Abstract
Superparamagnetism of magnetic adatoms and molecules—preferential alignment of their spins along an easy axis—is a useful effect for nanoscale applications as it prevents undesired spin reversal. The underlying magnetic anisotropy barrier—a quadrupolar energy splitting—originates from spin–orbit interaction and can nowadays be probed by electronic transport measurements. Here we predict that in a much broader class of systems, quantum dots with spins larger than 1/2, superparamagnetism can arise without spin–orbit interaction: by attaching them to ferromagnets, a quadrupolar spintronic exchange field is generated locally. It is observable by means of conductance measurements and leads to enhanced spin filtering even in a state with zero average spin. Analogously to the spintronic dipolar exchange field, giving rise to a local spin torque, the effect is susceptible to electric control and increases with tunnel coupling as well as with spin polarization.
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Acknowledgements
We acknowledge stimulating discussions with J. Barnaś, A. Cornia, S. Das, J. König, S. J. van der Molen, J. Splettstoesser, I. Weymann and H. van der Zant. The use of the SPINLAB computational facility and the open access Budapest flexible DM-NRG code35 (http://www.phy.bme.hu/~dmnrg/) is kindly acknowledged. We acknowledge the financial support from the DFG (FOR 912), the Foundation for Polish Science (M.M.) and the Alexander von Humboldt Foundation (M.M.).
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M.R.W. conceived the idea. M.H. and M.M. performed the analytic and numerical calculations, respectively. M.H. provided M.M. and M.R.W. with fitting formulas for the physical analysis of DM-NRG results. M.M. prepared the initial manuscript. All authors contributed to writing the manuscript.
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Misiorny, M., Hell, M. & Wegewijs, M. Spintronic magnetic anisotropy. Nature Phys 9, 801–805 (2013). https://doi.org/10.1038/nphys2766
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DOI: https://doi.org/10.1038/nphys2766
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