R. Burriel

SPIN-CROSSOVER IN THE FE(ABPT)2(NCX)2 (X = S, SE) SYSTEM : STRUCTURAL, MAGNETIC, CALORIMETRIC AND PHOTOMAGNETIC STUDIES

The compounds [Fe(abpt) 2 (NCS) 2 ] ( 1 ) and [Fe(abpt) 2 (NCSe) 2 ] ( 2 ) with abpt=4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P 2 1 / n with Z =2, a =8.538(8), b =10.246(8), c =16.45(2) A, β =93.98(9)° for 1 and a =8.623(2), b =10.243(3), c =16.585(3) A, β =93.19(2)° for 2 . In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS − ( 1 ) and NCSe − ( 2 ) groups in the trans -position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)↔high-spin (HS) conversion centered at T 1/2 around 180 and 224 K for 1 and 2 , respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Δ H =5.8±0.5 ( 1 ) and 8.6±0.8 kJ mol −1 ( 2 ); Δ S =32.5±3 ( 1 ) and 38±4 J mol −1 K −1 ( 2 ). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr + laser coupled to an optical fiber. The critical LIESST temperatures T liesst are around 40 and 32 K for 1 and 2 , respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS→LS relaxation kinetics have been investigated in the temperature range 6–40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1 . The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K.

Entropy determinations and magnetocaloric parameters in systems with first-order transitions: Study of MnAs

We present a study of the giant magnetocaloric effect in MnAs produced by a magnetostructural first-order phase transition. Results deduced from magnetization, M, and heat capacity, Cp,B(T), are compared and discussed. Some spurious effects are explained, and especially a spike in the isothermal entropy change, ΔST, occurring at TC when obtained via the Maxwell relation (∂S/∂B)T=(∂M/∂T)B. Alternative determination methods are given to circumvent this problem. The spike is explained as an artifact due to the incorrect application of the Maxwell relation to path dependent thermodynamic functions that are not state functions. The added wrong contribution to ΔST has been calculated using calorimetric data, giving a good agreement with the result from the magnetization measurements.

Spontaneous Magnetization in Ni-Al and Ni-Fe Layered Double Hydroxides

Layered double hydroxides containing paramagnetic Ni (II) and diamagnetic/paramagnetic Al (III)/Fe (III) ions have been prepared and characterized. Ni 2Al(OH) 6(NO 3). nH 2O ( 1), Ni 2Fe(OH) 6(NO 3). nH 2O ( 2), Ni 2Fe(OH) 6(C 6H 8O 4) 0.5. nH 2O ( 3), and Ni 2Fe(OH) 6(C 10H 16O 4) 0.5. nH 2O ( 4) were prepared by coprecipitation at controlled pH as polycrystalline materials with the typical brucite-like structure, with alternating layers of hydroxide and the corresponding anions, which determine the interlayer separation. Magnetic studies show the appearance of spontaneous magnetization between 2 and 15 K for these compounds. Interestingly, the onset temperature for spontaneous magnetization follows a direct relationship with interlayer separation, since this is the only magnetic difference between compounds 2, 3, and 4. Magnetic and calorimetric data indicate that long-range magnetic ordering is not occurring in any of these materials, but rather a freezing of the magnetic system in 3D due to the magnetic disorder and competing intra- and interlayer interactions. Thus, these hydrotalcite-like magnetic materials can be regarded as spin glasses.

Crystal structures and magnetic properties of the mono-µ-halogeno-bridged copper(II) chains Cu(pcpci)X [pcpci =N-(2′-pyridylcarbonyl)pyridine-2-carboximidate, X = Cl or Br]

Compounds Cu(pcpci)Cl (1) and Cu(pcpci)Br (2)[pcpci =N-(2′-pyridylcarbonyl)pyridine-2-carboximidate] are isostructural and crystallize in the monoclinic space group Pc with two formula units in cells of dimensions [(1)/(2)]: a= 3.963(2)/3.888(3), b= 8.48(3)/8.641(3), c= 18.21(12)/17.944(11)A, and β= 94.9(2)/92.63(6)°. The crystal and molecular structure of (2) has been determined by X-ray diffraction methods. Least-squares refinement of 1 351 reflections with I > 2.5σ(I) and 189 parameters gave a final R′= 0.065. The complex exhibits a chain arrangement built up of Cu(pcpci)Br units linked through the bromine atoms. The copper(II) ions are five-co-ordinate and the co-ordination geometry is near square pyramidal. Magnetic susceptibility measurements were analyzed in terms of the Heisenberg model yielding antiferromagnetic exchange interactions of J(1)=–0.4 and J(2)=–0.8 cm–1. These values are discussed on the basis of the structural features and correlated with published magnetostructural data.

Two-Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin-Crossover Compounds Fe(X-py)2[Ag(CN)2]2 (X=H, 3-methyl, 4-methyl, 3,4-dimethyl, 3-Cl)†

In the series of polymeric spin-crossover compounds Fe(X-py)(2)[Ag(CN)(2))](2) (py=pyridine, X=H, 3-Cl, 3-methyl, 4-methyl, 3,4-dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high-spin (HS) to the low-spin (LS) state occurs by two-step transitions for three out of five members of the family (X=H, 4-methyl, and X=3,4-dimethyl). The two other compounds (X=3-Cl and 3-methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin-crossover behaviour of the compound undergoing two-step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3-methyl undergo light-induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS-to-LS relaxation curves at different temperatures support the low-cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4-dimethyl the behaviour is well described by a double exponential function in accordance with the two-step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.

Magnetism in Polyoxometalates: Anisotropic Exchange Interactions in the Co Moiety of [Co3W(D2O)2(ZnW9O34)2]12−—A Magnetic and Inelastic Neutron Scattering Study

The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3.

Correlation of normal and superconducting transport properties on textured Bi-2212 ceramic thin rods

The electric and thermal properties well above and below Tc of Bi-2212 textured ceramics have been correlated through a careful analysis of the microstructure and the transport measurements. Thin rods with the same Bi-2122 stoichiometry and textured by a laser floating zone technique have been studied with that aim. By changing the growth parameters, it has been possible to produce strong changes in microstructure and critical current density, Jc, with small variations in the thermal conductivity. The existence of phase and composition gradients across the thin rods, which explains the variations of Tc, makes the relation difficult between the normal state resistivity and Jc (77 K). A simple qualitative analysis that takes into account the observed microstructure has been developed to correlate the electric transport properties in the normal and in the superconducting states.

Mechanochemical Effect in the Iron(III) Spin Crossover Complex [Fe(3-MeO-salenEt)2]PF6 as Studied by Heat Capacity Calorimetry.

Magnetic and thermal properties of the iron(III) spin crossover complex [Fe(3MeO-salenEt)(2)]PF(6) are very sensitive to mechanochemical perturbations. Heat capacities for unperturbed and differently perturbed samples were precisely determined by adiabatic calorimetry at temperatures in the 10-300 K range. The unperturbed compound shows a cooperative spin crossover transition at 162.31 K, presenting a hysteresis of 2.8 K. The anomalous enthalpy and entropy contents of the transition were evaluated to be Delta(trs)H = 5.94 kJ mol(-1) and Delta(trs)S = 36.7 J K(-1) mol(-1), respectively. By mechanochemical treatments, (1) the phase transition temperature was lowered by 1.14 K, (2) the enthalpy and entropy gains at the phase transition due to the spin crossover phenomenon were diminished to Delta(trs)H = 4.94 kJ mol(-1) and Delta(trs)S = 31.1 J K(-1) mol(-1), and (3) the lattice heat capacities were larger than those of the unperturbed sample over the whole temperature range. In spite of different mechanical perturbations (grinding with a mortar and pestle and grinding in a ball-mill), two sets of heat capacity measurements provided basically the same results. The mechanochemical perturbation exerts its effect more strongly on the low-spin state than on the high-spin state. It shows a substantial increase of the number of iron(III) ions in the high-spin state below the transition temperature. The heat capacities of the diamagnetic cobalt(III) analogue [Co(3MeO-salenEt)(2)]PF(6) also were measured. The lattice heat capacity of the iron compounds has been estimated from either the measurements on the cobalt complex using a corresponding states law or the effective frequency distribution method. These estimations have been used for the evaluation of the transition anomaly.

Linear chain antiferromagnetic interactions in Cs2CuCl4

The magnetic susceptibility of polycrystalline samples of Cs2CuCl4 has been measured over the temperature interval 1.1–30 K. A broad maximum in the data, centered around 2.6–2.7 K, is consistent with the existence of lower‐dimensional antiferromagnetic interactions in this compound. The model used for fitting the data is the Heisenberg linear chain of Bonner and Fisher, with a small molecular‐field correction. The parameters resulting from the data analysis are 〈g〉=2.20±0.02, −J/k=2.0±0.1 K, and −zJ’/k=0.10±0.05 K.

Specific heat of the cubic metallic perovskites Mn3ZnN and Mn3GaN

Abstract Heat capacity measurements of Mn3ZnN and Mn3GaN between 6 and 350 K are reported. Anomalies of first order character originating from simultaneous magnetic and crystallographic phase transitions are detected. The electronic γ values are derived.