CONICET | Buscador de Institutos y Recursos Humanos

Spin Crossover systems [1] are very interesting materials because of the ability to change their magnetic state, between a low-spin and a high spin configuration, under the influence of external stimuli (temperature, electromagnetic irradiation, pressure, etc.) being interesting candidates e.g. for memory devices. Among them, the [Fe(trz)3]X family (trz: triazole derivative) is a promising candidate for exhibiting bistability [2]. For the specific case of [Fe(Htrz)2(trz)](BF4), the Fe(II) ions are coordinated by the ligands forming a 1-D polymeric structure exhibiting a cooperative spin transition between the low spin (S=0) state to a high spin (S=2) state at 386 K (rising T) or 343K (lowering T). We have prepared [Fe(Htrz)2(trz)](BF4) as bulk material and as 11 nm nanoparticles (using conventional reverse micelle techniques [3])In order to observe a sufficiently detailed account of the spin-crossover transition, a selected Doppler energy ROI (region of interest) scan was performed. The experimental system developed in our laboratory allows the selection of ROIs in any spectral region, with a very high flexibility. The ROI can be composed of any number of points (up to 4098) with a freely selectable energy for each point. For the present case a ROI of 51 equally spaced points, approximately centred at the isomer shift of the high temperature phase was chosen; it also included one point at an energy corresponding to background transmission for control and normalization purposes. The ROI was periodically swept at 0.5 points/s while temperature was increased and then decreased at 0.23 K/min. This approach provided a detailed temperature and Doppler energy observation of the transition leading to a wealth of experimental information. Additional information on the experimental setup can be found elsewhere [4]. Figure 1 is a bidimensional Doppler velocity vs time representation of the scan performed on the bulk sample, where the transmission counts are indicated by the dots gray tone (in a colour scale in the original). The inset shows the temperature time dependence. The plot clearly shows the transition from a low spin configuration at low temperature to a high spin one at high temperature and vice versa. The low and high spin configurations correspond to isomer shifts of 0.31 and 0.85 mm/s, and quadrupole splitings of 0.3 and 2.2 mm/s, respectively, in agreement with previous results[5]. At this low heating rate it was observed that the transition displays hysteresis being centred at 370.7 K (heating up) and 340.5 K (cooling down) with a temperature transition region of about 5 K, in reasonable agreement with calorimetric, magnetic and X-ray absorption experiments [5] (not shown here). The data have been analysed as a whole with a temperature and energy dependent theoretical model which provides a quasi continuous description of the transition, in terms of subspectra intensities and hyperfine parameters variation. A detailed description of the used fitting function and results will be given.The nanoparticles size distribution was studied by Dynamic Light Scattering. A mean size of 11 nm was determined. The spectrum temperature evolution was followed by a Doppler energy ROI scan, similar to the one carried out for the bulk sample. Details of the spin-crossover transition in the nanoparticles system are discussed in comparison to the one observed in the bulk material.[1] P. Gütlich, P. J. van Koningsbruggen, F. Renz, Struct. Bonding, 107 (2004), 27[2] J. G. Haasnoot, Coord. Chem. Rev., 200 ( 2000) 131[3] E.Coronado, J.R.Galán-Mascarós, M.Monrabal-Capilla, J.García-Martínez, and P.Pardo-Ibáñez, Adv.Mater., 19 (2007) 1359[4] P. Mendoza Zélis, G.A. Pasquevich, A. Veiga, M.B. Fernández van Raap and F.H. Sánchez, Hyperfine Interact., in press.[5] J. Kroeber, J-P. Audiere, R. Claude, E. Codjovi, O. Kahn, J. G. Haasnoot, F. Groliere, C. Jay, A. Bousseksou, Chem.Mater. vol:6, (1994) 1404-1412

enviar mensaje