T. Tang

Magnetocaloric properties of Ag-substituted perovskite-type manganites

Abstract Magnetic entropy change larger than that of gadolinium has been observed near room temperature in polycrystalline of La 0.8 Ag 0.2 MnO 3 perovskite-type manganite. The large magnetic entropy change produced by the abrupt reduction of magnetization is associated with a first-order phase transition of the sample near the Curie temperature. This phenomenon indicates that perovskite manganites have some potential applications for magnetic refrigerants in an extended high-temperature range.

The magnetocaloric effect and magnetic phase transitions in Dy(Co1−xAlx)2 compounds

Abstract Polycrystalline samples of Laves-phase compounds Dy(Co 1− x Al x ) 2 ( x =0.0, 0.02, 0.04, 0.06, 0.08, 0.1) have been prepared and their magnetic properties were investigated by means of magnetization and ac susceptibility measurements. Results on the magnetocaloric effect (MCE) of these samples have been obtained from the magnetization data, which suggest their potential as working substance of magnetic refrigerators at cryogenic temperature from 142 to 200 K. Both the ac susceptibility results and Arrott plots were used to characterize the magnetic phase transitions in these compounds. By discussing the influence of partial replacement of Co by Al on the magnetic phase transitions and magnetocaloric properties of them, we conclude that the size of the MCE and the order of the magnetic phase transition are closely related.

Giant magnetoresistance of bulk polycrystalline La0.833Na0.167MnO3 with Ag2O addition

Abstract A series of bulk polycrystalline samples of La 0.833 Na 0.167 MnO 3 with Ag 2 O addition were prepared by conventional solid-state reaction processing in air. The mol proportion between La 0.833 Na 0.167 MnO 3 and Ag 2 O is 1: x ( x =0, 0.04, 0.08, 0.125, 0.25, and 0.50). X-ray diffraction patterns show that the sample for x =0, i.e., pure La 0.833 Na 0.167 MnO 3 , contains a single perovskite phase and that the samples for x >0 are composed of two phases (a magnetic perovskite phase and a nonmagnetic Ag-rich phase). It is found that, in this series of polycrystalline samples, a maximum magnetoresistance occurs for x =0.25, i.e., for a composite of the two phases. The corresponding magnetoresistance ratio is about 27% at room temperature. The enhancement of the magnetoresistance effect in such an inhomogeneous granular system can be attributed to the spin dependent scattering of electrons at the interface of the two phases.

The evolution of magnetic domain structure with magnetic history in amorphous film with perpendicular anisotropy

Amorphous rare-earth-Fe-B (RE-Fe-B) films were deposited on Si substrates at 300 °C by dc magnetron sputtering. The evolution of their magnetic domain structures with magnetic history was observed by magnetic force microscopy. It was found that the remanent domain configuration can be gradually transformed from stripes to bubbles after applying different magnetic fields along the out-of-plane direction, which indicates that the film has a perpendicular magnetic anisotropy. In addition, an in situ investigation on the evolution of domain structure under an in-plane field was carried out using a homemade sample holder. The results showed that the domain structure depends on both the applied field and the magnetic history. The present work provides a feasible approach for manipulating magnetic domains in amorphous RE-Fe-B film by employing a magnetic field.

The dependence on microstructure of the magnetotransport behaviour in Sm1/2Sr1/2MnO3

Two sets of polycrystalline manganites with the composition Sm1/2Sr1/2MnO3 were prepared using the sol–gel (SG) and the solid-state reaction (SSR) methods. It is found that the magnetotransport properties for both sets of samples with the same composition are very strongly dependent on their preparation methods. For the SG samples, insulating behaviour at zero magnetic field over a temperature range from 70 to 300 K, and a metal–insulator transition with a huge reduction in resistance near TC ~ 100 K under a sufficiently high magnetic field over 2 T, which lead to a maximum magnetoresistance ratio value up to 105% at 2 T and over 106% at 5 T near 65 K, were observed. However, for the samples prepared by the SSR method the resistance (R)–temperature (T) curve only showed a single insulator–metal transition. At the same time, their temperature-dependent magnetizations, M(T), are also totally different. The morphologies obtained by scanning electron microscopy demonstrate that there is a large difference in microstructure between the two sets of samples. The fine grains and a large number of grain boundaries are primary factors leading to the insulating characteristics at zero field and the unique transport properties induced by a sufficient magnetic field in the SG samples.

Amorphous to polycrystalline transformation of (BiDy)IG films (abstract)

(BiDy)Fe5O12 films in the amorphous state were made by rf magnetron sputtering from a sintered Bi2DyFe5O12 ceramic target onto quartz substrates and confirmed by x‐ray diffraction. The films were annealed at different temperatures ranging from 500 to 800 °C for 1 h in air. The crystallization temperature was about 650 °C, as measured by DTA method. Magnetization M versus temperature relation of well crystallized films showed both a compensation temperature Tcomp (80 K) and Curie temperature TC (563 K). The M‐T curve from 1.5 to 600 K of amorphous film showed no definite TC or Tcomp; M measured in a field of 20 kOe decreased monotonously. The Faraday rotation (500–700 nm) of amorphous films was small. The M and Faraday rotation (633 nm) increased rapidly at the crystallization temperature. The Bi ion induced peak (550 nm) in the Faraday spectra got higher with the increasing annealing temperature and was up to 9.14 degree/μm. The Faraday spectra of films annealed at 650 °C for different times showed that t...