Uniaxial magnetic anisotropy in Fe/GaAs(001): Role of magnetoelastic interactions

The origin of the in-plane uniaxial magnetic anisotropy of epitaxial $\mathrm{Fe}\left(001\right)$ films on $\mathrm{GaAs}\left(001\right)$ has been controversially attributed either to a magnetoelastic interaction or to oriented interface bonds. In order to check the relevance of magnetoelastic contributions we performed magneto-optic Kerr effect (MOKE) and magnetoelastic stress measurements on $\mathrm{Fe}\left(001\right)$ and ${\mathrm{Fe}}_{32}{\mathrm{Co}}_{68}\left(001\right)$ bcc epitaxial films on $\mathrm{GaAs}\left(001\right)$ in a thickness range from several monolayers (ML) up to 100 ML. The magnetoelastic coupling coefficient $B_2$ of 30 ML films is $+(8.4\pm 1.4){\mathit{MJ}/\mathrm{m}}^3$ for $\mathrm{Fe}\left(001\right)$ and $-(23.2\pm 3.7){\mathit{MJ}/\mathrm{m}}^3$ for ${\mathrm{Fe}}_{32}{\mathrm{Co}}_{68}\left(001\right)$, respectively. In spite of the opposite sign of $B_2$ for $\mathrm{Fe}$ and ${\mathrm{Fe}}_{32}{\mathrm{Co}}_{68}$, the same sign of the uniaxial anisotropy constant $K_U^{\mathrm{eff}}$ is experimentally observed. It is concluded that the magnetoelastic interaction is not the dominating contribution to the uniaxial magnetic anisotropy in these systems.

Figure 1

GaAs(001) surface after Ar ion sputtering ($E$ $=$ 1 keV, $j$ $=$ 25 μA/cm${}^2$, $t$ $=$ 1 h) at 550 °C under sample rotation and subsequent annealing in UHV at 550 °C for 15 min. (a) RHEED diffraction pattern along the [110] azimuth (first Laue circle) indicating a very flat surface; (b) 200 nm $\times$ 200 nm STM image and line scans showing atomically flat terraces separated by monoatomic steps. The stripe pattern is due to As dimer rows. The line scans identify the increased periodicity of the (4 $\times$ 6) surface reconstruction along [110].

Figure 2

Normalized in-plane magnetization loops at $T$ $=$ 300 K measured by longitudinal MOKE for a 14 ML epitaxial Fe(001) film on GaAs(001) along the [110] and [1$\overline{1}$0] axes (inset shows the [110] loop with enlarged field scale). The (red/dark gray) hatched area represents the magnetizing energy $W_{\mathrm{mag}}$ along [1$\overline{1}$0]. For the [110] orientation the anhysteretic $M$($H$) loop is obtained by averaging between the right and the left branch of the loop leading to $W_{\mathrm{mag}}\approx 0$.

Figure 3

Polar diagrams of the magnetizing energy, $W_{\mathrm{mag}}\left(\phi \right)$, at 295 K for 75 ML Fe (a) and 100 ML Fe${}_{32}$Co${}_{68}$ (b) on GaAs(001) in the (001) plane. The constant in Eq. (1) is set to $\frac{1}{4}{K}_1^{\mathrm{eff}}$ for Fe and to zero for Fe${}_{32}$Co${}_{68}$ in order to always keep $W_{\mathrm{mag}}>0$. The (red/dark gray) solid line in Fig. 3b represents a numerical fit assuming a superposition of a uniaxial and a fourfold in-plane magnetic anisotropy. The fourfold easy axes are along ⟨100⟩ for the Fe film and along ⟨110⟩ for Fe${}_{32}$Co${}_{68}$, respectively; the uniaxial easy axis is along [110] in both cases.

Figure 4

Change in curvature of a 12 mm $\times$ 2 mm GaAs stripe of 180 μm thickness with 100 ML Fe(001) (left) or Fe${}_{32}$Co${}_{68}$(001) (right) epitaxially grown on top when the magnetization is switched from [110] to [1$\overline{1}$0] and back.

Figure 5

Uniaxial anisotropy constant, $K_U^{\mathrm{eff}}$, versus inverse film thickness of epitaxial Fe(001) and Fe${}_{32}$Co${}_{68}$(001) films on GaAs(001) measured at 300 K. The slope is proportional to the interface anisotropy constant, $K_S$. Deviations from the straight line at small film thicknesses are due to reduced Curie temperature and the gradual decrease of the interface anisotropy constant $K_S$ below 6 ML (Refs. 6 and 19). Error bars are represented by the size of the symbols.