Enhancing magnetocrystalline anisotropy of the Fe70Pd30 magnetic shape memory alloy by adding Cu

Strained epitaxial growth provides the opportunity to understand the dependence of intrinsic and extrinsic properties of functional materials at frozen intermediate stages of a phase transformation. In this study, a combination of thin film experiments and first-principles calculations yields the binding energy and magnetic properties of tetragonal Fe70Pd30-xCux ferromagnetic shape memory thin films with x = 0, 3, 7 and structures ranging from bcc to beyond fcc (1.07<c/a_bct<1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarisation and Curie temperature are only moderately lowered as expected from our calculations. Beyond c/a_bct>1.41 the samples undergo structural relaxations through adaptive nanotwinning. For all tetragonal structures, we observe a significant increase of the magnetocrystalline anisotropy constant K1, which reaches a maximum of K1=-2.4*10^5 Jm^-3 at room temperature around c/a_bct=1.33 and is thus even larger than for binary Fe70Pd30 and the prototype Ni-Mn-Ga magnetic shape memory system. Since K1 represents the driving force for variant reorientation in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a promising route towards microactuators applications with significantly improved work output.