A. B. Davydov

Anomalous hall effect in Mn δ-doped GaAs/In0.17Ga0.83As/GaAs quantum wells with high hole mobility

Magnetic and magnetotransport properties of GaAs(δ〈Mn〉)/In0.17Ga0.83As/GaAs quantum wells with different Mn concentrations are studied. The delta-doped manganese layer has been separated from the GaAs quantum well with a spacer with an optimal thickness (3 nm), which has provided a sufficiently high hole mobility (≥103 cm2V−1 s−1) in the quantum wells and their effective exchange with Mn atoms. It is found that the anomalous Hall effect (AHE) is exhibited only in a restricted temperature range above and below the Curie temperature, while the AHE is not observed in quantum wells with quasi-metallic conductivity. Thus, it is shown that the use of the AHE is inefficient in studying magnetic ordering in semiconductor systems with high-mobility carriers. The features observed in the behavior of the resistance, magnetoresistance, and Hall effect are discussed in terms of the interaction of holes with magnetic Mn ions with regard to fluctuations of their potential, hole transport on the percolation level, and hopping conduction.

Growth of magnetic eutectic GaSb-MnSb films by pulsed laser deposition

Eutectic GaSb + MnSb films ranging in thickness from 80 to 130 nm have been grown on sapphire substrates by pulsed laser deposition using mechanical droplet separation. The films were similar in composition to the ablation target, consisting of the eutectic GaSb-MnSb alloy. According to atomic force and electron microscopy data, the films were homogeneous, with p-type conductivity. Their electrical properties depended significantly on deposition conditions. The best films had a resistivity of 7 × 10−3 Ω cm, carrier concentration of 8.1 × 1019 cm−3, and carrier mobility of 102 cm2/(V s). Characteristically, the films had a negative magnetoresistance. Their magnetization curves showed saturation in a magnetic field of ∼1 × 10−1 T. According to the magnetic-field dependences, the coercive force in the films was within 3 × 10−2 T; that is, the films were soft magnets with a small domain size.

Planar Hall effect and anisotropic magnetoresistance in layered structures Co0.45Fe0.45Zr0.1/a-Si with percolation conduction

Magnetic and magnetotransport properties of multilayered nanostructures Co0.45Fe0.45Zr0.1/a-Si obtained by ion-beam sputtering are investigated. The temperature dependence of the resistance obeys a law of the form Rxx ∝-logT, which is typical of metal-insulator nanocomposites on the metal side of the percolation transition. The magnetoresistance anisotropy effect, as well as the planar Hall effect, is observed for the first time for this type of nanocomposites in the vicinity of the percolation transition. The correlation of these two effects with the transverse (between Hall probes) magnetoresistive effect, which may reach 6–9%, is revealed. A weak negative magnetoresistance of the order of 0.15%, which is observed for subnanometer amorphous silicon layer thicknesses, is attributed to spin-dependent electron transitions between adjacent ferromagnetic layers in the case when the exchange interaction between these layers is of the antiferromagnetic type.

Ferromagnetic transition in GaAs/Mn/GaAs/InxGa1 − xAs/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/InxGa1 − xAs/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.

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.

Transport, magnetotransport, and ferromagnetism in diluted magnetic semiconductors

The magnetic properties, Hall effect, and magnetoresistance are discussed for various diluted magnetic semiconductors: In1−xGaxAs quantum well structures, delta-doped with Mn; layers of InAs quantum dots in GaAs, delta-doped with Mn; GaAs structures ion-implanted with Mn and Mg; the thermoelectrics Bi2Te3⟨Fe⟩ and Sb2−xCrxTe3. We investigate the influence of the ferromagnetism that arises in all of those systems on the transport properties, Hall effect, and low-temperature negative magnetoresistance. In Bi2Te3⟨Fe⟩ and Sb2−xCrxTe3 we investigate the influence of Fe and Cr magnetic impurities on the Seebeck coefficient and thermal conductivity.

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.

Properties of Structures Based on Laser-Plasma Mn-Doped GaAs and Grown by MOC-Hydride Epitaxy

A method of doping GaAs with Mn using the laser evaporation of a metal target during MOC-hydride epitaxy is developed. The method is used to form both homogeneously doped GaAs:Mn layers and two-dimensional structures, including a δ-doped GaAs:Mn layer and a InxGa1−xAs quantum well separated by a GaAs spacer with a thickness of d= 3–6 nm. It is shown that, at room temperature, the formed structures have magnetic and magnetooptical properties most probably caused by the presence of MnAs clusters. In the low-temperature region (∼ 30 K), the anomalous Hall effect is observed. This effect is attributed to the exchange interaction between Mn ions via 2D-channel holes.

Properties of InGaAs/GaAs quantum wells with a δ〈Mn〉-doped layer in GaAs

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an InxGa1−x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (Tm ∼ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.

Quantum, normal and anomalous Hall effect in 2D ferromagnetic structures: GaAs/InGaAs/GaAs quantum well with remote Mn delta-layer

We study experimentally the electronic transport and magnetism of GaAs/InGaAs/GaAs quantum wells (QW) with remote Mn δ-layer. The 2D energy spectrum of the carriers is revealed by quantum Hall Effect measurements. The ferromagnetic ordering is evidenced by anomalous Hall Effect and direct magnetization measurements. A signature of this state is also observed on the temperature dependence of the sample resistance. The dependence of the Curie temperature, TC, on manganese content, quantum well depth, and on the spacer thickness between QW and Mn δ-layer shed light on the mechanisms of exchange interaction in such structures.