The Preparation And Magnetic Properties Of LiNbO₃ Thin Films

Multiferroic materials have showed a big prospect in the devices of information storage,spintronics,magnetic sensor and capacitors-inductance integration. Magnetic-elements-doped LiNbO₃ is a promising system of multiferroic materials, which can have ferroelectric and ferromagnetic simultaneously.In this paper, the LiNbO₃/SiO₂/Si thin films has been deposited on Si(100) substrate and Si(111) substrate respectively by radio-frequency (RF) magnetron sputtering technique. The crystal orientation, composition and surface morphology of the LiNbO₃ thin films have been characterized by the X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), and scanning electron microscopy (SEM). The effect of the SiO₂ buffer layer on the c-axis orientation of LiNbO₃ film was specially and systematically investigated. Simultaneously, a LiNbO₃ thin films doped Magnetic elements(Fe,Co,Ni), which has the room temperature ferromagnetism, has also been deposited on Si(111) substrate by Co-sputtering technique based on it. The valence of the magnetic elements in the LiNbO₃ thin films and the magnetization of the LiNbO₃ thin films have been characterized by X-ray photoelectron spectroscopy(XPS) and vibrating sample magnetometer(VSM). The following results are obtained:1. When the SiO₂ buffer layer is not annealed, the lack of oxygen of SiO₂ buffer layer will be larger with the increase of the thickness of SiO₂ buffer layer. When the SiO₂ buffer layer is annealed, with the escalate of the annealing temperature of SiO₂ buffer layer, the crystallization of SiO₂ buffer layer enhanced, but the shortage of oxygen of SiO₂ buffer layer reduced.2. The c-axis orientation of LiNbO₃ thin films can be effectively improved and enhanced by adjust the crystalline quality and thickness of the SiO₂ buffer layer. With the increase of annealing temperature and thickness of SiO₂ buffer layer, when the LiNbO₃ thin films deposited on the Si (100) substrate, the c-axis orientation of LiNbO₃ thin films enhanced, while the c-axis orientation of LiNbO₃ thin films weakened, when the LiNbO₃ thin films deposited on the Si (111) substrate. On Si (111) substrate, when the thickness of the SiO₂ buffer layer is 10nm and not annealed, the LiNbO₃ thin films has the best c-axis orientation, the texture coefficient is 90%.3. Both of the Fe or Co-doped LiNbO₃ thin films show palpable ferromagnetism at 300K. It is found that Fe or Co in (Fe/Co):LiNbO₃ thin films exist as Fe³⁺ or Co²⁺ respectively. And there is not any impurity phase in the (Fe/Co):LiNbO₃ thin films, it indicates that the room temperature ferromagnetism is the intrinsic properties of (Fe/Co):LiNbO₃ thin films. The results of measurements show that Fe³⁺ in the Fe:LiNbO₃ has two occupational sites: pentahedral site and octahedral site. And the observed ferromagnetism of Fe:LiNbO₃ thin films originates from the super-exchange interactions between the Fe³⁺ in these two sites associated with the oxygen ions. And the saturation magnetization of the film weakened with the increasing of the Fe content. The Co²⁺ in Co:LiNbO₃ thin film has only on occupational site—octahedral site. The results of the magnetic measurements show that the observed ferromagnetism of Co:LiNbO₃ thin films originates from the super-exchange interactions between Co²⁺. And the saturation magnetization of the film enhanced with the increasing of the Co content.4. The Ni-doped LiNbO₃ thin films also exhibit obvious ferromagnetism at 300K. But the results of the magnetic mearsurements show that the Ni in the Ni: LiNbO₃ thin films has formed as Ni metal, and the observed ferromagnetism of Co:LiNbO₃ thin films originates from the Ni metal, but not the films.