Terahertz spin current pulses controlled by magnetic heterostructures

In spin-based electronics, information is encoded by the spin state of electron bunches 1 , 2 , 3 , 4 . Processing this information requires the controlled transport of spin angular momentum through a solid 5 , 6 , preferably at frequencies reaching the so far unexplored terahertz regime 7 , 8 , 9 . Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins 10 , 11 , 12 from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter 13 based on the inverse spin Hall effect 14 , 15 , which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states 16 . Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters 7 , 8 , 9 . Through properly designed heterostructures it is possible to control the shape and duration of spin current pulses generated by laser excitation.