SINGLE-ION-ANISOTROPY, CRYSTAL-FIELD EFFECTS, SPIN REORIENTATION TRANSITIONS, AND SPIN-WAVES IN R2CUO4 (R=ND, PR, AND SM)

We report a detailed study of single-ion anisotropy and crystal-field effects in rare-earth cuprates R2CuO4 (R=Nd Pr, and Sm). It is found t hat most of the magnetic properties are mainly due to the coupling bet ween the copper and rare-earth magnetic subsystem which exhibits a lar ge single-ion anisotropy. This anisotropy prefers ordering of rare-ear th moments along [100] for R = Pr and Nd and along [001] for X = Sm. C ombined with a pseudodipolar interaction arising from the anisotropy o f the R-Cu exchange, we can explain the magnetic structures of these m aterials. The spin reorientation transitions in Nd2CuO4 can be explain ed in terms of a competition between various interplanar interactions which arises because of the rapid temperature dependence of the Nd mom ent below about 100 K. Finally we introduce a simple two-dimensional m odel for the Nd spin-wave spectrum. For zero wave vector, this model g ives two optical modes involving Cu spins whose temperature-dependent energies agree with experimental results and an acoustic mode whose en ergy is predicted to be of order root 2k(4) Delta approximate to 5 mu eV, where k(4) is the fourfold in-plane anisotropy constant and Delta is the Nd doublet splitting.