Spin-Valve Effect in Magnetic Resonant Tunneling Devices
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Spin-Valve Effect in Magnetic Resonant Tunneling Devices A. N. Chantis, D. O. Demchenko, and A. G. Petukhov Physics Department, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995
ABSTRACT We propose a new electronic device utilizing resonant tunneling between two magnetic materials. The device is comprised of a semiconductor quantum well sandwiched between two insulating barriers and two ferromagnetic electrodes. The situation in which a resonant level fits in the energy interval where the minority density of states of a ferromagnetic emitter is zero can be considered as an almost ideal spin valve and leads to a great enhancement of magnetoresistance. This situation can be achieved by tuning the width of the quantum well. As an example we will consider GaMnAs/AlAs/GaAs/AlAs/GaMnAs double-barrier heterostructure. We can demonstrate that at a certain thickness of the quantum well and the barriers this system can significantly outperform conventional tunneling junctions comprised of one insulating barrier sandwiched between two ferromagnetic electrodes. ONE-BAND MODEL In this section we will briefly explain the operational principles of the proposed device using one-band spin-polarized model [1] as a generic example. Even though one-band spindependent resonant tunneling devices (RTDs) were considered in the literature [2, 3] the spin-valve effect described below has been overlooked. We will consider small biases when only the electrons at the Fermi surface contribute to the tunneling current, and will take into account elastic processes only. The double-barrier structure in question is shown in Fig.1, where w and L are barrier and well widths respectively. For the situation presented in Fig.1 the only option when the resonant level contributes to the current corresponds to the case (a) of the majority spins and of the ferromagnetic alignment of the magnetizations in the emitter and collector. For the parallel (ferromagnetic) alignment of the magnetizations in the leads (Fig.1 (a) (b)) the resonant conditions are: ∓
h ¯2 2 h ¯2 2 h ¯2 2 ∆ + k + k = E + k = EF R 2 2m∗ k 2m∗ z 2m∗ k
(1)
where signs “-” and “+” in the left-hand side correspond to the majority and minority spin channels respectively. The similar conditions for the antiparallel (antiferromemagnetic) alignment of the leads (Fig.1 (c),(d)) are: ∓
∆ h ¯2 2 h ¯2 2 h ¯2 2 ∆ h ¯2 2 h ¯2 2 + k + k = E + k = ± + k + k = EF R 2 2m∗ k 2m∗ z 2m∗ k 2 2m∗ k 2m∗ z
(2)
Here ER is the energy of the resonant level, ∆ is the magnetization-induced splitting of the majority and minority bands, kk is the in-plane wave-vector component (it is assumed to be conserved), and kz is the wave-vector component in the direction of the current. The
F7.4.1
Ferro w
w L
L
EF
EF ER
e-
ER
M
a) Majority spins (↑)
∆
b) Minority spins (↓)
Antiferro
w
w L
L
EF
EF ER
ER ∆
c) Majority spins (↑)
d) Minority spins (↓)
Figure 1: Schematic flat-band diagrams illustrating operational principle of the resonant tunneling spin valve resonant level contribute
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