Reorientation transition of the magnetic proximity polarization in Fe/(Ga,Mn)As bilayers

Recently, it has been observed that thin ferromagnetic Fe films deposited on top of (Ga,Mn)As layers induce a significant proximity polarization in the (Ga,Mn)As film even at room temperature. Furthermore, it was found that a thin interfacial region of the (Ga,Mn)As film is coupled antiferromagnetically to the Fe layer. Here we report a series of combined x-ray magnetic dichroism and superconducting quantum interference device magnetometer measurements for Fe/(Ga,Mn)As bilayers where the (Ga,Mn)As layer thickness is varied between 5 and 50 nm. We find a reorientation transition of the magnetic proximity polarization as a function of the (Ga,Mn)As thickness. The data are compared to results obtained performing ab initio calculations. A varying concentration of Mn interstitials as a function of (Ga,Mn)As layer thickness is responsible for this reorientation. Furthermore, exchange bias is studied in the fully epitaxial bilayer system. We find a rather strong ferromagnetic exchange bias. The strength of the exchange bias can be estimated by using a simple partial domain wall model.

Figure 1

Fe and Mn $L_{\text{2,3}}$ XAS/XMCD spectra recorded at room temperature for Fe/(Ga,Mn)As heterostructures with 12$\%$ Mn: (a) Fe/5 nm (Ga,Mn)As measured at the Mn $L_{\text{2,3}}$ edge. (b) Fe/5 nm (Ga,Mn)As measured at the Fe $L_{\text{2,3}}$ edge. (c) Normalized Mn $L_{\text{3}}$ dichroism as a function of (Ga,Mn)As thickness for 6$\%$ Mn (black square) and for 5 nm (Ga,Mn)As, 12$\%$ Mn (red circle). The green triangle denotes the induced dichroism for 50 nm (Ga,Mn)As with 6$\%$ Mn (data taken from measurements of Ref. 13).

Figure 2

Element specific hysteresis loops with an in-plane field along [110] direction: (a) XMCD hysteresis loop measured at the Mn $L_{\text{3}}$ edge (red line) for Fe/5 nm (Ga,Mn)As with 12$\%$ Mn at room temperature normalized to the hysteresis loop measured at the Fe $L_{\text{3}}$ edge (black line). (b) XMCD hysteresis loop measured at the Mn $L_{\text{3}}$ edge (red line) normalized to the hysteresis loop measured at the Fe $L_{\text{3}}$ edge (black line) for the same sample at $T=40$ K.

Figure 3

(a) Exchange coupling parameters $J_{ij}$ between Fe and Mn atoms for five different atomic structures at the (Ga,Mn)As/Fe interface: I, Ga terminated; II, Ga terminated with Mn interstitials at the interface; III, As terminated; IV, As terminated with Mn interstitials at the interface; and V, interface consisting of a single atomic plane of alternating Fe and As atoms.[23] The stars (*) for structure V correspond to the interactions of Mn${}_{\text{Ga}}$ from the interface with Fe atoms from the first fully occupied Fe layer. In each case, the parameters are given for the nearest-neighbor Mn${}_{\text{Ga}}$-Fe (Mn${}_{\text{I}}$-Fe) distances. (b) Results of Monte Carlo calculations: layer-resolved magnetization for a 7-ML Fe/8-ML (Ga${}_{\text{0.95}}$Mn${}_{\text{0.05}}$As) As bilayer with an As-terminated interface and Mn interstitials at the interface layer of (Ga,Mn)As (atomic structure IV). Each atomic layer is counted. In this case layer 8 contains As atoms. (c) Structure IV, the frame is around the interface layer. (d) Structure V, the AsFe layer at the interface is shown with the frame.

Figure 4

(a) Major magnetization loop $m\left(H\right)$ along (Ga,Mn)As [110] measured at $T=20$ K for a bilayer Fe/30 nm (Ga,Mn)As sample with 12$\%$ Mn showing a distinct two-step switching process (open circles); minor magnetization loop after saturating the Fe layer (red solid circles). (b) SQUID measurement of the field dependence of the magnetic moment $m\left(H\right)$ for Fe/5 nm (Ga,Mn)As, 12$\%$ Mn (open circles) and the corresponding (Ga,Mn)As reference (red solid circles) along [110] measured at 10 K. (c) Minor magnetization loops for Fe/30 nm (Ga,Mn)As with 12$\%$ Mn measured along [110] for different temperatures. (d) Dependence of the exchange field $H_E$ on temperature for Fe/30 nm (Ga,Mn)As with 12$\%$ Mn (black squares) and 6$\%$ Mn (red circle).

Figure 5

Schematic representation of a partial domain-wall configuration in (Ga,Mn)As. Directly at the interface an interfacial layer with thickness $d$ of (Ga,Mn)As is aligned antiparallel with respect to Fe. The arrows represent the spin direction, which continuously rotates with increasing distance from the interface. At a certain distance $t_{\text{1}}$, a complete domain with thickness $t_{\text{2}}$ is formed.