Structural behavior and magnetic properties of a Ni-Mn-Ga single crystal across the martensite/austenite two-phase region

The martensite/austenite two-phase region of a Ni-Mn-Ga single crystal is investigated using different analytical techniques (DSC, SQUID, and XRD) in order to accurately determine the maximum operating temperature for the magnetic shape memory (MSM) effect. It is found that the width ΔTA = Af − As of the two-phase region and the austenite start temperature As are significantly affected by the variant structure of the martensite prior transformation. The austenite finish temperature Af remains on the other hand constant. In the multi-variant martensite the width ΔTA measured as large as 20 K, while in the single-variant the width is <2 K. A huge shift of up to 16 K toward higher values was measured in the As temperature of the single-variant sample, which in turn determines the maximum MSM operating temperature. Magnetic and mechanical properties across the two-phase region were thoroughly investigated. A considerable improvement of the high-temperature MSM properties is demonstrated for the alloy examined in the present work.