You Wei Du

Dynamics of Magnetization Reversal in Thin Polycrystalline Co Films

The thin polycrystalline Co films with different annealed time were prepared by magnetron sputtering method. XRD and hysteresis loops of the samples were measured. A mean field equation with Heisenberg model for calculating dynamic scaling was derived. The experimental and simulated results indicate that, the scaling law, A=A0+H0 a ω b, describes well dynamic magnetization along easy and hard axes for the anisotropy magnetic films; the anisotropy scaling exponents exist clearly in the anisotropy films; the simulated results are consistent with the experimental those.

The Magnetic Entropy Changes in Gd1-xCrx Alloys

A series of Gd1-xCrx (x=0.01,0.03,0.05 and 0.07) alloys have been prepared by arc melting. After introducing a small quantity of Cr into Gd, the Curie temperatures of these alloys increase. Magnetic entropy changes at the Curie temperature of Gd1-xCrx (x=0.01, 0.03, 0.05) alloys are nearly the same as that of Gd. However, compared with Gd, the magnetic entropy changes of Gd1-xCrx (x=0.01, 0.03, 0.05) alloys remain at a high level in a wider temperature range. So Gd1-xCrx (x=0.01, 0.03, 0.05) alloys are more suitable as magnetic refrigerant to be used in Ericsson Recycle for room temperature magnetic refrigeration. Our results and the fact that Cr is quite cheaper than Gd, suggest that Gd1-xCrx alloys maybe utilized as refrigerant in room temperature magnetic refrigeration.

Enhanced Magnetic Properties and Spin-Phonon Coupling of Single-Crystalline BiFeO3 Nanorods Prepared by Hydrothermal Technique

By hydrothermal methods single-crystalline BiFeO3 (BFO) nanorods with diameters of 15-40nm and lengths of 100-400nm were synthesized. The rhombohedra phased nanorods grow along the [110] direction. The nanorods show weak ferromagnetism at room temperature and enhanced spin-glass type magnetism at low temperature. Around 100K, a strong spin-two-phonon coupling has been detected, confirming the spin-glass freezing behaviors. With emphasis on the size and morphology, the enhanced magnetism and the spin-glass freezing of the BFO nanorods have been discussed.

Reorientation Phase and Striped Domain Patterns of Thin Films

A ferromagnetic monolayer with competing long-range dipolar interaction, short-range interaction and magnetic uniaxial anisotropy is studied using Monte Carlo simulation. Striped domain patterns are found and reveal the microstructure of the magnetization transition at the ground state via the competing between dipolar interaction and uniaxial anisotropy. We present the finite temperature phase diagram and find a temperature-induced magnetization reorientation from out-of-plane to in-plane phase. The influence of the various interactions on the structure of the striped phase and reorientation behavior has been discussed.

Enhanced Magnetoelectricity and Large Phonon Abnormality of Feather-Like CoFe2O4-BaTiO3 Nanostructures

By cost-effective techniques of hydrothermal reaction and polymer-assisted deposition, Feather-like (CoFe2O4)0.3-(BaTiO3)0.7 (CFO-BTO) nanostructures have been synthesized. The nanostructures show an average diameter of 250 nm and lengths up to 5 μm, with CFO nanopillars embedded in BTO matrix. The three-dimensional microstructure and growth mechanism of the nanostructures have been investigated. A large magnetoelectric (ME) response at room-temperature and a strong phonon abnormality between 110 °C and 140 °C have been studied.

Cutoff Frequency Study on Nanocrystalline FeNbB Thin Films

A series of nanocrystalline FeNbB films were fabricated using ion-beam sputtering technique from FeNbB target. Pieces of these films were annealed for 1 hour at various temperatures up to 5730C. Room temperature soft magnetic properties of these films were measured. The influence of microstructure on magnetic behavior in nanocrystalline FeNbB films is investigated in a series of specimens with different film’s thickness. For the sample 120nm and 5000C annealed, cutoff frequency was found to be 5E7 Hz, which has the μf0=5E10.

Preparation and Characterization of Alumina Nanofibers by Gas-Solid Reaction Method

Using gas-solid reaction method, the non-crystalline alumina nanofibers with different morphology and aspect ratios could exuberantly grow by feeding air into one closed box or bubbling air into several organic liquids which contained the thin aluminum amalgam sheet at room temperature. Their diameters were about 2-10nm, whereas the length-diameter ratios were distributed widely. The diameters and length could be controlled by adjusting the kinds of the organic reagent. The as-grown nanofibers were then heat-treated at temperatures range from 300 to 1000°C. The results of X-ray diffraction and thermo-gravimetric analysis indicated the γ-Al2O3 nanofibers would be produced at 500°C and the α-Al2O3 particles could be obtained at about 1000°C. The electron micrographs suggested the nanofibers growth direction was randomly in the air or organic solutions, except for growing regular naonofibers in polyoxyalklene-400. The growth mechanics of alumina nanofibers were briefly discussed.

Monte Carlo Simulation of Magnetic Properties of Multilayer with Amorphous Surfaces

Monte Carlo simulation studies are performed to examine the effects of amorphous surfaces and the size of the system on magnetic critical properties of a face-centered cubic magnet modeled with a Heisenberg Hamiltonian. A Gaussian probability distribution of the exchange integrals is applied for the description of amorphous surfaces. The Spontaneous magnetization and magnetic susceptibility are calculated for both the inner and surface. A variety of phase diagrams have been also obtained. The simulated results can well explain the experimental facts.

The Influences of Size and Anisotropy Strength on Hysteresis Scaling for Anisotropy Heisenberg Multilayer Films

In this paper, we simulate the magnetization dynamic processes of the multilayer films, and calculate their hysteresis loop areas using Monte Carlo method. The simulated results indicate that, the size and anisotropy strength of the anisotropy multilayer films influence evidently the dynamic phase transition, and the phase transition temperature increases with enhancing values of the anisotropy constant and layer thickness. It is also found that, with increasing number of layers of films, the value of α decreases, while the magnitudes of β and γ increase. On the contrary, with increasing anisotropy strength, the value of α increases, while the magnitudes of β and γ reduce.

Magnetic Properties and Magnetostriction of ZnxNi1-x MnSb Alloys

We have investigated the magnetic properties and magnetostriction of ZnxNi1-xMnSb compounds prepared by solid state reaction method. It is found that for x less than 0.6 the magnetization of ZnxNi1-xMnSb almost remains unchanged. However, when x is larger than 0.6 the magnetization starts to drop linearly. Experimental result indicates that at low Zn concentrations, x < 0.7, the Curie temperature (TC) decreases with increasing Zn concentration x. However, when x > 0.7, the Curie temperature increases distinctly with increasing x. The Zn concentrartion dependence of magnetostrictive ceofficient is also studied. The experimental curve shows that when x < 0.6 the value of magnetostriction coefficient for ZnxNi1-xMnSb decreases linearly with the increasing Zn concentration x. However, when x is above 0.6, the magnetostrictive ceofficient raises distinctly. By analyzing the XRD pattern, the structures of the materials are examined and the relationship between the properties and the structures are discussed. A structural phase transition is observed. It has been indicated that the structure plays an important role in the magnetic and magnetostrictive properties of the system.