  Optical trapping of magnetic Fe-oxide particles is notoriously difficult due
to their high refractive indices, not to mention high absorptivity at the
trapping infra-red wavelengths. We synthesize Fe co-doped NaYF4:Yb,Er
ferromagnetic upconversion particles that not only have refractive indices
conducive for optical trapping, but also heat less than Haematite particles at
off-resonant wavelengths. These particles are hexagonal shaped with dimensions
of the order of 3 micrometer and also bear high coercivity of 20 mT and
saturation magnetisation of 1 Am^2/kg. This enables simultaneous use of optical
trapping and magnetic forces to generate micro-manipulation in all the six
degrees of freedom of a rigid body. We also show that these particles heat
significantly when illuminated on absorption resonance at 975 nm while emitting
visible light with possible implications for fluorescence microscopy and
photothermal therapy of cancer cells.
