Miguel Castro

A Switchable Molecular Rotator: Neutron Spectroscopy Study on a Polymeric Spin-Crossover Compound

A quasielastic neutron scattering and solid-state (2)H NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment--the pyrazine ligand--occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.

Accurate measurement of the specific absorption rate using a suitable adiabatic magnetothermal setup

Accurate measurements of the specific absorption rate (SAR) of solids and fluids were obtained by a calorimetric method, using a special-purpose setup working under adiabatic conditions. Unlike in current nonadiabatic setups, the weak heat exchange with the surroundings allowed a straightforward determination of temperature increments, avoiding the usual initial-time approximations. The measurements performed on a commercial magnetite aqueous ferrofluid revealed a good reproducibility (4%). Also, the measurements on a copper sample allowed comparison between experimental and theoretical values: adiabatic conditions gave SAR values only 3% higher than the theoretical ones, while the typical nonadiabatic method underestimated SAR by 21%.

Adiabatic magnetothermia makes possible the study of the temperature dependence of the heat dissipated by magnetic nanoparticles under alternating magnetic fields

Determining the low-temperature dependence of the specific absorption rate (SAR) of magnetic nanoparticles under alternating magnetic fields with amplitudes and frequencies similar to those used in applications such as magnetic fluid hyperthermia, becomes essential when theoretical expressions fail to extrapolate the behavior of nanoparticle arrangements. We prove that adiabatic magnetothermia is capable of providing SAR(T) data displaying an excellent continuity with those obtained from magnetic measurements at lower ac-field amplitudes and frequencies.

Calorimetric study of the green phases R2BaCuO5 (R = Gd, Dy, Ho, Er, Lu, Y)

Abstract Adiabatic heat-capacity measurements between 5 and 60 K have been performed on a series of green phases R2BaCuO5. Independent peaks are generally observed for the three-dimensional magnetic ordering of the Cu and the magnetic rare earth ions. Contrarily, the compounds with nonmagnetic rare earth ions show rounded anomalies characteristic of low-dimensional magnetic interactions.

Heat capacity study of La2NiO4 and Pr2NiO4

Abstract A calorimetric study of La 2 NiO 4 and Pr 2 NiO 4 has been carried out with adiabatic and ac calorimetry. Thermodynamic functions of both compounds have been calculated at rounded temperatures. Heat capacity anomalies corresponding to structural transitions from the low-temperature tetragonal P4 2 /ncm phase to the higher-temperature orthorhombic Bmab phase have been found at 56K for La 2 NiO 4 and at 121.2K for Pr 2 NiO 4 . The three-dimensional magnetic ordering of the Ni ions produces a small peak in the heat capacity at 328K annd 324K, respectively, for both compounds in agreement with the expected low-dimensional behavior. No long-range order of Pr 3+ has been detected down to 2K. A broad excess heat capacity in Pr 2 NiO 4 over the estimated lattice contribution was found to be due to the electronic energy levels of Pr 3+ . An energy gap of around 50K between the ground and the first-excited level with equal degeneracies has been deduced.

Magnetic properties of SmNiO3

Abstract SmNiO3 synthesized under high oxygen pressure has been studied from 1.8 to 650 K by means of both, DC magnetization and AC calorimetry. This compound shows a metal—insulator transition at 400 K and an antiferromagnetic ordering at lower temperatures. The DC magnetic susceptibility and the heat capacity clearly show the magnetic ordering at 220 K. In addition, a change in the slope of the M(T) curve is detected at the metal—insulator transition. The heat capacity measurements give an entropy content for the magnetic transition lower than the expected for a S = 1 2 magnetic ion. This result suggests strong antiferromagnetic correlations above TN in this material.

Character of the phase transition in liquid crystals by AC calorimetry

Abstract A method based on the analysis of the temperature sensor of a high-resolution AC calorimeter has been developed in order to determine the character of the phase transition in liquid crystals. A comparative study between this method, denoted as ‘thermistor method’, and the standard phase shift procedure is carried out in different kind of liquid crystal phase transitions. The sensitivity of the standard phase shift method has been studied through the thermal models for our system. Finally, the method, when it is applied to first-order transitions, provides additional thermal features as coexistence region, metastability and latent heat ratio between transitions, and it could allow to elucidate the assignment of the character of the transition lines in phase diagrams.

Heat-capacity measurement of R2BaNiO5 (R = Y, Ho, Er) oxides

Abstract Adiabatic heat-capacity measurements between 5 and 350 K have been performed on R 2 BaNiO 5 (R = Y, Er, Ho). Three-dimensional magnetic ordering transitions are observed at 32.5 and 47.6 K for the Er and Ho compounds, respectively. Additionally there are broad anomalies in the ordered temperature region centered at 8 and 16 K, respectively and other excess heat-capacity contributions at higher temperatures due to thermal population of low-energy levels of the rare earths. In Y 2 BaNiO 5 there is no evidence of any of these anomalies. The one- and three-dimensional magnetic interactions have been analyzed.

Anomalous Dielectric Behaviour in Orthopalladated Ferroelectric Liquid Crystals

Abstract A ferroelectric ortho-palladated dimer derived from chiral imines with two chiral terminal chains in the aromatic ring has been characterized by dielecctric permittivity, polarization, X ray and heat capacity measurements. An anomalous behaviour has been detected in the low temperature range of the SmC* phase. A new molecular arrangement has been found on cooling prior to the crystallization not detected before by DSC experiments.