Spin-Transfer Torque on a Single Magnetic Adatom

We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunneling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunneling. By changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunneling conductance.

Figure 1

(a) Scheme of the proposed setup: a magnetic STM tip and a magnetic adatom on an insulating monolayer deposited on a metal. (b) Two of the spin assisted tunneling events of Eq. (2). Spin transitions when electrons flow from tip to surface (c) and from surface to tip (d). Dashed transitions (III) and (IV) are less efficient when the tip is polarized (see text).

Figure 2

(a) Average magnetization $⟨S_z⟩$ versus applied bias for $T=1\mathrm{K}$ (dashed line) and $T=0.1\mathrm{K}$ (solid line). (b) Average magnetization after saturation ($eV=5\mathrm{meV}$) versus tip polarization for the same two temperatures. (c) $dI/dV$ for single Mn in ${\mathrm{Cu}}_2\mathrm{N}$ surface probed with a nonpolarized tip (solid line) and two different tip polarizations, ${\mathcal{P}}_T=-1/3$ (thin dashed line) and ${\mathcal{P}}_T=-1$ (thick dashed line) for a fixed temperature $T=0.1\mathrm{K}$. (d) Variation of the populations $P_M$ with applied bias for nonpolarized tip and $T=0.1\mathrm{K}$.

Figure 3

Switching time ${\tau }_{\mathrm{rel}}$ for ${\mathcal{P}}_T=-1$, $v_S=1$, and $v_T=0.5$. (a) ${\tau }_{\mathrm{rel}}$ as a function of applied bias $V$ for $T=0.1\mathrm{K}$ and (b) as a function of temperature for $eV=2|D|$.