Correlation-Mediated Processes for Electron-Induced Switching between Néel States of Fe Antiferromagnetic Chains

The controlled switching between two quasistable Néel states in adsorbed antiferromagnetic Fe chains has recently been achieved by Loth et al. [Science 335, 196 (2012)] using tunneling electrons from an STM tip. In order to rationalize their data, we evaluate the rate of tunneling electron-induced switching between the Néel states. Good agreement is found with the experiment, permitting us to identify three switching mechanisms: (i) low STM voltage direct electron-induced transitions, (ii) intermediate STM voltage switching via spin-wave-like excitation, and (iii) high STM voltage transitions mediated by domain-wall formation. Spin correlations in the antiferromagnetic chains are the switching driving force, leading to a marked chain-size dependence.

Figure 1

(a) Setup used in the experiment of Ref. 12. A few atoms of an Fe monoatomic chain on $\mathrm{CuN}/\mathrm{Cu}(100)$ are shown under an STM tip that injects spin-polarized electrons. The chain spins alternate along the chain axis to form a Néel state. (b) Efficient excitation and deexcitation schemes leading to the switching between Néel states. $D$ refers to the direct process between the quasidegenerate Néel states, and $I$ refers to the indirect one that involves excitation and deexcitation mechanisms. The up arrow on the left shows the excitation by a tunneling electron, whereas the down arrows on the right show the relaxation induced by substrate electrons. All possible deexcitation cascades are included in the numerical study, we schematically show a few deexcitation paths.

Figure 2

Tunneling electron-induced switching rate between Néel states in ${\mathrm{s}}^{-1}$ as a function of the STM voltage in mV. Lines: The present calculations using Eq. (5) for Fe chains of different lengths (see the inset for details); the STM current is set equal to 20 pA at 2 mV [12]. The higher rates correspond to the Fe dimer, and the rate decreases as the chain length increases. The experimental results [12] are plotted for transitions from low to high currents (dots) and from high to low currents (diamonds).

Figure 3

Excitation probability from the Néel state 1 to level $i$, $P(1\to i)$ (black circles); branching ratio toward state 2 once level $i$ is populated, $N_i(2)$ (red crosses); and the total indirect switching probability between Néel states 1 and 2, $P(1\to i)N_i(2)$ (green diamonds). Many states $i$ are excited by an electron, with a probability roughly exponentially decreasing with the excitation energy, but the switching probability is strongly modulated by the deexcitation branching ratio.