WSU Spintronics  Overview

What is Spintronics?

Once studied primarily for their fundamental importance, magnetic materials are now essential for making complex structures with unique magnetic properties. Devices based on these structures, which utilize electron's spin rather than charge are revolutionizing electronics, optical communications, and electronic data storage. Studies of the processes governing operation of these new devices are giving rise to an entire new field of physics, engineering, and materials science research, often called spintronics (spin transport + electronics). The discovery of Giant Magnetoresistance (GMR) and Tunneling Magnetoresistance (TMR) has made it possible to construct new devices, such as magnetic sensors and nonvolatile random access memories (MRAMs). All GMR, TMR, spin transistors and most other spintronics devices utilize the transfer of spin-polarized electrons across an interface from one ferromagnetic layer to another. This often results in dramatically different resistance for magnetically aligned and anti-aligned states. This effect is particularly important in a new class of materials, the so-called half-metals, which have only one type of spin at the Fermi level (and thus are 100% spin-polarized). To be a half-metal, a magnetic material must have a metallic band structure for one spin direction and a semi-conducting (with a band gap) for another. 100% spin polarization allows true on/off operation, with an essentially infinite ratio of impedance between the two states. If the development of half-metals succeeds, many of today's digital electronic devices could be replaced with much smaller, more rugged devices with an intrinsic advantage of nonvolatile memory. Then, for example, nonvolatile reprogrammable logic could be fabricated with magnetoelectronic elements, which would introduce an entirely new, software-driven computing.

Magnetic Semiconductors

Until very recently research on spintronics materials was almost entirely focused on metals and metallic oxides. However, the advancement of film deposition techniques, especially molecular beam epitaxy (MBE) changed the situation dramatically. The ability of MBE to dope III-V semiconductor compounds with Mn beyond its solubility level made it possible to realize an ultimate electronic material, spin-polarized semiconductor. Now it has become possible to explore the physics and applications of a unique combination of charge and spin degrees of freedom in semiconductor hetero-structures. Both resonant tunneling diodes and light-emitting diodes incorporating one layer of magnetic semiconductor have been demonstrated. Optical spin injection and detection allow one to incorporate magnetism, electronics and photonics in semiconductors and to fabricate a variety of high-performance devices such as optical switches, encoders and decoders. One of the most exciting applications of semiconductor spintronics, is Quantum Computing, which is based on our ability to create, control, and transfer coherent spin states.

Spin Polarization Measurements

The ability to measure the spin polarization P of magnetic materials (metals, oxides, and magnetic semiconductors) is of vital importance to spintronics. Measuring P requires a spectroscopic technique that can discriminate between spin-up and spin-down electrons at the Fermi level EF. Spin-polarized photoemission spectroscopy is technically capable of providing the most direct density of states (DOS) measurement of P but lacks the necessary energy resolution and is quite surface-sensitive. An effective alternative to photoemission is the use of spin-polarized tunneling in a planar junction geometry that does allow the electronic spectrum near EF to be probed with sub-meV energy resolution. This technique has been successfully used for a number of magnetic metals, but its drawback is the need of fabricating a layered device consisting of a thin ferromagnetic film on top of a uniform ~ 1 nm oxide layer which, in turn, is formed on top of a superconducting base. Many interesting materials cannot be made within this stringent constraint. On the other hand, (Point Contact) Andreev Reflection Spectroscopy, which is currently being developed by us and others, measures P of the transport current and has no materials constraints.

Andreev Reflection

(Point Contact) Andreev Reflection Spectroscopy is based on Andreev reflection, a process taking place at the interface between a normal metal and a superconductor, which allows the propagation of a single electron with the energy below the superconducting gap from the normal metal to the superconductor, or rather a conversion of a quasi-particle current into a super-current. This process can be similarly described as reflection or retro-reflection of a hole, because as the reflected hole (approximately) retraces the trajectory of the incident electron. In a non-magnetic normal metal the Andreev process is always allowed, because every energy state in a normal metal has both spin-up and spin-down electrons.

Point Contact Andreev Reflection Spectroscopy (PCAR)

In a magnetic metal Andreev reflection is limited by a minority spin population and is completely forbidden in the case of a half-metal (at T = 0K), as there are no states for a hole to get reflected to, resulting in zero conductance across the interface below the gap. The Point Contact Andreev Reflection Spectroscopy (PCAR) technique makes use of a correlation between the degree of suppression of Andreev reflection at a metal-superconductor interface and the spin polarization of the metal. Conversely, if one is able to measure a conductance of a point contact (the easiest geometry to observe Andreev reflection) at an interface with an unknown material, one can determine the spin polarization of this material from the overall shape of the conductance curve. These measurements can be done in a wide variety of ferromagnetic systems, including dilute magnetic semiconductors.

Scanning Hall Probe Microscopy

While the spin polarization measurements by the Andreev Reflection technique can be classified as a part of electrical spin detection, there are several advantages in doing alternative measurements, which do not require direct contact with the spin system and thus are non-invasive. Scanning Hall Probe Microscopy is a highly sensitive non-invasive technique that can be used to measure the spatial spin distribution in magnetic domains, magnetic phase transitions, and many other magnetic, as well as superconducting systems.

Copyright ©2006 Boris Edward Nadgorny