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Register a new userAnisotropic Magnetoresistance in Ga$_{1-x}$Mn$_x$As
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We have measured the magnetoresistance in a series of Ga$_{1-x}$Mn$_x$As samples with 0.033$le x le$ 0.053 for three mutually orthogonal orientations of the applied magnetic field. The spontaneous resistivity anisotropy (SRA) in these materials is negative (i.e. the sample resistance is higher when its magnetization is perpendicular to the measuring current than when the two are parallel) and has a magnitude on the order of 5% at temperatures near 10K and below. This stands in contrast to the results for most conventional magnetic materials where the SRA is considerably smaller in magnitude for those few cases in which a negative sign is observed. The magnitude of the SRA drops from its maximum at low temperatures to zero at T$_C$ in a manner that is consistent with mean field theory. These results should provide a significant test for emerging theories of transport in this new class of materials.
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We have investigated the electronic structure of the $p$-type diluted magnetic semiconductor In$_{1-x}$Mn$_x$As by photoemission spectroscopy. The Mn 3$d$ partial density of states is found to be basically similar to that of Ga$_{1-x}$Mn$_x$As. However, the impurity-band like states near the top of the valence band have not been observed by angle-resolved photoemission spectroscopy unlike Ga$_{1-x}$Mn$_x$As. This difference would explain the difference in transport, magnetic and optical properties of In$_{1-x}$Mn$_x$As and Ga$_{1-x}$Mn$_x$As. The different electronic structures are attributed to the weaker Mn 3$d$ - As 4$p$ hybridization in In$_{1-x}$Mn$_x$As than in Ga$_{1-x}$Mn$_x$As.
We perform a theoretical study, using {it ab initio} total energy density-functional calculations, of the effects of disorder on the $Mn-Mn$ exchange interactions for $Ga_{1-x}Mn_xAs$ diluted semiconductors. For a 128 atoms supercell, we consider a variety of configurations with 2, 3 and 4 Mn atoms, which correspond to concentrations of 3.1%, 4.7%, and 6.3%, respectively. In this way, the disorder is intrinsically considered in the calculations. Using a Heisenberg Hamiltonian to map the magnetic excitations, and {it ab initio} total energy calculations, we obtain the effective $JMn$, from first ($n=1$) all the way up to sixth ($n=6$) neighbors. Calculated results show a clear dependence in the magnitudes of the $JMn$ with the Mn concentration $x$. Also, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions, in the case of fixed Mn concentration. Moreover, theoretical results for the ground-state total energies for several configurations indicate the importance of a proper consideration of disorder in treating temperature and annealing effects.
We have studied the depth-dependent magnetic and structural properties of as-grown and optimally annealed Ga[1-x]Mn[x]As films using polarized neutron reflectometry. In addition to increasing total magnetization, the annealing process was observed to produce a significantly more homogeneous distribution of the magnetization. This difference in the films is attributed to the redistribution of Mn at interstitial sites during the annealing process. Also, we have seen evidence of significant magnetization depletion at the surface of both as-grown and annealed films.
We report on the magnetic and the electronic properties of the prototype dilute magnetic semiconductor Ga$_{1-x}$Mn$_x$As using infrared (IR) spectroscopy. Trends in the ferromagnetic transition temperature $T_C$ with respect to the IR spectral weight are examined using a sum-rule analysis of IR conductivity spectra. We find non-monotonic behavior of trends in $T_C$ with the spectral weight to effective Mn ratio, which suggest a strong double-exchange component to the FM mechanism, and highlights the important role of impurity states and localization at the Fermi level. Spectroscopic features of the IR conductivity are tracked as they evolve with temperature, doping, annealing, As-antisite compensation, and are found only to be consistent with an Mn-induced IB scenario. Furthermore, our detailed exploration of these spectral features demonstrates that seemingly conflicting trends reported in the literature regarding a broad mid-IR resonance with respect to carrier density in Ga$_{1-x}$Mn$_x$As are in fact not contradictory. Our study thus provides a consistent experimental picture of the magnetic and electronic properties of Ga$_{1-x}$Mn$_x$As.
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 $vec{H}$, revealing an antisymmetric dependence on the helicity of the field sweep.
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