NEWS & ANALYSIS MATERIALS NEWS

a b V DC Researchers magnetize single Tip copper atoms V RF

here is ongoing interest in creating Tmagnets as tiny as possible, to create N PRUHH൶FLHQWKDUGGULYHVWRUDJHWRSHU- 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 ment will ruin its internal magnetism. In amplifier 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 QHWLF¿HOG a7HVOD WRDOORZWKHDWRPV and can act as tiny magnets. The overall UHVHDUFKHUVPRGL¿HGDVFDQQLQJWXQQHO- WRUHWDLQWKHLUK\SHU¿QHVWDWHZLWKRXW HQHUJ\RIDQDWRPGL൵HUVVOLJKWO\GHSHQG- ing microscope (STM) to emit electrons succumbing to environmental perturba- ing on whether the electron and the nucleus ZLWKDVSHFL¿FPDJQHWLFDOLJQPHQW$Q 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 RSSRVLWHGLUHFWLRQV7KLVHQHUJ\GL൵HUHQFH erts a torque on an orbital electron in the would naturally be found in the desired LVPXFKVPDOOHUWKDQWKHHQHUJ\GL൵HUHQFH FRSSHUSXWWLQJLWLQDVSHFL¿FVSLQVWDWH state. Through their STM polarization EHWZHHQGL൵HUHQWHOHFWURQRUELWVDQGWKH This then dictates the opposite spin state technique, the researchers were able to GL൵HUHQWPDJQHWLFVWDWHVDUHFDOOHG³K\SHU- in the nucleus, to conserve overall angu- improve this by a factor of 17, resulting ¿QHVWUXFWXUH´7RDOORZDVLQJOHDWRPWR lar momentum. As the researchers write, in 30% of the atoms having the desired act as a magnet, it is necessary to control its ³DQHOHFWURQVSLQÀLSVDQGDQXFOHDUVSLQ nuclear magnetic state. K\SHU¿QHVWUXFWXUH²WRGLFWDWHWKHDOLJQ- ÀRSV´DQGWKLVSURFHVVDOORZVWKHQXFOHDU The researchers are hopeful that in PHQWRIWKHHOHFWURQDQGQXFOHXV²ZLWKRXW spin of atoms to be dictated one at a time. the future this technology can be used to KDYLQJWKHPGHRULHQWGXHWRWKHUPDOÀXF- A single-atom magnet is always in a develop spintronics, in which signals are tuations from the environment. SUHFDULRXVVWDWHEHFDXVHWKHK\SHU¿QHHQ- sent by angular momentum rather than The researchers wanted to stabilize HUJ\GL൵HUHQFHVDUHVRVPDOOFRPSDUHG charge, and be used for detecting mag- the magnetism of single atoms, and did to thermal agitation (in this case, elec- QHWLF¿HOGVRQH[WUHPHO\VPDOOVFDOHV 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 DEXUJHRQLQJ¿HOGGXHWRWKHLUH[RWLF 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.  $OWKRXJKJUDSKHQHR൵HUVH[FHOOHQW 7KLVLVDFRQVHTXHQFHRIWKH¿QLWHPDVV Massive charge-carrier mobilities have VSLQWUDQVSRUWLWVX൵HUVIURPORZVSLQ± 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 WKDWGH¿QHVWKHLQWHUDFWLRQEHWZHHQDQ 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

86 MRS BULLETIN • VOLUME 44 • FEBRUARY 2019 • www.mrs.org/bulletin NEWS & ANALYSIS MATERIALS NEWS

material incompatible with spintronic ap- photolithographically patterned graphene. induced strong SOC in the graphene layer, plications.Topological insulators (TIs), on The layered assembly, sitting on a highly DQGLVWKH¿UVWHYLGHQFHRIWKLVSKHQRP- the other hand, are endowed with a very doped silicon substrate, was topped with HQRQLQJUDSKHQH±7,VWUXFWXUHV strong SOC. A collaboration has now electrodes to complete the stack and enable Next, the spin signal was tuned by brought these two materials together. In spin transport measurements. applying a gate voltage across the het- an article published recently in Science The research group studied the electron- erostructure. These experiments, together Advances (doi:10.1126/sciadv.aat9349), ic spin lifetime and the magnitude of the with theoretical understanding developed a research team led by Saroj P. Dash of spin signal. It was found that the spin life- by the researchers, suggest that due to the Chalmers University of Technology and WLPHVRIWKHKHWHURVWUXFWXUHVZHUHVLJQL¿- proximity of the TI, the charge carriers in

Stephan Roche of Campus Universitat cantly reduced (22 ps for graphene:Bi2Se3 graphene gain mass, which leads to the Autònoma de Barcelona has reported a and 7 ps for graphene:Bi1.5Sb0.5Te1.7Se1.3) RSHQLQJRIDVPDOOEDQGJDS FD± heterostructure of graphene and bismuth- FRPSDUHGWRSULVWLQHJUDSKHQH ± meV) and the development of strong based TIs. To realize the van der Waals het- ps). This surprising reduction in spin life- SOC. This happens owing to hybrid- erostructure, the team transferred mechani- time was suggested to arise due to the ization between the electronic bands of FDOO\H[IROLDWHGÀDNHVRIWKH7,RQWRSRI proximity of the TI to the graphene, which graphene and TI. “Generation of a large SOC in graph- ene has been a long-standing goal. We PRGL¿HGWKHSURSHUWLHVRIJUDSKHQHYLD DSUR[LPLW\H൵HFW2XU'LUDFPDWHULDO heterostructure still supports spin trans- port while acquiring a strong SOC,” Dash TiO2/Co VD\V³7KHVH¿QGLQJVVKRXOGSDYHWKH way toward creation of novel topologi- TI FDOTXDQWXPH൵HFWVDQGKHOSREVHUYHQHZ Gr spintronic phenomena.” According to the researchers, it now becomes possible to concomitantly ma- nipulate the electronic spin and maintain VSLQWUDQVSRUWLQJUDSKHQH±7,KHWHURVWUXF- 2 μm tures, opening the door to deployment of these structures in next-generation spin-

Scanning electron microscope image of the Dirac material heterostructure. TI is topological tronic devices. insulator. Credit: Science Advances, AAAS. Ahmad R. Kirmani

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