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Thermal spintronics: spin-Seebeck effect in GaMnAs

Figure 1The charge Seebeck effect is a redistribution of charge due to an applied temperature gradient. It is also known as the thermoelectric effect. This physics is the basis for thermoelectric sensors and coolers. The spin Seebeck effect is a redistribution of spin due to an applied temperature gradient. It was first discoved in 2008 by Uchida et al. [Nature 455, 778 (2008)] in a ferromagnetic metal, permalloy (NiFe).

Recently, we discovered that the spin-Seebeck effect also occurs in GaMnAs, a ferromagnetic semiconductor. The spin Seebeck effect is detected as a voltage transverse to the applied thermal gradient in platinum electrodes deposited on top of the GaMnAs. To isolate the spin-Seebeck effect, we measure the change in voltage as the magnetization flips following the magnetic hysteresis as a function of magnetic field (Fig. 1).

The spin-Seebeck coefficient (Sxy) changes sign across the sample, something that no other thermal transport effects exhibit, and it persists at long length scales (~1cm) (Fig. 2).

Finally, the spin-Seebeck effect is not affected by longitudinal conduction. We cut the conducting GaMnAs to cause an electrical short across the sample and then measured the spin-Seebeck spatial dependence (Fig. 3). The spin distribution remains unaffected revealing that the spin-Seebeck effect is not caused by electron or hole spins moving along the thermal gradient.Figure 2

The ability to use heat to generate or move spins from magnetic materials (GaMnAs) into non-magnetic materials (Pt) opens an exciting new area of spin-based electronics research. This coupling of heat particles (phonons) with magnetic moments and spins provides a new tool to thermally control spins in materials.

Work in collaboration with: Prof. Heremans (OSU. Mech. E.) and Prof. Awschalom (UCSB, Physics)

For more information about this work, please see:

Figure 3C. M. Jaworski, J. Yang, S. Mack, D. D. Awschalom, J. P. Heremans and R. C. Myers. Observation of the Spin-Seebeck Effect in a Ferromagnetic Semiconductor. Nature Materials 9, 898-903 (2010).