Ferromagnetic Resonance Of CrO₂Thin Films Prepared By Chemical Vapor Deposition

As a conventional magnetic recording media, chromium dioxide (CrO2) was mainly used in magnetic recording tapes. Recently, it was found theoretically and experimently that CrO2is an important half-metallic material which have highest spin polarization(100%) of all materials to date; theory further predicted that CrO2have the minimum value of Gilbert damping parameter (about0.0005) among all ferromagnetic materials, these properties make CrO2an ideal system for the study of half-metallic or a potential material for spintronic devices.In this work, CrO2thin films were deposited on AI2O3(0001) and TiO2(110) substrates. Deposition conditions for CrO2thin films growth that affects the structure and magnetism of thin films were investigated by X-ray diffraction (XRD), Scanning electron microscope (SEM), Vibrating sample magnetometry (VSM) and Ferromagnetic resonance (FMR). The experimental data measured by FMR were numerically fitted to calculate Gilbert damping parameters of the CrO2thin films. Several factors that influence the Gilbert damping parameter were explored. The results are as follows:(1)It turns out that CrO2thin films cannot be deposited on glass. When Si(100) and A12O3(0001) were used as substrates, diffraction peaks of CrO2(200) and CrO2(400) had been discovered besides that of Cr2O3(006) and Cr2O3(0012) by XRD. SEM cross-sectional analysis showed that chromium oxides had columnar structure on A12O3(0001).The weaker magnetism and unconspicuous in-plane uniaxial magnetocrystalline anisotropy of the thin film have been proved by the VSM measurement. The resonance peak in FMR of the thin film was comprised of multiple peaks, which makes it impossible to be fitted accurately.(2)In order to get single crystal and pure phase of CrO2thin films, tetragonal single crystal TiO2(110) substrate was used because of the fine lattice match. The thin films were investigated by XRD and SEM and experimental results showed that before annealing, only CrO2(110) and (220) peaks were present with different thickness of CrO2films indicating a-axis growth; the thin films became more and more compact and flat with increasing the films thickness. In-plane apparent uniaxial magnetocrystalline anisotropy of the thin films has been proved by the VSM measurement at room temperature under atmospheric pressure. There was a90°angle between the easy axis and the hard axis. Periodic change of the remanence on dependence of angle φH is well described by|cosφH|.But periodic change of the coercivity versus the angles nearby the easy axis was different from that nearby the hard axis. With films thickness increasing, the coercivity diminished at the same location on the films and in-plane uniaxial magnetocrystalline anisotropy weakened. The dynamic magnetic behavior of CrO2thin films have been investigated by the Electron Spin Resonance (ESR) at the microwave frequency of9GHz. It turned out that the variation of resonance fields versus in-plane anglesφH (or out-plane angle θH)was monotonic between the easy and hard axis for all films. Orientation of easy axis of resonance field lies along0°of in-plane angle, while the variation of resonance fields on dependence of the out-plane angle was exactly opposite. However, the variation of FMR linewidths was obvious different from that of resonance fields for all of the films. FMR linewidths revealed the unimodality with the variation of magnetic fields versus the out-plane angles and occurred as minimum at the location of maximal resonance field. As the film thickness increased, the rate of change of resonance fields along with the out-plane angles enlarged nearby the easy axis, and the rate of change of resonance fields versus the in-plane angles diminished nearby the hard axis. Meanwhile, FMR linewidths diminished and the peak position of the linewidths curve shifted about5°. The Gilbert damping parameters of thin films were obtained by means of numerical fitting of the linewidths. It showed that the Gilbert damping parameter was anisotropy and their values diminished with the films thickness increasing, changing from0.07to0.009.(3) Same samples were investigated after annealing, it turned out that the structure of thin films was more compact, and the crystallization was enhanced. Besides, ESR linewidths decreased, and the Gilbert damping parameters became smaller.In this work, single crystal CrO2(110) thin films were successfully prepared on TiO2(110) substrates via exploring the technical parameters of chemical vapor deposition (CVD). Experimental and numerical fitting showed that the Gilbert damping parameters depend strongly on structures of the film, and it is anisotropic as well. Accordingly, the Gilbert damping parameters were modified by annealing or changing the thickness of the thin film.