M. Lampalzer

Spin injection, spin transport and spin coherence

We discuss advances, advantages and problems of spintronics through the example of a semiconductor laser whose emission intensity and polarization are modulated by the spin orientation of electrons. We show that spin transport should be feasible at room temperature and present possible concepts and first results concerning spin injection at high temperatures. Finally, we describe the coherent dynamics of coupled electron and hole spins in a quantum mechanical picture and measure the magnetic field-induced dynamics of localized excitons in a 3 nm GaAs quantum well. The system is capable of performing a quantum controlled not operation (CNOT), which realizes a basic two-qubit operation of quantum information processing in a semiconductor nanostructure.

Ferromagnetic resonance studies of (Ga,Mn)As with MnAs clusters

We report on the temperature and angular dependence of ferromagnetic resonances originating from MnAs clusters embedded in a Ga1−xMnxAs host matrix with x≈0.1%. The MnAs clusters were formed in situ during metal-organic vapour-phase epitaxy of (Ga,Mn)As on (001) and (111) GaAs substrates. The samples exhibit Curie temperatures of TC≈330K which are above room temperature and about 10K higher than in bulk MnAs and MnAs thin films. From the angular dependence of the ferromagnetic resonances it can be concluded that the crystallographic orientation of the clusters with respect to the matrix is the same for both substrates, i.e. MnAs[0001]||GaAs[111]. We observe only one orientation of the clusters for (111) and four different orientations for (001) substrates. The easy axis of magnetisation is perpendicular to the c-axis of the clusters. Only in the case of the (111) sample, all magnetic moments of the clusters can align parallel to an applied in-plane magnetic field at small fields.

Anisotropy of the magnetotransport in (Ga,Mn)As/MnAs paramagnetic-ferromagnetic hybrid structures

We investigated the temperature-dependent magnetoresistance of granular (Ga,Mn)As/MnAs hybrids grown on (100) GaAs in different transport geometries. The observed magnetoresistance effects are much bigger than for a corresponding (Ga,Mn)As reference sample without MnAs nanoclusters. We find that the magnetoresistance effects depend strongly on the chosen transport geometry. When the external field is perpendicular to the sample plane the effects are largest. The smallest effects occur when the external field is in the sample plane and parallel to the current. Furthermore, we have established by ferromagnetic resonance studies that the magnetic properties of the ensemble of ferromagnetic MnAs nanoclusters is similar for the magnetic field orientations studied. Therefore, the observed anisotropy of the magnetoresistance mainly reflects the difference in current path through the sample which leads to a variation of the degree of interaction between the free carriers in the matrix and nanoclusters.

Optical characterisation of MOVPE-grown Ga1−xMnxAs semimagnetic semiconductor layers

Abstract For the first time, a successful growth of Ga 1− x Mn x As layers on (100)GaAs substrates by metal-organic vapour-phase epitaxy is reported. Optical and magneto-optical spectroscopy of the E 0 , E 1 and E 1 +Δ 1 transitions is possible due to the high structural quality of the samples. A strong exchange interaction is found in the Ga 1− x Mn x As layers between the magnetic moments of manganese and the excitonic states, which exhibits an effective ferromagnetic coupling. The Mn concentrations in the layers can be estimated by two different and independent optical methods (i) from the Zeeman splitting of the E 0 excitons and (ii) from the red-shift of the E 1 transition due to the p-doping with Mn.

Overgrowth experiments of ferromagnetic (MnGa)As-cluster layers by MOVPE

We present the results of the epitaxial overgrowth of magnetic (MnGa)As-cluster structures with GaAs, (AlGa)As and AlAs using metal organic vapor phase epitaxy (MOVPE). The structural differences in the overgrowth are investigated by means of atomic force microscopy (AFM) combined with transmission electron microscopy (TEM), in particular, to proof the successful overgrowth of the cluster layers with AlAs. Out of these experiments a first model for the overgrowth is developed. Measurements using a SQUID-magnetometer confirm the existence of ferromagnetism above room temperature in the cluster layers after overgrowth; however, other magnetic properties as the coercitive field are influenced by the overgrowth process.

Monitoring the sign reversal of the valence band exchange integral in (Ga,Mn)As

Abstract We report the successful growth of magnetic Ga 1− x Mn x As layers on (1 0 0) GaAs substrates by metal-organic vapour-phase epitaxy. Depending on the growth parameters, two different magnetic phases of Ga 1− x Mn x As can be grown. (i) At low Mn-concentrations, Ga 1− x Mn x As alloys are obtained. These alloys exhibit a paramagnetic behaviour with a strong exchange interaction between the localised magnetic moments of the Mn 2+ ions and the extended excitonic states. (ii) At high Mn-concentrations, Mn(Ga)As clusters are formed within a Ga 1− x Mn x As host. The samples are ferromagnetic even above room temperature. The ferromagnetism has been investigated by SQUID and ESR measurements. The s–d and p–d exchange integrals have been determined independently by combining photoluminescence excitation and spin-flip Raman spectroscopy. A reversal of sign of the valence band exchange integral has been detected along with the transition from the paramagnetic to the ferromagnetic phase.

Influence of Codoping on the Magnetoresistance of Paramagnetic (Ga,Mn)As

We studied the correlation between magnetoresistance (MR) effects and the type of conducti- vity in paramagnetic Ga1−xMnxAs with x < 0.5%. Series of samples of (Ga,Mn)As with different codoping levels of Te have been prepared by metal-organic vapor-phase epitaxy (MOVPE). With increasing Mn-content from doping levels to x = 0.5% in paramagnetic Ga1−xMnxAs, the MR at 1.6 K changes continuously from positive to strongly negative. This manifests the complex interplay between p–d exchange induced valence band splitting and the metal–insulator transition. Codoping with Te leads eventually to a dominant conduction band transport. It is characterized by a small negative contribution at low magnetic fields to the parabolic MR similar to that found in highly n-doped diamagnetic semiconductors. It shows that the Mn-induced conduction band splitting is much smaller than the valence band splitting, i.e., |N0α| ≪ |N0β|.

Magneto-Optical Spectroscopy on (Ga,Mn)As Based Layers—Correlation Between the p–d Exchange Integral and Doping

It is well established that the ferromagnetism of (Ga,Mn)As is mediated by holes. An important parameter in this context is the p–d exchange integral N0β as the RKKY interaction parameter JRKKY is proportional to (N0β)2. In magnetic semiconductors, magnitude and sign of N0β are strongly dependent on the local electronic configuration of the Mn-ion and on the free carrier density. To investigate the correlation between the p–d exchange interaction and doping in Ga1−xMnxAs, paramagnetic samples with x ≈0.1% have been intentionally n-doped with Te. High-field Hall measurements at 1.6 K confirm that with increasing degree of Te co-doping, the samples change from p- to n-type. The magnetic field induced splitting of the excitonic and electronic states has been studied by various magneto-optical spectroscopic techniques at 1.8 K and external magnetic fields up to 7.5 T. The p–d exchange integral changes from N0β > 0 (ferromagnetic coupling) to N0β < 0 (antiferromagnetic coupling) with increasing degree of Te co-doping. Thus, N0β can be tuned and can even change its sign in dependence on the doping conditions of the Mn-containing layers.