.The spin reorientation and magnetocaloric effects of some rare earth elements are still worthy of in-depth study.Part of the content in this paper mainly focuses on the magnetic phase transition and magnetocaloric effects of single perovskite oxide RFe_1-xMn_xO₃(R=Gd,Tb,Dy,Ho and Tm).In addition,the double perovskite oxide YBaCuFeO₅ with oxygen deficiencies is the second type of multiferroic compound,due to its special spiral magnetic structure,electric polarization occurs at relatively high temperatures.Therefore,it is still very meaningful to further increase this transition temperature.Another part of this article is devoted to replacing Ba ions with a small amount of Ca ions and replacing Y ions with rare earth Gdand Ho in YBaCuFeO₅ material,and to study the influence of doping on their transition temperature,dielectric properties and magnetocaloric effect.It is hoped that while carrying forward the advantages of the material itself,we will dig into its new physical phenomena in order to realize the versatility in the application of material technology.The main innovative findings of this paper are divided into the following three points:1.By measuring the thermomagnetic curves of high-quality GdFe_1-xMn_xO₃(0?x?0.3)single crystals in different directions,it is more intuitive to find that compared with the parent oxide GdFeO₃,Mn³⁺ions with larger magnetic anisotropy are introduced,breaking the balance of original magnetic interaction,when the Mndoping amount is 0.2,the spin orientation transition from?₄ to?₁ occurs at a temperature of?150 K.Moreover,this transition temperature increased to 220 K with the further increase of Mndoping content to 0.3,and the spin reorientation transition temperature remained stable under a magnetic field of 10 kOe.The study of the low-temperature magnetocaloric effect found that when an external magnetic field of70 kOe is applied,although the introduction of Mnmakes the maximum magnetic entropy change value of GdFe_0.7Mn_0.3O₃ slightly lower than that of GdFeO₃,it is also as high as 34.2 J/kg K under a 70 kOe magnetic field.Therefore,GdFe_0.7Mn_0.3O₃single crystal can become a multifunctional material that not only has a high magnetic phase transition temperature,but also has a large magnetic entropy change at extremely low temperatures.2.Based on the above research results,the spin reorientation temperature in the Mnion-doped orthoferrite can be adjusted by adjusting the Mnion content.When the heavy rare earth ions R=Gd,Tb,Dy and Ho,the parent materials RMnO₃ and RFeO₃at both ends have the same lattice structure.Studies have shown that the lattice symmetry of the four perovskite oxides RFe_0.5Mn_0.5O₃(R=Gd,Tb,Dy and Ho)are all orthorhombic structure.In the experiment,it was found that the antiferromagnetic order temperature of all samples was close to room temperature.At the same time,three compounds other than Ho Fe_0.5Mn_0.5O₃ also exhibit spin reorientation behavior near room temperature.Then we calculated the low-temperature magnetic entropy changes of RFe_0.5Mn_0.5O₃(R=Gd,Tb,Dy and Ho),and obtained the maximum entropy changes of the four oxides in a 70 kOe magnetic field of 23.3 J/kg K,9.3 J/kg K,9.5 J/kg K and 12.6 J/kg K.For Tm FeO₃ and Tm MnO₃ which are orthorhombic and hexagonal structures,respectively.We further studied the influence of Mnion doping on the polycrystalline lattice structure,magnetic properties and low-temperature magnetocaloric effects of Tm FeO₃.X-ray powder diffraction technology proved that Mnion doping does not change the lattice structure of Tm FeO₃.The study of the magnetic properties after doping found that the gradual introduction of Mnions into the Fesite can effectively adjust the magnetic phase transition of Tm FeO₃.It is mainly reflected in the spin reorientation temperature range from the original 90.3 K-73.2 K temperature range of the Tm FeO₃ sample to the 180.0K-156.0 K temperature range of the Tm Fe_0.7Mn_0.3O₃ sample.In addition,under a magnetic field of 70 kOe,the maximum magnetic entropy change values of the Tm Fe_1-xMn_xO₃(x?0.3)samples are calculated to be 6.29 J/kg K,6.56 J/kg K,6.79J/kg K and 7.22 J/kg,respectively.Our experimental results can provide a valuable reference for the development of RFe_1-xMn_xO₃(R=rare earth ion)as a multifunctional material.3.For the multiferroic double perovskite compound YBaCuFeO₅ with oxygen defects,its magnetic and dielectric transition temperature is higher than 200 K.The magnetic phase transition temperature of the compound can be adjusted not only by changing the distortion of the material or chemical pressure,but also by replacing the non-magnetic element at the Ba position or replacing Y with other rare earth ions.We replaced Ba ions with a small amount of Ca ions,and through the magnetic measurement data of YBa_1-xCa_xCuFeO₅(x=0,0.025,0.05)polycrystalline samples,we find that the phase transition temperature of the spiral antiferromagnetic structure to the collinear antiferromagnetic structure is increased from 206 K to 301 K,but the temperature of dielectric anomalies decreased.These results indicate that the magnetic phase transition and dielectric transition of YBa_1-xCa_xCuFeO₅(x=0,0.025,0.05)samples are not related.When the heavy rare earth ions Gdand Ho are used to replace Y ions,the DC magnetic measurement results show that there is no magnetic phase change.The study of the magnetocaloric effect of RBaCuFeO₅(R=Gdand Ho)found that GdBaCuFeO₅ has a large magnetic entropy change at low temperatures.