Fluctuation breakdown of metastable states of magnets in phase transitions
Abstract
The problem of inhomogeneous states of magnets in the neighborhood of a first order phase transition is examined. The structure and energy of the critical fluctuations are investigated for any degree of metastability for paramagnetic-antiferromagnetic, paramagnetic-ferromagnetic, and antiferromagnetic-ferrimagnetic transitions. The limiting case of critical nuclei is studied for a weak metastability. It is shown that because of magnetostriction there is a definite temperature (or external field) range in which metastable states can exist for an arbitrarily long time in the neighborhood of a first order phase transition point. These ranges are found and a comparison is made with available experimental results for certain magnets on the minimal magnitude of the hysteresis originating at the phase transition. The influence of the specimen surface on nucleus formation in magnets is investigated. It is shown that the near-surface nuclei can diminish the range in which the metastable states are not broken down. The energetic barriers for the formation of near-surface nuclei are determined. The influence of point defects of a crystal on the nucleus formation in a magnet is investigated, and it is shown that their presence can shift the whole phase transition picture to either side in the temperature or the external field depending on the kind of defects and on their concentration in the crystal. Kinetics of the metastable-state breakdown process are investigated. Reduced dynamical equations describing the evolution of fluctuations in magnets are obtained, from which it follows that the subcritical fluctuations will collapse while the post-critical fluctuations will grow; solutions of the fluctuation evolution equations are found without and with magnetostriction taken into account. The times of metastable state breakdown of magnets are estimated.