In experiments and applications usually the spin magnetic moment of magnons is considered. In this Paper we identify an additional degree of freedom of magnons: an \emph{ orbital} magnetic moment brought about by spin-orbit coupling.Our microscopic theory uncovers that spin magnetization $\vec{ M}^\mathrm{ S}$ and orbital magnetization $\vec{ M}^\mathrm{ O}$ are independent quantities. They are not necessarily collinear; thus, even when the total spin moment is compensated due to antiferromagnetism ($\vec{ M}^\mathrm{ S} = \vec{ 0}$), $\vec{ M}^\mathrm{ O}$ may be nonzero. This scenario of orbital weak ferromagnetism is realized in paradigmatic kagome antiferromagnets with Dzyaloshinskii-Moriya interaction. We demonstrate that magnets exhibiting a magnonic orbital moment are omnipresent and propose transport experiments for probing it.
Categories: cond-mat.str-el cond-mat.stat-mech