Effect of magnetic field on early stage of ordering process of FePd alloys

Abstract

With a disorder-order transition of FePd alloys from A1 to L10, characteristic micro-twin structures are formed, and marked uniaxial magnetic anisotropy is revealed, and much work has been done regarding the relation between microstructures and magnetic properties of the alloys. Few work, however, has been reported as to the effect of magnetic field on the morphology change at the beginning of order reactions of the alloys. The present study has been carried out to examine microstructure changes of an early stage of ordering of FePd alloys with heating at various temperatures near the magnetic Curie temperature under magnetic fields by transmission electron microscopy. The results are summarized as follows: (1) By in-situ transmission electron microscope observations with a standard objective pole piece particular variant domains grow preferentially, while every variant domain nucleates homogeneously with an anti-magnetic objective pole piece. (2) Under a magnetic field parallel to the specimen surface, a variant crystal with c-axis near the direction of the magnetic field starts to grow from grain boundaries, and {011} macrotwins are formed as accommodation defects. (3) Magneto-heat treatment effect is present in a certain temperature range of ferromagnetic short range order just above the magnetic Curie temperature. (4) Magnetostriction calculated from elastic constants is an order of 10-5, which seems small but still plays an important role in the initiation of variant domains.