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An experimental study of the magnetocaloric effect in tetraaquabis(hydrogen maleato)nickel(II), [Ni(C4H3O4)2(H2O)4] powder sample is presented. The magnetocaloric properties of the studied sample were investigated using specific heat and magnetization measurements in magnetic fields up to 9 T in the temperature range from 0.4 to 50 K. A large conventional magnetocaloric effect was found at a temperature of about 3.5 K, where −ΔSM = 8.5 Jkg−1K−1 and 11.2 Jkg−1K−1 for a magnetic field of 5 T and 7 T, respectively. Assuming a substantial role of the crystal field, the temperature dependence of the magnetic specific heat in a zero magnetic field was compared with an S = 1 model with single-ion anisotropy parameters D and E (axial and rhombic). The best agreement was found for the parameters D/kB = −7.82 K and E/kB = −2.15 K. On the other hand, the experimental temperature dependence of −ΔSM shows higher values compared to the theoretical prediction for the mentioned model, indicating the presence of additional factors in the system, such as an exchange interaction between magnetic ions. The first exchange pathway can be realized through maleic rings between the nearest Ni(II) ions. The second exchange pathway can be realized through water molecules approximately along the a crystallographic axis. Broken-symmetry DFT calculations performed using the computational package ORCA provided the values of ferromagnetic exchange interactions, J1/kB = 1.50 K and J2/kB = 1.44 K (using B3LYP functional). The presence of such ferromagnetic correlations in the studied system may explain the enhanced magnetocaloric effect compared with the model of an anisotropic spin-1 paramagnet. [ABSTRACT FROM AUTHOR]

• Metal-organic framework ZIF-8 (1) and Cu-ZIF (2) were prepared as nanocrystallites. • Vibration spectra and lattice specific heat of (1) and (2) are the same. • Boson peak identified in the specific heat of (1). • Short-range correlations between surface spins manifest in specific heat below 6 K. • Short-range correlations between bulk spins manifest in susceptibility above 100 K. Applying the procedure of a rapid synthesis in the aqueous system, nanocrystalline form of ZIF-8 and Cu-ZIF was obtained. The composition of the latter was inferred from the comparison of vibration spectra and lattice specific heats of both compounds which become nearly identical. The nanocrystalline character of both compounds prevented direct comparison of powder XRD patterns and manifested by the appearance of a boson peak in the diamagnetic ZIF-8 specific heat. The low-temperature magnetic contribution in Cu-ZIF specific heat and a Curie-Weiss tail in low-temperature susceptibility may originate from the effect of surface spins while the round maximum observed in the susceptibility at 135 K indicates the presence of strong low-dimensional correlations with the exchange coupling ∼200 K. Corresponding magnetic contribution cannot be observed in the specific heat due to the huge overlap by the high-temperature lattice contribution. Negligible magnetization values at low temperatures correspond to the aforementioned strong antiferromagnetic coupling. Future experimental scenarios are discussed which should provide better understanding of the bulk and surface magnetism in this unique quantum system. [ABSTRACT FROM AUTHOR]

• 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]

[Ni(fum)(phen)] (fum = fumarato, phen = 1,10 -phenanthroline) has been identified as a system consisting of antiferromagnetic S = 1 dimers with single-ion magnetic anisotropy D comparable to the intradimer exchange coupling J , D / J ≈ 1 and D / k B = −6 K. We present reclassification of the physical model of [Ni(fum)(phen)] magnetic subsystem and the magnetocaloric investigation of a [Ni(fum)(phen)] powder sample at temperatures above 2 K in magnetic fields up to 7 T. The analysis of magnetic specific heat and magnetocaloric effect in [Ni(fum)(phen)] has shown that the temperature dependence of the isothermal entropy change under different magnetic fields is in good agreement with theoretical predictions for the S = 1 ferromagnetic dimer with D / k B = −11.5 K, E / k B = −3.4 K and intradimer exchange coupling J / k B = 2.5 K. Sizeable conventional magnetocaloric effect was found around 5.8 K leading to - ΔS max = 8.2 Jkg−1K−1 for ΔB = 7 T. • Magnetocaloric effect in [Ni(fum)(phen)] – ferromagnetic dimer. • The maximal entropy change –Δ SM ≈ 8.2 J/(Kkg) is achieved in 7 T. • The intradimer ferromagnetic coupling improves the magnetocaloric properties. [ABSTRACT FROM AUTHOR]

• The glasses of As x S (100-x) with x = 20, 28.6, 45, 40, 50 were prepared. • Specific heat C p / T 3 shows a nonlinear dependence of boson peak (BP) on composition. • Longitudinal elastic moduli, Raman spectroscopy and C p / T 3 depend on composition. • Correlation between type of nanocluster and C p / T 3 was found in As x S (100-x) glasses. • Ring-like and branchy-like nanoclusters are a possible origin of BP. The combination of mean coordination and concentration dependencies of elastic modules and low-frequency Raman scattering of As-S glasses brings new information about the spectroscopic boson peak (SBP) behavior. The analysis of the characteristic temperature positions of the thermometric boson peak (TBP) and the magnitude of heat capacity of As-S glasses was performed. This study was focused on the As-S compounds from the first glass-forming region with particular attention to the compositions of flexible, intermediate, and stressed-rigid phases. The origin of SBP and TBP is expected to have a structural nature. The spectral features found in the super low-frequency region of Raman spectra were assigned to quasi-localized "soft" modes. The sulfur-rich glass, g -As 2 S 5 , represents the intermediate phase with As n S m clusters containing closed ends and several "soft" nanoclusters. These soft clusters can be responsible for intrinsic quasi-localized "soft" modes, the overall rigidity of the system, and quasi-elastic light scattering. [ABSTRACT FROM AUTHOR]

• Neutron powder diffraction experiments in temperature range 20 mK–300 K. • Completing the magnetic phase diagram of NdMn 1−x Fe x O 3 substitutional system. • X-rays and neutrons used as complementary probe. The low temperature crystal and magnetic structures of NdMn 0.1 Fe 0.9 O 3 were determined on the basis of X-ray and neutron powder diffraction experiments as well as from specific heat and magnetization measurements. The compound crystallizes in the orthorhombic crystal structure, space group Pnma, in the entire temperature interval 20 mK < T < 300 K. The lattice parameters exhibit standard thermal expansion effects for T > 20 K and at lower temperatures the anomalies due to magnetostriction effect were observed. The iron sublattice orders magnetically into 3 different magnetic structures, namely into Γ 5 = (A x , F y , G z) for 130 K < T < T N (Néel temperature); Γ 1 = (G x , C y , A z) for 15 K < T < 130 K and Γ 3 = (C x , G y , F z) for 20 mK < T < 15 K. The Nd ions order at temperatures below 1.6–1.75 K into the same Γ 3 phase as the iron sublattice at these temperatures. The obtained magnetic structures fit perfectly in between the magnetic phases of closely related NdFeO 3 and NdMn 0.5 Fe 0.5 O 3 compounds. Our study, together with all previously published data, completes the entire magnetic phase diagram of NdMn 1− x Fe x O 3 (0 ≤ x ≤ 1) solid solution substitutional system. [ABSTRACT FROM AUTHOR]

Organo-metallic compounds Cu( en )(H 2 O) 2 SO 4 ( en =C 2 H 8 N 2 ) and Cu( tn )Cl 2 ( tn =C 3 H 10 N 2 ) representing S =1/2 quasi-two-dimensional Heisenberg antiferromagnets with an effective intra-layer exchange coupling J/ k B ≈3 K, have been examined by specific heat measurements at temperatures down to nominally 50 mK and magnetic fields up to 14 T. A comparative analysis of magnetic specific heat in zero magnetic field revealed nearly identical contribution of short-range magnetic correlations and significant differences were observed at lowest temperatures. A phase transition to long-range order was observed in Cu( en )(H 2 O) 2 SO 4 at T C =0.9 K while hidden in Cu( tn )Cl 2 . A response of both compounds to the application of magnetic field has rather universal features characteristic for a field-induced Berezinskii–Kosterlitz–Thouless transition theoretically predicted for ideal two-dimensional magnets. [ABSTRACT FROM AUTHOR]