Magnetic field-induced phase transitions in Cu(en)2SO4 – A dimerized S = 1/2 quantum antiferromagnet

Abstract

Magnetic susceptibility, magnetization and specific heat of powder and single crystal of Cu(en)2SO4 were investigated. The analysis of the data confirmed that Cu(en)2SO4 can be treated as a quasi-two-dimensional array of magnetic dimers with singlet-ground state persisting up to about 6.5 T. Magnetization and susceptibility studies were performed in the singlet phase and the analysis revealed that g-factor anisotropy is the main mechanism responsible for the different behavior in the magnetic field applied along the b and c axis. The investigation of powder and single crystal specific heat in magnetic fields applied along all three crystallographic directions revealed only small anisotropy associated with the g-factor anisotropy. All mentioned data sets resemble main features of the spin 1/2 HAF dimer model with J/kB = -5.52 K. The observed deviations can be ascribed to the presence of inter-dimer interactions with the effective strength z’J’/kB = -2.7 K. In fields above 6.5 T the system passes from the singlet phase to the ordered state stable up to about 11 T. The expectation of a dome shape magnetic phase diagram is based on the strong dimerization of the magnetic lattice. Future experiments and calculations are discussed to identify the origin of the quantum phase transition associated with the gap closing.