L. N. Oveshnikov

Berry phase mechanism of the anomalous Hall effect in a disordered two-dimensional magnetic semiconductor structure.

L. N. Oveshnikov, V. A. Kulbachinskii, 2 A. B. Davydov, B. A. Aronzon, 3 I. V. Rozhansky, 5 N. S. Avkeriev, K. I. Kugel, and V. Tripathi 8 National Research Center Kurchatov Institute, Moscow 123182, Russia Low Temperature Physics Department, M.V. Lomonosov Moscow State University, Moscow 119991, Russia P.N. Lebedev Physical Institute, Russian Acad. Sci., Moscow 119991, Russia Ioffe Institute, Russian Acad. Sci., St. Petersburg 194021, Russia Peter the Great St. Petersburg Polytechnic University, St. Petersburg 195251, Russia Institute for Theoretical and Applied Electrodynamics, Russian Acad. Sci., Moscow 125412, Russia Materials Science Division, Argonne National Laboratory, Lemont, IL 60439, USA Department of Theoretical Physics, Tata Institute of Fundamental Research, Mumbai 400005, IndiaThe anomalous Hall effect (AHE) arises from the interplay of spin-orbit interactions and ferromagnetic order and is a potentially useful probe of electron spin polarization, especially in nanoscale systems where direct measurement is not feasible. While AHE is rather well-understood in metallic ferromagnets, much less is known about the relevance of different physical mechanisms governing AHE in insulators. As ferromagnetic insulators, but not metals, lend themselves to gate-control of electron spin polarization, understanding AHE in the insulating state is valuable from the point of view of spintronic applications. Among the mechanisms proposed in the literature for AHE in insulators, the one related to a geometric (Berry) phase effect has been elusive in past studies. The recent discovery of quantized AHE in magnetically doped topological insulators - essentially a Berry phase effect - provides strong additional motivation to undertake more careful search for geometric phase effects in AHE in the magnetic semiconductors. Here we report our experiments on the temperature and magnetic field dependences of AHE in insulating, strongly-disordered two-dimensional Mn delta-doped semiconductor heterostructures in the hopping regime. In particular, it is shown that at sufficiently low temperatures, the mechanism of AHE related to the Berry phase is favoured.

Experimental determination of the electron effective masses and mobilities in each dimensionally-quantized subband in an InxGa1 − xAs quantum well with InAs inserts

HEMT structures with In0.53Ga0.47As quantum well are synthesized using molecular-beam epitaxy on InP substrates. The structures are double-side Si δ-doped so that two dimensionally-quantized subbands are occupied. The effect of the central InAs nanoinsert in the quantum well on the electron effective masses m* and mobilities in each subband is studied. For experimental determination of m*, the quantum μq and transport μt mobilities of the two-dimensional electron gas in each dimensionally-quantized subband, the Shubnikov-de Haas effect is measured at two temperatures of 4.2 and 8.4 K. The electron effective masses are determined by the temperature dependence of the oscillation amplitudes, separating the oscillations of each dimensionally-quantized subband. The Fourier spectra of oscillations are used to determine the electron mobilities μq and μt in each dimensionally-quantized subband. It is shown that m* decreases as the InAs-nanoinsert thickness d in the In0.53Ga0.47As quantum well and electron mobilities increase. The maximum electron mobility is observed at the insert thickness d = 3.4 nm.

Anomalous hall effect in a 2D heterostructure including a GaAs/InGaAs/GaAs quantum well with a remote Mn δ-layer

For heterostructures with a GaAs/InGaAs/GaAs quantum well and a magnetic (Mn) δ-layer spatially separated from it (remote Mn δ-layer), the magnetic field and temperature dependences of the anomalous component of the Hall resistivity have been analyzed. The comparison with the temperature dependence of the longitudinal electrical resistance reveals three temperature ranges where three different mechanisms of the anomalous Hall effect are manifested. The reported results can be treated as experimental evidence of the essential role of the intrinsic mechanism of the anomalous Hall effect in a two-dimensional system.

Anomalous hall effect in the In1 − xMnxSb dilute magnetic semiconductor with MnSb inclusions

We study InSb:Mn polycrystals with different values of the Mn content. In these samples, two ferromagnetic phases have been found: MnSb nanoinclusions with TC ≈ 600 K and an InMnSb magnetic host with TC below 10 K. The magnetic field dependence obtained for the measured Hall resistance exhibits a nonlinear behavior within a wide temperature range. At high temperatures, such a behavior can be attributed to the existence of two types of charge carriers, namely, light and heavy holes. At temperatures below the Curie point of the InMnSb host, the anomalous Hall effect contributing to the nonlinearity of the Hall resistance has been observed. Ferromagnetic MnSb inclusions do not contribute to the anomalous Hall effect. They do not lead to any spin polarization of charge carriers owing to the Schottky barrier, which surrounds these inclusions and prevents their interaction with charge carriers. A method has been proposed for distinguishing the anomalous Hall component in the case where the Hall resistance includes a nonlinear contribution of a different nature.

Influence of buffer-layer construction and substrate orientation on the electron mobilities in metamorphic In0.70Al0.30As/In0.76Ga0.24As/In0.70Al0.30As structures on GaAs substrates

The influence of construction of the buffer layer and misorientation of the substrate on the electrical properties of In0.70Al0.30As/In0.76Ga0.24As/In0.70Al0.30As quantum wells on a GaAs substrate is studied. The temperature dependences (in the temperature range of 4.2 K < T < 300 K) and field dependences (in magnetic fields as high as 6 T) of the sample resistances are measured. Anisotropy of the resistances in different crystallographic directions is detected; this anisotropy depends on the substrate orientation and construction of the metamorphic buffer layer. In addition, the Hall effect and the Shubnikov–de Haas effect are studied. The Shubnikov–de Haas effect is used to determine the mobilities of electrons separately in several occupied dimensionally quantized subbands in different crystallographic directions. The calculated anisotropy of mobilities is in agreement with experimental data on the anisotropy of the resistances.

High-temperature magnetism and microstructure of a semiconducting ferromagnetic (GaSb)1−x(MnSb)x alloy

We have studied the properties of relatively thick (about 120 nm) magnetic composite films grown by pulsed laser deposition using the eutectic compound (GaSb)0.59(MnSb)0.41 as target for sputtering. For the studied films we have observed ferromagnetism and an anomalous Hall effect above room temperature, confirming the presence of spin-polarized carriers. Electron microscopy, atomic and magnetic force microscopy results suggest that the films under study have a homogenous columnar structure in the bulk while MnSb inclusions accumulate near the surface. This is in good agreement with the high mobility values of charge carriers. Based on our data we conclude that the magnetic and magnetotransport properties of the films at room temperature are defined by the MnSb inclusions.

Ferromagnetism in GaAs/InAs/GaAs Quantum Dot Layer Delta-Doped with Mn

Transport, magnetotransport and magnetic properties of structures GaAs/InAs/GaAs with a InAs quantum dot (QD) layer have been investigated in the temperature interval 4.2<T<300 K. The structures were delta-doped by Mn from a one side to provide magnetic properties and by carbon from the other side to enhance a p-type conductivity. The ferromagnetic phase up to 400 K was detected by SQUID magnetometer. Anomalous Hall-effect was observed at low T. The role of additional disorder in conducting channel due to the QD layer was investigated. It is shown a principal role of a fluctuation potential of Mn layer separated from conducting QD layer by a spacer in anomalous transport properties of structures. The negative magnetoresistance was observed at low T due to the reduction of the spin-flip scattering by aligning spins by magnetic field.

Anomalous Hall Effect in 2D DMS

Transport and magnetic properties of δ-Mn doped GaAs/InGaAs/GaAs quantum wells (QW) with various In content were studied at temperatures 4.2K≤T≤300K. Fluctuation potential (FP) appeared to be crucial for transport characteristics of structures under investigation. The magnetic percolation transition was observed at temperature Tp in the range 20 - 40K. The Tp dependence on the In content is nonmonotonic due to the peculiarities of free-carrier mediated exchange interaction mechanisms. The change of the anomalous Hall effect (AHE) sign with decreasing temperature was detected at temperatures close to the Tp. The main reason of the AHE sign change is the variation of contributions of different AHE mechanisms (intrinsic and side-jump) caused by the reduction of spin-dependent scattering intensity with temperature decrease. We believe that our results are the experimental observation of the AHE intrinsic mechanism in 2D.

Shubnikov de Haas Effect and Electron Mobilities in the Isomorphic InGaAs Quantum Well With the InAs Insert on the InP Substrate

The Shubnikov - de Haas (SdH) effect at T=4.2 and 8.4 K and the Hall effect have been investigated in isomorphic In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As/InP quantum well with InAs inserts to the center of the quantum well. The structures were both- sides delta-doped by silicon. The effective mass m* was measured by SdH effect by a new method which allowed experimentally to determine m* in the every dimensionally quantized subband using the temperature dependence of the SdH effect amplitude. Central InAs inserts in a quantum well lead to a decreasing of m* by about 20% as compared with the uniform In0.53Ga0.47As lattice-matched quantum well. We also calculated the transport and quantum mobility of electrons in dimensionally quantized subbands using the SdH effect. The calculated and experimentally determined values are in a good agreement.