Reading and writing single-atom magnets
Reading and writing single-atom magnets

        
        
            
            
                
                    
                        
        Letter
    
    

                        Published: 09 March 2017
                    

                    
                    Kai Yang1,3, William Paul1, Philip Willke1,4, Taeyoung Choi1, Thomas Greber1,5, Andreas J. Heinrich6,7 & …Christopher P. Lutz1 

                    

                    
                        
    Nature

                        volume 543, pages 226–228 (2017)Cite this article
                    
                    
        
            
                
                    
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            AbstractThe 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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                Download referencesAcknowledgementsWe 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 informationAuthors and AffiliationsIBM Almaden Research Center, San Jose, 95120, California, USAFabian D. Natterer, Kai Yang, William Paul, Philip Willke, Taeyoung Choi, Thomas Greber & Christopher P. LutzInstitute of Physics, École Polytechnique Fédérale de Lausanne, Lausanne, CH-1015, SwitzerlandFabian D. NattererSchool of Physical Sciences and Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing, 100049, ChinaKai YangIV. Physical Institute, University of Göttingen, Friedrich-Hund-Platz 1, Göttingen, D-37077, GermanyPhilip WillkePhysik-Institut, Universität Zürich, Winterthurerstrasse 190, Zürich, CH-8057, SwitzerlandThomas GreberInstitute of Basic Science, Center for Quantum Nanoscience, Seoul, South KoreaAndreas J. HeinrichPhysics Department, Ewha Womans University, Seoul, South KoreaAndreas J. HeinrichAuthorsFabian D. NattererYou can also search for this author in
                        PubMed Google ScholarKai YangYou can also search for this author in
                        PubMed Google ScholarWilliam PaulYou can also search for this author in
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                        PubMed Google ScholarContributionsF.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 authorsCorrespondence to
                Fabian D. Natterer, Andreas J. Heinrich or Christopher P. Lutz.Ethics declarations
              
                Competing interests
                The authors declare no competing financial interests.
              
            Additional informationReviewer Information Nature thanks N. Lorente, R. Sessoli and W. Wernsdorfer for their contribution to the peer review of this work.Extended data figures and tablesExtended 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.Related audioPowerPoint slidesSource dataRights and permissionsAbout this articleCite this articleNatterer, F., Yang, K., Paul, W. et al. Reading and writing single-atom magnets.
                    Nature 543, 226–228 (2017). https://doi.org/10.1038/nature21371Download citationReceived: 20 August 2016Accepted: 09 January 2017Published: 09 March 2017Issue Date: 09 March 2017DOI: https://doi.org/10.1038/nature21371
            

            

        


    

    
        Editorial SummaryReading and writing of single atoms (Natterer, Physics Letter, Liesbeth Venema)The ultimate limit of miniaturized classical data storage would be to use single-atom magnetic bits. Holmium atoms are seen as promising candidates because they have long magnetic relaxation times, so they do not easily lose their information. Fabian Natterer et al. now achieve the reading and writing of the magnetism of single holmium atoms, using a scanning tunnelling microscope, and show that individual atoms keep their state for many hours. They use these atoms to make a two-bit memory, to which they write the four possible states. They then use nearby magnetic iron atoms as sensors to confirm the magnetic states. This work suggests that single-atom magnetic memory should be possible.
    

    
    
        
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