Planar approximation for the frequencies of spin transfer oscillators
  A large class of spin transfer oscillators use the free layer with a strong easy plane anisotropy, which forces its magnetization to move close to the plane. We show that in this situation the effective planar approximation provides a fast and accurate way of calculating the oscillator frequency. 
Ya. B. Bazaliy and F. Arammash
cond-mat.mtrl-sci