  We examine the effects of a relativistic wind on the spin down of a neutron
star and apply our results to the study of Soft Gamma Repeaters (SGRs), thought
to be neutron stars with magnetic fields > 10^{14} G. We derive a spin-down
formula that includes torques from both dipole radiation and episodic or
continuous particle winds. We find that if SGR1806-20 puts out a continuous
particle wind of 10^{37} {\rm erg} {\rm s}^{-1}, then the pulsar age is
consistent with that of the surrounding supernova remnant, but the derived
surface dipole magnetic field is only 3 \times 10^{13} G, in the range of
normal radio pulsars. If instead, the particle wind flows are episodic with
small duty cycle, then the observed period derivatives imply magnetar-strength
fields, while still allowing characteristic ages within a factor of two of the
estimated supernova remnant age. Close monitoring of the periods of SGRs will
be able to establish or place limits on the wind duty cycle and thus the
magnetic field and age of the neutron star.
