Research On Preparation And Magnetic Properties Of CaB₆ Thin Films

Alkaline-earth boride calcium hexaboride has the excellent property of high hardness,high strength and high chemical stability because of their structure characteristics.In addition,it still has low electron work function and high neutron absorption rate,which make it widely used in many fields.The boron and calcium in CaB₆ don’t have unfilled shells of 3d,4f and 5f,which Is different with the traditional ferromagnetic materials,But,the research has found that the CaB₆ doped with La has high temperature ferromagnetism.This discovery doesn’t only make the research of CaB₆ become a hot spot,but also provide the research direction for the application of electron spin components.There are many theories explaining the origin of the ferromagnetism,and some theoretical models（low density electron gas,deficit theory）are already presented.However,the researches on the structure and magnetism of CaB₆ film are scanty,especially on the stoichiometric CaB₆ film,and it indicates the necessity of making relevant research.In this paper,different composition CaB₆ films with different atomic ratio of boride and calcium are prepared by DC magnetron sputtering,and the targets with different atomic ratio of boron and calcium are sintered to get the stoichiometric CaB₆ films.When the rest parameters remain constant,this paper researches the effect of the substrate temperature and annealing temperature on structure and morphology of films,and magnetic properties of CaB₆ films,discovers the change rule of magnetism,and discusses the origin of magnetism.X-Ray photoelectron spectroscopy（XPS）is used to test the CaB₆ films before annealing,and the result shows that there are two characteristic peaks of Ca 2p in films and the characteristic peak of cubic B in 188.5eV,which indicates there are B-Ca atomic group.The result of X-ray diffraction shows the existence of crystallite,and the two kinds of results indicate that there is a certain degree of crystallinity in the CaB₆ films prepared by direct current magnetron sputtering.In order to improve the films5 crystallinity,different temperatures（5000C,600℃,700℃,800℃）,according to the result of DSC,are used to heat the films prepared at the room temperature,and the films’ morphology changes as temperature changes.The increase of crystallization temperatures have a strong influence to the growth process of grains,and two kinds of B-rich films all present the maximum surface roughness at 700℃,which relates to atomic migration,nucleus formation,grains coalescence.In order to know the growth process,the work about how the substrate temperatures affect the films prepared DC magnetron sputtering is done,and analyzing the structure of films prepared at different substrate temperature（500℃,600℃,700℃,800℃）,discovers the growth pattern of films with substrate temperature consists with the annealed films.However,substrate temperature has more advantage impact on the growth of thin films,because deposited atoms directly contact with high temperature substrate and obtain lots of energy,which promotes the migration rate and makes the surface roughness decrease.As the temperature increases,the films’ magnetron has a trend that it decreases,increases and decreases again,and that is related to films internal structure.The defects have an immediate effect on magnetism in films,and the B6 vacancies have the greatest impact on magnetism.In the films,when the number of B6 vacancy is large,the magnetism would increase,but when vacancy and defect density are large,the magnetism from them is small,so the films’ maximum saturation magnetic moment would go down.The effect of temperature on CaB₆ thin films’ magnetic moment is mainly dependent on the density of B6 vacancies and other defects.