Superconductivity and magnetism in a novel La0.85Ga0.15FeAsO compound

Abstract

We report magnetization measurements on samples of La0.85Ga0.15FeAsO in the wide temperature range of 10 K-400 K, up to the field of 90 kOe having been prepared by solid-state reaction. This novel compound crystallizes with the tetragonal ZrCuSiAs-type structure. The crystal structure was determined by X-ray powder diffraction (P4/nmm: a = 4.032 angstrom, and c = 8.741 angstrom). Temperature dependence of magnetization for this compound is measured with zero-field cooling (ZFC) and field cooling (FC) with an application of H = 50 Oe. The large bifurcation between the ZFC and FC curves has been observed starting at around 370 K. At lower temperatures below T-sc(set-on) similar to 37 K, superconductivity and magnetism coexist. Intermediate temperature range (37 K < T < 300 K), the sample is in a mixed state of the ferromagnetic clusters with paramagnetic moments, described well by the Modified Curie Weiss expression. At higher temperatures above 360 K, it is a pure paramagnetic, described perfectly by the simple Curie-Weiss expression. The natures of the superconducting flux-pinning centers are revealed based on analysis of the pinning force, F-pin at T = 10 K. The normalized pinning force, F-p/F-p,F-max versus H indicates the filamentary character owing to the pinning parameters, p = 0.5, q = 2. These results point out that, in the superconductivity, the dominance of grain boundary with single-vortex pinning (mu = 1/7) mechanism The Curie temperature, T-curie was estimated using the modified Arrot plots. The critical exponents, beta, (sic), and delta were estimated, and their values are in close agreement with those predicted for a mean-field-like behavior at a magnetic phase transition.