Search

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.

Abstract: 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 J ex 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. [Copyright &y& Elsevier]

Complementary experimental methods such as &mgr;SR spectroscopy, ac magnetometry, and relaxation calorimetry have been employed as a probe of critical behavior of a unique ferrimagnetic molecular magnet {[MnII(pydz)(H2O)2][MnII(H2O)2][NbIV(CN)8]-2H2O}n with Tc ≈ 42 K. A full set of critical exponents is determined. Static exponents α, &bgr;, &ggr;, and the dynamic exponent &ohgr; are extracted directly from the measurements. Further critical exponents &ngr;, &eegr;, 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 &kgr; and &kgr;′. 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. [ABSTRACT FROM AUTHOR]

Abstract: AC susceptibility, , for ferromagnetic CuII[WV(CN)8]- and CuII[MoV(CN)8]-based molecular magnets with of the unique bilayered structure is investigated. 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 ; second harmonic of 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 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 there is almost no frequency dependence of . 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. [Copyright &y& Elsevier]

A series of diluted molecular magnets {[MnII(H2O)2]2[MIV(CN)8]·4H2O}n (M = Nb/Mo) exhibiting a ferrimagnetic phase have been synthesized and investigated to demonstrate the first example of site percolation in magnetic coordination polymers. It is the first case of a magnetic percolation study with selective site substitution, where magnetic ions of only one type are replaced by their diamagnetic analogues leaving the other magnetic sublattice untouched. By modifying the molecular field model the NbIV concentration dependence of the magnetic ordering temperature was reproduced and the percolation threshold was determined.

Cyanide-bridged {Mn II 9 [W V (CN) 8 ] 6 } clusters with the ground state spin S SG = 39/2 were connected by a 4,4'-dipyridyl disulfide (4,4'-dpds) linker into 2-D double-connected coordination layers of the I 0 O 2 type, {Mn II 9 (4,4'-dpds) 4 (MeOH) 16 [W V (CN) 8 ] 6 }·12MeOH (1). The intercluster contacts are controlled by the bridging Mn II -(4,4'-dpds)-Mn II coordination modes and direct hydrogen bonds W-CN···HO MeOH -Mn in three crystallographic directions, with the vertex-to-vertex contact unprecedented in {Mn 9 W 6 }-based networks dominating over the typical edge-to-edge contacts. The resulting 3D supramolecular network of high-spin clusters was subjected to a thorough magnetic characterization in context of two critical issues. First, the intracluster W V -CN-Mn II exchange coupling and intercluster interaction were successfully modeled through the combination of dc measurements, Quantum Monte Carlo simulations, and mean-field calculations, yielding a reasonable J ap = -8.0 cm -1 , J eq = -19.2 cm -1 (related to apical and equatorial CN bridges, depending on the angle they form with the S 4 axis of dodecahedral [W(CN) 8 ] 3- units, respectively), and zJ' = 0.014 cm -1 with the average g W = g Mn = 2.0 parameter set. Continuing this approach, we simulated the magnetocaloric effect (MCE) and compared it to the experimental result of ΔS max for fields >5.0 T. Second, two relaxation processes were induced by a relatively weak magnetic field, H -1 K -1 for fields >5.0 T. Second, two relaxation processes were induced by a relatively weak magnetic field, H dc = 500 Oe, at an H ac field frequency range of up to 10 kHz, which are related to dipole-dipole interactions between high-spin (39/2) moieties. The observed relaxation times significantly differ from each other, the slow process with τ slow at tenths of a second being temperature independent and the faster process being 3-5 orders of magnitude faster with the effective energy barrier Δ eff = 17.6 K. These dynamic properties are surprising, since the compound is made up of isotropic high-spin molecules.

The rotating magnetocaloric effect (RMCE) is a new issue in the field of magnetic refrigeration. We have explored this subject on the two-dimensional (2D) enantiopure {[Mn II (R-mpm) 2 ] 2 [Nb IV (CN) 8 ]}·4H 2 O (where mpm = α-methyl-2-pyridinemethanol) coordination ferrimagnet. In this study, the magnetic and magnetocaloric properties of single crystals were investigated along the bc//H easy plane and the a*//H hard axis. The observed small easy plane anisotropy is due to the dipole-dipole interactions. For fields higher than 0.5 T, no significant difference in the magnetocaloric effect between both geometries was noticed. The maximal magnetic entropy change for conventional effect was observed at 32 K and the magnetic field change μ 0 ΔH = 5.0 T attaining the value of ∼5 J mol -1 K -1 . The obtained maximal value of -ΔS m is comparable to previously reported results for polycrystalline octacyanidoniobate-based bimetallic coordination polymers. A substantial anisotropy of magnetocaloric effect between the easy plane and hard axis appears in low fields. This includes the presence of inverse magnetocaloric effect only for the a*//H direction. The difference between both geometries was used to study the rotating magnetocaloric effect. We show that the inverse part of magnetocaloric effect can be used to enhance the rotating magnetic entropy change up to 51%. This finding is of key importance for searching efficient materials for RMCE.

Two high-spin pentanuclear NiW clusters with diimine blocking ligands have been obtained: {[Ni(4,4'-MeObpy)2]3[W(CN)8]2}·12H2O (1) and {[Ni(phen)2(H2O)][Ni(phen)2]2[W(CN)8]2}·7H2O (2) (4,4'-MeObpy = 4,4'-dimethoxy-2,2'-bipyridine, phen = 1,10-phenanthroline). Despite the similarity of the building blocks and synthetic conditions the compounds show different topologies of the cluster core: 1 is a trigonal bipyramid while 2 is a decorated square. Both cluster structures are chiral with either ΔΔΔ or ΛΛΛ configuration around all three Ni centres. In 1 spontaneous resolution occurs and it crystallises in the P212121 space group forming a conglomerate containing both types of enantiomorphic crystals. 1Δ and 1Λ are the first pair of enantiomorphic structures of cyano-bridged clusters of trigonal bipyramidal topology obtained with achiral bidentate blocking ligands. 2 crystallises as a racemic compound in a centrosymmetric space group P1[combining macron] with both enantiomers present in the structure. 2 is an exceptional square-motif containing structure with an identical stereoconfiguration of all complex cations within one cluster. Ferromagnetic interactions are present in both clusters resulting in the ground spin state S = 4.

The syntheses, X-ray structures, and magnetic behaviors of two new cyano-bridged assemblies, the molecular [Mn(III)(salen)H2O]3[W(V)(CN)8].H2O (1) and one-dimensional [Mn(salen)(H2O)2]2[[Mn(salen)(H2O)][Mn(salen)]2[Mo(CN)(8)]].0.5ClO4.0.5OH.4.5H2O (2), are presented. Compound 1 crystallizes in the monoclinic system, has space group P2(1)/c, and has unit cell constants a = 13.7210(2) A, b = 20.6840(4) A, c = 20.6370(2) A, and Z = 4. Compound 2 crystallizes in the triclinic system, has space group P, and has unit cell dimensions a = 18.428(4) A, b = 18.521(3) A, c = 18.567(4) A, and Z = 2. The structure of 1 consists of the asymmetric V-shaped Mn-NC-W-NC-Mn-O(phenolate)-Mn molecules, where W(V) coordinates with [Mn(salen)H2O] and singly phenolate-bridged [Mn(salen)H2O]2 moieties through the neighboring cyano bridges. The [W(V)(CN)8]3- ion displays distorted square-antiprism geometry. The structure of 2 consists of the cyano-bridged [Mn3(III)Mo(IV)]n- repeating units linked by double phenolate bridges into one-dimensional zigzag chains. The Mn(III) centers are bound to Mo(IV) of square-antiprism geometry through the neighboring cyano bridges. The magnetic studies of 1 reveal the antiferromagnetic intramolecular interactions through the CN and phenolate bridges and the relatively weak intermolecular interactions. Compound 1 becomes antiferromagnetically ordered below TN = 4.6 K. The presence of the magnetic anisotropy is documented with the MH measurements carried out for both polycrystalline and single-crystal samples. At T = 1.9 K, the spin-flop transition is observed in the field of 18 kOe applied parallel to the bc plane, which is the easy plane of magnetization. Field dependence of magnetization of 1 shows field-induced metamagnetic behavior from the antiferromagnetic ground state of ST = 3/2 to the state of ST = 5/2. The magnetic properties of 2 indicate a weak antiferromagnetic interaction between Mn(III) centers in double-phenolate-bridged [Mn(III)(salen)]2 dinuclear subunits and a very weak ferromagnetic interaction between them through the diamagnetic [Mo(IV)(CN)8]4- spacer.