MRI Study of Nucleation and Growth During the Field Induced First Order Phase Transition Between U2D2 and CNAF 3He near Absolute Zero

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The field-sweep-induced first order phase transition between two nuclear antiferromagnetic states of solid 3He was studied at the temperatures near absolute zero. The phase transition proceeded in two stages. The first stage was governed by a nucleation process, in which many small seeds of the stable phase appeared in many places throughout a crystal. The measured nucleation rate was essentially temperature independent, once we corrected the data for weak temperature dependence due to limited growth of the seeds after nucleation. With the help of MRI measurements, we understand that the nucleation occurs at peculiar heterogeneous nucleation sites, which are distributed in the entire crystal. Magnetic planar defects in the nuclear ordered spin structure are proposed as a candidate for the nucleation sites. The second stage is understood as a process, which is controlled by the flow of released latent heat across the solid-liquid interface.