Magnetic Coercivity Control by Heat Treatment in Heusler Ni-Mn-Ga(-B) Single Crystals

Abstract

The combination of enlarged magnetic coercivity and magnetic shape memory (MSM) functionality is essential for novel magnetomechanical effects in MSM alloys. We found that increasing the density of thermal antiphase boundaries (APBs) provides a method to increase the magnetic coercivity without deteriorating the MSM functionality. APB density was controlled by different heat treatments in Ni-Mn-Ga(-B) MSM single crystals with five-layered modulated martensite structure. Slow cooling ~1 K/min of Ni-Mn-Ga through the B2'-L21 transition resulted in a low density (<1/micrometer) of APBs observed by magnetic force microscopy and low coercivity <2 mT. Water quenching resulted in fine magnetic and APB patterns and enlarged the coercivity to 24 mT at room temperature, and this further increased with decreasing temperature up to 41 mT at 10 K. The analysis of magnetization approach to saturation indicated the antiferromagnetic character of APBs on which the magnetic domain walls were pinned. Despite the one to two order increase of coercivity, the twinning stress remained low, between 0.7 to 1.4 MPa, and about 6% MSM effect was observed. The best ratio between coercivity and twinning stress (17 mT / 0.7 MPa) was obtained for the sample quenched in air. Contrary to previous reports, 100 ppm B doping of Ni-Mn-Ga had no or weak effect on the magnetic coercivity. Instead, the major effect originated from the high density of antiphase boundaries.