506 CHAPTER 10. COHERENT TRANSPORT AND MESOSCOPIC DEVICES
possible to develop electronic devices that are dependent on the spin of the electrons much like
optical devices are dependent on polarization of light. In optics the use of a polarizer, analyzer
and modulator allow one to make switches. The same can be possible in electronics if electrons
can be injected and extracted with spin selection.
In magnetic semiconductors it is possible to use ferromagnetic contacts to inject electrons
with spin selectivity. Notable examples of magnetic semiconductors are InGaAsMn, CdMnTe,
ZnMnSe, and HgMnTe. These semiconductors, known as diluted magnetic semiconductors, and
their heterostructures with other semiconductors can now be fabricated and they offer a unique
opportunity for the combined studies of semiconductor physics and magnetism. The magnetic
semiconductors are fabricated by the usual epitaxial techniques like MBE or MOCVD and Mn
is introduced as an extra ingredient. The Mn composition is usually≤ 20 %.
In recent years there has been a growing interest in a field known as “spintronics” (after spin
and electronics). In conventional electronics, electron density is modulated to create devices for
digital and analog applications. In spintronics the expectation is that one modulates the spin
of electrons. As in quantum interference devices discussed in section 10.4, such a possibility
promises very low power, high density devices. An important point to note in spin dependent
devices is that usual scattering mechanisms that impact transport cause only very weak spin
scattering. Thus an electron can maintain its spin value for several microns (or even 100 microns
at low temperature). However, this does not mean that spin based transistors can function at
high temperatures or for long channel lengths. Non-spin altering scattering processes are still
important in spintronic devices.
In conventional electronic devices we ignore the electron spin. As noted above the main
reason we have not worried about electron spin is that usually density of spin-up and spin-down
electrons is the same and the spin splitting in the presence of a magnetic field is small. However,
it is possible to prepare a semiconductor sample in a state where electrons in the conduction band
have a much higher density of spin-down electrons. This can be done by using optical injection
or electronic injection. Electrons (or other charged particles) interact with a magnetic field via a
magnetic moment which is written as
μs=−gμBS=γS (10.6.1)where S is the spin of the particle;gis known as theg-factor and characterizes the particle. The
constantμBis the Bohr magneton and has a value
μB=e
2 m(10.6.2)
The constantγis called the gyromagnetic or magnetogyric ratio. The magnetic interacction
associated with the spin is
Hspin=−μs·B (10.6.3)
Spin Injection and Spin Transistor
In ferromagnetic materials, once the material is magnetized, there is a strong selection of spin
orientation (below the Curie temperature). If a ferromagnetic contact is used in a semiconductor