Generally of the atomic spin, however one in principle may also deal with a coarse-grained model where one deals with resultant spins. Operators for quantum spins form a Lie algebra, however at the level of spin-wave approximation, the problem is essentially no different from that of, say, phonons (which are bosons - as opposed to hard-core bosons).

Thanks.

Behnam Farid

Rajnandini Sharma : My pleasure.