Y.W. Du

Magnetic properties of ultrafine nickel particles

Ultrafine nickel particles have intriguing physical and chemical properties, which are interesting both in fundamental and applied research. The size of the particle was controlled by gas pressure. X‐ray diffraction studies showed that fine Ni particles have fcc crystal structure and are coated with thin Ni oxide on the surface. Electron micrographs showed a spherical particle shape, forming a long chain. Size dependence of magnetic properties were studied. The specific magnetic moment drastically decreases when reducing the diameter d of particles <15 nm. The coercivity Hc also approaches zero when d is about 15 nm. Therefore, we can suppose that the critical size for superparamagnetism at room temperature is about 15 nm. According to the superparamagnetic formula KV=25 kT, the value of the magnetic anisotropy constant can be determined, K = −5.8 × 105 (erg/cc). It is found to be larger than bulk Ni [K=−3.4–5.1 × 104 (erg/cc)], the same as Fe particles. The maximum of coercivity at room temperature is ab...

Magnetic properties of fine iron particles

Iron particles were prepared by a gas evaporation method in a nitrogen atmosphere. The particle size, shape, and magnetic properties were influenced by the nitrogen pressure. Particles prepared by this method were relatively stable in air and were not seriously oxidized. The recoilless fraction is low for the oxide layer, so that it cannot be detected at room temperature by Mossbauer spectrum. A superparamagnetic peak associated with the oxide microcrystal layer did not occur in the Mossbauer spectra due to the action of the magnetic field from the iron core and the interaction between iron particles and the low Debye temperature. From the electron micrography, the magnetic reversal process seems to be determined by a chain‐of‐spheres mechanism but Hc∼T curve cannot coincide with the Ms∼T curve at low temperatures. An increase of Hc may be associated with magnetic anisotropy. It is found that the magnetization versus temperature shows an anomalous increase in M(T) at higher T. We think that this means tha...

Magnetoresistance of FeSiO2 granular films

A new type of magnetoresistance (MR) was observed in Feue5f8SiO2 granular films. The samples were prepared on glass substrates using ion-beam sputtering technique. MR ratios of the order of 2.7% at room temperature and 4.5% at 210 K were obtained when a magnetic field of 1.2 T was applied parallel to the film plane. We have measured MR as a function of Fe volume fraction (fV) of the films. It was found that MR appears only in samples with fV less than the percolation threshold (fC) and has a peak value at fV ≈ 0.28. We also found that the enhancement of MR always appears together with the increase of resistance (R) of samples in the region 0.28 ≤ fV < 0.5. Our results indicate that MR in these materials arises from the spin-dependent tunneling effect and imply that the size of Fe particles and the thickness of SiO2 tunnel barriers may play an important role in the MR effect of Feue5f8SiO2 granular films.

Electric field control of the magnetocaloric effect.

Through strain-mediated magnetoelectric coupling, it is demonstrated that the magnetocaloric effect of a ferromagnetic shape-memory alloy can be controlled by an electric field. Large hysteresis and the limited operating temperature region are effectively overcome by applying an electric field on a laminate comprising a piezoelectric and the alloy. Accordingly, a model for an active magnetic refrigerator with high efficiency is proposed in principle.

The effect of nitrogen incorporation on the magnetic properties of carbon-doped ZnO

Samples of carbon-doped ZnO powders were prepared by the standard solid-state reaction method and sintered separately in argon and nitrogen atmospheres. According to the results of Raman spectroscopic investigation, the samples sintered in nitrogen showed lower D-bond (disordered) and G-bond (graphitic) concentrations, plausibly a result of nitrogen incorporation into the carbon-doped ZnO sample. All the samples are ferromagnetic at room temperature, and compared with those sintered in argon, those sintered in nitrogen have a lower magnetic moment. We found that the electrons-mediated mechanism is more suitable than the holes-mediated one for the explanation of ferromagnetism of carbon-doped ZnO materials.

Ferromagnetism in Tb doped ZnO nanocrystalline films

Nanocrystalline Tb-doped ZnO films have been prepared by ion-beam sputtering technique. Magnetic characterization showed that the films are ferromagnetic with Curie temperature (TC) higher than room temperature. By further treated with a rapid thermal annealing process, both the grain size and the carrier concentration of the films increase, while the saturation magnetization of the films decreases. This magnetic behavior can be hardly explained by either bound magnetic polaron model or free carrier mediation model, thus suggests that the grain boundaries play a key role for the origin of ferromagnetism in these films.

A study of cobalt nanowire arrays

Ordered arrays of cobalt nanowires have been fabricated by electrodepositing the corresponding materials into the pores of anodic aluminum oxide (AAO) membranes. Magnetic measurements illustrate their easy magnetization axis is along the nanowire. This kind of array can be used as a vertical storage medium. The ideal storage density is about 170xa0Gxa0bitxa0in-2.

Lattice effect in Pr doped LaSrMnO perovskite

Abstract We have measured resistivity, magnetization and magnetoresistance in polycrystalline La 0.7− x Pr x Sr 0.3 MnO 3 with fixed carrier concentration. With decreasing the average ionic radius of the La site 〈 r A 〉, we found a decrease in T c , an increase in resistivity at T c , and an increase in the magnitude of the magnetoresistance. The samples show a metal-insulator transition in the high-temperature paramagnetic phase with the variation of 〈 r A 〉. These results demonstrate the close relationship between 〈 r A 〉 and the band structure.

Ferromagnetic resonance study on Fe‐SiO2 granular films

Ferromagnetic resonance (FMR) measurement was performed at the frequency of 9.8 GHz and at the room temperature on a series of Fe‐SiO2 granular films fabricated by the Ar+‐beam sputtering technique. Our results indicated that when the iron volume fraction (fv) of the films exceeds 0.28, the FMR spectra are superpositions of volume modes and surface modes. The analysis on the effective anisotropy field suggests that the intrinsic anisotropy (including the volume and interface one) becomes maximal when fv near the percolation threshold (fp). In particular, it was found for the films with fv=0.42 that the spin wave resonance modes satisfy the relation Hres∝n with a separation field of about 174 Gs between the successive modes. Our study reveals that Fe particles are coupled by a yet unknown interparticle interaction when fv is just below fp.

Magnetic-field-induced dielectric anomaly and electric polarization in Mn4Nb2O9

Magnetic, dielectric, and magnetoelectric properties have been investigated in the polycrystalline Mn4Nb2O9. Under zero magnetic fields, no dielectric anomaly and electric polarization are observed in this compound. When the sample is exposed in magnetic field, finite dielectric peaks and electric polarization are induced, which increase with increasing magnetic field, showing magneto-dielectric and magnetoelectric effects. The origin of magnetoelectric coupling of this compound has been discussed.