Universal behavior of magnon-mediated spin transport in disordered NM/NM/FI heterostructure

Abstract

We numerically investigate the magnon-mediated spin transport and scaling behaviors of a nonmagnetic metal/nonmagnetic metal/ferromagnetic insulator (NM/NM/FI) heterostructure in the presence of Anderson disorders. For the two-dimensional (2D) NM/NM/FI system, an enhancement of spin conductance in the weak disorder regime is found due to the increasing of the interfacial density of states (DOS) at NM/FI interface. As a result, a new scaling regime is uncovered in the metallic regime where the spin conductance fluctuation rms(G) scales linearly with the average spin conductance <G>, independent of system parameters such as Fermi energies and temperatures. The competition between the disorder-enhanced interfacial DOS and disorder-suppressed spin transport results in a non-monotonic dependence of average spin conductance on disorder strength. In the localized regime, the variance of lnG for different system parameters follows a universal function that depends linearly on the average of logarithm of spin conductance <lnG>.The distribution of lnG in the localized regime is non-Gaussian whose deviation from Gaussian can be characterized by the third and fourth order cumulants of lnG, denoted as k3 and k4, respectively. We find that k3~<-lnG>^3/2 and k4~<lnG>^2. This suggests that the spin conductance mediated by magnon in NM/NM/FI hybrid systems belongs to a different universality class. For fixed <lnG>, the lnG distribution in the localized regime for different system parameters is found to collapse onto a single curve, suggesting the distribution P(lnG;<lnG>) is a universal function that depends only on <lnG>. Furthermore, spin thermopower S in the presence of disorder is studied and the variance of lnS is found to scale linearly with <lnS> for different system parameters in the localized regime. Spin conductance and spin thermopower for 1D NM/NM/FI are also studied.