Ben Li Sheu

Onset of Ferromagnetism in Low-Doped Ga1 xMnxAs

We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of Ga1-xMnxAs. We compare it with measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originate from Mn disorder and As antisite defects. The experimentally determined hopping energy is an excellent predictor of the Curie temperature, in agreement with the theory.

Antisite effect on hole-mediated ferromagnetism in (Ga,Mn)As

We study the Curie temperature and hole density of (Ga,Mn)As while systematically varying the As-antisite density. Hole compensation by As-antisites limits the Curie temperature and can completely quench long-range ferromagnetic order in the low doping regime of 1-2% Mn. Samples are grown by molecular beam epitaxy without substrate rotation in order to smoothly vary the As to Ga flux ratio across a single wafer. This technique allows for a systematic study of the effect of As stoichiometry on the structural, electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than 1.5% Mn, a strong deviation from Tc ~ p^0.33 is observed. Our results emphasize that proper control of As-antisite compensation is critical for controlling the Curie temperatures in (Ga,Mn)As at the low doping limit.

Magnetoresistance anomalies in (Ga,Mn)As epilayers with perpendicular magnetic anisotropy

We report the observation of anomalies in the longitudinal magnetoresistance of tensile-strained (Ga,Mn)As epilayers with perpendicular magnetic anisotropy. Magnetoresistance measurements carried out in the planar geometry (magnetic field parallel to the current density) reveal spikes that are antisymmetric with respect to the direction of the magnetic field. These anomalies always occur during magnetization reversal, as indicated by a simultaneous change in sign of the anomalous Hall effect. The data suggest that the antisymmetric anomalies originate in anomalous Hall effect contributions to the longitudinal resistance when domain walls are located between the voltage probes. This interpretation is reinforced by carrying out angular sweeps of

Scaling theory of magnetoresistance and carrier localization in Ga1-xMnxAs

vec{H}

Width dependence of annealing effects in (Ga,Mn)As nanowires

, revealing an antisymmetric dependence on the helicity of the field sweep.

Scaling analysis of the magnetoresistance in Ga1 xMnxAs : Evidence for strong fluctuation and interaction effects

We compare experimental resistivity data on Ga_{1-x}Mn_xAs films with theoretical calculations using a scaling theory for strongly disordered ferromagnets. All characteristic features of the temperature dependence of the resistivity can be quantitatively understood through this approach as originating from the close vicinity of the metal-insulator transition. In particular, we find that the magnetic field induced changes in resistance cannot be explained within a mean-field treatment of the magnetic state, and that accounting for thermal fluctuations is crucial for a quantitative analysis. Similarly, while the non-interacting scaling theory is in reasonable agreement with the data, we find clear evidence in favor of interaction effects at low temperatures.

Non-collinear giant magnetoresistance in (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayers

We study the time dependence of annealing on a series of GaAs-capped (Ga,Mn)As nanowires of varying widths. For different annealing times, our measurements indicate that decreasing the wire width monotonically increases the Curie temperature enhancement associated with annealing, as well as the drop in resistivity. These results are consistent with the lateral diffusion of interstitial Mn ions, which constitute an important source of defects in these materials. Furthermore, the thinner wires show a higher rate of change of conductivity with annealing time, suggesting a more efficient removal of Mn interstitials in thinner wires.