Exchange bias in a ferromagnetic semiconductor induced by a ferromagnetic metal: Fe/(Ga,Mn)As bilayer films studied by XMCD measurements and SQUID magnetometry

We demonstrate an exchange bias in (Ga,Mn)As induced by antiferromagnetic coupling to a thin overlayer of Fe. Bias fields of up to 240 Oe are observed. Using element-specific x-ray magnetic circular dichroism measurements, we distinguish a strongly exchange-coupled (Ga,Mn)As interface layer in addition to the biased bulk of the (Ga,Mn)As film. The interface layer remains polarized at room temperature.

Figure 1

(Color online) SQUID magnetometry measurements of a Fe (2 nm)/(Ga,Mn)As (20 nm) bilayer film and a (Ga,Mn)As (20 nm) control sample. (a) Moment versus temperature for the Fe/(Ga,Mn)As film at remanence (squares, left axis) and under a 500 Oe applied field (dots, left axis), and for the control sample under 100 Oe (circles, right axis). (b) Major (dots, thick line) and minor (circles) hysteresis loops at 5 K for the Fe/(Ga,Mn)As film for magnetic field applied along the [110] crystalline axis, and the hysteresis loop for the control sample (thin line) along the same axis. The inset of (b) shows the exchange bias field versus the thickness $d$ of the (Ga,Mn)As layer in a series of bilayer films (dots) and a fit showing $1/d$ dependence (dashed line).

Figure 2

(Color online) XMCD asymmetry versus applied field along the [110] axis at 2 K for a Fe (2 nm)/(Ga,Mn)As (10 nm) film. (a) $\text{Fe}{L}_3$, total electron yield; (b) $\text{Mn}{L}_3$, total electron yield; and (c) $\text{Mn}{L}_3$, fluorescent yield. Dots and circles are data for increasing and decreasing fields, respectively; lines are to guide the eye.

Figure 3

(Color online) (a) Polarization-averaged $\text{Mn}{L}_{2,3}$ spectrum for a Fe/(Ga,Mn)As film; (b) XMCD spectra measured in remanence at 2 K; (c) XMCD spectra measured under a 1000 Oe applied field at 2 K; and (d) XMCD spectrum measured under a 2000 Oe applied field at 300 K. XMCD spectra are obtained using TEY (thick red lines) and FY (thin blue lines) detection.