Full Tunability of Strain along the fcc-bcc Bain Path in Epitaxial Films and Consequences for Magnetic Properties

Strained coherent film growth is commonly either limited to ultrathin films or low strains. Here, we present an approach to achieve high strains in thicker films, by using materials with inherent structural instabilities. As an example, 50 nm thick epitaxial films of the ${\mathrm{Fe}}_{70}{\mathrm{Pd}}_{30}$ magnetic shape memory alloy are examined. Strained coherent growth on various substrates allows us to adjust the tetragonal distortion from $c/a_{\mathrm{bct}}=1.09$ to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. Magnetometry and x-ray circular dichroism measurements show that the Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy change over broad ranges.

Figure 1

(a) A considerable shift of the ${\mathrm{Fe}}_{70}{\mathrm{Pd}}_{30}$ $(002{)}_{\mathrm{bct}}$ peak (arrow) is observed in x-ray diffraction patterns that have been measured on the films grown on different substrate materials ($\mathrm{Co}{K}_{\alpha }$). Peak positions expected for fcc and bcc structure are marked by dotted lines. They represent the boundaries of the Bain transformation. (b) Sketch of the different stages during the Bain transformation between fcc (top, dark blue) and bcc structure (bottom, red). The black lines mark the bct unit cell used to describe this transformation. (c) Variation of the ${\mathrm{Fe}}_{70}{\mathrm{Pd}}_{30}$ crystal lattice (substrate materials are marked on top). The in-plane lattice parameter $a_{\mathrm{bct}}$ (open symbols) follows the straight line representing identity with the substrate’s lattice spacing $d$. The $c/a_{\mathrm{bct}}$ ratio (solid symbols) is varied almost from fcc to bcc structure by the coherent epitaxial growth. The dotted curve illustrates the expected change of $c/a_{\mathrm{bct}}$ at constant volume of the unit cell.

Figure 2

(a) Calculated energy required to induce tetragonal distortions ($c/a_{\mathrm{bct}}$) along the Bain path. Compared to literature data of the common metals Fe [23] and Pd [24] only little elastic energy is required for a tetragonal distortion of Fe-Pd. (b) Change of the magnetocrystalline anisotropy constants $K_1$ and $K_3$ and the ratio of orbital to spin momentum (${\mu }_{\mathrm{orb}}/{\mu }_{\mathrm{spin}}$) of Fe $3d$ electrons along the Bain path. For each quantity, the experimental error is represented by an error bar.

Figure 3

The curvature of the relative spontaneous magnetization versus temperature decreases with increasing tetragonal distortion. Inset: Curie temperature $T_C$ and shape parameter $s$ that have been extracted using Kuz’min’s model [26]. $T_C$ increases considerably when approaching the bcc structure. The shape parameter only shows minor changes.