The highest-density magnetic storage media will code data in single-atom bits. To date, the smallest individually addressable bistable magnetic bits on surfaces consist of 5-12 atoms 1,2 . Long magnetic relaxation times were demonstrated in molecular magnets containing one lanthanide atom [3][4][5][6][7][8][9][10][11] , and recently in ensembles of single holmium (Ho) atoms supported on magnesium oxide (MgO) 12 . Those experiments indicated the possibility for data storage at the fundamental limit, but it remained unclear how to access the individual magnetic centers. Here we demonstrate the reading and writing of individual Ho atoms on MgO, and show that they independently retain their magnetic information over many hours. We read the Ho states by tunnel magnetoresistance 13,14 and write with current pulses using a scanning tunneling microscope. The magnetic origin of the long-lived states is confirmed by single-atom electron paramagnetic resonance (EPR) 15 on a nearby Fe sensor atom, which shows that Ho has a large out-of-plane moment of (10.1 ± 0.1) µB on this surface. In order 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 remotely by EPR. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is possible. 2The demonstration of magnetic bistability in single molecule magnets containing one rare earth atom 3,5,8,10 illustrated the potential of single-atom spin centers in future storage media 4,6,7,9,11 . A ligand field that provides a barrier against magnetization reversal by lifting the Hund degeneracies in single molecule magnets 3-10 can also be realized for atoms bound to a surface [16][17][18][19] . While a break junction probes the quantum states of one isolated molecule 20 , a surface enables preparation of and access to numerous spin centers. Magnetic lifetimes in the milliseconds range were accordingly obtained for single 3d atoms on magnesium oxide (MgO) 21 but the report of magnetic bistability for Ho atoms on a platinum surface is debated [22][23][24][25] . A major advance of observing magnetic remanence was recently achieved with an ensemble of isolated Ho atoms on MgO 12 , yet the question remained whether electrical probing of the highly localized f orbitals of individual rare earth atoms is possible 23,26,27 .Here we address the magnetic switching of individual Ho atoms on MgO, which we control by current pulses and detect by changes to the tunnel magnetoresistance using a spinpolarized scanning tunneling microscope (STM) 13 . We prove the magnetic origin of the switching in the tunneling resistance by STM-enabled single-atom electron paramagnetic resonance (EPR) on an adjacent iron (Fe) sensor atom. Additionally, we determine by this method the out-of-plane component of the Ho magnetic moment, and use the long lifetime to store two bits of information in an array of two Ho atoms whose magnetic state can be read remotely ...

Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1/2 centers are particularly desirable because they readily manifest coherent quantum fluctuations. Here we introduce a controllable spin-1/2 architecture consisting of titanium atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM), and subsequently perform electron spin resonance (ESR) on individual atoms to drive transitions into and out of quantum eigenstates of the coupled-spin system. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling, to strong exchange coupling. The local magnetic field of the magnetic STM tip serves to precisely tune the superposition states of a pair of spins. The precise control of the spin-spin interactions and ability to probe the states of the coupled-spin network by addressing individual spins will enable exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.Building networks of spin-1/2 objects with adjustable interactions represents a versatile approach for quantum simulation of model Hamiltonians [1, 2] because it provides direct experimental access to quantum emergent phenomena, such as topologically generated gapped excitations [3], spin liquids [4] and anyon excitations [5]. However, the precise control of spin interactions and integration beyond a few spins, while maintaining the ability to address individual spins, remains notoriously challenging [6]. Atomically engineered spin networks on surfaces, such as coupled atomic dimers, chains [7,8], ladders [9] and arrays [10], provide a bottom-up realization of tailored spin systems, by using STM to position and address individual atoms [9,11]. Atoms with large spin S generally exhibit strong magnetocrystalline anisotropy that results in Ising-like interactions [4,12]. In contrast, quantum fluctuations scale in proportion to 1/S, so they are maximal for the smallest possible spin, S = 1/2 [4].Spins interact via exchange and dipolar interactions. At the scale of a few coupled spins, shortrange exchange coupling can give rise to magnetic ordering such as magnetic bistability [9,13] and

Phase coherence of single-atom spins on surfaces is investigated in a scanning tunneling microscopy experiment.

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Spin resonance provides the high-energy resolution needed to determine biological and material structures by sensing weak magnetic interactions. In recent years, there have been notable achievements in detecting and coherently controlling individual atomic-scale spin centres for sensitive local magnetometry. However, positioning the spin sensor and characterizing spin-spin interactions with sub-nanometre precision have remained outstanding challenges. Here, we use individual Fe atoms as an electron spin resonance (ESR) sensor in a scanning tunnelling microscope to measure the magnetic field emanating from nearby spins with atomic-scale precision. On artificially built assemblies of magnetic atoms (Fe and Co) on a magnesium oxide surface, we measure that the interaction energy between the ESR sensor and an adatom shows an inverse-cube distance dependence (r). This demonstrates that the atoms are predominantly coupled by the magnetic dipole-dipole interaction, which, according to our observations, dominates for atom separations greater than 1 nm. This dipolar sensor can determine the magnetic moments of individual adatoms with high accuracy. The achieved atomic-scale spatial resolution in remote sensing of spins may ultimately allow the structural imaging of individual magnetic molecules, nanostructures and spin-labelled biomolecules.