Effect Of Grain Size On The Structure And Magnetic Properties Of Double Perovskite La_1.5Sr_0.5CoMnO₆

Double perovskite manganese oxides have attracted much attention in the fields of information technology and spin electronics due to their different magnetic and electrical properties.In recent years,it has been found that La_1.5Sr_0.5Co Mn O₆,as a member of the double perovskite oxide family,exhibits significant spontaneous exchange bias effects and can achieve magnetic control without external magnetic field induction,providing new research ideas for spintronics applications.At present,the research on the spontaneous exchange bias effect is still in the initial stage,the investigation of influencing factors is not comprehensive and the formation principle is not unified.Therefore,the relationship between the exchange bias effect and the grain size was studied by controlling the grain size of La_1.5Sr_0.5Co Mn O₆,and the physical mechanism was discussed.In the experiment,polycrystalline powder samples with different particle sizes（46 nm～309 nm）were successfully prepared by controlling different annealing temperatures using the sol-gel method.The crystal structure of La_1.5Sr_0.5Co Mn O₆ shows that the annealing temperature induces a phase transition:the cubic phase changes into a distorted rhombohedral phase.Raman spectroscopy analysis indicates a significant degree of disorder in the sample.The magnetic analysis shows that the decrease of particle size reduces the paramagnetic to ferromagnetic transition temperature from 228 K to 219K.The singularity of the Griffith phase is reduced,and the spontaneous exchange bias effect and the conventional exchange bias effect are changed(the exchange bias field increases with the increase of grain size.When the grain size is 309 nm,H_CEB and H_SEB reach the maximum value of 1.2 k Oe and0.4 k Oe,respectively.)The core-shell model is used to explain the reasons for the change of the exchange bias field,that is,the antiferromagnetic arrangement in the core and the exchange anisotropic coupling between the interface of the ferromagnetic clusters in the shell play a central role.In order to further investigate the effect of ferromagnetic/antiferromagnetic ratio on exchange bias effect,the effect of annealing in different atmospheres（argon,air and oxygen）on exchange bias effect is also studied.The result shows that when the sample is annealed in oxygen,the spontaneous exchange bias field and the conventional exchange bias field are the largest（1.62 k Oe and 1 k Oe,respectively）,which are analyzed to be related to the symmetry of the crystal structure and the uncompensated antiferromagnetic coupling.