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Toward Single Atom Magnets

April 6, 2015 - 11:00am
Ileana Rau
IBM Research, San Jose, CA

Magnetic anisotropy is a fundamental property of magnetic materials that governs the stability
and directionality of their magnetization. The ability to control the magnetic anisotropy of
nanoscale systems will open novel avenues for spintronics, magnetic memory devices, and
quantum computation. At the atomic level, magnetic anisotropy originates from the spin-orbit
coupling that connects the spin moment of a magnetic atom to the spatial symmetry of its
ligand or crystal field environment. In the case of 3d transition metal atoms, the same crystal
field that is necessary for the anisotropy usually quenches the orbital moment and reduces the
total magnetic moment of the atom to its spin component. As a result, single molecule
magnets and magnetic tunnel junctions show an anisotropy energy per atom that is typically
one to two orders of magnitude smaller than the maximal value allowed by the spin-orbit
coupling. We have overcome this limitation by carefully designing the coordination geometry
of magnetic atoms on a surface to preserve the orbital moment while inducing uniaxial
anisotropy. I will present scanning tunneling spectroscopy and x-ray absorption spectroscopy
measurements that show that single Cobalt atoms deposited on a thin MgO layer retain most
of their free-atom orbital moment L=3. Because Cobalt adsorbs on top of the Oxygen atom,
the resulting crystal field is effectively cylindrical and leads to a strikingly large magnetic
anisotropy energy, at the theoretical limit. Spin-polarized tunneling measurements reveal a
stable magnetic groundstate with a large total moment of ~5.5µB and a long-lived excited state
of opposite magnetic moment with a relaxation time of 0.2 ms. These results offer a strategy,
based on symmetry arguments and careful tailoring of the interaction with the environment
for the rational design of nanoscopic permanent magnets and single atom magnets.

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