Abstract
The understanding the spin transport has been a critical prerequisite for development of current spintronic applications, such as giant-magnetoresistance sensors or spin-torque-based magnetic random-access memories [1]. Nowadays, novel concepts of spintronic devices operating beyond the gigahertz clock rate, based on optical spin-orbit torque switching [2] or antiferromagnetic recording media, are promising sub-picosecond base operation times [3] and require new insights into the ultrafast spin transport. Althought this topic has already attracted considerable attention in past years [4,5,6], the dynamics, propagation and theoretical models of the femtosecond spin transport are still not completely understood.
In this contribution, we report on a new robust method for investigation of out-of-plane ultrafast spin transport based on the principle of the spintronic emission of pulses of terahertz (THz) radiation [7], complemented with a new theoretical description. Using time-domain THz spectroscopy, we reveal spectral dependence of spin current relaxation and speed of propagation in copper, and compare it to the diffusive, super-diffusive and ballistic models. Moreover, our new experimental technique is universal and can be used for investigation of THz spin conductance of any metallic thin layer, which can help future development of new devices based on ultrafast spin transport.