Anisotropic paramagnetic susceptibility of crystalline ruby at cryogenic temperatures

The static paramagnetic susceptibility of chromium ions in sapphire (Cr3+ in Al2O3) in zero applied static magnetic field has been determined between 4.2 and 100 K using the whispering gallery mode technique at microwave frequencies. Although sapphire exhibits a uniaxial anisotropy in permittivity, in contrast the magnetic susceptibility has been measured to have a susceptibility component only perpendicular to the crystal axis. This component showed a strong Curie law dependence below 30 K.