Structural and magnetic anisotropies of $\mathrm{Fe}∕\mathrm{ZnSe}\left(001\right)$ thin films

The morphology, the atomic structure, and the strain of very thin $\mathrm{Fe}$ epilayers grown on $\mathrm{ZnSe}\left(001\right)$ have been studied by combined scanning tunnel microscope (STM) and extended x-ray-absorption fine-structure experiments. It turns out that the crystal structure of the flat and anisotropic islands (up to 5 monolayers) and terraces (higher coverages) observed by STM is close to bulk iron but affected by a weak out-of-plane tetragonal distortion. Strain measurements and magnetic anisotropy constants of very thin films obtained by ferromagnetic resonance measurements allow us to conclude that the important out-of-plane anisotropy term is determined by the interface atomic structure. Finally, we put forward two hypotheses to explain why the measured out-of-plane anisotropy is one order of magnitude larger than the [110] in-plane anisotropy. The former is related to a disordered configuration of iron and zinc atoms between bonding and antibonding sites at the interface. The latter hypothesis is related to the direction of the demagnetization field due to the morphology of iron films.

Figure 1

STM images at different $\mathrm{Fe}$ coverages (1, 3, 5, and $7\mathrm{ML}$’s) on a $c(2\times 2)$ reconstructed $\mathrm{Zn}$-terminated $\mathrm{ZnSe}\left(001\right)$ surface. RHEED diagrams are also presented. (a) At $1\mathrm{ML}$ we observe flat discs, with an average diameter of $\sim 30\mathrm{\AA }$ and a maximum thickness of $2\mathrm{ML}$’s. (b) At $3\mathrm{ML}$’s, islands present more straight edges along the ⟨110⟩ directions of the substrate. (c) At $5\mathrm{ML}$’s, the corrugation is released by a two-dimensional coalescence: large, atomically flat rectangular terraces cover the surface. The atomic resolution can be achieved on this surface, where the reconstruction corresponds to a $\mathrm{Fe}(2\times 2)$ $\mathrm{Se}$-induced reconstruction. At $7\mathrm{ML}$’s the island coalescence is accomplished and the film presents long-range order. The terrace step height is about 1.6 Å. The $(2\times 2)$ reconstruction of the segregating $\mathrm{Se}$ is still well identified.

Figure 2

Absorption spectra recorded on the five samples at the normal (a) and grazing incidences (b). We notice that, except at very low coverage $\left(0.8\mathrm{ML}\right)$, the main EXAFS $\alpha \text{−}\mathrm{Fe}$ structures are observed in all the collected spectra.

Figure 3

Fourier transform of the measured EXAFS oscillations at different coverages at (a) normal incidence and (b) grazing incidence.

Figure 4

Analysis of the phase derivatives of the experimental signal $(4\mathrm{ML}\text{'}\mathrm{s})$. We notice that minima of the phase derivatives of NI and $\mathrm{GI}$ signals are located at distinct positions, indicating a tetragonal distortion of the thin film.

Figure 5

(Color online) Comparison between measured and simulated spectra at the (a) normal and (b) grazing incidences. For simulated spectra obtained by FEFF 6.0: $T_{\mathrm{debye}}=470°\mathrm{C}$. Bulk iron contribution is calculated by a cluster of iron atoms in a bct structure (see parameters in Table , $4\mathrm{ML}$’s). In the case of thin-film simulations: iron is epitaxied on $\mathrm{ZnSe}\left(001\right)$ $(a_{\mathrm{ZnSe}}∕2=2.825\mathrm{\AA })$; the lattice parameters are the same of Table ($4\mathrm{ML}$’s) ; distance nearest-neighbor $\mathrm{Fe}\text{−}\mathrm{Se}$ atoms at the interfaces $=2.48\mathrm{\AA }$. We notice that the FT spectra are slightly modified with respect to bulk. Simulations called “with islands” are obtained taking into account the size and the morphology of iron clusters as observed by STM.

Figure 6

Angular dependence of the resonance field of 25- and $4\text{−}\text{ML}\text{−}$ thick layers. Out-of-plane measurements performed at the frequency $9.25\mathrm{GHz}$ of the microwave field. At $\theta =0$ the magnetic field is along [100] for $25\mathrm{ML}$’s and $\left[1\overline{1}0\right]$ for $4\mathrm{ML}$’s, easy and hard axis, respectively.

Figure 7

Angular dependence of the resonance field of $6\text{−}\text{ML}\text{−}$thick layers. In-plane measurements were performed at the frequency $9.25\mathrm{GHz}$ of the microwave field. At $\varphi =0$ the magnetic field is along [100].