Evidence for a Magnetic Proximity Effect up to Room Temperature at $\mathrm{Fe}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ Interfaces

We report x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry experiments to study magnetic order and coupling in thin $\mathrm{Fe}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}(100)$ films. We observe induced magnetic order in the $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ layer that extends over more than 2 nm, even at room temperature. We find spectroscopic evidences of a hybridized $d$ configuration of Mn atoms in $\mathrm{Fe}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$, with negligible Mn diffusion and/or MnFe intermixing. We show by experiment as well as by theory that the magnetic moment of the Mn ions couples antiparallel to the moment of the Fe overlayer.

Figure 1

XAS/XMCD spectra measured in total electron yield at room temperature and at remanence. All thicknesses in the Figures are given in nanometers. (a) Mn $L_{2,3}$ edges of $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ (Mn 5%): as grown oxidized sample (black curve), after mild sputtering (blue curve), after Fe deposition (red curve). (b) Mn $L_{2,3}$ edges of pure $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ (Mn 5%) after HCl etching (green curve), 0.02 nm $\mathrm{Mn}/\mathrm{GaAs}(001)$ (blue curve), 0.02 nm $\mathrm{Mn}/\mathrm{GaAs}(001)$ with overgrowth of 1.4 nm Fe (yellow curve), 3.2 nm thick ${\mathrm{Fe}}_{0.97}{\mathrm{Mn}}_{0.03}$ alloy grown on GaAs(100) (orange curve), 0.01 nm Mn sandwiched between two thin Fe layers grown onto GaAs(001) (turquoise curve). (c) Room temperature magnetization dependent XAS ($I^{-},I^{+}$) and XMCD (red squares) spectra for the sample with 2 nm $\mathrm{Fe}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ (Mn 5%). The yellow curve is the XMCD spectrum of 0.02 nm thick $\mathrm{Mn}/\mathrm{GaAs}(001)$ film with 1.4 nm Fe overlayer. (d) Mn XMCD line shapes of different Mn samples corresponding to the spectra in (b). (e) XAS (black dots, black curve) and XMCD (red curve) spectra of 2 nm $\mathrm{Fe}/\mathrm{GaMnAs}(\mathrm{Mn}5\%)$ at the Fe $L_{2,3}$ edges. (f) Hysteresis curves measured at the maximum XMCD signal on Fe (red) and Mn (green).

Figure 2

(a) Ratio between Mn and Fe remanent XMCD signals in various samples as a function of Fe thickness (top) and temperature (bottom). Square symbols represent room temperature data, triangles represent low temperature data. All samples have a common temperature dependent slope as a function of Fe thickness. The Mn concentration ranges from $S1=3\%$, $S2=2\%$ to $S4=6\%$. The arrow indicates the temperature dependent measurements shown in  (b). (b) Evolution of the Fe and Mn remanent XMCD signal and of their ratio as a function of temperature for a $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ (Mn 5.5%) sample covered by a 2 nm thick Fe film.

Figure 3

(a) Spontaneous $M(T)$ curves of sample 3 (6% Mn concentration) (red circles) and from a pure $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ substrate (blue squares), measured during field cooling. Each point is determined by a linear extrapolation of $M(H)$ ($H=0.475–0.7\mathrm{kOe}$) to zero field. The field is high enough to saturate both the Fe and $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ films. The Mn fraction located in the bulk is not affected by the surface and has the same $T_C$ as the pure $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ substrate (blue squares). (b) Sketch of a simple model for the Mn distribution $\rho (x)$ and the fraction of ferromagnetic Mn at room temperature $\delta (x)$ as discussed in the text.

Figure 4

Monte Carlo simulation of a $\mathrm{Fe}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ film consisting of 7 ML of Fe and 7 ML of $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ ($x_{\mathrm{Mn}}=5\%$) compared to the calculation at $T=0\mathrm{K}$. A reduction of about 10% of the Fe moment is observed. The $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ layer remains FM at room temperature and is coupled antiparallel to Fe.