Magnetic Nanostructures by Adaptive Twinning in Strained Epitaxial Films

Abstract

We exploit the intrinsic structural instability of the Fe(70)Pd(30) magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials.

Twinning Phenomena along and beyond the Bain Path

“…The tetragonal distortion of the parent unit cell can reach values up to c/a| fct = 1.09 with a preferred out-of-plane orientation of the c-axis. In DFT calculations of the energy landscape as a function of the tetragonal distortion, a second minimum was observed close to these experimental values [17,34]. Instead of an expected strong increase in energy beyond the Bain path, these supercell calculations show a further minimum of energy, due to the formation of {101} fct twin boundaries perpendicular to the substrate.…”

“…Close to this composition, the energy for straining the unit cell is relatively low, and all tetragonal distortions along the Bain path can be stabilized in thin films [13]. Due to the essentially flat energy landscape even beyond the limits of the Bain path, Fe 70 Pd 30 films can be strained with c/a| fct > 1.0 [17].…”

“…Fe 70 Pd 30 films were deposited from an alloy target on different epitaxially-grown metallic buffer layers (e.g., Cu and Cr) at room temperature (base pressure < 10 −8 mbar) [13,17]. The 50 nm Cr buffer layer was deposited at 300…”

“…Recently, it has been shown that introducing Cu as a buffer layer causes the unit cell of Fe-Pd and its alloy, Fe-Pd-Cu, to be highly strained beyond the limits of the Bain path [17,34]. The tetragonal distortion of the parent unit cell can reach values up to c/a| fct = 1.09 with a preferred out-of-plane orientation of the c-axis.…”

“…This is only possible with a geometrically-defined twinning relation. The c/a| fct -ratio of the nanotwins adjust to the boundary conditions of the parent unit cell by the following geometrically relation [17]. c a…”