Z.B. Guo

Large magnetic entropy change in La0.75Ca0.25MnO3

Magnetic entropy change of La0.75Ca0.25MnO3 with different particle size has been investigated. Magnetic entropy change ∼4.7 J/kg K larger than that of Gd has been observed at 224 K under a magnetic field of 1.5 T in comparatively big particles. This phenomenon of large magnetic entropy change indicates that the perovskite-type manganese oxides have potential applications for magnetic refrigerants in high temperature.

Preparation and magnetic properties of SrFe12O19 particles prepared by the salt-melt method

Abstract The salt-melt method was adopted to prepare high-performance Sr ferrite without further pulverizing. The reaction path, particles size and magnetic properties have been investigated by using high-purity starting raw materials to synthesize Sr ferrite by replacing SrSO 4 with celestite. The magnetic properties are compared with those of materials prepared by the ceramic process. Due to the lower reaction temperature, no further pulverization is needed and due to low-cost minerals, the process should be more economical than the conventional ceramic process.

Magnetic entropy change in La0.75Ca0.25-xSrxMnO3 perovskites

Abstract Magnetic entropy change has been studied in a series of polycrystalline samples of La 0.75 Ca 0.25− x Sr x MnO 3 with fixed carrier concentration and gradual change in the average ionic radius of the La site 〈 r A 〉. In a fixed crystal structure with orthorhombic or rhombohedral phase, we found that the magnetic entropy change decreases with increasing 〈 r A 〉. This phenomenon has been attributed to the difference in the volume thermal expansion at T c which is influenced by 〈 r A 〉. A smaller 〈 r A 〉 would be beneficial for obtaining larger magnetic entropy change.

Electrical properties of La0.7−xPrxSr0.3MnO3 perovskite

We present the electrical properties of polycrystalline La0.7−xPrxSr0.3MnO3 in which the average ionic radius of the A site 〈rA〉 is systematically varied while keeping the carrier concentration fixed. Below the Curie temperature Tc, the spin-disorder scattering process has been regarded to play an important role in the resistivity. With decreasing 〈rA〉, the structure transition from rhombohedral to orthorhombic phase accompanied with the sharp increase in resistivity at Tc and the increase of the activation energies has been observed.

Large magnetic entropy change in La0.67−xGdxCa0.33Mno3

Abstract A magnetic entropy change larger than that of gadolinium has been observed in perovskite manganites La 0.67− x Gd x Ca 0.33 MnO 3 ( x = 0, 0.05, 0.1, and 0.15) with fixed carrier concentration. The large magnetic entropy change produced by the abrupt reduction of magnetization is attributed to an anomalous thermal expansion at the Curie temperature. This phenomenon indicates that perovskite manganites have some potential applications for magnetic refrigerants in an extended high-temperature range.

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.

Giant uniaxial piezoresistance in GMR perovskite La0.85Sr0.15MnO3

Abstract The uniaxial-pressure dependences of electric and magnetic properties have been investigated in perovskite-type LaSrMrO. A giant piezoresistance (GPR) effect and a remarkable pressure-induced increase of T p , the onset of the metal-semiconductor-like (M-S) transition, have been observed. The piezoresistance value (PR) and the pressure coefficient of T p are all larger by two orders of magnitude than that in the case of hydrostatic pressure. Piezoresistance and magnetoresistance peaks were found to locate at almost the same temperature. This fact suggests that GMR and GPR may originate from the same mechanism(s) and may be relative to lattice transformation(s).

Thermal hysteresis in magnetization and conductivity for polycrystalline La0.55Dy0 12Ca0.33MnO3

Abstract A considerable thermal hysteresis in magnetization and conductivity and a large magnetoresistance ratio ( Δπ π 0 ) in excess of 80% (with applied magnetic field H = 15 kOe) over a wider temperature span (∼50 K) has been obtained in the sample of polycrystalline La0.5Dy0.12Ca0.33MnO3, indicating that a particular magnetic structure, the canted ferromagnetic structure, should be present. The metamagnetic transition from canted ferromagnetic ordering to ferromagnetic ordering is possible to occur at relatively high temperatures and under a small applied magnetic field.

Field induced antiferromagnetic ordering in NiS cluster confined in zeolite Y

An interesting field induced antiferromagnetic ordering in NiS clusters confined in Y molecular sieve was observed. The NiS cluster-zeolite Y composite exhibits an unusual antiferromagnetic ordering state under a magnetic field of 0.724–1.164 T and paramagnetic behavior out of this field region. The antiferromagnetic ordering can be stable up to 180 K. The phenomenon may be understood by the concept of energy level crossing due to intracluster interaction at a certain magnetic field.