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a b VDC Researchers magnetize single Tip copper atoms

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here is ongoing interest in creating Tmagnets as tiny as possible, to create N more efficient hard-drive storage, to per- S form MRI with injected particles rather than giant rings, and to bring quantum computing closer to reality. As a mag- net gets smaller and smaller, it is more MgO/Ag(001) likely that interactions with its environ- Current amplifier ment will ruin its internal magnetism. In a recent study in Nature Nanotechnology (a) The nucleus of an atom acting as a magnet, with the magnet of the electron pointing in the (doi:10.1038/s41565-018-0296-7), a opposite direction. (b) A scanning tunneling microscope tip interacts with a Cu atom on a surface. group of researchers from IBM and sev- Credit: Nature Nanotechnology. eral universities reported on the small magnets they made by stabilizing the magnetism of a single atomic nucleus. in the 1980s. Individual atoms of copper The experiment was conducted at a very Both an electron and a nucleus in an deposited on a magnesium oxide surface low temperature (~1 K) and strong mag- atom have intrinsic angular momentum on top of silver substrate were used. The netic field (~1 Tesla) to allow the atoms and can act as tiny magnets. The overall researchers modified a scanning tunnel- to retain their hyperfine state without energy of an atom differs slightly depend- ing microscope (STM) to emit electrons succumbing to environmental perturba- ing on whether the electron and the nucleus with a specific magnetic alignment. An tions. Even under these conditions, it is are magnetically pointing in the same or electron jumping from the STM tip ex- expected that fewer than 2% of the atoms opposite directions. This energy difference erts a torque on an orbital electron in the would naturally be found in the desired is much smaller than the energy difference copper, putting it in a specific spin state. state. Through their STM polarization between different electron orbits, and the This then dictates the opposite spin state technique, the researchers were able to different magnetic states are called “hyper- in the nucleus, to conserve overall angu- improve this by a factor of 17, resulting fine structure.” To allow a single atom to lar momentum. As the researchers write, in 30% of the atoms having the desired act as a magnet, it is necessary to control its “an electron spin flips and a nuclear spin nuclear magnetic state. hyperfine structure—to dictate the align- flops,” and this process allows the nuclear The researchers are hopeful that in ment of the electron and nucleus—without spin of atoms to be dictated one at a time. the future this technology can be used to having them de-orient due to thermal fluc- A single-atom magnet is always in a develop spintronics, in which signals are tuations from the environment. precarious state because the hyperfine en- sent by angular momentum rather than The researchers wanted to stabilize ergy differences are so small compared charge, and be used for detecting mag- the magnetism of single atoms, and did to thermal agitation (in this case, elec- netic fields on extremely small scales. so using technology developed by IBM trons scattering from the silver below). Alex Klotz

materials, however, in which the charge on the surface that is “protected” from Dirac material heterostructures carriers become massless, allowing them scattering. As such, Dirac materials are lead to next-generation spintronics to move at relativistic speeds. The Dirac a burgeoning field, due to their exotic equation is required to capture the phys- physics and the promise of revolutionary lassical materials, such as metals, ics of such quantum materials. Graphene applications, such as quantum computing Chave electrons that exhibit a parabol- is a Dirac material that comprises a sin- and spintronic-logic devices. ic dependence of energy on momentum. gle sheet of sp2-bonded carbon atoms. Although graphene offers excellent This is a consequence of the finite mass Massive charge-carrier mobilities have spin transport, it suffers from low spin– of electrons, which limits their speed to been reported for graphene. Topological orbit coupling (SOC), a phenomenon

well below the speed of light. The phys- insulators (such as Bi2Se3), are another that defines the interaction between an ics of such materials can be comprehen- exciting Dirac material, and are bulk electronic spin and its orbital motion. A sively explained using the Schrödinger insulators with electronic surface states low SOC results in poor external con- equation. There also exists a class of that allow high-mobility charge transport trol over the electronic spin, making the

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