The Effect Of Ca Doping At A-site On Magnetism In Perovskite Chromium-based Oxides ReCrO₃（Re=La,Y）

The perovskite rare earth chromium-based oxide Re CrO₃ has become a research hotspot,due to its unique physical properties and great application prospects in functional materials.Previous studies on chromium-based oxidation have focused on its catalytic properties and electrical transport properties at high temperatures,but there is no systematic study of its magnetic properties.In this paper,we selected the LaCrO₃ and YCrO₃ as the research object and doped with divalent Ca ions in their A-site.The effect of Ca doping on crystal structure and magnetic properties was studied by adjusting the doping content,trying to explore the mechanism of magnetic changes in the system.Details are as follows:In chapter one:We introduced some basic concepts related to the perovskite chromium-based oxide by beginning from the basic structure,properties and preparation methods.Meanwhile,the recent developments and studys on the basic structure and properties of chromium-based oxide have been reviewed and discussed.In addition,the basic concepts and theories of magnetism involved in the study are introduced.In chapter two:Describes the sample preparation methods,experimental reagents,experimental equipment,sample analysis methods involved in the experimental process.In chapter three:Firstly,Samples of La CrO₃ and La_0.67Ca_0.33CrO₃ prepared at different sintering temperatures were selected as the research object,and the optimum sintering temperature for the preparation of La_1-x-x Ca_xCrO₃（0≦x≦0.5）samples were 1400℃.Subsequently,a series of La_1-xCa_x CrO₃（0≦x≦0.5）samples with different doping levels were prepared by sintering at 1400℃using a solid-state reaction method.It is found that,the crystal structure and space group of La_1-xCa_xCrO₃ samples are the same as the matrix La CrO₃,and the lattice parameters a,b,c gradually decrease with increasing doping content.Doping Ca²⁺in the A-site of LaCrO₃ will result in a mixed state of Cr³⁺/Cr⁴⁺ions in the system.At this time,the system will not only have a double exchange effect which from Cr³⁺-O-Cr⁴⁺,but also have a super-exchange effect which frome Cr³⁺-Cr³⁺and Cr⁴⁺-Cr⁴⁺.The double exchange effect and the super exchange effect compete with each other and lead to antiferromagnetic and ferromagnetic coexistence in the system.Magnetic test results show that LaCrO3 is a G type antiferromagnet.0<x<0.15,the system coexists with the ferromagnetic phase and the antiferromagnetic phase below the T_C.As the doping amount increases,the sample changes from G type antiferromagnetic to cant antiferromagnetic.0.15≦x≦0.2,the system is an cant antiferromagnetic state,and the ferromagnetic phase below the T_C temperature dominates.0.2<x<0.4,the system coexists with the ferromagnetic phase and the antiferromagnetic phase below the T_C.The sample changes from cant antiferromagnetic to A type antiferromagnetic.0.4≦x≦0.5,the sample is A-type antiferromagnetic structure,and the system has antiferromagnetic phase below the T_N.x≧0.33,the system exhibit spin glass state at low temperature.These phenomenon can be attributed to the strong competition between the ferromagnetic phase and the antiferromagnetic phase that destroys the long-range magnetic ordering of the system.In addition,we have drawn a simple magnetic phase diagram of La_1-xCa_xCrO₃（0≦x≦0.5）.In chapter four:In this work,we study YCrO₃ in a similar way to the third chapter.First,the optimum temperature for the preparation of Y_1-x-x Ca_xCrO₃ in solid-state reaction was determined as 1400℃.Subsequently,a series of Y_1-xCa_xCrO₃（0≦x≦0.5）samples with different doping levels were prepared by sintering at 1400℃using a solid-state reaction method.The crystal structure and space group of Y_1-xCa_xCrO₃ samples are the same as the matrix YCrO₃.However,when x≧0.2,the impurity phase CaCrO₄ appears.Same as La_1-xCaxCrO₃ system,doping Ca²⁺in the A-site of YCrO₃ will lead to the compete between double exchange effect and the super exchange effect.But YCrO₃ is cant antiferromagnetic,the antiferromagnetic and ferromagnetic coexistence in the system.In addition,we have drawn a magnetic phase diagram of Y_1-xCa_xCrO₃（0≦x≦0.5）.