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• Magnetization, susceptibility and specific heat experimentally investigated. • Only g -factor anisotropy is responsible for anisotropy in all data. • All data resemble main features of antiferromagnetic dimer model. • Field-induced ordered state stabilized between 6.5 T and 11 T. • Assumption of a dome shape magnetic diagram due to strong dimerization. Magnetic susceptibility, magnetization and specific heat of powder and single crystal of Cu(en) 2 SO 4 were investigated. The analysis of the data confirmed that Cu(en) 2 SO 4 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 / k B = -5.52 K. The observed deviations can be ascribed to the presence of inter-dimer interactions with the effective strength z'J' / k B = -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. [ABSTRACT FROM AUTHOR]

The analysis of specific heat, magnetic susceptibility and magnetization identified the studied compound Cu[C 6 H 2 (COO) 4 ][H 3 N–(CH 2) 2 –NH 3 ]·3H 2 O as a quasi-two-dimensional S = 1/2 Heisenberg antiferromagnet on the rectangular lattice with the intrachain coupling J 1 /k B ≈ 7.39 K and the interaction ratio R ≈ 0.44. A phase transition to a magnetically ordered state was observed in zero magnetic field at T N = 1.28 K. The analysis of magnetic specific heat in non-zero magnetic fields revealed features characteristic of the field-induced Berezinskii-Kosterlitz-Thouless phase transition theoretically predicted for ideal two-dimensional magnets. The electron paramagnetic resonance studies of Cu[C 6 H 2 (COO) 4 ][H 3 N–(CH 2) 2 –NH 3 ]·3H 2 O revealed the increase of g x and g y and decrease of g z below 25 K due to the presence of dipolar coupling and the exchange anisotropy. The upturn of linewidth appearing below 30 K can be ascribed to the development of intralayer magnetic correlations. • The system represents S = 1/2 Heisenberg antiferromagnet on the rectangular lattice. • A phase transition to a magnetically ordered state occurs in B = 0 T at T N = 1.28 K. • The field-induced Berezinskii-Kosterlitz-Thouless phase transition was observed. [ABSTRACT FROM AUTHOR]

• 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]

Magnetic properties of three variants of MOF-76(Gd), {[Gd(BTC)(H2O)]·G}n (BTC = benzene-1,3,5-tricarboxylate, G = guest molecules) were investigated by static susceptibility, isothermal magnetization and specific heat capacity measurements. In the study we used as synthesized MOF-76(Gd)-DMF (1) (G = DMF = dimethylformamide), containing DMF molecules in the cavity system, compound MOF-76(Gd) (2), activated complex without solvents in the cavities and water exchanged sample MOF-76(Gd)-H2O (3). A pronounced change in the magnetic entropy was found near the critical temperature for all three compounds. It was shown, that magnetic entropy change depends on the solvatation of the MOF. The highest value entropy change, ΔSMpk(T) was observed for compound 2 (ΔSMpk(T) = 42 J kg−1 K−1 at 1.8 K for ΔH = 5 T). The ΔSMpk(T) for the compounds 1, 2 and 3 reached 81.8, 88.4 and 100% of the theoretical values, respectively. This suggests that in compound 3 Gd3+···Gd3+ antiferromagnetic interactions are decoupled gradually, and higher fields promote a larger decoupling between the individual spin centers. The observed entropy changes of compounds were comparable with other magnetic refrigerants proposed for low-temperature applications. To study the magnetothermal effect of 2 (the sample with largest −ΔSMpk), the temperature-dependent heat capacities (C) at different fields were measured. The value of magnetic entropy S obtained from heat capacities (39.5 J kg−1 K−1 at 1.8 K for an applied magnetic field change of 5 T) was in good agreement with that derived from the magnetization data (42 J kg−1 K−1 at 1.8 K). [ABSTRACT FROM AUTHOR]