Control of quantum magnets by atomic exchange bias

Abstract

Mixing of discretized states in quantum magnets has a radical impact on their properties. Managing this effect is key for spintronics in the quantum limit. Magnetic fields can modify state mixing and, for example, mitigate destabilizing effects in single-molecule magnets. The exchange bias field has been proposed as a mechanism for localized control of individual nanomagnets. Here, we demonstrate that exchange coupling with the magnetic tip of a scanning tunnelling microscope provides continuous tuning of spin state mixing in an individual nanomagnet. By directly measuring spin relaxation time with electronic pump-probe spectroscopy, we find that the exchange interaction acts analogously to a local magnetic field that can be applied to a specific atom. It can be tuned in strength by up to several tesla and cancel external magnetic fields, thereby demonstrating the feasibility of complete control over individual quantum magnets with atomically localized exchange coupling.

Extremely long-lived magnetic excitations in supported Fe chains

“…When B is increased, the two states split almost linearly and the present study, as well as the earlier experimental study by Yan et al 14 , is devoted to the decay of…”

“…The 6.15 % change is chosen to bring the computed results in agreement with the unperturbed experimental data at 2T (the three parameters, J, D and E, act in a similar way and agreement with experiment can also be found when modifying only one of the three parameters). This small change is to be compared with the accuracy of the spectroscopic fit reported by Yan et al 14 on the energies which amounts to around 5% for D and 10% for J. As a consequence, the discrepancy between theory and experiment seen in …”

“…Yan et al 14 recently reported measurements of the first excited state ( 14 found different values for the state lifetime when the STM tip was placed on the central or on an edge atom of the chain. They showed that this was due to the perturbation of the chain by the polarized tip that disappears when the tip is moved at larger distances from the chain.…”

“…Indeed, the different approaches were adapted to different time scales for the lifetime. In particular, the direct measurements, adapted to long time scales, revealed extremely long lifetimes, up to a few hours, for the Fe chains, that quickly increase in the even chain case as the chain length increases 14 . One can also mention measurements of excitation profiles 26,27,28 ; indeed a finite lifetime leads to broadening of the peaks in an excitation spectrum.…”

“…When a magnetic field B is applied, the two Néel states of an odd chain split. The recent study by Yan et al 14 reported i) that a polarized magnetic STM tip very easily alters the system and ii) that for the Fe 3 chains, the upper Néel state has a very long lifetime, increasing roughly linearly with the applied magnetic field. It is the aim of the present work to compute the lifetime of the upper Néel state in a series of odd Fe chains on Cu 2 N using the method introduced by Novaes et al 31 .…”