Terahertz spin conductance probes of coherent and incoherent spin tunneling through MgO tunnel junctions

We study femtosecond spin currents through MgO tunneling barriers in CoFeB(2 nm)|MgO(d)|Pt(2 nm) stacks by terahertz emission spectroscopy. To obtain transport information independent of extrinsic experimental factors, we determine the complex-valued spin conductance G_d of the MgO layer (thickness d= 0-6 {\AA}) over a wide frequency range (0.5-8 THz). In the time domain, G_d has an instantaneous and delayed component that point to (i) spin transport through Pt pinholes in MgO, (ii) coherent spin tunneling, and (iii) incoherent resonant spin tunneling mediated by defect states in MgO. Remarkably, a unique signature of (iii) is its relaxation time that grows monotonically with d to as much as 270 fs at d= 6 {\AA}, in full agreement with an analytical model. We expect that terahertz spin conductance spectroscopy will yield new insights into ultrafast spin transport in a wide range of materials.