Using microwave ultrasonic interferometry we have studied the relaxation dynamics of an order‐order transition in an Ising antiferromagnet. The system is prepared in one antiferromagnetic phase and relaxes to its thermodynamically inequivalent time‐reversed conjugate, driven by an induced staggered field. The time‐reversed phases induce equal and opposite changes in the velocity of transverse polarized ultrasonic waves and this is exploited by the interferometer to monitor the volume fractions of the conjugate phase during the relaxation process. The results indicate that the relaxation is dominated in the initial stages by domain growth of the stable phase as opposed to nucleation‐dominated growth. This process can be closely fitted by a time‐cubed growth law for the domains of stable phase. The final stages are well approximated by exponential behavior.

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In the experiments described, the relaxations were initiated by increasing the magnetic field to the final value at which it was desired to observe the relaxation. However, owing to the requirement that the field should not overshoot and exceed the critical field Bc this approach to the final value is necessarily slow and allows the relaxation to commence before the final field value is reached, thereby rendering the time origin uncertain. The value of the 100% metastable phase signal is likewise uncertain since it varies as order parameter×magnetic field, and the latter is still changing when the relaxation begins. Work is in progress on a refinement of the experimental technique designed to eliminate these uncertainties.
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