Plastic and magnetoplastic deformation of Dy single crystals

Abstract

Several authors have published reports of plastic deformation in Dy and Tb single crystals magnetized in fields of about 100 kOe parallel to the magnetically hard direction at cryogenic temperatures [1–3]. The mechanism for this deformation has not been described in literature. We have investigated the nature of this phenomenon in Dy crystals, using optical and scanning electron microscopy. X-ray diffraction, and magnetic measurements. Plastic deformation was generally found to occur at 70–90 kOe at 4.2 K in Dy and the mechanism is {101̄2} 〈101̄1̄〉 mechanical twinning, which was determined by independent mechanical tests to be a primary twinning system. The most likely driving force for the deformation appears to be the decrease in magnetostatic and magnetocrystalline anisotropy energies on twinning, since a relatively easy 〈101̄0〉 direction in the twinned region lies almost colinear with the field. A simple calculation shows that this energy difference leads to an equivalent tensile stress comparable to the yield stress of Dy.