Geo-Myung Shin

Giant magnetic entropy change in La0.7Ca0.3MnO3 in low magnetic field

A giant magnetocaloric effect was found in La0.7Ca0.3MnO3 manganite at low magnetic fields. The magnetic entropy changes caused by a magnetic field change of 0.5 and 1.0 T were as large as 5.04 and 6.25 J kg−1 K−1, respectively, which exceed those of perovskite-like magnetic refrigerants or elemental Gd by a factor of 2–3. The extremely high values of the magnetocaloric effect are attributed to a sharp drop in the magnetization at a first-order ferromagnetic–paramagnetic phase transition and S-shaped magnetization versus field dependence.

Magnetotransport properties of c-axis oriented La0.7Sr0.3MnO3 thin films on MgO-buffered SiO2/Si substrates

c-axis oriented La0.7Sr0.3MnO3 (LSMO) films on MgO-buffered SiO2/Si substrates were prepared, and their texture, microstructure, and magnetotransport properties were studied and compared to epitaxial LSMO/MgO (001) and polycrystalline LSMO/SiO2/Si films. c-axis oriented MgO buffer layers were obtained on amorphous SiO2 layer through rf sputter deposition at low substrate temperature and consequent postannealing processes. In situ pulsed laser deposition-grown LSMO films, deposited on the MgO layer, show strong c-axis texture, but no in-plane texture. The c-axis oriented LSMO films which are magnetically softer than LSMO/SiO2/Si films exhibit relatively large low field magnetoresistance (LFMR) and sharper MR drop at lower field. The large LFMR is attributed to a spin-dependent scattering of transport current at the grain boundaries.

Fabrication of high-jc YBCO films by the TFA-MOD Process using YBCO powder as precursor

YBCO thin films were prepared on LaAlO/sub 3/ (100) single-crystal substrates using the metal-organic deposition of trifluoroacetates (TFA-MOD). Unlike the normal TFA-MOD process, TFA-based solutions were prepared dissolving YBCO powder into the trifluoroacetic acid solvent. The solutions were coated on the substrates by spin coating, fired at the temperature up to 400/spl deg/C in 4.2% humid and oxygen atmosphere, fired again at the temperature region of 725-850/spl deg/C for 2 h in 4.2% humidity and 1000 ppm oxygen partial pressure, and finally oxygenated at 450/spl deg/C for 1 h. Similar to the normal TFA-MOD process, both microstructure and superconducting properties of YBCO films were highly sensitive to the firing temperature. Optimally processed YBCO films of /spl sim/250 nm thickness exhibited T/sub C,zero/ of 90 K and J/sub C/ of 2.9 MA/cm/sup 2/ at 77 K in a self-field. The present results clearly evidence that our cost-effective modified TFA-MOD process is applicable to the fabrication of YBCO coated conductors.

Magneto-Transport Properties of

We report the magneto-transport properties of La_0.7Ca_0.3-xSr_xMnO₃ (<i>x</i>=0 , 0.075, 0.1, 0.125, 0.15, 0.3) thin films on (100) LaAlO₃ substrates prepared by pulsed electron-beam deposition (PED). With increasing <i>x</i> from 0 to 0.3, both Curie temperature (<i>TC</i>) and metal-insulator transition temperature (<i>TMI</i>) of the films were increased from 206 and 212 K to 339 and 366 K, respectively. Except <i>x</i>=0.125 film, with increasing <i>x</i> , all other LCSMO films showed a tendency of gradual decrease in the maximum MR ratios (<i>MR</i> _MAX) at increased maximum MR temperature (<i>T</i> _MAX,) values. The film of <i>x</i>=0.125 exhibited extraordinary high <i>MR</i> _MAX value of 16% at 306 K. After post-annealing in a pure oxygen atmosphere at 900^degC, this film showed more enhanced <i>MR</i> _MAX of 18% at 325 K.

\hbox{La}_{0.7}\hbox{Ca}_{0.3-{\rm x}}\hbox{Sr}_{\rm x}\hbox{MnO}_{3}

SmBa₂Cu₃O_7-delta(SmBCO) thin films were successfully prepared on the LaAlO₃(100) substrate by the TFA-MOD method in an extremely reduced oxygen atmosphere. The precursor solution was prepared dissolving SmBCO powder into the trifluoroacetic acid solution. The precursor solution was spin-coated on the LaAlO₃(100) single crystalline substrates, calcined at the temperature up to 400degC, and fired at various high temperatures for 2 h in reduced oxygen atmospheres. Optimally processed SmBCO thin film exhibited T_C,zero value of 90.2 K and J_C value of 0.8 MA/cm² at 77 K, respectively. Relatively lower J_C values in SmBCO films compared with MOD-processed YBCO films are attributed to the existence of a-axis oriented grains.

Thin Films Deposited by Pulsed Electron Deposition

We report the effects of barium-based perovskite particle inclusions of BaCeO₃ (BCO), BaZrO₃ (BZO), and YBa₂SnO_5.5 (YBSO) on the thickness dependence of critical current density (J_c) in metal organic deposition (MOD)-processed YBa₂Cu₃O_7-δ (YBCO) films on the SrTiO₃ (STO) single-crystal substrate. With increasing film thickness, while BZO-doped YBCO films show a serious degradation in J_c, caused by the interlayer at the boundaries of multicoated layers, BCO- and YBSO- doped films exhibit a significant enhancement of J_c (77 K, self-field) over ~1 μm thickness even though BCO- or YBSO- doped YBCO films have degraded in-plane textures compared with undoped films, implying that these perovskite dopants affect not only the pinning properties but also the growth of multicoated layers. The highest critical current (I_c) value over 440 A/cm-width, corresponding to J_c value of 1.8 MA/cm², was obtained from the 2.4-μm-thick YBSO-doped film.