Ferroelectric And Ferromagnetic Properties Of Nonmagnetic Elements （Ca、Ba、Cu） Doped LiNbO₃Films

Multiferroic materials as a new type of multi-functional material have attracted greatattention in recent years due to potential applications in the research field of electronics.However, the single-phase multiferroics with simultaneous ferroelectric and ferromagneticordering are rare, so people could obtain the multiferroics by doped. LiNbO3is a veryexcellent ferroelectric materials, so the transition metal elements can be doped into theLiNbO3thin films, which has ferromagnetism. But, it easily produces clusters or magneticsecondary phases, when the transition metals with traditional magnetic3d are doped inLiNbO3.Nonmagnetic elements doped in LiNbO3can avoid the disadvantages, which is idealsystem to study magnetic mechanism and origin for doped dilute magnetic semiconductor.The nonmagnetic elements Ca、Ba、Cu doped LiNbO3films were deposited on SiO2/Si(111)substrates by RF-magnetron sputtering. The composition, crystal structure, valence state andthe local structure have been characterized by the X-Ray energy dispersive spectroscopy(EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) as well as X-rayabsorption fine structure (XAFS) technique. Magnetization was studied by vibrating samplemagnetometer (VSM). Ferroelectric properties were investigated by TF Analyner2000andimpedance analyzer. In this paper, the results show that:(1) The local structural, ferroelectric and ferromagnetic of Ca-doped LiNbO3films havebeen studied. The Ca ions exhibit+2valence states and there does not exist secondary phasesin samples. SQUID results show that the Ca-doped LiNbO3films have room temperatureferromagnetism and a maximum saturation magnetization of4.8emu/cm3at the3at.%Cadoping concentration. The saturated magnetic moment of Ca-doped LiNbO3films afterannealing at different temperature increases with the decreasing of annealing temperature.Due to the increase of annealing temperature, the crystalline structure of the films will beperfect. The concentration of oxygen vacancy will decrease with the rising of temperature,therefore, the oxygen vacancy is the main factors influencing the ferromagnetism of theCa-doped LiNbO3films. The pure LiNbO3thin films (733k) is higher than the FerroelectricCurie temperature (675k) of the Ca doped LiNbO3thin film, the dielectric constant anddielectric loss decreases with the increase of doping concentration.(2) The local structural, ferroelectric and ferromagnetic of Ba-doped LiNbO3films havebeen studied. The Ba ions exhibit+2valence states and there does not exist secondary phasesin samples. By comparing experimental dopant’s L3-edge XANES spectrum of Ba-codopedLN films with the calculated spectra showed that, Ba replaces Li lattice sites and Li vacancieswere produced to compensate the charge imbalance in LiNbO3films. All Ba-doped LiNbO3films have room temperature ferromagnetism and a maximum saturation magnetization of2.2emu/cm3at the6.2at.%Ba doping concentration. The pure LiNbO3thin films (733k) is higher than the Ferroelectric Curie temperature (649k) of the Ba doped LiNbO3thin film, thedielectric constant and dielectric loss decreases with the increase of doping concentration.(3) The local structure, composition, and magnetic of the films were investigated. Bycomparing experimental dopant’s K-edge XANES spectrum of Cu-codoped LN films with thecalculated spectra showed that Cu ions substitute for Nb5+sites of the LiNbO3lattice in thevalence of+2states and Cu-related secondary phases or clusters as the source offerromagnetism is safely ruled out. SQUID measurement shows that the Cu-doped films haveroom-temperature ferromagnetism and a maximum saturation magnetization of2.4emu/cm3at the3.2at.%Cu doping concentration. It can be concluded that the ferromagnetism ofCu-doped LiNbO3films is intrinsic and can be described by the bound magnetic polaronsmechanism based on defects. The pure LiNbO3thin films (733k) is higher than theFerroelectric Curie temperature (649k) of the Ba doped LiNbO3thin film, the dielectricconstant and dielectric loss decreases with the increase of doping concentration.