Reading and writing single-atom magnets

• Published in: Nature (London) 2017-03, Vol.543 (7644), p.226-228
• Main Authors: Natterer, Fabian D., Yang, Kai, Paul, William, Willke, Philip, Choi, Taeyoung, Greber, Thomas, Heinrich, Andreas J., Lutz, Christopher P.
• Format: Article
• Language: English
• Subjects: 639/301/1005/1008; 639/301/119/1001; 639/766/119/997; 639/925/927/1062; Atomic physics; Atoms & subatomic particles; Crystals; Data storage; Holmium; Humanities and Social Sciences; Lanthanides; letter; Magnesium; Magnetic fields; Magnetism; Methods; multidisciplinary; Resonance; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature21371

A two-bit magnetic memory is demonstrated, based on the magnetic states of individual holmium atoms, which are read and written in a scanning tunnelling microscope set-up and are stable over many hours. Reading 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. 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 atoms 1 , 2 , 3 . Long magnetic relaxation times have been demonstrated for single lanthanide atoms in molecular magnets 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , for lanthanides diluted in bulk crystals 13 , 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 magnetoresistance 15 , 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 resonance 17 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