Room-Temperature Nanoseconds Spin Relaxation in WTe₂ and MoTe₂ Thin Films

The Weyl semimetal WTe₂ and MoTe₂ show great potential in generating large spin currents since they possess topologically protected spin-polarized states and can carry a very large current density. In addition, the intrinsic non-centrosymmetry of WTe₂ and MoTe₂ endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe₂ and MoTe₂. Here, a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe₂ (MoTe₂) thin film using the time-resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate, and suppression of backscattering required by time-reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature.