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.
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