Du Youwei

Martensitic transformation and related magnetic effects in Ni—Mn-based ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni—Mn-based alloys are reviewed based on our recent research results.

High coercivity in large exchange-bias Co/CoO-MgO nano-granular films*

We present a detailed study on the magnetic coercivity of Co/CoO-MgO core-shell systems, which exhibits a large exchange bias due to an increase of the uncompensated spin density at the interface between the CoO shell and the metallic Co core by replacing Co by Mg within the CoO shell. We find a large magnetic coercivity of 7120 Oe around the electrical percolation threshold of the Co/CoO core/shell particles, while samples with a smaller or larger Co metal volume fraction show a considerably smaller coercivity. Thus, this study may lead to a route to improving the magnetic properties of artificial magnetic material in view of potential applications.

Magnetic properties and Mössbauer spectra of X type hexagonal ferrites

Abstract The process of formation and the chemical instability of Ba2Fe22+Fe283+O46 were studied by magnetic and X-ray analyses. The magnetic properties, resistivity and Mossbauer spectra were measured. A compound with X structure was also obtained by firing 1 2 yGd2O3 + (1 - y)BaO + 6Fe2O3(y ≤ 0.15) at 1400°C in air. It was found that the uniaxial anisotropy constant K1 increased with y at 293 K.

W-type hexagonal ferrites RxBa-xFe18O27

Abstract W-type ferrites RxW = RxBa1-xFe18O27 (R = Ce, La) wer prepared. Their magnetic properties and Mossbauer spectra were measured. It was found that the substitution of Ce or La for Ba in BaW improves partially the chemical stability of W-type ferrites.

The magnetic phase transition and the low-field Arrott plots of (GDxDy1−x)Co2 compounds

Abstract A series of (GdxDy1−x)Co2 (0–0.55) compounds have been prepared by arc-melting method. In order to determine the phase transition order in these compounds, we have performed some magnetization measurements. Only from the plots of temperature dependence of magnetization and the low-field magnetization isotherms, it is difficult to judge the phase transition order. The low-field Arrott plot is proved to be a more reliable method to characterize the first-order transition and the second-order transition. No first-order transition is observed in (GdxDy1−x)Co2 compounds (x≠0). The result is discussed in the Inoue-Shimizu like model.

Exchange coupled Nd2Fe14B/α-Fe nanocomposite magnets with fine α-Fe grains obtained by low wheel speed spinning

Abstract Exchange coupled Nd2Fe14B/α-Fe nanocomposite magnets have been synthesized by melt-spinning. The effect of wheel speed on the magnetic properties and microstructure has been studied. The results show that a uniform Nd2Fe14B/α-Fe nanocomposite structure with fine α-Fe grains can be developed at an optimum wheel speed of about 12 m/s. After a low temperature annealing, the magnetic properties can reach maximum values: iHc=432.2 kA/m, Jr=1.08 T, (BH)max=115 kJ/m3. With further increasing the wheel speed, the volume fraction of an amorphous phase increases. The microstructure of the ribbons becomes nonuniform after a subsequent high temperature crystallization, and several α-Fe grains abnormally grow to about 100 nm, deteriorating the magnetic properties of the ribbons.

Surface critical properties of a face-centered cubic magnetic films

Abstract The novel properties of a Heisenberg model on a face-centered cubic lattice (FCC) are investigated by Monte Carlo simulation. Spontaneous magnetization, magnetic susceptibility, specific heat and spin configuration are calculated for both the bulk and surface. The dependence of the critical temperature τc on the ratio of surface to bulk coupling Js/Jb and the size of the system are studied. It is found that a critical value (Js/Jb)c, where the surface and bulk magnetization simultaneously vanish, depends on the specific model. A variety of phase diagrams have also been obtained. The simulated results are consistent with those derived by mean field theory with transfer matrix method, and well explain the experimental facts.

The origin of the large magnetocaloric effect in RCo2 (R=Er, Ho and Dy)

The phase transitions of the RCo2 (R=Er, Ho and Dy) compounds are of first-order type and a metamagnetic transition is observed above the Curie temperature. Owing to their large magnetic entropy change, the RCo2 compounds are competitive candidates as the working substance for magnetic refrigeration. In this paper we discuss the origin of this large magnetic entropy change.

Surface effect of magnetization of nanosized magnetic clusters

Abstract Based on the Monte Carlo method, magnetizations and spin-wave excitations at the surface, in the core for the clusters with different surface interaction, surface thickness and atom number are studied. It is found that, for the pure cluster, the Bloch T3/2 law is well satisfied at low temperature (T

Multiferroic properties in terbium orthoferrite

Multiferroic properties in a polycrystalline terbium orthoferrite are investigated. Different thermomagnetic behaviors are observed in different magnetic fields, which is attributed to the suppression of the low temperature magnetic phase by an external magnetic field. Further studies reveal that the ferroelectricity originates from the spin configuration below 3.5 K. In addition, the magnetic field control of electric polarization and dielectric constant is observed, which suggests a magnetoelectric effect in TbFeO3. The origin of ferroelectricity in this rare-earth orthoferrite is discussed.