SPINTRONICS

CHARGE AND SPIN-POLARIZED TRANSPORT

Spintronics, or Magnetoelectronics, is the study of the behaviour of the

electron –the spin–, in addition to the degree of freedom studied in previous simulations, the charge.

Spin transport: "

of an elementary particle, such as the electron. [...] Any charged object possessing spin also possesses an

intrinsic magnetic moment. It has been known for decades that in ferromagnetism the spins of electrons

are preferentially aligned in one direction. Then, in 1988, it was demonstrated that currents flowing from

a ferromagnet into an ordinary metal retain their spin alignment for distances longer than interatomic

spaces, so that spin and its associated magnetic moment can be transported just as charge. This

means that magnetization as well can be transferred from one place to another.

"

D. Treger, S. A. Wolf, , AccessScience, McGraw-Hill Companies (2008)

Spintronics (spin transport)

A dc voltage biased II-VI semiconductor multi-quantum-well structure with normal contacts exhibits self-sustained

spin polarized current oscillations if one or more of its wells are doped with magnetic impurities Mn.

Self-sustained current oscillations may appear or not depending on the spin splitting Δ induced by the exchange

interaction. From our results, we propose how to design a device behaving as a spin polarized current oscillator.- Self-sustained current oscillations:
Polarized current density
*J*and^{+}(t)*J*(left), and electric field^{–}(t)*E*(right)_{i}(t)

Quantum wells magnetization: Δ_{i}= 0 meV, Δ_{1}= Δ_{16}= Δ_{34}= 12 meV, Δ_{50}= 2 meV.

- Effect on the electric field of the index of the first magnetic quantum well:

Polarization:*i*_{1}and*i*_{2}are the first and second magnetic quantum wells.

*i*_{2}is fixed = 30, and*i*_{1}moves from 2 to 9, 13 and 20

Magnetoswitching

In Gunn diodes, the parameter values leading to a type of oscillatory behaviour cannot be modified once the diode has been made.

Similarly, in a conventional III-V weakly coupled n-doped semiconductor superlattice (SL), the boundary condition at the injector, the

SL configuration, and the doping density at the quantum wells determine whether the system exhibits current oscillations mediated by

charge dipole or monopole waves, or multistable static electric field domains. But again, once the SL has been made and contacted,

the stable solutions can be selected only by changing the bias and this limits the type of attractors present in a particular SL.

The situation is different in the case of a dilute magnetic semiconductor multiquantum well structure: any self-oscillations that may

appear are due to triggering of dipoles at the magnetic quantum well. However, by changing the external magnetic field we can select

either stable stationary states or self-sustained current oscillations as the dilute magnetic semiconductor multiquantum well response.

Charge and spin responses to voltage and magnetic field switching of n-doped dc voltage biased II-VI dilute magnetic semiconductor

multiquantum well structures are analysed.

- Voltage switching:

Current-voltage characteristic (branches 2 and 3 of 10) of a multi-quantum well structure of 12 quantum wells

Labels (A) to (D) denote the initial and final values of the voltage in the (sudden) switching

Stationary/oscillatory behaviour of current density and corresponding electric field wave

- Magnetic field switching:

Multi-quantum well structure of 10 quantum wells

Constant voltage and magnetic field switching from*B*= 2 to 6 T.

Simultaneous voltage switching*V/V*_{0}= 2 → 10 and magnetic switching Δ = 10 → 15 meV - Phase diagram

Zeeman level splitting Δ vs. dimensionless applied voltage φ =*V/V*_{0}for different multiquantum well structure lengths.

Each color corresponds to a structure with a different number of quantum wells,

from*N*= 4 (cyan) to 10 (red). Colored regions correspond to stable self-sustained

current oscillations; outside colored regions: stable stationary states.

- Voltage switching:

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