Jinguang Cheng

Enhanced electron correlation in rare-earth doped Ca3Co4O9

Thermoelectric and thermodynamic properties of a series of Ca-site rare-earth R3+ doped Ca3Co4O9 were investigated. For a fixed doping level, with the reduction in R3+ radius, thermopower is found to gradually increase despite the carrier concentration remains unchanged. The scaling behavior between thermopower and electron specific heat coefficient reveals that the additional increase in thermopower originates from enhanced electron correlation which may be attributed to doping-induced chemical pressure.

The thermal-transport properties of the Ca3−xAgxCo4O9 system (0≤x≤0.3)

Polycrystalline Ca3−xAgxCo4O9 (0≤x≤0.3) samples were prepared by solid-state reaction and their thermo-transport properties were studied from 5 K to room temperature. With the substitution of Ag+ for Ca2+, the internal chemical pressure induced by Ag+ doping has a strong effect on the transport properties of such a strongly correlated Fermi liquid system. The electrical conductivity and the thermoelectric power increase simultaneously because of the enhancement of carrier concentration and the change of carrier mobility. The thermal conductivity decreases monotonically up to x = 0.3 due to the Ag ion acting as a rattler in the system. These results showed that the thermoelectric performance of the Ca3Co4O9 system can be improved by doping with Ag.

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.

Efficient room temperature thermoelectric characteristics of Ca(3-x)Ag(x)Co(4)O(9+delta)/Ag(y) composites

[Wang, Yang; Sui, Yu; Cheng, Jinguang; Wang, Xianjie; Su, Wenhui] Harbin Inst Technol, Dept Phys, CCMST, Harbin 150001, Peoples R China. [Sui, Yu; Su, Wenhui] Chinese Acad Sci, Int Ctr Mat Phys, Shenyang 110016, Peoples R China.;Sui, Y (reprint author), Harbin Inst Technol, Dept Phys, CCMST, Harbin 150001, Peoples R China;suiyu@hit.edu.cn

Magnetoresistance in Sr2FeMoO6 : x glass composites

The effects of interfacial states on the temperature dependence of the magnetoresistance (MR) of Sr2FeMoO6-glass composites have been studied. X-ray diffraction analyses show that the glass is most likely located at the grain boundary without causing a change of the crystal structure of Sr2FeMoO6. The variation of the resistance with temperature and magnetic field indicates that the added glass layer has profound influence on the MR properties. At low temperature, the MR in low fields is enhanced notably because the insulating barrier for the intergranular tunneling is improved by adding the glass layer at the grain surface. However, at high temperature, the MR decreases rapidly with the increase of temperature due to, in addition to the enhancement of spin-independent hopping of electrons through the localized states, the fast decay of spin polarization at the surfaces of the grains. This decay is induced by the separation of the ferromagnetic grains with the nonmagnetic glass layer at the grain boundaries.

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

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.

Large reversible magnetocaloric effect in TmTiO3 single crystal

The magnetic properties and magnetocaloric effect (MCE) of TmTiO3 single crystal were studied by magnetization and heat capacity measurements. A large reversible MCE was observed at the Curie temperature T-C of 65 K, which is related to a second-order magnetic transition from paramagnetic to ferrimagnetic state. The values of maximum magnetic entropy change can reach 7.2 and 13.5 J kg(-1) K-1 for a field change of 2 and 5 T, respectively, with no obvious hysteresis loss in the vicinity of the Curie temperature. The corresponding maximum adiabatic temperature changes are found to be 3.5 and 6.8 K. The magnetic transition and the origin of large MCE in TmTiO3 single crystal are discussed