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. 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 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.

/pdf/reading-and-writing-single-atom-magnets-5ft2pfwefb.pdf

Reading and writing single-atom magnets

Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom's magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.

http://science.sciencemag.org/content/sci/344/6187/988.full.pdf

Reaching the magnetic anisotropy limit of a 3d metal atom

A permanent magnet retains a substantial fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom allows for storing and processing information in the smallest unit of matter. We show that individual holmium (Ho) atoms adsorbed on ultrathin MgO(100) layers on Ag(100) exhibit magnetic remanence up to a temperature of 30 kelvin and a relaxation time of 1500 seconds at 10 kelvin. This extraordinary stability is achieved by the realization of a symmetry-protected magnetic ground state and by decoupling the Ho spin from the underlying metal by a tunnel barrier.

https://science.sciencemag.org/content/sci/352/6283/318.full.pdf

Magnetic remanence in single atoms

Two-dimensional layered graphene-like crystals including transition-metal dichalcogenides (TMDs) have received extensive research interest due to their diverse electronic, valleytronic, and chemical properties, with the corresponding optoelectronics and catalysis application being actively explored. However, the recent surge in two-dimensional materials science is accompanied by equally great challenges, such as defect engineering in large-scale sample synthesis. It is necessary to elucidate the effect of structural defects on the electronic properties in order to develop an application-specific strategy for defect engineering. Here, two aspects of the existing knowledge of native defects in two-dimensional crystals are reviewed. One is the point defects emerging in graphene and hexagonal boron nitride, as probed by atomically resolved electron microscopy, and their local electronic properties, as measured by single-atom electron energy-loss spectroscopy. The other will focus on the point defects in TMDs and their influence on the electronic structure, photoluminescence, and electric transport properties. This review of atomic defects in two-dimensional materials will offer a clear picture of the defect physics involved to demonstrate the local modulation of the electronic properties and possible benefits in potential applications in magnetism and catalysis.

Atomic Defects in Two-Dimensional Materials: From Single-Atom Spectroscopy to Functionalities in Opto-/Electronics, Nanomagnetism, and Catalysis

Using density functional theory (DFT), we have explored the structural, electronic, magnetic and optical properties of two-dimensional 3d-transition metal (TM)-embedded graphitic carbon nitride (g-C3N4) sheets. g-C3N4 sheets are structurally modified in different ways depending upon the radius of embedded-TM atoms and the crystal field stabilization energy gained by the corresponding geometry. Bare g-C3N4, which is a wide-gap semiconductor, becomes metallic upon TM inclusion. The d-orbitals of TMs hybridize with the pπ-orbitals of the g-C3N4 framework and close the band gap in TM-embedded g-C3N4 (TM-g-C3N4). Interestingly, for V, Cr and Fe embedded g-C3N4, the TM atoms interact ferromagnetically to each other and result in a ferromagnetic ground state. However, Mn couples antiferromagnetically and Cu and Zn are nonmagnetic in the ground state of their corresponding TM-g-C3N4 sheets. Because of structural distortion, Co- and Ni-g-C3N4 do not have a well-ordered magnetic orientation. Performing Heisenberg-model-based Monte Carlo simulations, we predict that V-, Cr- and Fe-g-C3N4 would possess Curie temperatures (Tc) of 205 K, 170.5 K, and 115 K, respectively. Furthermore, these modified g-C3N4 sheets also show prominent absorption at low energy, which evidently confirms their efficient photoabsorption capacity. The present study demonstrates the multifunctional behavior of TM-g-C3N4, which shows significant promise for application in various fields such as in memory devices or for photocatalysis.

Computational studies on magnetism and the optical properties of transition metal embedded graphitic carbon nitride sheets

Single magnetic atoms on non-magnetic surfaces have magnetic moments that are usually destabilized within a microsecond, too speedily to be useful, but here the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The magnetic moments of individual magnetic atoms are attractive components for both memory and quantum computing applications. But interactions between such atoms and the substrates on which they are mounted tend to destabilize the magnetic moments, giving them lifetimes of typically less than a few milliseconds. Toshio Miyamachi and colleagues have now identified a system consisting of single atoms of the lanthanide series rare earth element holmium on a highly conductive surface, in which intrinsic symmetries related to the properties of both the atom and the substrate combine to minimize these destabilizing interactions. As a result, the magnetic moments of the atoms can achieve lifetimes of several minutes. Single magnetic atoms, and assemblies of such atoms, on non-magnetic surfaces have recently attracted attention owing to their potential use in high-density magnetic data storage and as a platform for quantum computing1,2,3,4,5,6,7,8. A fundamental problem resulting from their quantum mechanical nature is that the localized magnetic moments of these atoms are easily destabilized by interactions with electrons, nuclear spins and lattice vibrations of the substrate3,4,5. Even when large magnetic fields are applied to stabilize the magnetic moment, the observed lifetimes remain rather short5,6 (less than a microsecond). Several routes for stabilizing the magnetic moment against fluctuations have been suggested, such as using thin insulating layers between the magnetic atom and the substrate to suppress the interactions with the substrate’s conduction electrons2,3,5, or coupling several magnetic moments together to reduce their quantum mechanical fluctuations7,8. Here we show that the magnetic moments of single holmium atoms on a highly conductive metallic substrate can reach lifetimes of the order of minutes. The necessary decoupling from the thermal bath of electrons, nuclear spins and lattice vibrations is achieved by a remarkable combination of several symmetries intrinsic to the system: time reversal symmetry, the internal symmetries of the total angular momentum and the point symmetry of the local environment of the magnetic atom.

/pdf/stabilizing-the-magnetic-moment-of-single-holmium-atoms-by-4bprzksohk.pdf

Stabilizing the magnetic moment of single holmium atoms by symmetry

Dynamic magnetic excitations in 3d and 4f atoms and clusters

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Stabilizing the magnetic moment of single holmium atoms by symmetry

Dynamic magnetic excitations in 3d and 4f atoms and clusters