Magnetic semiconductor

Magnetic semiconductors are materials that exhibit both ferromagnetism (or a similar response) and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of charge carriers (n- or p-type), practical magnetic semiconductors would also allow control of quantum spin state (up or down). This would theoretically provide near-total spin polarization (as opposed to iron and other metals, which provide only ~50% polarization), which is an important property for spintronics applications, eg. spin transistors.

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While many traditional magnetic materials, such as magnetite, are also semiconductors, materials scientists generally predict that magnetic semiconductors will only find widespread use if they are similar to well-developed semiconductor materials. To that end, dilute magnetic semiconductors have recently been a major focus of magnetic semiconductor research. These are based on traditional semiconductors, but are doped with transition metals instead of, or in addition to, electronically active elements.

Hideo Ohno and his group at the Tohoku University were the first to measure ferromagnetism in transition metal doped compound semiconductors such as indium arsenide and gallium arsenide doped with manganese referred to as GaMnAs. These materials exhibited reasonably high Curie temperatures (yet below room temperature) that scales with the concentration of p-type charge carriers. Ever since, ferromagnetic signals have been measured from various semiconductor hosts doped with different transition atoms.

Materials

The manufacturability of the materials depend on the thermal equilibrium solubility of the dopant in the base material. Eg, solubility of many dopants in zinc oxide is high enough to prepare the materials in bulk, while some other materials have so low solubility of dopants that to prepare them with high enough dopant concentration thermal nonequilibrium preparation mechanisms have to be employed, eg. growth of thin layers.

A flurry of research in the past few years has shed some light on the crucial factors that are needed to achieve high-Curie temperature (above room temperature) ferromagnetic semiconductors, which can explain the so-called controversy in the field and lack of reproducibility in the magnetic properties for the same materials. Indeed, the first great discovery in the field was in 1986 by T. Story and co-workers where they demonstrated that the ferromagnetic Curie temperature of Mn²⁺-doped Pb_1-xSn_xTe can be controlled by the carrier concentration.[1] The theory proposed by Dietl required charge carriers in the case of holes to mediate the magnetic coupling of manganese dopants in the prototypical magnetic semiconductor, Mn²⁺-doped GaAs. If there is an insufficient hole concentration in the magnetic semiconductor, then the Curie temperature would be very low or would exhibit only paramagnetism. However, if the hole concentration is high (>~10²⁰ cm^-3), then the Curie temperature would be higher, between 100-200 K.[2]

The controversy that you are discussing has arisen because researchers for the most part have neglected the importance of carriers in their materials. Recent research by the University of Washington group led by Daniel Gamelin has shed some light for instance on the importance of interstitial zinc (a shallow donor) for controlling the ferromagnetism in a high-Curie temperature, Co²⁺-doped ZnO.[3][4]

Several examples of ferromagnetic semiconductor materials are eg.:

• Manganese-doped indium arsenide and gallium arsenide (GaMnAs), with Curie temperature around 50–100 K and 100–200 K, respectively
• Manganese-doped indium antimonide, which becomes ferromagnetic even at room temperature and even with less than 1% Mn. [5]
• Oxide semiconductors [6]
  • Manganese- and iron-doped indium oxide, ferromagnetic at room temperature
  • Manganese-doped zinc oxide
  • n-type Cobalt-doped zinc oxide[7]
  • Titanium dioxide:
    • Cobalt-doped titanium dioxide (both rutile and anatase), ferromagnetic above 400 K
    • Chromium-doped rutile, ferromagnetic above 400 K
    • Iron-doped rutile and iron-doped anatase, ferromagnetic at room temperature
    • Nickel-doped anatase
  • Manganese-doped tin dioxide, with Curie temperature at 340 K
  • Iron-doped tin dioxide, with Curie temperature at 340 K