Spin torque driven electron paramagnetic resonance of a molecular spin

Current-induced spin torques are routinely used to control the magnetization of magnetic memory devices and drive magnetic resonance in nanooscillators [1-3]. Such current-based spin manipulation schemes are now emerging as a promising new direction also for spin qubit manipulation and entanglement. Molecules offer an ideal platform to obtain a full understanding of spin manipulation through spin-polarized electronic transport at the quantum level. Recent advances combine scanning tunneling microscopy (STM) and electron paramagnetic resonance (EPR) to access dynamic spin responses with neV energy and pm spatial resolution [4,5]. Thereby, EPR-STM is an excellent tool for the controllable injection of spin currents from a magnetic tip into the spin-hosting electronic states of individual atomic and molecular systems.

Here we demonstrate EPR driving of a single spin in a pentacene molecule using a time-dependent spin current injected from a magnetic STM tip, i.e., by a spin torque [6]. We use the STM bias to control the interaction of the spin-polarized tunnelling current with the molecular spin states, providing first experimental insights into the action of spin torques on the quantum state of spins. Additionally, spin torques enable EPR studies even when the thermal polarization of the studied spin is too low to yield an observable EPR signal. Our work introduces a new paradigm for spin qubit manipulation harnessing spin-polarized currents.

[1] Ralph, Stiles, Journal of Magnetism and Magnetic Materials 320, 1190–1216 (2008).

[2] Manchon et al., Reviews of Modern Physics 91, 035004 (2019).

[3] Chen et al., Proceedings of the IEEE 104, 1919–1945 (2016).

[4] Baumann et al., Science 350, 417–420 (2015).

[5] Seifert et al., Physical Review Research 2, 013032 (2020).

[6] Kovarik, Schlitz et al., Science 384, 1368–1373 (2024).