Anisotropic paramagnetic susceptibility of crystalline ruby at cryogenic temperatures

The static paramagnetic susceptibility of chromium ions in sapphire (Cr
³⁺
in Al
₂
O
₃
) in zero applied static magnetic field has been determined between 4.2 K 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 only have a susceptibility component perpendicular to the crystal axis. This component showed a strong Curie law dependence below 30 K.