Spin echo without an external permanent magnetic field

We suggest a sequence of instantaneous spin rotations, that reverses the time evolution of a spin subject to an arbitrary built-in magnetic field. The sequence does not require the knowledge of the direction or strength of the built-in field, and is applicable as long as that field is constant in time. The advantage of the suggested sequence compared to the known spin-echo sequences is that it does not involve the application of a permanent strong magnetic field. The spin rotations may be achieved by application of short magnetic field pulses.