A comprehensive study of thermodynamic properties of three samples of bimetallic molecular magnets [Co-II(pyrazole)(4)](2x)[Fe-II(pyrazole)(4)](2(1-x))[Nb-IV(CN)(8)]center dot 4H(2)O with x = 1 (Co2Nb), 0.5 (CoFeNb), and 0 (Fe2Nb) is reported. The three samples display the same crystallographic structure crystallizing in the tetragonal system with space group I4(1/a). Their heat capacities are measured in the temperature range 0.36-100 K without applied field as well as in the field of mu H-0 = 0.1, 0.2, 0.5, 1, 2, 5, and 9 T. The results imply the presence of the second-order phase transitions to magnetically ordered phases at 4.87(8), 7.1(2), and 8.44(3) K for x = 1, 0.5, and 0, respectively. The corresponding thermodynamic functions are analyzed to discuss the stability of the mixed compound and the magnetocaloric effect (MCE). The Gibbs energy of mixing is found to be positive but smaller in magnitude than the energy of thermal fluctuations indicating that the mixed sample is most probably metastable in the full detected temperature range. The enthalpy of mixing is negative, which points to favoring a direct neighborhood of the Co(II) and Fe(II) ions in the solid solution CoFeNb. The negative values of the entropy of mixing are explained by considering the enhanced rigidity of the crystal lattice of the solid solution sample. To extract the magnetic contribution to the heat capacity, an approach based on a reasonable frequency spectrum is adopted Taking advantage of the in-field heat capacity measurements, MCE was described in terms of the isothermal entropy change AS M and the adiabatic temperature change Delta T-ad. The magnitudes of these quantities are typical for the class of molecular magnets. The values of vertical bar Delta S-M vertical bar(max) detected for mu(0)Delta H = 5 T amount to 7.04, 5.26, and 4.93 J K-1 mol(-1) for Co2Nb, CoFeNb, and Fe2Nb, respectively, and are on the order of those obtained for the same field change in the isostructural compounds. The values of Delta T-ad detected for mu(0)Delta H = 5 T amount to 4.16, 2.47, and 2.01 K for Co2Nb, CoFeNb, and Fe2Nb, respectively, and are larger or comparable with those observed for the isostructural compounds. Temperature dependences of exponent n, quantifying the field dependence of Delta S-M, display minima close to the transition temperatures, implying through their values that the studied compounds belong to the universality class of the three-dimensional Heisenberg model. The regeneration Ericsson cycles employing the studied compounds as the working substance were considered. Most surprisingly, the Ericsson cycle operating between the temperatures corresponding to the full width at half maximum of the vertical bar Delta S-M vertical bar signal (T-C, T-H) turns out to be totally ineffective. Through shifting the temperature of the hot reservoir T-H down to the temperature T-max corresponding to vertical bar Delta S-M vertical bar(max), the coefficient of performance is rendered positive and comparable with that of the Carnot cycle. A detailed analysis indicates that the regeneration Ericsson cycle operating between T-C and T-max should be most efficient for the maximal studied value of the applied field (=9 T) with irrelevant differences between the studied compounds.

Magnetic measurements of a three-dimensional (3D) molecular magnet built of Mn9[W(CN)8]6 clusters have been carried out to study its static and dynamic properties.Measurements of ac susceptibility in the presence of static magnetic field revealed slow magnetic relaxations.It was found that for the 120 Hz wave frequency the optimal static field which maximizes the imaginary component of the ac susceptibility is about 500 Oe.

Magnetic properties of copper dimolybdate trihydrate CuMo2O7·3H2O have been studied from room temperature down to 2K with the use of ac and dc magnetometry. There was no sign of long range order; however, the compound deviates from the classical Curie law for paramagnetic systems. This situation is due to exchange interactions between Cu ions forming dimers, whereas Mo ions do not possess magnetic moments. The estimated value of Jex is −2.74(1)K from fitting the dc susceptibility and −3.01(1) K from the fit of magnetization versus magnetic field. The antiferromagnetic coupling is confirmed by the value of θ=−4.0(3)K from the Curie–Weiss analysis. The dynamic properties measured by ac susceptibility as a function of applied dc field indicate three regions of different magnetic relaxation behaviors.

Magnetocaloric effect in {[Fe(pyrazole)$_4$]$_2$[Nb(CN)$_8$]$\cdot$4H$_2$O}$_n$ molecular magnet is reported. It crystallizes in tetragonal I4$_1$/a space group. The compound exhibits a phase transition to a long range magnetically ordered state at $T_\mathrm{c}\approx$8.3 K. The magnetic entropy change $\Delta S_\mathrm{M}$ as well as the adiabatic temperature change $\Delta T_\mathrm{ad}$ due to applied field change $\mu_0\Delta H$=0.1, 0.2, 0.5, 1, 2, 5, 9 T as a function of temperature have been determined by the relaxation calorimetry measurements. The maximum value of $\Delta S_\mathrm{M}$ for $\mu_0\Delta H=5$ T is 4.9 J mol$^{-1}$ K$^{-1}$ (4.8 J kg$^{-1}$ K$^{-1}$) at 10.3 K. The corresponding maximum value of $\Delta T_\mathrm{ad}$ is 2.0 K at 8.9 K. The temperature dependence of the exponent $n$ characterizing the field dependence of $\Delta S_\mathrm{M}$ has been estimated. It attains the value of 0.64 at the transition temperature, which is consistent with the 3D Heisenberg universality class.

The critical behaviour of the three dimensional (3D) molecular magnet {[FeII(pirazol)4]2[NbIV(CN)8]·4H2O}n has been studied with the use of experimental techniques such as ac magnetometry and zero field μSR spectroscopy. The sample orders magnetically below Tc=7.8 K. The measurements allowed to determine static exponents β, γ, and the dynamic exponent w. The resulting exponent values indicate that the studied system belongs to the universality class of the 3D Heisenberg model.

Different aspects of critical behaviour of magnetic materials are presented and discussed. The scaling ideas are shown to arise in the context of purely magnetic properties as well as in that of thermal properties as demonstrated by magnetocaloric effect or combined scaling of excess entropy and order parameter. Two non-standard approaches to scaling phenomena are described. The presented concepts are exemplified by experimental data gathered on four representatives of molecular magnets.

In this paper magnetic properties of hybrid inorganic-organic compound {[FeII (pyrazole)4 ]2 [NbIV (CN)8 ]∙4H2 O}n are presented. This is a three dimensional molecular magnet with well localized magnetic moments, which make it a suitable candidate for testing magnetic models. In order to characterize the magnetic properties of the above compound we performed the AC/DC magnetometry in the range 0-5 T. The special attention was paid to the phase transition at 7.9 K. The study in magnetic field supports magnetic ordering below 7.9 K.

In this paper magnetic properties of hybrid inorganic-organic compound {[FeII (pyrazole)4 ]2 [NbIV (CN)8 ]∙4H2 O}n are presented. This is a three dimensional molecular magnet with well localized magnetic moments, which make it a suitable candidate for testing magnetic models. In order to characterize the magnetic properties of the above compound we performed the AC/DC magnetometry in the range 0-5 T. The special attention was paid to the phase transition at 7.9 K. The study in magnetic field supports magnetic ordering below 7.9 K.

In this paper magnetic properties of hybrid inorganic-organic compound {[FeII(pyrazole)4]2[NbIV(CN)8]∙4H2O}n are presented. This is a three dimensional molecular magnet with well localized magnetic moments, which make it a suitable candidate for testing magnetic models. In order to characterize the magnetic properties of the above compound we performed the AC/DC magnetometry in the range 0-5 T. The special attention was paid to the phase transition at 7.9 K. The study in magnetic field supports magnetic ordering below 7.9 K.

Three possible distributions of temperature points in the μSR measurement of local field (order parameter) are discussed. The least square method is applied to estimate the scale of the deviations of the fitted parameters from the true values. It indicates that the distribution corresponding to a uniform section of the order parameter values (uniform-in-signal) incurs the smallest errors. The distribution constructed on the basis of the zeros of the Chebyshev polynomials yields comparable uncertainties, while the uniform-in-temperature distribution turns out to be least effective incurring considerably larger errors. These findings can be useful while planning an order parameter measurement in the μSR experiment.

Complementary experimental methods such as mu SR spectroscopy, ac magnetometry, and relaxation calorimetry have been employed as a probe of critical behavior of a unique ferrimagnetic molecular magnet {[Mn-II(pydz)(H2O)(2)][Mn-II(H2O)(2)][Nb-IV(CN)(8)]center dot 2H(2)O}(n) with T-c approximate to 42 K. A full set of critical exponents is determined. Static exponents alpha, beta, gamma, and the dynamic exponent w are extracted directly from the measurements. Further critical exponents nu, eta, and z are derived on the basis of scaling or hyperscaling relations. The knowledge of the thermal dependence of the order parameter combined with the results of the calorimetric measurements allowed for the determination of two further static critical exponents kappa and kappa'. The system shows a close affinity to the three-dimensional (3D) Heisenberg model. Ferrimagnetism of the compound leads to a coexistence of typically ferro-and antiferromagnetic characteristics.

The rapidly developing field of molecular magnetism supplies a multitude of novel compounds of unprecedented properties and structure. Molecular magnets predominantly belong to the class of compounds involving well localized magnetic moments. This feature together with the fact that the nature and symmetry of magnetic interactions is encrypted in the critical behaviour makes them a perfect testing ground of the existing theoretical spin models. It is demonstrated that the experimental technique of the μSR spectroscopy is perfectly suited to study magnetic fluctuations and spin dynamics in the neighbourhood of a phase transition. This unique method can even dispense with the complementary measurements of the AC susceptibility or heat capacity to supply a complete set of the static and dynamic critical exponents. It can thus be used to pinpoint the universality class of the material of interest.

The fast developing field of molecule-based magnets involving organic and coordination chemistry provides the physicist with a multitude of novel compounds of unprecedented structure. The magnetic structure of Cu-4(tetren)[W(CN)(8)](4) (1) was shown to consist of weakly coupled double layers. By contrast, in the structurally similar compound Cu2+xCu4[W(CN)(8)](4) (2) the free spaces between the double layers are filled with paramagnetic copper (II) ions leading to a unique magnetic network. Both compounds exhibit the transition to a magnetically ordered phase at T-c approximate to 33 K and T-c approximate to 40 K, respectively. The critical behavior of 1 and 2 is investigated using complementary methods: ac magnetometry, relaxation calorimetry, and muon spin-rotation spectroscopy. Apart from alpha, beta, and gamma, critical exponents kappa and kappa' describing the combined scaling of excess entropy and order parameter are determined for both compounds. This type of scaling is verified for 1, the system revealing the signatures of the Berezinskii-Kosterlitz-Thouless transition. For 2 their values imply that the system is close to the universality class of the three-dimensional Heisenberg model. The relatively small value of exponent gamma = 1.05 for 2 indicates the presence of noncollinearity in the spin arrangement. Exponents kappa and kappa'for 2 are also found consistent with noncollinear models. The shift of the heat-capacity anomaly toward higher temperatures with increasing applied field indicates the presence of ferromagnetic interactions in 2.

Muon spin relaxation has been used to study the magnetic properties of a low-dimensional molecular magnet with a structure consisting of bilayers of [W(CN)8]3− and Cu2+. In the magnetically ordered state a spontaneous precession signal was found to contain two main components and the temperature dependence of the characteristic internal fields was followed up to the critical temperature. The critical exponent obtained for the magnetic order parameter β = 0.237(12) points to the two-dimensional character of the transition and reflects its underlying Berezinski–Kosterlitz–Thouless nature. Experiments performed in the longitudinal magnetic field demonstrate clearly a spin-flip phenomenon associated with the weak inter-bilayer coupling, that takes place in the magnetic field region below 100 G. The muon precession signals measured in the vicinity of this transition provide detailed local information about the corresponding rearrangements of the spin structure.

The experimental study of photo-induced magnetization of hybrid molecular magnet containing cobalt(II) and tungsten(V) magnetic centers bridged by 4,4'-bpy and CN- is presented. The observed increase in magnetization rate is attributed to the defects due to inter-valence transfer between (CoWv)-W-II reversible arrow (CoWIV)-W-III. The time evolution of magnetization is parameterized by the power law rather than exponential function.

Two quasi-one-dimensional compounds [Ln(III)(terpy)(DMF4][W-V(CN)8] center dot 6H(2)O, where Ln stands for Gd or Sm, were synthesized and the measurements of their magnetic features were carried out. Magnetization was measured at 2 K in the field range 0-5 T. To extract physical information from the experimental data a generalization of the theoretical approach given by Verdaguer et al. (Phys. Rev. B 29, 5144 (1984)) [1] is put forward. That theoretical model is found to fit the data well. It allows for the determination not only of the coupling constant but also of the zero-field splitting parameter.

AC susceptibility, χAC, for ferromagnetic CuII[WV(CN)8]- and CuII[MoV(CN)8]-based molecular magnets with TC≈30K of the unique bilayered structure is investigated. χAC(T) is measured with different frequency and amplitude of the oscillating field for various magnitudes of static applied field. CuII[WV(CN)8]-based compounds show a sharp anomaly at the transition and extremely low effect below TC; second harmonic of χAC and DC magnetization curves suggest some weak antiferromagnetic interactions while critical behaviour points to the Ising anisotropy in these samples. Susceptibility of these compounds is strongly enhanced by applying DC field of about 50 Oe. For the CuII[MoV(CN)8]-based compounds χAC is more typical and the critical exponent γ is close to that of 3D Heisenberg magnets. It is suggested that the different behaviour of these isostructural and isospin compounds is related to the more diffuse 5d shell of tungsten centres that may enhance the possible antiferromagnetic exchange through the CN-bridges. At HDC=0 there is almost no frequency dependence of χAC. Weak glass-like character of tungstate compounds which appears in the applied field is probably due to random weak interactions through the hydrogen bonds disordered in the space between the bilayers and random anisotropy coming from the countercations.

The photo-induced magnetic effects (PME) have been observed during the investigation into magnetic properties of [MnR4TPP][TCNE]*2PhMe (R = OC12H25, TPP tetraphenylporphyrin, TCNE tetracyanoethylene) charge transfer salt molecular magnet. The study of magnetic properties of [MnR4TPP][TCNE]*2PhMe and PME were performed in both static (DC) and dynamic (AC) modes with Lake Shore 7225 equipment using a sample probe with optical fiber. The Xe lamp with optical cut-off filters or the He–Ne laser was used. Results of DC magnetization relaxation measurements obtained for different magneticfields in He bath temperature (4.2 K) during illumination differ significantly from those obtained before illumination. Both the value and the time dependence of magnetization changed during illumination. Hysteresis loop measured before and during illumination was also found to undergo considerable change. On the other hand the AC measurements revealed only slight alteration in the real and imaginary components of the susceptibility. Simple theoretical description of the magnetization process recorded at 4.2 K, was performed and the relaxation data were found to be consistent with the model.

The photo-induced magnetic properties of [MnR4TPP] [TCNE], (TPP=tetraphenylporphyriyn, TCNE=tetracyanoethylene, R=OC12H25) charge transfer molecular magnet were investigated. Results of AC susceptibility and DC magnetization as well as relaxation measurements were obtained. The substance under study, composed of pseudo 1D chains, forms a long range ordered 3D magnetic system at Tc=22 K. Magnetic moments of MnIII (S=2) and those located at TCNE− anions are coupled antiferromagnetically in the chain. At temperatures above ∼8 K the interchain interaction favors the ferromagnetic orientation of the resulting chain moments, while at lower temperatures the coupling is negative and at the initial magnetization curve the spin flop transition is observed. The magnetization of the sample as a function of the applied field is strongly influenced by the illumination with light of different wavelength. Both the value of magnetization and the time dependence of magnetization changed during the irradiation. On the other hand, the AC susceptibility results show no significant change upon irradiation.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.