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Posted: Sep 20, 2011
Ferromagnetism can be induced electrically at room temperature
(Nanowerk News) Inducing and controlling magnetization in ferromagnetic semiconductors using electric rather than magnetic fields could lead to smaller and more energy-efficient spintronic devices. Until now, however, this electrical control has only been achieved at cryogenic temperatures in magnetic semiconductors. Tomoteru Fukumura and colleagues from the University of Tokyo, Tohoku University and other institutions in Japan have now extended electrical control all the way up to ambient temperature in cobalt-doped titanium dioxide, paving the way for room-temperature spintronics ("Electrically Induced Ferromagnetism at Room Temperature in Cobalt-Doped Titanium Dioxide").
The ferromagnetism in magnetic semiconductors typically originates from an interplay between magnetic 'dopants' and the electrons that jump from atom to atom in the semiconductor. "These itinerant electrons mediate the ferromagnetic exchange interaction between the spins of electrons on the magnetic dopant ions," explains Fukumura.
Cobalt-doped titanium dioxide is nonmagnetic without voltage (left). When subject to an externally applied voltage in an electric double-layer transistor structure (right), however, the material becomes ferromagnetic.
The exchange interaction is a quantum mechanical effect that occurs when spatial or spin coordinates are exchanged among electrons. This type of 'crosstalk' of spins among magnetic cobalt dopants is facilitated by the itinerant electrons in this system, causing the spins to become polarized and generating the type of pure 'spin current' needed for spintronics. This effect is apparent in a number of material systems at very low temperatures. The exchange interaction can also be induced in cobalt-doped titanium oxide at room temperature, but producing a sufficiently high density of itinerant electrons for the effect to be usable requires the application of high voltages that tend to cause electrical breakdown in conventional metal-oxide field-effect transistors.
Fukumura and his team overcame this difficulty by constructing an electric double-layer transistor structure (see image). "This type of transistor uses a liquid electrolyte as a gate insulator, and because a nanometer-thin electric double layer is formed just above the semiconductor, a small applied voltage is sufficient to generate a very high electric field," says Fukumura.
Electrical measurements revealed that the cobalt-doped titanium oxide displays a distinct ferromagnetic response at modest gate voltages due to an increase in itinerant electron density. Importantly, the ferromagnetism is induced and controlled at ambient, rather than cryogenic, temperatures, although the reason for its appearance at such high temperatures remains a mystery. Nevertheless, the achievement marks a crucial step forward in developing spintronics as a practical technology.