C. W. Su

Periodic reversal of magneto-optic Faraday rotation on uniaxial birefringence crystal with ultrathin magnetic films

An experimental approach of inclined incidence magneto-optic Faraday effect observed in the polar plane is applied. Three samples containing ferromagnetic cobalt ultrathin films on a semiconductor zinc oxide (0001) single crystal substrate with in-plane and out-of-plane anisotropy are evaluated. Through the fine adjustment of crossed polarizers in the magneto-optic effect measurement completely recorded the detail optical and magneto-optical responses from the birefringent crystal substrate and the magnetic film, especially for the signal induced from the substrate with uniaxial optical axis. The angle dependency of interference phenomena periodically from the optical and magneto-optical responses is attributed to the birefringence even in the absence of a magnetic field. The new type of observation finds that the transmission Faraday intensity in the oblique incidence includes a combination of polarization rotations, which results from optical compensation from the substrate and magneto-optical Faraday effects from the film. The samples grown at different rates and examined by this method exhibit magnetic structure discriminations. This result can be applied in the advanced polarized-light technologies to enhance the spatial resolution of magnetic surfaces with microstructural information under various magnetic field direction.

MAGNETO-OPTIC FARADAY EFFECT ON SPIN ANISOTROPIC Co ULTRATHIN FILMS AND POST-NITRIDIZATION ON ZnO(002) CRYSTAL

Perpendicular magnetic anisotropy in the initial growth of epitaxial in-plane anisotropic Co ultrathin films on the ZnO(002) crystal surface was discovered. The critical thickness of weak spin reorientation transition phenomenon in deposition process from in-plane to out-of-plane magnetic anisotropy is around 2 nm. Ultrathin 1.2 nm Co film was stabilized by post-irradiation of low-energy N ions from the observation of hysteresis loop transforming from an S-shape soft magnetic state to a square hard magnetic state. The addition of N affected the magnetic behavior. The time-dependent nitridization process was observed using Auger electron spectroscopy. Magneto-optical Faraday effect measurements were used to observe the magnetic properties of high-transmission Co/ZnO(002) and Co–N/ZnO(002) surface. A strong polar magneto-optic Faraday effect was dominated in the 2.0–4.0 nm Co/ZnO(002) surfaces. From the N+ implantation that reduces the corresponding coercivity, a hexagonal c-axis lattice structure of Co with a preferred perpendicular anisotropy on the ZnO(002) surface may be explained.

Effects of N-Doping on Magnetic Properties of ZnCoO Diluted Magnetic Semiconductor Thin Films

Radio-frequency magnetron sputtering technique was used in manufacturing thin films of Co doped and (Co, N) co-doped ZnO (Co:ZnO and Co:Zn(N, O)). The sputtering target was a commercial Zn-Co-O compound with 5% Co in weight. The sputtering gas of pure argon was used for Co:ZnO films; and two other different gases, a mixture of Ar and N2 (gas flow-rate ratio 15 : 5 seem) and a pure N2, was used for (Co, N):ZnO films. Without N-doping, the films show n-type electrical conduction. Upon N-doping, films become poorly conductive and even exhibit p-type conduction for pure N2 sputtering-gas. Paramagnetism is observed in the film with no N-doping (Co:ZnO); but superparamagnetism and then a mixture of ferromagnetism plus paramagnetism were observed with increasing N-doping. Based on the magnetic phase diagram proposed by Coey, we explain in detail the distinct magnetic properties which are closely related with the microstructure and electrical properties of each sample.

Determination of composition in stoichiometric Co-N ultrathin films by nitrogen plasma sputtering

This work utilizes low-energy sputtering to incorporate the generated nitrogen plasma into an epitaxial 1.4nm Co film on the surface of a ZnO(002) substrate. In this method, ultrathin Co–N amorphous films were formed. Interestingly, Co is key to the formation of Co–N films. Without the deposition of Co on the ZnO(002), nitride films cannot be formed. Observations of the surface composition of the Co–N films after the firing of a N+ ion beam onto it demonstrated that the surface concentration of Co reduced at the same rate as the reduction in the concentration of N upon successive sputtering. Theoretical calculations based on the Auger peak-to-peak amplitudes established that the composition of the amorphous Co–N thin films may be Co3N2.

Influence of Vacuum Annealing on Structural, Optical, Electrical, and Magnetic Properties of Zn

The Zn_0.94Co_0.05Al_0.01O diluted magnetic semiconductor (DMS) thin films were made by rf magnetron co-sputtering, and annealed in vacuum at 500°C (for 0, 20, 40, and 80 min, respectively). The films have a single ZnO wurtzite structure with -axis preferred orientations, without any segregated secondary phase, and Co²⁺ ions substitute tetrahedrally coordinated Zn²⁺ ions site. The as-grown film is insulating and in a state of tensile stress parallel to the c-axis. After annealing in vacuum, the films have tensile stresses parallel to the (002) plane, and become conductive; moreover, the carrier concentrations increase with increasing the annealing time. The as-grown film possesses the room-temperature ferromagnetism (RTFM) and the saturation magnetization (M_s) is 0.8 μ_B/Co. As the film is annealed in vacuum, the value of M_s is reduced first then enhanced with the increase of annealing time.

_{0.94}

Determination of magnetic anisotropy on perpendicular and longitudinal fields in most magneto-optical materials is usually essential in magnetic measurements. However, 3D information is still insufficient and may be misled due to only two spin vectors. The vacuum magneto-optical Faraday effect measurement (the transmission mode of magneto-optics technique) in an ultrahigh vacuum system, a new concept for the reconstruction of 3D magnetic anisotropy is introduced. The Faraday rotation in the ultrathin (magnetic film)/(optical crystal) system exhibits a polar plane oscillation as a function of incidence angle. The crystal birefringence is responsible for causing the oscillation. The Faraday rotation, which consists of crystal optics and magneto-optics, originates from the crystal and the ultrathin film, respectively. Alternatively, we clarify a debate that the easy axis of the Co/ZnO(0001) film is only located at the plane. Through the observation of the angle-dependent coercivity, the magnetic easy axis in the proposed multilayer structure including double anisotropy is proposed.

Co

The magnetic properties of 1 nm thick in-plane anisotropic Co ultrathin film on ZnO(0001) were investigated through successive 500 eV nitrogen-ion sputtering. Magneto-optical Faraday effects were used to observe the evolution of the ion-irradiated sample in longitudinal and perpendicular magnetic fields. The ferromagnetic phase of the initial in-plane anisotropic fcc β-Co phase transformation to β-Co(N) phase was terminated at paramagnetic CoNx phase. In-plane anisotropy with weak out-of-plane anisotropy of the Co/ZnO sample was initially observed in the as-grown condition. In the sputtering process, the N+ ions induced simultaneous sputtering and doping. An abrupt spin reorientation behavior from in-plane to out-of-plane was found under prolonged sputtering condition. The existence of perpendicular anisotropy measured from the out-of-plane Faraday effect may be attributed to the co-existence of residual β-Co and Co4N exchange bonding force by the gradual depletion of Co-N thickness.

_{0.05}

The Zn_1-xCo_xO and Zn_1-xCo_xN_yO_1-y thin films were grown by RF magnetron co-sputtering, but in different sputtering gases (Ar and N₂O, respectively). X-ray diffraction analysis indicated that all the samples had <i>c</i>-axis preferred orientation, without any segregated secondary phase. UV-visible spectroscopy and X-ray photoelectron spectroscopy showed that Co²⁺ ions substituted tetrahedrally coordinated Zn²⁺ ions site for the films. The electrical and magnetic properties were investigated by the Hall effect and superconducting quantum interference device (SQUID) measurements, respectively. The Zn_1-xCo_xO had <i>n</i>-type conduction with electron concentration of 10¹⁹ cm^-3, and exhibited paramagnetism. Due to the charge compensation by N doping, the Zn_1-xCo_xN_yO_1-y possessed large resistivity, and also presented paramagnetism, besides the film Zn_{1- x}Co_xN_yO_1-y (x = 0.0162, y = 0.0004) , which possessed room temperature ferromagnetism (RTFM).