Spin dynamics in a tunnel junction between ferromagnets

The dynamics of a single spin embedded in the tunnel junction (quantum point contact) between ferromagnets is addressed. Using the Keldysh technique, we derive a quantum Langevin equation. As a consequence of the spin-polarization in the leads, the spin displays a rich and unusual dynamics. Parallel configured and equally strong magnetic moments in the leads yield an ordinary spin precession with a Larmor frequency given by the effective magnetic field. Unequal and/or non-parallel configured magnetization, however, causes nutation of the spin in addition to the precession. Our predictions may be directly tested for macroscopic spin clusters. The interest in a number of techniques that allow one to detect and manipulate a single spin in the solid state remains tremendous both experimentally [1]–[4] and theoretically [5]–[11]. Being a crucial element in spintronics and spin-based quantum information processing, such studies are also of fundamental importance. So far, most of the efforts [6], [8]–[11] have been focused on understanding the mechanism for the tunneling current modulation, which is the hallmark of a single spin detection using scanning tunneling microscopy (STM) [1]–[3]. Recently, the coupling between a single spin and supercurrent in Josephson junctions has also been studied [12, 13]. In particular, a single spin nutation induced by an ac supercurrent in a dc biased Josephson junction was shown for the first time in [12]. From the viewpoint of single spin manipulation, such a kind of nutation provides a significant implication for the control of spin dynamics electrically. The question is whether the spin dynamics (or even switching) can be realized 3 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 013017 1367-2630/08/013017+09$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft