Nonthermal Photocoercivity Effect in a Low-Doped (Ga,Mn)As Ferromagnetic Semiconductor

We report a photoinduced change of the coercive field, i.e., a photocoercivity effect (PCE), under very low intensity illumination of a low-doped (Ga,Mn)As ferromagnetic semiconductor. We find a strong correlation between the PCE and the sample resistivity. Spatially resolved dynamics of the magnetization reversal rule out any role of thermal heating in the origin of this PCE, and we propose a mechanism based on the light-induced lowering of the domain wall pinning energy. The PCE is local and reversible, allowing writing and erasing of magnetic images using light.

Figure 1

(a) MOKE hysteresis loops probed by a HeNe laser. (b) MOKE hysteresis loops recorded for two laser powers after subtracting the linear background. Arrows indicate the light-assisted magnetization reversal process in an external magnetic field $H_0$. (c) Zero-field magnetization profile obtained after initialization in a negative field ($-1\mathrm{kOe}$), byte $\{00000000\}$. (d) As (c) but following a write procedure (performed by 1 ms pulses with $P=100\mu \mathrm{W}$), $\{01110100\}$. (e) As (d) following a partial deletion procedure, $\{00100100\}$. (f) As (e) following a rewrite, $\{11101110\}$. Thick lines in lower parts of (c)–(f) are cross sections of the magnetization profiles.

Figure 2

Kerr angle in saturation ${\theta }_s$ as a function of (a) illumination power (at $T=2\mathrm{K}$ and $T=7\mathrm{K}$) and (b) temperature (for $P=10\mu \mathrm{W}$). Coercive field $H_c$ (c) as a function of illumination power at different temperatures ($T=2$, 5, 7, and 11 K) and (d) as a function of temperature ($P=10\mu \mathrm{W}$). In all panels, the lines are guides to the eye.

Figure 3

(a) The light-induced change in coercive field ($H_{\mathrm{cd}}-H_{\mathrm{cl}}$) normalized to the coercivity in dark $H_{\mathrm{cd}}$. (b) Resistance versus temperature. (c) Field-induced switching measurement with (open symbols) and without (solid symbols) additional illumination from a xenon lamp with an intensity of about $1\mathrm{W}{\mathrm{cm}}^{-2}$. (d) Difference in coercive field scaled by the illumination intensity as function of photon energy, shown along with the penetration depth $\Lambda ={\alpha }^{-1}$ of light into GaAs.

Figure 4

Spatially resolved dynamics of the PCE. (a) Magnetization profile of a $100\mu \mathrm{m}\times 100\mu \mathrm{m}$ area obtained after an exposure time $\tau ={10}^{-3}\mathrm{s}$ (in a magnetic field $H_0=480\mathrm{Oe}$) with a focused beam with $P=500\mu \mathrm{W}$. (b) The same after exposure time $\tau =1\mathrm{s}$. (c) Cross sections of the magnetization profiles recorded for various exposure times in a magnetic field $H_0=350\mathrm{Oe}$. The $y$ axis is scaled such that 0 and 1 correspond to the Kerr angles obtained for the layer uniformly magnetized downwards and upwards, respectively. The solid lines are fits to Eq. (1). (d) Size of the reversed domain $S$ as a function of the exposure time $\tau$. Solid line is a fit to Eq. (2) with $\Delta =6\mu \mathrm{m}$ and ${\tau }_0=0.2\mathrm{s}$.