Spin separation in digital ferromagnetic heterostructures

Abstract

In a study of the ferromagnetic phase of a multilayer digital ferromagnetic semiconductor in the mean-field and effective-mass approximations, we find the exchange interaction to have the dominant energy scale of the problem, effectively controlling the spatial distribution of the carrier spins in the digital ferromagnetic heterostructures. In the ferromagnetic phase, the majority-spin and minority-spin carriers tend to be in different regions of the space ͑spin separation͒. Hence, the charge distribution of carriers also changes noticeably from the ferromagnetic to the paramagnetic phase. An example of a design to exploit these phenomena is given here. DOI: 10.1103/PhysRevB.66.073312 PACS number͑s͒: 75.70.Cn, 75.50.Pp, 75.10.Ϫb The research in proactive use of the spins of the carriers to add a new dimension to electronics starts a new area known as spintronics.1 The recent discoveries 2,3 of ferromagnetism with high Curie temperatures in a number of conventional semiconductors doped with magnetic impurities hold promise for the implementation of spintronics in semiconductors.Inhomogeneously doped semiconductors, such as the p-n junction, play a crucial role in conventional electronic devices. Their properties depend on the distribution of the itinerant carriers, which is governed by the Coulomb interaction of the carriers with the impurities and with other carriers. In this paper we study the charge and spin distributions of the itinerant carriers in semiconductors ␦ doped with magnetic impurities, such as GaMnAs.4,5 Our theory is within the mean-field, effective-mass, and virtual-crystal approximations. In addition to the electrostatic forces, the itinerant carriers have an exchange interaction with the magnetic impurities. Our calculation shows that the magnetization of the high Mn concentration in the ␦ layers in the ferromagnetic phase gives rise to a spin-dependent potential experienced by the carriers comparable in order of magnitude to the charge potential of the ␦ layer. The effect of the spin-dependent potential on the inhomogeneous spin distribution of the carriers was noted by Loureiro da Silva et al. 6 in multilayered GaMnAs with 5% magnetic impurities. By contrast, the ␦ doping with a nominal concentration per atomic plane of 25-50% Mn atoms gives rise to a qualitatively different phenomenon of spin separation. This creates the possibility of the magnetic control of the itinerant carriers by manipulating the magnetization of the Mn ions. In particular, the spindependent potential may be used to influence the spin dependence of the distribution of itinerant carriers. The majority carriers 7 accumulate in the region of the Mn layers whereas minority carriers are repelled from the Mn region. In the paramagnetic phase, the spin potential averages to zero and the magnetic influence disappears. To show the potential of the spin separation for device applications, we give an example of how heterostructures may be designed to engender spin separation. This paper is organized as follows. A brief review...

Single-exciton spectroscopy of semimagnetic quantum dots

“…Hence, the exact many-spin wave functions of the Mn coupled to zerodimensional exciton feature ferromagnetic correlations, very much like bulk carrier mediated ferromagnetic order 23,25 and in agreement with experimental observations in photoexcited QD.…”

“…The Mn spins interact also with each other via short-range superexchange coupling. This Hamiltonian has been succesfully used to describe bulk, 15,23,24 two-dimensional, 25 and zero-dimensional QD. [18][19][20][21][22] In general, the Hamiltonian cannot be solved exactly and in most instances the mean field or some other approximations are used.…”

Self-sustained Magnetoelectric Oscillations in Magnetic Resonant Tunneling Structures

“…If the quantum well is made of a ferromagnetic DMS [13,14], resonant tunneling conditions should influence magnetic ordering as well. It has already been predicted that the critical temperature T c of the well can be strongly modified electrically [15][16][17]. Here we show that the magnetic ordering affects back the tunneling current, in a peculiar feedback process, leading to interesting dynamic transport phenomena.…”

Semiconductor spintronics

“…Investigation based on mean field models reveal that p-d exchange interaction in δ-doped layers tends to be further enhanced by an additional confinement of the carriers, e.g., in a quantum well, leading accordingly to higher Curie temperatures (Fernández-Rossier and Sham, 2001;Kim and Yi, 2002;Lee et al, 2002). In such digital ferromagnetic heterostructures the occurrence of spin separation (the majority and minority-spin carriers reside in different spatial regions) has been predicted (Fernández-Rossier and Sham, 2002). Furthermore, it has been shown that the Curie temperature can be controlled over a broad range by external electric fields, which modulate the overlap of the subband wave functions with the Mn δ-doping profile (Nazmul et al, 2004;Lee et al, 2000), as schematically sketched in Fig.…”