Valley-Mediated and Electrically Switched Bipolar-Unipolar Transition of the Spin-Diode Effect in Heavy Group-IV Monolayers

Spin and valley are two coupled degrees of freedom relevant for low-energy electrons in heavy group-IV monolayers with strong spin-orbit couplings, such as silicene, germanene, and stanene. Using these materials, we propose that a spin-diode effect driven by longitudinal bias can be realized in a ferromagnetic/antiferromagnetic junction, where an interlayer electric field with strength E-z is applied in the antiferromagnetic region. This effect shows a unique bipolar-unipolar transition strongly depending on E-z. When E-z = 0, only electrons from one valley contribute to electric current, with one spin only moving forward and the opposite spin only moving backward, thus corresponding to a bipolar spin diode. When E-z is increased, the active valley can be switched to the other valley, and only electrons of one spin orientation are involved in transport and only move forward, corresponding to a unipolar spin diode. This transition is attributed to the characteristic band-to-band tunneling mechanism of spin-valley matching. We also find that, by reversing the direction of the interlayer electric field, the bipolar or unipolar regime of the spin diode does not change as a result of the symmetry in transport. By electrically adjusting the longitudinal bias and E-z, we further prove that the signals produced by the spin diode are strong enough to be observed even when the magnetic exchange fields are on the order of 1 meV, which is fully available in experimentally reported samples with Eu (Gd)-intercalated materials or those in proximity to ferromagnetic insulators. Our findings on the valley-mediated controllability and switching between the bipolar and unipolar regimes of the spin-diode operation suggest promising applications of heavy group-IV monolayers in improvement of reprogrammable spin logic and nonvolatile memory.