Yi-hua Liu

Low-field MR behaviour in La0.67Ca0.33MnO3/ZrO2 composite system

La0.67Ca0.33MnO3 (LCMO) nanoparticles were prepared by the sol–gel method. The composite samples of (LCMO)x/(ZrO2)1−x were obtained by mixed LCMO and ZrO2 powders and sintered at 1300°C. Detailed studies of magnetic and magnetotransport properties for composite samples have been performed. The x-ray diffraction and scanning electronic microscopy observations indicated that no reaction happened between LCMO and ZrO2 grains. The low-field magnetoresistance (LFMR) can be greatly improved by ZrO2 addition close to the percolation threshold of x = 60%. The field sensitivity of MR at 77 K in the low field region for (LCMO)0.4/(ZrO2)0.6 is 7%/100 Oe, which is one order of magnitude larger than that obtained by pure LCMO. The enhancement of the spin-dependent tunnelling and scattering of electrons at the interfaces of LCMO and ZrO2 grains is responsible for the enhanced LFMR.

Giant magnetoimpedance effect in sandwiched films

The giant magnetoimpedance (GMI) effect in films with a sandwiched structure has been studied. FeNiCrSiB/Cu/FeNiCrSiB sandwiched films with a magnetic closed-loop structure were deposited onto a glass substrate by rf sputtering and then annealed at an optimum temperature. GMI ratios as large as 63% and 77% were obtained at 13 MHz in longitudinal and transverse fields, respectively. The ratios are almost twice as large as those obtained in FeNiCrSiB single films.

Current spin polarization and spin injection efficiency in ZnO-based ferromagnetic semiconductor junctions

[FeNi(3 nm)/Zn1−xCoxO(3 nm)]2/ZnO(d nm)/[Zn1−xCoxO(3 nm)/Co(3 nm)]2 (d=3 and 10) semiconductor junctions were prepared by magnetron sputtering system and photolithography. The spin valve effiect was observed in these junctions because the utility of the ferromagnetic composite layers acted as soft and hard magnetic layers. The electrical detection was performed by measuring the magnetoresistance of these junctions to investigate the current spin polarization αsc in the ZnO layer and the spin injection efficiency η of spin-polarized electrons. αsc was reduced from 11.7% (and 10.5%) at 90 K to 7.31% (and 5.93%) at room temperature for d=3 (and d=10). And η was reduced from 39.5% (and 35.5%) at 90 K to 24.7% (and 20.0%) at room temperature for d=3 (and d=10).

Characterization of activation energy for flow in metallic glasses

The molar volume (V{sub m}) scaled flow activation energy ({Delta}E), namely as the activation energy density {rho}{sub E}={Delta}E/V{sub m}, is proposed to describe the flow of metallic glasses. Based on the energy landscape, both the shear and bulk moduli are critical parameters accounting for the {rho}{sub E} of both homogeneous and inhomogeneous flows in metallic glasses. The expression of {rho}{sub E} is determined experimentally to be a simple expression of {rho}{sub E}=(10/11)G+(1/11)K. The energy density perspective depicts a realistic picture for the flow in metallic glasses and is suggestive for understanding the glass transition and deformation in metallic glasses.

Magnetic properties and giant magneto-impedance in amorphous FeNiCrSiB films

The giant magneto-impedance effect has been observed at room temperature in amorphous FeNiCrSiB films prepared by RF sputtering followed by an annealing treatment. The frequency spectra and field dependence of the effect are studied in the frequency range from 100 kHz to 13 MHz and the magnetic field range from 0 to 70 Oe. The longitudinal, transverse and perpendicular effects are observed in detail. A transverse magneto-impedance effect with a magnitude similar to the longitudinal case has been obtained, and a large perpendicular MI effect is also obtained. A maximum GMI ratio of about 38% is obtained at 13 MHz in the longitudinal and transverse cases. The field sensitivity of the longitudinal GMI is 3.4% at 13 MHz. A large perpendicular MI ratio of 11% is obtained at 11 MHz. The effect of annealing on the effective permeability and GMI is reported in this paper.

Magnetic properties and giant magnetoresistance in Fe0.35(In2O3)0.65 granular film

Fe/In2O3 granular films have been prepared by the radio frequency sputtering method. The magnetic and transport measurements of a representative sample, Fe0.35/(In2O3)0.65, showed that there exist different magnetic states in different temperature regions. At room temperature, the film shows superparamagnetic behaviour, and a 5.2% magnetoresistance (MR) ratio was obtained. The susceptibility measurements showed that the blocking temperature is 50 K. Below a certain freezing temperature Tf of about 10 K, the film transits from the ferromagnetic state to the particle-spin-cluster state. In this event, the MR ratio of the film increases dramatically with decreasing temperature. A maximum giant magnetoresistance (GMR) ratio up to 506% is obtained at the metal–semiconductor transition temperature of about 2.2 K. The mechanism of this GMR is related to the interaction with the impurities influencing the local magnetization which is quite different to the spin-dependent tunnelling effect at room temperature.

Substituting effects of Sm in polycrystalline La-Ca-Mn-O

The Sm substituting effects for La in La0.67Ca0.33MnO3 have been studied systematically. With increasing Sm doping amount, the metal-isolator phase transition temperature for the samples decreases monotonically, the corresponding peak resistivity increases dramatically and the Curie temperature decreases monotonically. The substitution of La-Ca-Mn-O with 0.13% Sm for La improved the magnetoresistance ratio by an order of magnitude. The effects of doping with Sm can be explained in terms of the lattice effects. An irreversible MR behaviour was observed in the Sm-doped compound. This effect was enhanced with increasing Sm doping amount.

Magnetic Properties and Giant Magneto-Impedance in Nanocrystalline FeCuCrVSiB Films

The giant magneto-impedance (GMI) effect has been observed in nanocrystalline FeCuCrVSiB films prepared by rf sputtering followed by an annealing treatment. The magnetic properties and the longitudinal and transverse GMI have been studied in detail. Field dependence and frequency spectra of GMI effect have been studied in the frequency range from 100 kHz to 13 MHz. Maximum GMI ratios as large as 50% and 25% were obtained at 13 MHz in the longitudinal and transverse cases, respectively. The field sensitivity of the longitudinal GMI is about 0.11% m/A at 13 MHz. The dependence of effective permeability on the frequency and magnetic field has been investigated.

Polarization effects and interlayer coupling in (Fe-Si)/Pd multilayered films

(Fe-Si)/Pd multilayered films with different layer thicknesses have been obtained by RF sputtering. When the thickness of the Pd layers is fixed, the specific saturation magnetization Ms of films increases with decrease in the thickness of Fe-Si layers. This result is caused by the polarization effect of Pd atoms. On varying the thickness dp of the Pd layers, with a fixed Fe-Si layer thickness, it is found that for Pd layers thinner than 36 AA the polarization effect of Pd is reduced and even disappears at dp=18 AA; then magnetic coupling between the magnetic layers appears on further decrease in dp.

The giant-magneto-impedance effect in FM/Ag/FM (FM≡FeCuCrVSiB) sandwich films

Sandwich films with the structure FM/Ag/FM have been prepared by radio-frequency sputtering and then annealed at an optimized temperature (here FM represents the FeCuCrVSiB amorphous soft magnetic layer). The giant-magneto-impedance (GMI) effects for the samples have been studied as a function of alternating-current frequency (50 kHz to 13 MHz) and external magnetic field. When the frequency increases above a critical frequency f* (about 150 kHz), the GMI ratio increases quickly with increasing frequency, and reaches a maximum value at a characteristic frequency fm of about 4 MHz. GMI ratios at fm as large as 109% and 135% were obtained for longitudinal and transverse applied magnetic fields, respectively. These results are much better than those obtained for single-layer films, and the frequencies used in producing the effect are very low.