Yi-Hua Liu

Ferromagnetism and magnetoresistance of Co–ZnO inhomogeneous magnetic semiconductors

Co–ZnO inhomogeneous magnetic semiconductor thin films were synthesized on the subnanometer scale by sputtering. Room temperature ferromagnetism with high magnetization was found. Large negative magnetoresistance of 11% was found at room temperature, and its value increased with a decrease in temperature up to 36% at 4.8 K. The mechanism for large negative magnetoresistance is discussed.

Large room-temperature magnetoresistance in Ag–Ti-added La0.67Ba0.33MnO3

The effects of Ag addition in Ti-doped La0.67Ba0.33MnO3 (LBT) (abbreviated as LBT/Agx, where x is the nominal molar ratio of Ag) on the electric and magnetoresistance (MR) properties have been studied systematically. The results show that the resistivity of the samples decreases dramatically with increasing Ag addition and reaches its minimum for x = 0.27. The room-temperature MR ratio of LBT/Agx increases with increasing x and reaches its maximum at x = 0.27. The MR ratio at 280 K for the x = 0.27 sample at H = 10 kOe is as large as 41%, which is about 3.4 times larger than that for LBT. The Ag addition makes the LBT crystallites more perfect and uniform, and suppresses the magnetic scattering at the grain boundaries. The Ag conduction channels penetrating into the insulating regions can also lower the resistivity. The near room-temperature TC and low resistivity of the Ag-added LBT are responsible for the large intrinsic room-temperature MR.

Magnetic and transport properties of homogeneous MnxGe1−x ferromagnetic semiconductor with high Mn concentration

Homogeneous MnxGe1−x ferromagnetic semiconductor films with high Mn concentration were prepared, contrasting with dilute inhomogeneous MnxGe1−x magnetic semiconductors. The saturation magnetization of Mn0.57Ge0.43 films is high, up to 327emu∕cm3 (1.04μB∕Mn) at 5K, and the Curie temperature is about 213K. The Mn0.57Ge0.43 films show semiconducting resistance, but the magnetoresistance is negligibly small. The anomalous Hall effect was observed below the Curie temperature, which is consistent with the magnetic measurements. The global ferromagnetism was discussed based on s,p-d exchange coupling between the weakly localized s,p hole carriers and the strongly localized d electrons of the Mn atoms.

Large room-temperature magnetoresistance in Pd-added manganites

A series of La0.67(Ca0.65Ba0.35)0.33MnO3∕Pdx composites were synthesized using a sol-gel method followed by a conventional solid-state reaction route. The results show that the resistivity of the composites decreases dramatically with Pd addition, while the room-temperature magnetoresistance (MR) increases remarkably. A large MR of about 170% is obtained at room temperature and 1T applied magnetic field for x=0.27 sample. The large enhancement of the MR can be attributed to the decrease in resistivity caused by the good conductive metal Pd, which improves the disordered atomic structure and magnetic property on the grain surfaces/boundaries. In addition, the polarization of Pd atoms near the Mn ions on the grain surfaces/boundaries also plays a very important role on the enhancement of the MR, which induces a large number of spin clusters.

Electronic transport and extra large magnetoresistance in LCBMO/Pdx composites

Two-phase composites LCBMO/Pdx were synthesized using the sol?gel technique followed by the solid-state reaction. Pd addition induces a remarkable decrease in resistivity, which is mainly related to the improvement of grain boundaries/surfaces caused by the segregation of Pd. The resistivity data for all samples follow the adiabatic small-polaron-hopping model at high temperature above the Curie temperature TC, while in the low temperature region, they are proportional to T2, reflecting that the electron?electron scattering mechanism is dominant in the ferromagnetic metallic state. In addition, Pd addition induces a large enhancement of room temperature magnetoresistance (MR). Especially for the x = 0.27 sample, an extra large magnetoresistance over 170% is obtained at 10?kOe and 289?K. This is the largest MR obtained at room temperature in all kinds of colossal magnetoresistance (CMR) materials. The good conductivity and polarizing effect of Pd are responsible for the enhancement of MR. The former leads to the decrease in resistivity and the latter induces a large number of spin clusters.

Electrical properties and room temperature magnetoresistance of La0.67(Ca0.65Ba0.35)0.33MnO3/Agx composites

The electrical properties of the La0.67(Ca0.65Ba0.35)0.33MnO3/Agx composite system are systematically investigated as a function of Ag-added content. Ag addition induces a decrease in resistivity (ρ) and an increase in temperature TP where the ρ peak is located. This is due to the improvement of grain boundaries caused by the segregation of Ag on the grain surfaces. The T2 dependence of the resistivity data in the low temperature region of T < TP reflects that the conductive mechanism in the ferromagnetic (FM) metallic state mainly arises from electron–electron scattering. At high temperatures, above the Curie temperature TC, the resistivity data for all samples follow the adiabatic spin polaron hopping model. Moreover, the ρ –T curves for x = 0.27 and 0.30 samples fit well to the phenomenological percolation model based on phase segregation. This result confirms the coexistence of FM clusters and paramagnetic regions near TC, and indicates the prominent intrinsic behaviour of the La0.67(Ca0.65Ba0.35)0.33MnO3 matrix. Note that a 27% molar ratio of added Ag induces a large room temperature magnetoresistance (MR) ratio of up to 41%. This value is attractive for the study of applications. The good agreement of experimental data with the Brillouin function indicates that the MR behaviour in this composite system accounts for the spin-dependent hopping of the electrons among the spin clusters.

The preparation of barium ferrite nanocrystalline powders by a stearic acid gel method

Abstract A new stearic acid gel method has been shown to be suitable for preparing BaFe12O19 nanostructured powders with narrow particle size distribution. Each state of the synthetic process was followed by use of DTA, TG, IR and X-ray analysis. The particle size and morphology were determined by transmission electron microscopy. The chemical composition of these particles was consistent with that of the stoichiometric bulk material analyzed by ion emission spectroscopy. Using this method, smaller, uniform-size and high purity BaFe12O19 nanocrystals could be easily obtained.

GIANT MAGNETOIMPEDANCE BEHAVIOR IN MAGNETIC FIELD-DEPOSITED AMORPHOUS FeCuCrVSiB SINGLE AND MULTILAYERED FILMS

The magnetic and magnetoimpedance behavior of FeCuCrVSiB and (F/SiO2)3/Ag/(SiO2/F)3(F = FeCuCrVSiB) soft magnetic amorphous films deposited in a magnetic field have been investigated systematically. The samples were deposited by the RF sputtering with or without a static magnetic field of 900 Oe applied in the film plane. It is found that the magnetic field applied during the deposition process improves the soft magnetic properties of the sample significantly. The coercive force of the sample is about 0.8 Oe, which is much smaller than that deposited without a field. Moreover, remarkable GMI effects were obtained at relatively low frequencies, which contrasts with the samples deposited without field. In the magnetic field-deposited state, the maximum GMI ratios of 21.5% and 19.5% were achieved for the single-layer films in longitudinal and transverse fields at a frequency of 13 MHz, while for the multilayered films, the maximum values of longitudinal and transverse GMI ratios are 45% and 44% at a frequency of 6.81 MHz, respectively. These superior GMI behaviors of the as-deposited films are related to the magnetic field used in the preparation process and the inserted SiO2 layers.

GIANT MAGNETOIMPEDANCE EFFECT IN AS-DEPOSITED (Fe88Zr7B5)0.97Cu0.03 AMORPHOUS FILMS

The giant magnetoimpedance (GMI) effect has been observed in as-deposited (Fe88Zr7B5)0.97 Cu0.03 amorphous film prepared by radio frequency sputtering. The maximum magnetoimpedance ratio of 17% was obtained at a frequency of 13 MHz. The field and frequency dependences of magnetoimpedance, magnetoresistance, magnetoreactance and transverse permeability have been investigated in detail. Our results show that a close correlation between longitudinal GMI and transverse permeability exists. Our results indicate that the amorphous (Fe88Zr7 B5)0.97Cu0.03 films have good soft magnetic properties and remarkable GMI effect even though in the as-deposited state.

LARGE ROOM TEMPERATURE MAGNETORESISTANCE IN (La0.7Sm0.3)0.7Sr0.3MnO3/Agx COMPOSITES

The effects of Ag addition in the composite (La0.7Sm0.3)0.7Sr0.3MnO3/Agx (abbreviated as LSSMO/Agx) has been studied. The results showed that Ag addition induces the decrease in resistivity (ρ) due to the improvement of grain boundaries resulted from the segregation of Ag on the grain surfaces. In addition, 27% molar ratio of Ag addition induces a large room temperature magnetoresistance (MR) ratio of 35%. The good agreement of experimental data with Brillouin function indicates that the MR behavior in this composite system accounts for the spin-dependent hopping of the electrons between the spin clusters.