Metastable tetragonal structure of Fe${}_{100-x}$Ga${}_x$ epitaxial thin films on ZnSe/GaAs(001) substrate

We investigated by x-ray diffraction the Ga concentration dependences of the structural properties of Fe${}_{100-x}$Ga${}_x$ (galfenol) thin films grown on a ZnSe/GaAs(001) substrate, a material known for its high magnetostriction. By molecular beam epitaxy (MBE) we grew a series of (001)-oriented layers without in-plane misorientation, ranging from pure Fe up to $x=29.4\%$ Ga. We find a strong Ga-induced tetragonal distortion that conserves the pristine Fe in-plane lattice parameters for all Ga compositions. Supported by theoretical predictions [R. Wu, J. Appl. Phys. 91, 7358 (2002)], we attribute this unusual tetragonal distortion to short-range ordering of Ga-Ga pairs along the [001]-growth direction. The low-temperature and out-of-equilibrium MBE growth regime tends to stabilize a strong deformed tetragonal phase (up to $c/a\sim 1.05$ for $x\sim 29\%$). This tetragonal structure is fully released by postgrowth annealing.

Figure 1

Reciprocal space mapping around the (002), (202), and (112) Bragg reflection of Fe film (a)–(c), two representative alloys film with 15.6% (d)–(f) and 29.4% Ga (g)–(h), and the associated reflections of the ZnSe/GaAs (001) substrate. The axes of $2\omega /\theta$ and $\Delta \omega$ are centered on the GaAs substrate.

Figure 2

Area scans around the asymmetric (202) and $(-202)$ (a),(b); (112) and $(-1-12)$ (c),(d) Bragg peaks of an alloy film with 15.6% Ga; (e) variation in the angle $[202,000,-202]$ and the ratio of $c/a=\tan [202,000,-202]$ (full circles) of the alloy film as a function of the Ga content.

Figure 3

Out-of-plane (up triangles), in-plane (down triangles), and the relaxed lattice parameters (full circles) of the films vs the Ga content, as compared with the bulk lattice parameters (open squares) from Ref.3.

Figure 4

Area scans around the (002) and (202) Bragg peaks of an alloy film at 15.6% Ga content for (a),(c) as-grown and after postannealing at 300${}^{\circ }$C (b),(d).