  We theoretically and numerically investigate the linear and nonlinear
magneto-optical contributions to the refractive index of metal nanospheres. The
analysis is in good agreement with the experimental extinction spectra of gold
nanocolloid solutions, with threshold magnetic fields less than 1 mT when
illuminated with light intensities less than 1 W/cm^2. Plasmonic current loops
and vortex power flows provide a new framework for the dynamical interaction
between material magnetization, light polarization and nano-surfaces. The
photo-induced magneto-optical interaction of non-magnetic metal nanocolloids
imparts significant, non-negligible torque forces in nanofluids.
