Terahertz (THz) vortex beams with various combinations of the orbital angular
momenta l=±1, ±2, ±3, and ±4 and the spin angular momentum s = ±1, or
conventional circular polarization, were used for studies of the magnon spectra
at the antiferromagnetic (AFM) resonance conditions in TbFe3(BO3)4 . We
observed strong vortex beam dichroism for the magnon doublet which is split in
the external magnetic field of ±0.5T. The absorption conditions at the magnon
energy depend on the total angular momentum of the light j that is determined
by the combination of the spin and orbital angular momenta: j=s+l. For the
higher orders of l, the selection rules for AFM resonances dictated by l
completely dominate over the ones for conventional circular polarization.