Anisotropy of Superconducting ${\mathrm{MgB}}_2$ as Seen in Electron Spin Resonance and Magnetization Data

We observed the conduction electron spin resonance (CESR) in fine powders of ${\mathrm{MgB}}_2$ both in the superconducting and normal states. The Pauli susceptibility is ${\chi }_s=2.0\mathrm{}\times {10}^{-5\mathrm{}}\mathrm{emu}/\mathrm{mole}$ in the temperature range of 450 to $600\mathrm{K}$. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below $T_c$ at several frequencies suggests that ${\mathrm{MgB}}_2$ is a strongly anisotropic superconductor with the upper critical field, $H_{c2\mathrm{}}$, ranging between 2 and $16\mathrm{T}$. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that ${\mathrm{MgB}}_2$ is an anisotropic superconductor with $H_{c2\mathrm{}}^{\mathrm{ab}}{/H}_{c2\mathrm{}}^c\approx 6–9$.