Magnetic anisotropy of high-Tcsuperconductors

Abstract

The magnetic anisotropies of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals have been investigated by simultaneously measuring the longitudinal (${\mathit{M}}_{\mathit{L}}$) and transverse (${\mathit{M}}_{\mathit{T}}$) components of the equilibrium magnetization of crystals oriented at arbitrary angles with respect to the applied field direction. The variations of ${\mathit{M}}_{\mathit{L}}$ and ${\mathit{M}}_{\mathit{T}}$ as a function of orientation, field, and temperature have been measured. In the regime where the simple three-dimensional anisotropic London theory is valid, it is shown that the measurement of ${\mathit{M}}_{\mathit{T}}$/${\mathit{M}}_{\mathit{L}}$ with an applied field in the range ${\mathit{H}}_{\mathit{c}1}$≪H≪${\mathit{H}}_{\mathit{c}2}$ yields the anisotropic mass ratio (${\mathit{m}}_3$/${\mathit{m}}_1$) directly. The possible breakdown of the simple theory is discussed.