  We calculate the number density, energy density, transverse pressure,
longitudinal pressure, and magnetization of an ensemble of spin one-half
particles in the presence of a homogenous background magnetic field. The
magnetic field direction breaks spherical symmetry causing the pressure
transverse to the magnetic field direction to be different than the pressure
parallel to it. We present explicit formulae appropriate at zero and finite
temperature for both charged and uncharged particles including the effect of
the anomalous magnetic moment. We demonstrate that the resulting expressions
satisfy the canonical relations, Omega = - P_parallel and P_perp = P_parallel -
M B, with M = - d Omega/d B being the magnetization of the system. We
numerically calculate the resulting pressure anisotropy for a gas of protons
and a gas of neutrons and demonstrate that the inclusion of the anomalous
magnetic increases the level of pressure anisotropy in both cases.
