Transport properties of GaAs/{delta}<Mn>/GaAs/IntimesGa1-timesAs/GaAs structures containing InxGa1-xAs (times {approx} 0.2) quantum well (QW) and Mn delta layer (DL) with relatively high, about one Mn monolayer (ML) content, are studied. In these str
uctures DL is separated from QW by GaAs spacer with the thickness ds = 2-5 nm. All structures possess a dielectric character of conductivity and demonstrate a maximum in the resistance temperature dependence Rxx(T) at the temperature {approx} 46K which is usually associated with the Curie temperature Tc of ferromagnetic (FM) transition in DL. However, it is found that the Hall effect concentration of holes pH in QW does not decrease below TC as one ordinary expects in similar systems. On the contrary, the dependence pH(T) experiences a minimum at T = 80-100 K depending on the spacer thickness, then increases at low temperatures more strongly than ds is smaller and reaches a giant value pH = (1-2)cdot10^13 cm^(-2). Obtained results are interpreted in the terms of magnetic proximity effect of DL on QW, leading to induce spin polarization of the holes in QW. Strong structural and magnetic disorder in DL and QW, leading to the phase segregation in them is taken into consideration. The high pH value is explained as a result of compensation of the positive sign normal Hall effect component by the negative sign anomalous Hall effect component.
We study experimentally and theoretically the effects of disorder, nonlinear screening, and magnetism in semiconductor heterostructures containing a $delta$-layer of Mn, where the charge carriers are confined within a quantum well and hence both ferr
omagnetism and transport are two-dimensional (2D) and differ qualitatively from their bulk counterparts. Anomalies in the electrical resistance observed in both metallic and insulating structures can be interpreted as a signature of significant ferromagnetic correlations. The insulating samples turn out to be the most interesting as they can give us valuable insights into the mechanisms of ferromagnetism in these heterostructures. At low charge carrier densities, we show how the interplay of disorder and nonlinear screening can result in the organization of the carriers in the 2D transport channel into charge droplets separated by insulating barriers. Based on such a droplet picture and including the effect of magnetic correlations, we analyze the transport properties of this set of droplets, compare it with experimental data, and find a good agreement between the model calculations and experiment. Our analysis shows that the peak or shoulder-like features observed in temperature dependence of resistance of 2D heterostructures $delta$-doped by Mn lie significantly below the Curie temperature $T_{C}$ unlike the three-dimensional case, where it lies above and close to $T_{C}$. We also discuss the consequences of our description for understanding the mechanisms of ferromagnetism in the heterostructures under study.
A detailed study of the magnetic and transport properties of Si1-xMnx (X = 0.35) films is presented. We observe the anomalous Hall effect (AHE) in these films up to room temperature. The results of the magnetic measurements and the AHE data are consi
stent and demonstrate the existence of long-range ferromagnetic (FM) order in the systems under study. A correlation of the AHE and the magnetic properties of Si1-xMnx (X = 0.35) films with their conductivity and substrate type is shown. A theoretical model based on the idea of a two-phase magnetic material, in which molecular clusters with localized magnetic moments are embedded in the matrix of a weak itinerant ferromagnet, is discussed. The long-range ferromagnetic order at high temperatures is mainly due to the Stoner enhancement of the exchange coupling between clusters through thermal spin fluctuations (paramagnons) in the matrix. Theoretical predictions and experimental data are in good qualitative agreement.
We report results of investigations of structural and transport properties of GaAs/Ga(1-x)In(x)As/GaAs quantum wells (QWs) having a 0.5-1.8 ML thick Mn layer, separated from the QW by a 3 nm thick spacer. The structure has hole mobility of about 2000
cm2/(V*s) being by several orders of magnitude higher than in known ferromagnetic two-dimensional structures. The analysis of the electro-physical properties of these systems is based on detailed study of their structure by means of high-resolution X-ray diffractometry and glancing-incidence reflection, which allow us to restore the depth profiles of structural characteristics of the QWs and thin Mn containing layers. These investigations show absence of Mn atoms inside the QWs. The quality of the structures was also characterized by photoluminescence spectra from the QWs. Transport properties reveal features inherent to ferromagnetic systems: a specific maximum in the temperature dependence of the resistance and the anomalous Hall effect (AHE) observed in samples with both metallic and activated types of conductivity up to ~100 K. AHE is most pronounced in the temperature range where the resistance maximum is observed, and decreases with decreasing temperature. The results are discussed in terms of interaction of 2D-holes and magnetic Mn ions in presence of large-scale potential fluctuations related to random distribution of Mn atoms. The AHE values are compared with calculations taking into account its intrinsic mechanism in ferromagnetic systems.
