Zhengnan Qian

Enhancement of low-field magnetoresistance in polycrystalline Sr2FeMoO6 with Al doping

Polycrystalline Sr2Fe1−xAlxMoO6 compounds with x=0, 0.05, 0.10, 0.15, and 0.30 were fabricated and their low-field magnetoresistance (LFMR) performance studied. The LFMR was greatly enhanced as x increased from nil to 0.15, and its origin was found to be intragranular spin-dependent scattering. The replacement of Fe by Al weakened the antiferromagnetic exchange in the antiphase boundary arising from the antisite defect and acted as a barrier for electron transport along the Mo–O–Fe–O–Mo–O–Fe chain in the ferromagnetic segregation and weakened the ferromagnetic exchange. When the external magnetic field was applied, the spin in the antiphase boundary appeared to align more easily to the external field after Al doping and the transport of the electron was easier due to the weakening of the double exchange barrier.

Effect of Surface Modifications on the Magnetic Properties of Ni0.5Zn0.5Fe2O4 Nanoparticles

Two different silica sources have been used for the preparation of Ni 0.5 Zn 0.5 Fe 2 O 4 /SiO 2 nanocomposites, and their effects on the particle size, crystallinity and the magnetic properties have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and physical property measurement system (PPMS). In both cases, Ni 0.5 Zn 0.5 Fe 2 O 4 nanoparticles of 11 nm in mean particle size were well dispersed in silica matrix. The coercivity and the blocking temperature of Ni 0.5 Zn 0.5 Fe 2 O 4 nanoparticles modified with the silicon coupling agent show little decrease, whereas the saturation magnetization increases obviously with respect to nanoparticles dispersed directly in silica matrix. It is considered that the decrease of the surface anisotropy resulting from the change in the surface coordination is responsible for the observed behavior.

A new semi-Heusler ferromagnet NiFeSb: electronic structure, magnetism and transport properties

We report on structural, magnetic, transport, and spin polarization measurements of a new semi-Heusler alloy NiFeSb, which was synthesized successfully by means of electron melt-spinning technique although its ferromagnetic state and crystal structure was predicted to be unstable by the band calculation. The temperature-dependent measurement of magnetizations can both be interpreted well by the spin-wave theory. The electrical resistivity follows a T-5 behavior at low temperatures. Point contact Andreev reflection measurements of the spin polarization yield moderate polarization of 52%

Magnetic and magneto-transport properties of double perovskite Sr2FeMoO6 with Co doping

The electrical, magnetic and magnetoresistance properties of polycrystalline Sr2Fe1-xCoxMoO6 compounds with x = 0.0, 0.05, 0.15 and 0.25 were investigated. With increasing Co content, both the degree of cationic order and the saturation magnetic moment per formula unit of these compounds increase, while their resistivity decreases monotonically and exhibits an enhanced metallic behaviour for x >= 0.15. The low-field magnetoresistance was greatly enhanced at x = 0.15, and the field dependence of magnetoresistance for x >= 0.15 almost saturates above 1.5 T. These results can be well explained by the change of magnetic structure of Sr2FeMoO6 after replacing partial Fe ions by Co ions. The enhanced metallic behaviour and low-field magnetoresistance also suggest a great application potential in spin electronic devices.

Enhanced low field magnetoresistance in Sr2FeMoO6-glass composites

In this paper, we report the enhancement of the low field magnetoresistance in Sr2FeMoO6-glass composites with different wt % percents of glass. The crystal structure of Sr2FeMoO6 does not change by adding glass, and the glass is most likely located at the grain boundaries. The low field magnetoresistance up to 1 T of Sr2FeMoO6-glass composites at 10 K all shows obvious butterfly-shaped field dependence other than the pure Sr2FeMoO6 bulk sample. The magnetoresistance of Sr2FeMoO6-glass composites at 10 K is enhanced gradually with increasing the glass concentration and reaches 39% with a wt % of 50%. The enhancement of low field magnetoresistance in Sr2FeMoO6-glass composites can be well explained by the spin-dependent tunneling at the glass boundaries, and it also allowed us to conclude that the spin polarization of Sr2FeMoO6 is at least 80% at low temperature

Superparamagnetic behaviour in melt-spun Ni2FeAl ribbons

Heusler alloy Ni2FeAl has been synthesized by the melt-spinning technique. The dc magnetization and frequency-dependent ac susceptibility measurements reveal that this alloy exhibits the characteristic feature of superparamagnetism. This behaviour may be associated with a structural disorder stemming from the fast quenching after the heat treatment. The small frequency-dependent ac susceptibility shifts in the blocking temperature and the existence of a pronounced peak in FC magnetization as well as Vogel–Fulcher activation processes indicate that intergranular interactions dominate in the melt-spun ribbons of Ni2FeAl.

Influence of nonmagnetic Al ions on magnetoresistance of double-perovskite Sr2Fe1−xAlxMoO6 (0≤x≤0.30)

The structural, magnetic, and magnetoresistance properties of the double-perovskite series Sr2Fe1-xAlxMoO6 (0 <= x <= 0.30) were systematically investigated in order to clarify the influence of nonmagnetic Al ions on the magnetoresistance. The structural refinements of these samples show that the degree of cationic order increases gradually from 88.5% for x=0 to 92% for x=0.30 without any change in the crystal structure. The magnetization measurements reveal that the substitution of nonmagnetic Al ion for Fe ion enhances the magnetic moment per Fe ion significantly. In addition, the magnetic-field dependence of magnetization and magnetoresistance of these Sr2Fe1-xAlxMoO6 samples were all fitted excellently by taking into account the contributions from ferromagnetic-coupled Fe-O-Mo region and nonferromagnetic-coupled regions. The fitting results indicate that the low-field magnetoresistance can be greatly enhanced due to the separation of the cationic-ordered Fe-O-Mo regions by the paramagnetic Mo-O-Al-O-Mo chains introduced through Al doping. Furthermore, doping nonmagnetic Al ions also suppress the formation of antiferromagnetic Fe-O-Fe antiphase boundaries, and then lead to the improvement of cation ordering and the reduction of magnetoresistance under high field. (c) 2005 American Institute of Physics.

Influence of hot pressure on the magnetoresistance of CrO2

In this paper, we investigate the influence of high temperature and high pressure (hot pressure) on the magnetic and transport properties of polycrystalline CrO2 samples compacted under high pressure and high temperature of up to 5GPa and 600°C, respectively. The magnetic moment increases with compacting temperature, and a metal-semiconductor transition is observed in hot-pressed samples, different from the cold-pressed samples. These results indicate that the formation of Cr2O3 at the grain boundaries of CrO2 is suppressed by hot pressure. The magnitude of low field magnetoresistance of up to 1T at 5K is enhanced first with the increase of compacting temperature and then decreased under higher compacting temperature. This result can be well explained by the change of spin-dependent tunneling at the modulated grain boundaries of CrO2 due to the transformation from Cr2O3 to CrO2 under hot pressure.