  We report the magneto optic Kerr effect (MOKE) angle of Au magnetically
excited by spin accumulation. We perform time resolved polar MOKE measurements
on Au/Co heterostructures. In our experiment, the ultrafast optical excitation
of the Co drives spin accumulation into an adjacent Au layer. The spin
accumulation, together with spin-orbit coupling, leads to non-zero terms in the
off-diagonal conductivity tensor of Au, which we measure by recording the
polarization and ellipticity of light reflected from the Au surface for photon
energies between 1.3 and 3.1 eV. At energies near the interband transition
threshold of Au, the Kerr rotation per A/m exceeds 1 urad. Typical values for
Kerr rotation per moment in transition ferromagnetic metals like Ni are < 10
nrad per A/m, while predicted values for heavy metals like Pt or W are < 13
nrad per A/m. The exceptional sensitivity of the optical properties of Au to
spin magnetic moments make Au to be an exceptionally sensitive optical
magnetometer, with potential applications in the development of optospintronic
technologies.
