Dr. Ileana Rau -
IBM Almaden Research Center
Toward Single
Atom Magnets
|
:
|
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 ~5.5mB 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.
|
No comments:
Post a Comment