W. Wulfhekel
Physikalisches Institut, Universität Karlsruhe,
Wolfgang-Gaede-Str. 1, 76131Karlsruhe, Germany
When
passing an electric current through a magnetic material, magnetic excitations
can be created. In these inelastic
scattering events, the electron spin of the current is flipped and angular
momentum is transferred to the magnetic system in form of a magnon. Energy is required for such excitations,
making inelastic scanning tunneling spectroscopy (ISTS) the method of choice to
study these excitations in nanoscale systems. We have successfully applied ISTS
to investigate the nature of the inelastic spin-torque effect, to obtain magnon
dispersions in thin films with precision similar to neutron scattering and to
measure the magnetic anisotropy of single magnetic atoms and clusters.
The
results on magnetic excitations in bulk Fe and thin films of Co on Cu(100) and
Cu(111) will be presented and discussed. They show that the probability of
current-assisted magnon creation is proportional to the spin polarization of
the local density of states of the sample. They also indicate that magnon
excitation in magnetic tunnel junctions should not be neglected, as most
minority electrons scatter inelastically leading to magnon creation. The
experiments further identify the interaction responsible for the spin
scattering to be the RKKY interaction.
A
thickness-dependent study of inelastic tunneling spectra of thin films of Co on
Cu(100) allowed experimental determination of the magnon dispersion in fcc Co
with high resolution. The results show no deviation from bulk behavior down to
the film thickness of 8 monolayers. The determined magnon stiffness is 667±3 eV·Å2.
The
high lateral resolution of the ISTS enables addressing single atoms on
surfaces. Further, atoms may be laterally manipulated with the STM tip to form
well-defined atomic clusters of magnetic material. Using both techniques
combined, the magnetic anisotropies of single atoms, dimers and trimers of Fe
and Co on Pt(111) were determined. The results are compared to XMCD data and
theoretical calculations.