Dimerized nature of magnetic interactions in the S = 1/2 quantum antiferromagnet Cu(en)2SO4.

• Crystal structure suggests formation of magnetic dimers between Cu(II) ions. • Good agreement of experimental data with antiferromagnetic dimer model was observed. • The intra-dimer exchange coupling was estimated J/k B = −5.52 K. • Spin gap ∼11 K is expected to vanish at the critical field 7.8 T. • Absence of phase transition to magnetic long-range order down to 0.4 K. Specific heat, magnetic susceptibility and magnetization of Cu(en) 2 SO 4 single crystal were investigated in the field applied along the b- axis. The analysis of the susceptibility within the Curie-Weiss law indicates the antiferromagnetic nature of the exchange coupling between Cu(II) ions with zJ/k B = −6.4 K. Considering the structure of this material, the formation of magnetic dimers can be expected. Very good description of susceptibility data was achieved using the Heisenberg antiferromagnetic dimer model with intra-dimer coupling J / k B = −5.52 K, effective inter-dimer coupling J ' / k B = −0.3 K and g b = 2.12. The energy gap in the spin excitation spectrum was estimated Δ/ k B ≈ 11 K and corresponding critical field B c b ≈ 7.8 T required for closing the gap. The dimerized nature of magnetic interactions was also confirmed by specific heat and isothermal magnetization data which manifest qualitatively the same behavior as the aforementioned dimer model. The possibility of further low-temperature experiments in the closed gap regime is discussed to obtain more detailed concept of magnetic interactions in this interesting compound. [ABSTRACT FROM AUTHOR]