High-field magnetism of the spin-ladder material Na ₂Fe ₂(C ₂O ₄) ₃(H ₂O) ₂

We report the results of our magnetic susceptibility, magnetization, and multi-frequency electron spin resonance (ESR) measurements on single crystals of the two-leg spin-ladder material Na ₂Fe ₂(C ₂O ₄) ₃-(H ₂O) ₂. Magnetization processes in magnetic fields up 53 T at 1.3 K paralell (H || a) and perpendicular (H ⊥ a) to the leg direction show a gradual increase compared with those of the isomorphous compound Na ₂Co ₂(C ₂O ₄) ₃(H ₂O) ₂, which was well explained by an isolated antiferromagnetic dimer model. Thus, we calculated the magnetic susceptibilities and magnetizations on the basis of a two-leg ladder model with a fictitious spin (S) one by a quantum Monte Carlo method and a density-matrix renormalization group method, respectively. These calculated results, however, did not agree well with the experimental ones. Therefore, we regarded Na ₂Fe ₂(C ₂O ₄) ₃(H ₂O) ₂ as an isolated S = 1 antiferromagnetic dimer with not only the anisotropy of the exchange interactions due to the truncation of spin space but also single-ion anisotropy. Consequently, we obtained good agreement between the experimental and calculated results on the magnetic susceptibilities, magnetizations, ESR resonance modes, and specific heat.