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| Fig. 1: An example of a spin transistor. |
In recent years, there has been a lot of excitement generated by materials called magnetic semiconductors due to their potential application to spintronic devices. These materials not only exhibit ferromagnetism, but also several useful semiconductor properties as well. Spintronic devices introduce a new type of conduction, by controlling the quantum spin state, as opposed to the charged carriers in conventional electronic semiconductors. In theory, this will enable almost total spin polarization, allowing the development of spin transistors which may then replace the current standard silicon transistors.
The research on ferromagnetic semiconductors began in the 1960's with the goal of combining electrical transport and magnetism. Typical materials used were EuO, GdN, CuCrTiS4. A special feature of these magnetic semiconductors is that carriers (s electrons) interact strongly with the moments of partly filled d or f shells of magnetic ions. Therefore, magnetic order affects strongly the carrier motion and the carriers themselves may exert a strong influence on the magnetic order. Many properties of magnetic semiconductors can be explained by the fact that the electron energy is minimal in the case of complete ferromagnetic order and that it increases on deviation from this order. Therefore, conduction electrons tend to establish and maintain the ferromagnetic order. At low densities, electrons cannot influence the state of a crystal as a whole. However, electrons may be localized in some part of a crystal and they can then establish there a sufficiently high degree of ferromagnetic order to achieve a strong reduction in their energy. The main problem with these types of magnetic semiconductors is their very low Curie point Tc (typically less than 100K), which makes them impractical for commercial use.
The second generation of magnetic semiconductors were developed starting in the late 1980s. These are based on traditional semiconductor materials like Si and GaAs which are then doped with transition metal elements instead of, or in addition to, electronically active elements to provide the magnetic properties. Examples of these so-called dilute magnetic semiconductors include Ga1-xMnxAs, Mn and Co doped ZnO, etc. Even though these show higher Curie temperatures above 100K, they still fall below room temperature and thus remain impractical for commercial use.
Quite recently, Mn- and Fe-doped indium oxide, Co-doped titanium dioxide, Mn- and Fe-doped tin dioxide and many similar materials have brought the Curie temperature up to room temperature or higher. The process involved in making these materials depend largely on the thermal equilibrium solubility of the dopants in the base material. In some materials, the low solubility limit forces the use of thin film techniques along with laser deposition to produced the desired dopant concentrations and high Curie temperature properties (largely determined by hole concentration).
The spin transistor is a magnetically-sensitive device made out of magnetic semiconductors, originally proposed in 1990. It is currently a hot area of research and is basically an advanced version of the conventional electronic transistor. The operation of the spin transistor relies on the ability of electrons (as fermions) to naturally exhibit one of two states of spin, essentially using electrons set in particular states of spin to store information. One advantage over conventional transistors is that these spin states can be detected and altered without necessarily requiring the application of an electric current. This in turn allows for the design of much smaller but even more sensitive detection hardware, unlike the current ones which rely on noisy amplifiers to detect the charges in data storage devices. In short, these devices are cheap, can store more data in less space and consume less power.
A second advantage of the spin transistor is that the spin of an electron is effectively permanent if undisturbed, so it can be used as means of non-volatile solid state storage that is not only cheap, but is also operationally cost effective since it does not require the constant application of current to refresh the memory state.
© 2007 Paul Lim. The author grants permission to copy, distribute and display the work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] E. L. Nagaev et al., "Ferromagnetic and Antiferromagnetic Semiconductors. Sov. Phys. Usp. 19, 863 (1975).
[2] Y. Z. Peng et al., "Room Temperature Diluted Magnetic Semiconductor Synthesized by Dual Beam Laser Deposition," Appl. Phys. A 80, 565 (2005).