  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.
