Anisotropic magnetoresistive and magnetic properties of La_{0.5}Sr_{0.5}CoO_{3-\delta} film

The magnetic and transport properties of La_{0.5}Sr_{0.5}CoO_{3-\delta} film grown on a LaAlO_3 substrate by pulsed-laser deposition are studied. The properties are found to be influenced by the magnetic anisotropy and inhomogeneity. Magnetoresistance anisotropy is determined by the shape anisotropy of the magnetization and the strain-induced magnetic anisotropy due to the film-substrate lattice interaction. Indications of the temperature-driven spin reorientation transition from an out-of plane orderded state at low temperatures to an in-plane ordered state at high temperatures as a result of competition between the mentioned sources of magnetic anisotropy are found.Comment: 5 pages, 8 figures, submitted to Fiz. Nizk. Temp, an extended version of short communication in cond-mat/020734

Influence of high-energy electron irradiation on the transport properties of La_{1-x}Ca_{x}MnO_{3} films (x \approx 1/3)

The effect of crystal lattice disorder on the conductivity and colossal magnetoresistance in La_{1-x}Ca_{x}MnO_{3} (x \approx 0.33) films has been examined. The lattice defects are introduced by irradiating the film with high-energy (\simeq 6 MeV) electrons with a maximal fluence of about 2\times 10^{17} cm^{-2}. This comparatively low dose of irradiation produces rather small radiation damage in the films. The number of displacements per atom (dpa) in the irradiated sample is about 10^{-5}. Nethertheless, this results in an appreciable increase in the film resistivity. The percentage of resistivity increase in the ferromagnetic metallic state (below the Curie tempetature T_{c}) was much greater than that observed in the insulating state (above T_{c}). At the same time irradiation has much less effect on T_{c} or on the magnitude of the colossal magnetoresistance. A possible explanation of such behavior is proposed.Comment: RevTex, 22 pages, 3 Postscript figures, submitted to Eur. Phys. J.

Transport, thermal and magnetic properties of RuSr_2(Gd_{1.5}Ce_{0.5})Cu_2O_{10-\delta}, a magnetic superconductor

Resistivity, thermoelectric power, heat capacity and magnetization for samples of RuSr_2(Gd_{1.5}Ce_{0.5})Cu_{2}O_{10-\delta} were investigated in the temperature range 1.8-300 K with a magnetic field up to 8 T. The resistive transitions to the superconducting state are found to be determined by the inhomogeneous (granular) structure, characterized by the intragranular, T_{c0}, and intergranular, T_{cg}, transition temperatures. Heat capacity, C(T), shows a jump at the superconducting transition temperature T_{c0}\approx 37.5 K. A Schottky-like anomaly is found in C(T) below 20 K. This low temperature anomaly can be attributed to splitting of the ground term $^{8}S_{7/2}$ of paramagnetic Gd^{3+} ions by internal and external magnetic fields.Comment: 3 pages (4 figs. incl.), reported at 50th Magnetism and Magnetic Materials Conference, San Jose, CA, USA, 200

The upper critical field in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated in magnetic fields up to 16 T. The upper critical field of the major fraction of the investigated sample was determined from ac susceptibility and resistance data and was found to increase up to Hc2(0) = 13 T at T = 0 corresponding to a coherence length of 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at intermediate temperatures. This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit.Comment: 8 pages with 4 figure

Characteristic crossing point (T∗≈2.7T_{\ast}\approx 2.7 K) in specific-heat curves of samples RuSr2_2Gd1.5_{1.5}Ce0.5_{0.5}Cu2_2O10−δ_{10-\delta} taken for different values of magnetic field

Magnetic properties of polycrystalline samples of RuSr$_2$(Gd$_{1.5}$Ce$_{0.5}$)Cu$_{2}$O$_{10-\delta}$, as-prepared (by solid-state reaction) and annealed (12 hours at 845$^{\circ}$C) in pure oxygen at different pressure (30, 62 and 78 atm) are presented. Specific heat and magnetization were investigated in the temperature range 1.8--300 K with a magnetic field up to 8 T. Specific heat, $C(T)$, shows a jump at the superconducting transition (with onset at $T\approx 37.5$ K). Below 20 K, a Schottky-type anomaly becomes apparent in $C(T)$. This low-temperature anomaly can be attributed to splitting of the ground term ${^8}S_{7/2}$ of paramagnetic Gd$^{3+}$ ions by internal and external magnetic fields. It is found that curves $C(T)$ taken for different values of magnetic field have the same crossing point (at $T_{\ast}\approx 2.7$ K) for all samples studied. At the same time, $C(H)$ curves taken for different temperatures have a crossing point at a characteristic field $H_{\ast}\approx 3.7$ T. These effects can be considered as manifestation of the crossing-point phenomenon which is supposed to be inherent for strongly correlated electron systems.Comment: 10 pages, 7 figures, submitted to J. Phys.: Condens. Matte