Preparation, Magnetism, Damping Factor And Stress-driven Magnetization Variation Of Single Crystal CrO₂ Films

With the development of electronic information technology, microelectronic devices turn to be miniaturization, integration and low power consumption. But with the decreasing of the device size, the electrical function of the material will be under restrictions. Therefore, the introduction of multifunctions in the material is so needed. CrO2 is a half-metallic material with almost 100% electron spin polarization, which can meet the functional requirements of basic spintronics devices. Our work focuses on the basic issue of CrO2 film in the magnetic information storage.Our work is divided into three parts:the preparation and characterization of CrO2 film, the damping factor of CrO2 film with different thicknesses and substrates, and the stress control of the magnetization switching of CrO2 film both in-plane and out of plane. The main results are as follows:1.The epitaxial CrO2 single crystal films were fabricated on single crystal TiO2 substrates under 380℃ by chemical vapor deposition method (CVD), because the single crystal rutile TiO2 has almost the same lattice constant with CrO2. The polycrystalline CrO2 particles were obtained on single crystal Si substrate, glass and polycrystal TiO2 film substrate. The morphology, crystal structure and magnetic properties of the CrO2 single crystal films with different thicknesses were studied.(a). If the single crystal TiO2 substrate with acetone and dilute HF acid treatment (named as B-type CrO2 film), the surface roughness of CrO2 film decreases with the thickness increasing. While if the substrate without chemical treatment (named as A-type CrO2 film), the surface roughness of CrO2 film increases with thickness increasing.(b). The XRD results show that the diffraction peaks of A, B-type (110) CrO2 and A-type (100) CrO2 are not shift with the thickness increasing, indicating that there is a lattice distortion in these CrO2 films. The diffraction peaks of B-type (100) CrO2 film have a shift compared with the diffraction peaks of bulk CrO2, and the shift decreases with the thickness increasing. This deviation of diffraction peaks in B-type (100) CrO2 films may be due to lattice mismatch, and the mismatch decreases with the thickness increasing.(c). Epitaxial (110) and (100)CrO2 films show an in-plane uniaxial anisotropy when the film thickness is more than 10 nm. The coercivity decreases with thickness increasing. For B-type (100) CrO2 film, when the thickness is less than 60nm, stress anisotropy introduced by lattice distortion overcomes the magnetocrystalline anisotropy, and resulting in magnetization flip between hard axis and easy axis.2. The damping factor and Meff of CrO2 films were obtained from the permeability spectra and out-of-plane electron spin resonance spectra, and the results of these two methods show the similar law. The surface unevenness of CrO2 films is the main reason of the damping factor variation. In addition, the stress caused by lattice mismatch of B-type (100) CrO2 film is also specially acting on damping factor. The fitting parameter variation showed as following:(a). For A-type (110) CrO2 film, the damping factor firstly increases and then decreases with the thickness increasing. The maximum of the damping factor presents when the CrO2 film thickness reaches to 230 nm. The Meff monotonically increases with the thickness increasing.(b). For B-type (110) CrO2 film, the damping factor decreases and the Meff monotonically increases with the thickness increasing.(c). For A-type (100) CrO2 film, the damping factor firstly increases and then decreases with the thickness increasing. The maximum of the damping factor presents when the CrO2 film thickness reaches to 200 nm. The Meff monotonically increases with the thickness increasing.(d). For B-type (100)CrO2 film, the damping factor firstly increases and then decreases with the thickness increasing. The Meff firstly decreases and then increases with the thickness increasing. The maximum of the damping factor and the minimum of Meff present when the CrO2 film thickness reaches to 70 and 53 nm, respectively. The transitional state of internal stress anisotropy and the magnetocrystalline anisotropy in CrO2 film of about 60nm obviously affect the the damping factor and Meff.3. The stress induced magnetization switching was studied in the CrO2/PZT actuator heterostructures.(a). Anisotropy field in the film plane slightly increases with the voltage increasing. Theoretically, the magnetocrystalline anisotropy of the CrO2 film can be overcome under a 6.5×109 Pa stress induced stress anisotropy.(b). Perpendicular magnetic anisotropy exists in B-type CrO2 film, which derived from the columnar growth of the CrO2 on film surface. For B-type (110) films with thickness of 138 nm, the variation of the saturation magnetization and the coercivity of the phase with perpendicular magnetic anisotropy are 18.4% and -15.1% under a 70 V voltage, respectively. But for B-type (100) CrO2 film, no significant regulatory effect since the applied stress is less than the stress induced by lattice mismatch.