Zhang Xiang-Qun

Photoinduced Magnetization Change in Multiferroic YbFe2O4

We investigate the influence of laser illumination on the magnetization in multiferroic YbFe2O4 single crystals. A photoinduced magnetization change is confirmed in both ab plane and the c-axis directions. The temperature dependence of the photoinduced magnetization reduction excludes laser heating as the cause. In terms of the breakdown of charge order driven by laser illumination, the photoinduced magnetization change provides strong evidence for the spin-charge coupling in YbFe2O4. This photomagnetic effect based on charge-order-induced multiferroicity could be used for the non-thermal optical control of magnetization.We have studied the influence of laser illumination on the magnetization in multiferroic YbFe2O4 single crystals. A photoinduced magnetization change has been confirmed in both ab plane and c axis direction. The temperature dependence of the photoinduced magnetization reduction excludes laser heating as the cause. In terms of the breakdown of charge order driven by laser illumination, the photoinduced magnetization change provides a strong evidence for the spin-charge coupling in YbFe2O4. This photomagnetic effect based on charge-order-induced multiferroicity could be used for non-thermal optical control of magnetization.

Electron spin resonance investigation of the substitution of Fe3^＋ for Ti4^＋ ions in rutile TiO2 single crystal

A Fe doped rutile TiO2 single crystal is grown in an O2 atmosphere by the floating zone technique. Electron spin resonance (ESR) spectra clearly demonstrate that Fe3+ ions are substituted for the Ti4+ ions in the rutile TiO2 matrix. Magnetization measurements reveal that the Fe:TiO2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K. The Fe3+ ions possess weak magnetic anisotropy with an easy axis along the c axis. The annealed Fe:TiO2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.

Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a CoFeB/MgO/CoFeB magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect (TR-MOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.

Temperature dependence of multi-jump magnetic switching process in epitaxial Fe/MgO (001) films

Temperature dependence of magnetic switching processes with multiple jumps in Fe/MgO (001) films is investigated by magnetoresistance measurements. When the temperature decreases from 300 K to 80 K, the measured three-jump hysteresis loops turn into two-jump loops. The temperature dependence of the fourfold in-plane magnetic anisotropy constant K1, domain wall pinning energy, and an additional uniaxial magnetic anisotropy constant KU are responsible for this transformation. The strengths of K1 and domain wall pinning energy increase with decreasing temperature, but KU remains unchanged. Moreover, magnetization reversal mechanisms, with either two successive or two separate 90° domain wall propagation, are introduced to explain the multi-jump magnetic switching process in epitaxial Fe/MgO(001) films at different temperatures.

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si (001) substrate

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films. In this paper, Co films with different thickness of CoSi2 buffer layers were grown on Si (001) substrates. In order to investigate morphology, structure, and magnetic properties of films, scanning tunneling microscope (STM), low energy electron diffraction (LEED), high resolution transmission electron microscopy (HRTEM), and surface magneto-optical Kerr effect (SMOKE) were used. The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of CoSi2 buffer layers. Few CoSi2 monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality. Furthermore, the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process, which is the typical phenomenon in cubic (001) films.

Fe-doping induced Griffiths-like phase in La0.7Ba0.3CoO3

The effect of Fe-doping on the magnetic properties of the ABO3-type perovskite cobaltites La0.7Ba0.3Co1–yFeyO3 (0 ≤ y ≤ 0.80) is reported. With no apparent structural change in any doped sample, the Curie temperature (TC) and the magnetization (M) are greatly suppressed for y ≤ 0.30 samples, while a distinct increase in TC for the y = 0.40 sample is observed. With the further increase of Fe concentration, TC increases monotonically. Griffiths-like phases in 0.40 ≤ y ≤ 0.60 samples are confirmed. The formation of the Griffiths-like phase is ascribed to B-site disordering induced isolation of ferromagnetic (FM) clusters above TC.

High-frequency properties of oil-phase-synthesized ZnO nanoparticles

Monodispersive ZnO nanoparticles each with a hexagonal wurtzite structure are facilely prepared by the high-temperature organic phase method. The UV-visible absorption peak of ZnO nanoparticles presents an obvious blue-shift from 385 nm of bulk ZnO to 369 nm. Both the real part and the image part of the complex permittivity of ZnO nanoparticles from 0.1 GHz to 10 GHz linearly decrease without obvious resonance peak appearing. The real parts of intrinsic permittivity of ZnO nanoparticles are about 5.7 and 5.0 at 0.1 GHz and 10 GHz respectively, and show an obvious size-dependent behavior. The dielectric loss angle tangent (tanδ) of ZnO nanoparticles with a different weight ratio shows a different decreasing law with the increase of frequency.

Magnetization reversal process in Fe/Si (001) single-crystalline film investigated by planar Hall effect

A planar Hall effect (PHE) is introduced to investigate the magnetization reversal process in single-crystalline iron film grown on a Si (001) substrate. Owing to the domain structure of iron film and the characteristics of PHE, the magnetization switches sharply in an angular range of the external field for two steps of 90° domain wall displacement and one step of 180° domain wall displacement near the easy axis, respectively. However, the magnetization reversal process near the hard axis is completed by only one step of 90° domain wall displacement and then rotates coherently. The magnetization reversal process mechanism near the hard axis seems to be a combination of coherent rotation and domain wall displacement. Furthermore, the domain wall pinning energy and uniaxial magnetic anisotropy energy can also be derived from the PHE measurement.