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The interfacial perpendicular magnetic anisotropy in W/CoFeB (1.2 ∼ 3 nm)/MgO thin film structures is strongly dependent on temperature, and is significantly reduced at high temperature. The interfacial magnetic anisotropy is generally proportional to the third power of magnetization, but an additional factor due to thermal expansion is required to explain the temperature dependence of the magnetic anisotropy of ultrathin CoFeB films. The reduction of the magnetic anisotropy is more prominent for the thinner films; as the temperature increases from 300 K to 400 K, the anisotropy is reduced ∼50% for the 1.2-nm-thick CoFeB, whereas the anisotropy is reduced ∼30% for the 1.7-nm-thick CoFeB. Such a substantial reduction of magnetic anisotropy at high temperature is problematic for data retention when incorporating W/CoFeB/MgO thin film structures into magneto-resistive random access memory devices. Alternative magnetic materials and structures are required to maintain large magnetic anisotropy at elevated temperatures.

Reversible-magnetization data have been studied for YBa2Cu408 in order to determine the thermodynamic properties and the magnetic-field dependence of the superQuid density. By fitting the Hao-Clem variational model to the reversible magnetization data below T, superconducting parameters such as the Ginzburg-I andau parameter e, the thermodynamic critical field H, and the superconducting order parameter are determined. The upper and lower critical fields H 2, 0 i, the penetration depth A, and the coherence length f are derived. Within the model there is a direct connection between the magnetization and the superBuid density, so the vortex structure, the behavior of the average order parameter in (H, T) plane, and the magnetic-field dependence of the super8uid density are obtained. In the high-field region, the evidence of strong vortex Quctuation effects is observed and explained by vortex Huctuation for a three-dimensional superconductor. The mean-field transition temperature T (H) and dH 2, /dT near T, derived from the analysis of vortex Huctuation effects are comparable with fitting results.