Current Research Projects

Spin Caloritronics

The new field of spin caloritronics deals with the interplay of spin and heat transport. It is a spin analogue to the field of thermoelectricity. It is driven by the vision of generating spin currents out of heat and the interest in this field has been stimulated by the observation of the spin Seebeck effect.

Transverse Spin Seebeck Effect

The Spin Seebeck effect has first been observed in a transverse set up. Here, a spin voltage is generated in response to an applied temperature gradient in a transverse direction. Difficulties in reproducing these initial findings have lead to a controversy about the existence or non-existence of this effect.

In order to help clarify this controversy, we calculate the wavevector-dependent transport of coupled magnons and phonons in insulating ferromagnets on the basis of a Boltzmann approach. We find that quasiballistic magnons can indeed contribute to a finite transverse Spin Seebeck signal. However, this contribution is tiny at room temperature and might be observable only at low temperatures.
(F.B. Wilken and T.S. Nunner "Quasiballistic contribution to the transverse spin Seebeck effect" (in preparation)).

Longitudinal Spin Seebeck Effect and Spin Pumping

In contrast to the transverse Spin Seebeck effect the longitudinal Spin Seebeck effect is well established by now. In order to address the longitudinal Spin Seebeck effect we derive macroscopic transport equations from a microscopic Boltzmann approach. This enables us to find a magnon temperature and to calculate the resulting wave vector dependent transport of magnons along the ferromagnet and across the interface between the ferromagnet and the normal conductor (spin pumping).
(R. Schmidt and T.S. Nunner "Boltzmann approach to the longitudinal Spin Seebeck effect" (in preparation);
R. Schmidt, F.B. Wilken and T.S. Nunner "Boltzmann approach to spin pumping" (in preparation)).

Ultrafast Spin Transport

The origin of the ultrafast demagnetization in ferromagnets after femtosecond laser irradiation is still not understood. In order to contribute to an understanding of the ultrafast magnetization dynamics, we are working on an approach, which includes superdiffusive spin currents as well as ultrafast spin-flip processes. In a first step we have considered spinless electrons and we have worked out the ultrafast dynamics of electrons without spin after laser excitation.
(C.Meyer zu Rheda and T.S. Nunner "Boltzmann approach to ultrafast electron dynamics" (in preparation)).