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Reading and writing single-atom magnets

Abstract

The single-atom bit represents the ultimate limit of the classical approach to high-density magnetic storage media. So far, the smallest individually addressable bistable magnetic bits have consisted of 3–12 atoms1,2,3. Long magnetic relaxation times have been demonstrated for single lanthanide atoms in molecular magnets4,5,6,7,8,9,10,11,12, for lanthanides diluted in bulk crystals13, and recently for ensembles of holmium (Ho) atoms supported on magnesium oxide (MgO)14. These experiments suggest a path towards data storage at the atomic limit, but the way in which individual magnetic centres are accessed remains unclear. Here we demonstrate the reading and writing of the magnetism of individual Ho atoms on MgO, and show that they independently retain their magnetic information over many hours. We read the Ho states using tunnel magnetoresistance15,16 and write the states with current pulses using a scanning tunnelling microscope. The magnetic origin of the long-lived states is confirmed by single-atom electron spin resonance17 on a nearby iron sensor atom, which also shows that Ho has a large out-of-plane moment of 10.1 ± 0.1 Bohr magnetons on this surface. To demonstrate independent reading and writing, we built an atomic-scale structure with two Ho bits, to which we write the four possible states and which we read out both magnetoresistively and remotely by electron spin resonance. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is indeed possible.

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Figure 1: Experimental set-up and magnetic switching of holmium.
Figure 2: Controlling and measuring the magnetic states of holmium.
Figure 3: Example of a stable two-bit atomic Ho array.

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Acknowledgements

We thank B. Melior for technical assistance and F. Donati for discussions. We acknowledge financial support from the Office of Naval Research. F.D.N. appreciates support from the Swiss National Science Foundation under project numbers P300P2_158468 and PZ00P2_167965, and help from A. Natterer. K.Y. acknowledges support from National Natural Science Foundation of China (grant number 61471337). W.P. thanks the Natural Sciences and Engineering Research Council of Canada for fellowship support. P.W. acknowledges financial support from the German academic exchange service. T.G. thanks B. Delley for discussions and IBM Research for its hospitality.

Author information

Authors and Affiliations

Authors

Contributions

F.D.N. conceived the experiment and wrote the manuscript. F.D.N., K.Y., W.P. and P.W. analysed the data. W.P. conceived the atom-switching routine. C.P.L. and A.J.H. enabled and supervised the project. All authors carried out measurements, discussed the results and contributed to the manuscript.

Corresponding authors

Correspondence to Fabian D. Natterer, Andreas J. Heinrich or Christopher P. Lutz.

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Competing interests

The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks N. Lorente, R. Sessoli and W. Wernsdorfer for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Switching-rate dependence of Ho atoms on MgO/Ag(100).

a, Switching rate at a constant current of 1.5 nA and a total field of 0.495 T (Bz ≈ 50 mT). The three threshold values (listed in c) are identical for positive and negative bias. The light blue circles represent the switching rate that was directly measured from current traces and the dark blue squares show a reduced-duty-cycle measurement for higher switching rates. The black line stems from a piecewise linear fit. b, The current-dependent switching rates follow a power law that has an exponent close to unity for all field and bias conditions. This scaling behaviour indicates a single-electron rate-limiting process in the reversal of the Ho moment by energetic electrons. c, Fit parameters for the switching rate in a. The uncertainties represent the standard deviation in the fit value.

Source data

Extended Data Figure 2 Temporal stability of a Ho bit at 4.3 K measured with STM-ESR.

a, ESR spectra for up (top, red) and down (bottom, blue) alignment of the Ho bit at 4.3 K. The sweeps were normalized to the ESR peak height and vertically offset for clarity. b, Monitoring the ESR signal at the two frequencies corresponding to up (blue) and down (orange) alignment of the Ho bit (arrows in a) versus time t (V = 60 mV, I = 20 pA, VRF = 25 mV). The Ho bit spontaneously switches to the up state at t = 1.55 h and remains in that state for the remainder of the sweep. A polynomial background was subtracted from each trace to correct for lateral tip drift. The gap at t = 1.1 h is an interruption that was used to realign the tip onto the Fe sensor to correct for tip drift.

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Natterer, F., Yang, K., Paul, W. et al. Reading and writing single-atom magnets. Nature 543, 226–228 (2017). https://doi.org/10.1038/nature21371

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