Ultrafast spin-dependent and spin Seebeck effect: beyond diffusive spin transport, toward a spin-caloritronic terahertz emitter

• Applicant: Professor Dr. Tobias Kampfrath
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257737198

The longitudinal spin-type Seebeck effect, the driving of a spin current antiparallel to a temperature gradient, is a key phenomenon in spin caloritronics. In ferromagnetic metals, it is termed the spin-dependent Seebeck effect as it arises from the different conductivity of majority- and minority-spin electrons. In ferromagnetic insulators, it is called the spin Seebeck effect and is due to magnons. So far, both effects have mostly been investigated under quasi-static conditions or in the diffusive regime. Such conditions blur elementary processes and provide little insight into applications where very fast spin-current variations are required.In this proposal, we will study both the spin-dependent and the spin Seebeck effect on ultrafast time scales. For this purpose, strong heat gradients will be induced by illuminating a ferromagnet (such as metallic Fe, Co, Ni or insulating Y3Fe5O12) with femtosecond laser pulses. Our optical approach permits simple sample geometries, that is, plane thin films or multilayers, without the need for electrical contacts or micro-structuring. Following generation of the heat-density gradient, the resulting spin, charge, and heat transport will be monitored by (i) a femtosecond probe pulse taking advantage of magneto-optic and thermo-optic effects and by (ii) detecting the terahertz electromagnetic pulse emitted by the time-varying spin and charge currents. As our time resolution is comparable to the velocity relaxation time of the electrons (~10fs in d-type metals) and magnons (~10ps to ~100ns in insulators) carrying the spin angular momentum, we will gain direct insight into elementary transport steps. For example, in insulators, how strongly are phonon heat and magnon spin transport correlated with each other? What happens when we drive optical phonons instead of electrons? In metals, how does the spin current evolve when we gradually increase the pump photon energy from ~50meV to ~1.5eV, thereby tuning the initial local electron distribution from almost Fermi-Dirac-type to highly non-equilibrium-like? Can we control the ultrafast spin transport with spin valves? Finally, what is the optimum sample structure to generate spin currents with large transient peak magnitudes? By using such intense spin-current bursts in conjunction with the inverse spin Hall effect, we will build an efficient spin-caloritronic source of terahertz electromagnetic radiation that in particular covers the so far still elusive frequency gap from 5 to 10THz.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1538: Spin Caloric Transport (SpinCaT)