Tuesday, September 18, 2012

1209.3405 (Milan Agrawal et al.)

Magnon-phonon coupling unmasked: a direct measurement of magnon

Milan Agrawal, Vitaliy I. Vasyuchka, Alexy D. Karenowska, Alexander A. Serga, Gennadiy A. Melkov, Burkard Hillebrands
Thermoelectric phenomena in magnetic materials attract much contemporary interest on account of the tantalizing possibilities they present for controlling and manipulating spin-information using heat currents in future 'spin caloritronic' devices. Thermoelectric effects have been observed in many magnetic systems but much of their underlying physics remains to be unraveled. Key is to understand how thermal magnons interact with phonons. Here, we present the first measurements of the spatial distribution of magnon temperature in a magnetic system subject to a lateral phonon temperature gradient. In Brillouin light scattering experiments on a magnetic insulator, we find that, contrary to theoretical predictions, the apparent temperature profiles of magnons and phonons track each other closely. This result supports three important conclusions. Firstly, that even in low-damping magnetic materials, the magnon-phonon interaction is significantly stronger than previously thought. Secondly, that short wavelength exchange magnons, which dominantly populate the thermal magnon spectrum at room temperature, must make a negligible contribution to the spin Seebeck effect. Thirdly, that this effect-which is perhaps the most important thermoelectric magnetic phenomenon of all-can only be explained if it is assumed that different regions of the magnon spectrum have distinct characteristic temperatures. This work gives new insight into the physics of magnon relaxation and magnon-phonon interactions and suggests that the synthesis of new magnetic materials combining low magnetic damping and low magnetoelastic coupling is crucial if the full technological potential of thermoelectric effects is to be realized.
View original: http://arxiv.org/abs/1209.3405

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