Ferromagnetism of low-dimensional Mn-doped III-V semiconductor structures in the vicinity of the insulator-metal transition

Abstract

The structural and transport properties of GaAs/Mn/GaAs/InxGa1−xAs/GaAs quantum wells (x≈0.2) with Mn δ-layer (4–10 at. %), separated from the well by a GaAs spacer, have been studied. The hole mobility in the investigated structures has exceeded the values known for magnetic III-V heterostructures by two orders of magnitude. For structures with the conductivity of the metal type, we have succeeded to observe at low temperatures Shubnikov–de Haas oscillations just confirming the two dimensionality (2D) of the hole energy spectrum. Exactly those 2D holes promote the ferromagnetic ordering of the Mn layer. That has been proven by (i) observing maxima (at 25–40 K) in temperature dependencies of the resistance, which positions agree with calculated values of Curie temperatures (for structures with the indirect interaction of Mn atoms via 2D holes), and (ii) revealing the negative spin-dependent magnetoresistance (NMR) as well as the anomalous Hall effect (AHE), which values are also in good agreement with calculations relating to ferromagnetic 2D III-V systems. As for the structures with the insulator type of the conductivity, their NMR and AHE features evidence the phase separation—the sample fragmentation with the formation of mesoscopic ferromagnetic areas separated by paramagnetic strata of the high tunnel conductivity.

Spin-polarized half-metallic state of a ferromagneticδlayer in a semiconductor host

“…As far as the physics of systems with a channel is concerned, [4][5][6] in the absence of clear experimental indications, we can provide some indicative predictions about the behavior of the FM critical temperature T c with varying distance d between the channel and the δ layer. In Sec.…”

“…For example, III-V DMHs of this type have been grown and studied, in which a δ layer of Mn was deposited close to a quantum well (also called channel) at some distance (called spacer) d from it. [4][5][6] The effect of the channel on magnetism in such multicomponent systems is threefold. First, the quantum magnetic proximity effect (interpenetration of the wave function tails both into the channel and into the δ layer) modifies the effective exchange integral between the local spins of magnetic ions in the δ layer and polarizes the free carrier spins in the channel.…”

“…Further lines of investigation include the mechanism of exchange coupling among different δ layers when they are organized into a periodic structure, 2,3 and the feedback of the channel on the magnetic properties of the δ layer. [4][5][6][7] As far as the first aspect is concerned, to evaluate the exchange interaction between two magnetic δ layers, in Ref. 18 we introduced a phenomenological model which assumed FM order within each δ layer as the starting point, and showed that there are two main mechanisms, which lead to the competition of FM and AFM coupling between two neighboring δ layers.…”

“…2,3 Little attention has been devoted, so far, to DMHs in which 3d-magnetic metal δ layers are inserted into a IV-group semiconductor (Si or Ge) host, although these systems are the more natural candidates to realize full integration of spintronics with standard Si-based electronics. While initially the magnetic properties of DMHs were attributed to the interplay of the different δ layers, or to the presence of free carriers in systems with a quantum-well carrier channel parallel to the δ layer, [4][5][6] increasing experimental evidence indicates that magnetism can be ascribed even to an individual δ layer, 2,7 while the channel or the periodic arrangement of δ layers may introduce some interesting modifications.…”

Ferromagnetic ordering in Mn-doped quantum wells GaAs-AlGaAs resulting from the virtual Anderson transition

“…[1]). While the systems of such a sort were studied in some detail, it is not a case for the structures without "external" delocalized carriers supplied by non-magnetic dopants, where the system of Mn dopants is close to the metal-insulator transition.…”