The Research Of Magnetism And Magnetocaloric Effects In Rare Earth Transition Metal Oxides

Magnetic refrigeration technology based on the magnetocaloric effect（MCE）has been considered as one of the new solid refrigeration technologies,which can potentially replace the widely used gas compression refrigeration technology due to its advantages of green environmental protection,high efficiency and energy saving,stability,and reliability.The design and development of high-performance magnetic refrigeration materials is one of the key factors for the practical application of magnetic refrigeration technology.Recent studies have shown that some rare earth transition metal oxides have excellent magnetocaloric effect properties.Thus,in this paper,rare earth double perovskite oxides RE₂CuTiO₆（RE=Gd,Dy,Ho,Er）,rare earth transition metal oxides Dy₂BaCu_1-xZn_xO₅（x=0.25,0.5,0.75）and Er₂BaNi_1-xTM_xO₅（x=0.33,0.67;TM=Cu,Zn）were studied as the research objects,and the magnetic phase transition types of rare earth transition metal oxide materials were adjusted by doping different transition metal elements,and the crystal structure,microstructure,magnetic phase transition,magnetic and magnetocaloric properties of the presentation materials were systematically studied.The main obtained findings are as follows:The investigation on the crystal structure of the rare earth double perovskite oxides RE₂CuTiO₆（RE=Gd,Dy,Ho,Er）illustrates that Gd₂CuTiO₆ oxide crystallizes in monoclinic crystal system（space group P2₁/c）,whereas Dy₂CuTiO₆,Ho₂CuTiO₆,and Er₂CuTiO₆crystallize in hexagonal crystal system（space group P（?）2）.RE₂CuTiO₆（RE=Gd,Dy,Ho,Er）oxides undergo a second-order magnetic phase transition from paramagnetic to ferromagnetic,and the magnetic phase transition temperatures are estimated to be 2.4 K,2.7 K,2.2 K,and 2.2 K,respectively.Under the applied magnetic field of 0-7 T,the maximum values of magnetic entropy change(-ΔS_M^max)of Gd₂CuTiO₆,Dy₂CuTiO₆,Ho₂CuTiO₆,and Er₂CuTiO₆are 48.3,22.2,15.2,and 16.0 J/kg K,respectively.The values of temperature averaged entropy change[TEC（5 K）]are43.0,20.9,15.0,and 15.6 J/kg,respectively.The corresponding values of the relative cooling capacity（RCP）and refrigeration capacity（RC）are 477.0 and 359.2 J/kg,462.9and 354.9 J/kg,442.5 and 344.0,as well as 392.0 and 225.4 J/kg,respectively.The present results illustrate that RE₂CuTiO₆ oxides have excellent magnetocaloric properties in the low-temperature region,especially Gd₂CuTiO₆,which has great application potential in the field of low-temperature magnetic refrigeration.The crystal structure of Dy₂BaCu_1-xZn_xO₅（x=0.25,0.5,0.75）oxides belong to the orthogonal crystal with the space group Pnma.The magnetic measurement results show that Dy₂BaCu_0.75Zn_0.25O₅and Dy₂BaCu_0.5Zn_0.5O₅ undergo a first-order magnetic phase transition from paramagnetic to antiferromagnetic,while Dy₂BaCu_0.25Zn_0.75O₅ oxide undergoes a second order magnetic phase transition from paramagnetic to ferromagnetic.The magnetic phase transition temperatures are 12.1 K,9.8 K,and 2.1K for Dy₂BaCu_0.75Zn_0.25O₅,Dy₂BaCu_0.5Zn_0.5O₅,and Dy₂BaCu_0.25Zn_0.75O₅,respectively.With the increasing amount of Zn incorporation,Dy₂BaCu_1-xZn_xO₅（x=0.25,0.5,0.75）oxides will undergo the process from antiferromagnetic to ferromagnetic at low temperature,which is an effective method to adjust the magnetic phase transition of this series of oxides.Under the magnetic field of 0-7 T,the values of-ΔS_M^maxand TEC（5K）are calculated to be 8.5 and 8.4 J/kg K,9.7 and 9.3 J/kg K,as well as 13.2 and 13.0J/kg K for Dy₂BaCu_0.75Zn_0.25O₅,Dy₂BaCu_0.5Zn_0.5O₅,and Dy₂BaCu_0.25Zn_0.75O₅,respectively.The corresponding values of RCP and RC are 227.4 and 178.0 J/kg,283.6and 213.6 J/kg,as well as 343.0 and 270.5 J/kg,respectively.The Er₂BaNi_1-xTM_xO₅（x=1/3,2/3;TM=Cu,Zn）oxides crystallize in the orthogonal crystal with space group Pnma that is different from the crystal structure of Er₂BaNiO₅,which indicates the doping of Cu and Zn elements can change the crystal structure.The magnetic measurement results show that all four oxides undergo a second-order magnetic phase transition from paramagnetic to ferromagnetic,among which the magnetic phase transition temperature of Er₂BaNi_0.67Cu_0.33O₅,Er₂BaNi_0.67Zn_0.33O₅,and Er₂BaNi_0.33Zn_0.67O₅oxides are all about 2.4 K,and the magnetic phase transition temperature of Er₂BaNi_0.33Cu_0.67O₅ oxide may be less than 2K.Under the magnetic field of 0-7 T,the values of-ΔS_M^maxand TEC（5 K）are 7.8 and7.6 J/kg K,9.6 and 9.5 J/kg K,10.5 and 10.4 J/kg K,12.6 and 12.3 J/kg K for Er₂BaNi_0.67Cu_0.33O₅,Er₂BaNi_0.33Cu_0.67O₅,Er₂BaNi_0.67Zn_0.33O₅,and Er₂BaNi_0.33Zn_0.67O₅,respectively.Accordingly,the values of RCP and RC are 251.9 and 199.8 J/kg,266.3and 206.0 J/kg,respectively.276.5 and 215.7 J/kg,268.7 and 207.0 J/kg,respectively.