We study dynamic and thermodynamic properties of a classical spin chain driven by imaginary magnetic field, which is shown to correctly describe the action of Slonczewski spin-transfer torque (STT). We consider the particular case of a parity-time (PT) symmetric system with mutually perpendicular applied real and imaginary magnetic fields, perdicting a new type of non-equilibrium phase transitions. It corresponds to a transition from precessional to exponentially damped dynamics of the spin chain and is associated with SST-driven spontaneous PT symmetry breaking. We show how the properties of this new phase transitions can be derived from the system’s partition function, demonstrating the remarkable connection between the system’s dynamics and its statistical properties. Our results establish the physical role of imaginary magnetic fields in the description of dynamical and statistical properties of open non-equilibrium spin systems.