Magnetic shape-memory (MSM) Heuslers are among the most promising materials for thermo-magneto-mechanical applications. However, the knowledge about the martensitic transformation (which is the basis of the multifunctionality in these materials) as a function of size reduction in the submicron scale is still very limited. Here, we aim to bridge this knowledge gap by investigating the behavior of these materials upon nanoscale confinement. We customize a top-down approach by patterning arrays of submicron epitaxial Ni-Mn-Ga structures with lateral sizes down to ∼70 nm, using a Cr hard mask on MgO(001) substrate. The structures include straight stripes, radial stripes, squares and triangles. The martensitic transformation temperature, sharpness, thermal hysteresis and magnetic characteristics of the material are investigated upon spatial confinement. Transmission electron microscopy techniques including Geometric Phase Analysis (GPA) algorithm, and quantitative theoretical analysis of stress help us to evaluate the martensitic transformation of Ni-Mn-Ga starting from continuous films and down to sub-micron patterns. We show that the size-dependent internal stress relaxation plays a primary role in broadening the martensitic transformation of the material, reducing thermal hysteresis, and pushing the transformation toward higher temperatures in the sub-micron structures. These findings highlight the importance of stress considerations upon incorporation of MSM Heusler materials into nanoscale functional devices.

中文翻译：

---

空间受限磁性形状记忆 Heuslers：对纳米级器件的影响


磁性形状记忆 （MSM） Heusler 是热磁机械应用最有前途的材料之一。然而，关于马氏体转变（这是这些材料多功能性的基础）作为亚微米尺度尺寸减小函数的知识仍然非常有限。在这里，我们的目标是通过研究这些材料在纳米级限制下的行为来弥合这一知识差距。我们在 MgO（001） 衬底上使用 Cr 硬掩模，通过对横向尺寸低至 ∼70 nm 的亚微米外延 Ni-Mn-Ga 结构阵列进行图案化，定制了一种自上而下的方法。结构包括直条纹、径向条纹、正方形和三角形。研究了在空间限制下材料的马氏体相变温度、锐度、热滞和磁特性。透射电子显微镜技术，包括几何相分析 （GPA） 算法和应力定量理论分析，帮助我们评估 Ni-Mn-Ga 的马氏体转变，从连续薄膜开始，一直到亚微米图案。我们表明，尺寸依赖性的内应力松弛在扩大材料的马氏体转变、减少热滞后以及推动亚微米结构向更高温度转变方面起主要作用。这些发现强调了在将 MSM Heusler 材料掺入纳米级功能器件时考虑应力的重要性。