Reversible magnetization and superconducting-state thermodynamic parameters for underdoped La1.90Sr0.10CuO4
Magnetization studies have been made of single-crystal La1.90Sr0.10CuO4 with H||c in order to determine the magnitude of the flux expulsion and free energy in a material that has substantially less than optimal doping. Well above the superconducting transition temperature, the normal-state magnetization exhibits a two-dimensional Heisenberg antiferromagnetic behavior. Below Tc, there is a large portion of the H-T plane where the sample shows reversible behavior so that thermodynamic variables such as the free energy and the shape of the magnetization curves can be determined. At low temperature, the vortices have a well defined Abrikosov regime that transforms to two-dimensional fluctuation behavior at higher temperatures. The magnetization vs temperature curves show a unique crossing point at 22 K where the magnetization is independent of magnetic field. From this value of the crossing point, the effective layer spacing s is derived to be 1.6 nm compared to the CuO2 lattice spacing of 0.66 nm. The fluctuations are found to obey two-dimensional scaling in that M/(TH)1/2 is a universal function of [T-Tc(H)]/(TH)1/2. Below 12 K, the data fit the Hao-Clem theory rather well and give κc values of about 175 and thermodynamic critical fields ranging from 112 mT at 12 K to 133 mT at 6 K.