Zhaocong Huang

Ferromagnetic resonance studies of Fe thin films with dilute heavy rare-earth impurities

The structure and magnetic properties of Fe1−xGd(Tb)x films prepared by sputtering have been investigated using x-ray diffraction, vibrating sample magnetometer, and ferromagnetic resonance. Magnetic measurements show that the saturation magnetization decreases as the increase of Gd(Tb) content, which agrees with that Gd(Tb) atomic moments align antiparallel with Fe moments. Theoretical fittings of the angular dependence of ferromagnetic resonance field yield the second- and forth-order perpendicular anisotropy constants K⊥1 and K⊥2, which show a spin reorientation with increasing Gd(Tb) doping. The intrinsic contribution of ferromagnetic resonance linewidth as a result of Gilbert damping is separated from the extrinsic contribution because of the inhomogeneity of the films. It is found that the additions of Gd additives do not significantly improve the damping constant, whereas the Tb dopants increase the damping constant from 0.04 without Tb to 0.11 with 19% Tb, thus a remarkable enhancement of 175%.

Texture induced magnetic anisotropy in Fe3O4 films

This letter reports a free energy density model for textured films in which the related physical concept and expression of magneto-texture anisotropy energy are presented. The structural characterization and out-of-plane angular dependence ferromagnetic resonance of strongly textured Fe3O4 films were systematically investigated. We found that the typical free energy density model for polycrystalline film cannot be applied to the textured films. With the introduction of magneto-texture anisotropy energy in the free energy density model for thin films, we simulated and quantitatively determined the competing anisotropies in (111)-textured Fe3O4 films.

Enhancement of magnetic moment in ZnxFe3−xO4 thin films with dilute Zn substitution

Highly (111)-textured ZnxFe3−xO4 thin films were grown by pulsed laser deposition on silicon substrates. The spin and orbital magnetic moments of the ZnxFe3−xO4 thin films have been obtained by X-ray magnetic circular dichroism (XMCD) and sum rule analysis. The total magnetic moments thus extracted are in good agreement with the values obtained by vibrating sample magnetometer. Both the unquenched orbital moment and the ratio of orbital-to-spin moment first increase significantly with increasing Zn substitution at a low concentration range ( 0≤x≤0.1), and then decrease at a higher concentration (x = 0.3). The underlying site-specific doping mechanisms involved here have been elucidated by detailed analysis of the XMCD of ZnxFe3−xO4 films. Our work demonstrates a practical means to manipulate the spin-orbit coupling in the ZnxFe3−xO4 thin films via Zn impurity doping.

Enhancing the Spin–Orbit Coupling in Fe3O4 Epitaxial Thin Films by Interface Engineering

By analyzing the in-plane angular dependence of ferromagnetic resonance linewidth, we show that the Gilbert damping constant in ultrathin Fe3O4 epitaxial films on GaAs substrate can be enhanced by thickness reduction and oxygen vacancies in the interface. At the same time, the uniaxial magnetic anisotropy due to the interface effect becomes significant. Using the element-specific technique of X-ray magnetic circular dichroism, we find that the orbital-to-spin moment ratio increases with decreasing film thickness, in full agreement with the increase in the Gilbert damping obtained for these ultrathin single-crystal films. Combined with the first-principle calculations, the results suggest that the bonding with Fe and Ga or As ions and the ionic distortion near the interface, as well as the FeO defects and oxygen vacancies, may increase the spin-orbit coupling in ultrathin Fe3O4 epitaxial films and in turn provide an enhanced damping.

Current-induced multiple domain wall motion modulated by magnetic pinning in zigzag shaped nanowires

Using micromagnetic simulation, we investigate the current-induced multiple domain wall motion (CIDWM) in zigzag nanowires with different bar angles (θ=90°, 120° and 150°). Two dynamic processes of single DWM and double DWM are found in different regimes of current density identified by two thresholds in all zigzag nanowires. The decreasing threshold current is found in the zigzag nanowires with increased bar angles, indicating the angular-dependence of the magnetic pinning. This work suggests a possibility of manipulating the single/multiple DWM in future DW devices by introducing the shape anisotropy.

Investigation on transformation of spindle-like Fe3O4 nanoparticles from self-assembling α-Fe2O3

Porous monodisperse spindle-like α-Fe2O3 nanomaterials are first synthesized successfully by a hydrothermal method, and then the as-prepared nanoparticles are annealed at different temperatures under various atmospheres to achieve the spindle-like Fe3O4 nanoparticles. The evolution of the features of nanoparticles, including the changes of the structures and microstructures as well as the magnetic properties, during the reduction process has been investigated by using the Raman spectrum and Mossbauer spectrum. Our research reveals that the α-Fe2O3 nanoparticles annealed by covering of the C powder become a mixture of α-Fe2O3 and Fe3O4 in the range of annealing temperature from 300 °C to 800 °C. With reduced atmospheric H2, spindle-like α-Fe2O3 nanoparticles are transferred to mixture of α-Fe2O3, Fe3O4 and Fe as temperature increases. They are also converted from a typical rhombohedral structure to a cubic α-Fe phase at 500 °C. Finally, with the atmosphere of H2/Ar (5%/95%), a pure Fe3O4 phase, and its exc...

The magnetic properties of well-aligned nickel nanochains synthesized by magnetic field-induced assembly approach

Highly uniform one-dimensional Ni chains with controllable diameters and lengths have been synthesized at 70 °C by a hydrothermal process under a 0.35 T induced magnetic field. The diameter of the spheres in the magnetic Ni chains is adjusted from 80 nm to 1000 nm with the chain length changed from 1.2 μm to 50 μm by varying the concentration of ethylene glycol and potassium hydroxide in the solution. The Ni chains with different length-to-diameter aspect ratio show the different particle shape and interparticle spacing. Magnetic hysteresis loop measurements demonstrate a uniaxial magnetic anisotropy (UMA) on the coercivity (Hc), and saturation field (Hs). The ferromagnetic resonance (FMR) shows that the difference between demagnetizing fields in the direction of easy and hard increases with increasing the length-to-diameter aspect ratio of nanochains, which is close to then that in Hs. From FMR measurements and theoretical simulation, the difference of the demagnetizing field between the length and width...

Ab initio understanding of magnetic properties in Zn2+ substitution of Fe3O4 ultra-thin film with dilute Zn substitution

The mechanism of the magnetic properties on the Zn2+ substituted Fe3O4 film have been investigated based on first principle calculations. It is found that the surface effect plays an important role in the occupation of Zn ion, and in turn changes the magnetic moment. It may also destroy the half metallic behavior of Fe3O4 film even if the Zn2+ concentration only is one Zn2+ per unit cell (4%), which is different from that in bulk material.

Investigation of magnetization dynamics damping in Ni80Fe20/Nd-Cu bilayer at room temperature

Focusing on the Ni80Fe20 (Py)/Nd-Cu bilayers, the magnetization dynamic damping from spin pumping effect is investigated systematically by doping itinerant Cu in rear earth metal Nd. Various Ta/Py/Nd1-xCux/Ta/Si films with x = 0%, 16%, 38%, 46% and 58% are prepared by magnetron sputtering. For every content of Cu, the thickness of Nd-Cu layer is changed from 1 nm to 32 nm. The damping coefficient increases with increasing the thickness of Nd-Cu layer, which shows the trend of the spin pumping behavior. Also, with increasing Cu concentration in the Nd-Cu layer, the damping coefficient decreases, implying that the spin-orbit coupling in Nd-Cu layer is indeed cut down by high itinerant of Cu dopants. It is interesting that the spin diffusion length (λSD) in the Nd-Cu layer for different Cu dopants is not found to increase monotonously.