Engineering the magnetic properties of nanoscale structures requires
understanding and controlling spin-coupling at the atomic scale. We
combine the imaging, manipulation, and spectroscopic capabilities of a
low-temperature scanning tunneling microscope (STM) to build atomic-scale
magnetic structures with tunable magnetic coupling. The nanoscale magnetic
systems are built from individual 3d transition-metal atoms such as Mn and
Fe, which are isolated from the conduction electrons of the
underlying bulk metal by a thin decoupling layer of copper nitride (CuN).
The nitrogen atoms in the CuN surface also play a crucial role in
determining the nature of the magnetic coupling inside the structures. By
varying the distance between the magnetic atoms along a row of
nitrogen-connected copper sites we are able to tune their magnetic
interaction, enabling us to construct and study systems well-described by
Ising or Heisenberg spin Hamiltonians.
We use STM-based inelastic electron tunneling spectroscopy to study the
excitation spectra of these spin-coupled systems. For Heisenberg coupling,
the spectra are interpreted as excitations of the total spin, which have
very little spatial variation along the magnetic nanostructures. In
contrast, the excitation spectra of Ising-coupled systems have strong
spatial variation, allowing us to address the spin excitations of each
individual atom in the spin-coupled structure. |