Size effect in spin-crossover systems investigated by FORC measurements, for surfacted [Fe(NH_2-trz)_3](Br)_2?3H_2O nanoparticles: Reversible contributions and critical size

摘要

We investigated the thermal transition of coated nano-particles of the title compound, on a set of samples of average diameter 〈d〉 ～ 30, 50, 70, 110 nm, with rather broad size distributions. As expected, the width of the major hysteresis loop was an increasing function of 〈d〉. We recorded first-order reversal curves (FORC), the initial parts of which displayed a finite slope, revealing the presence of reversible contributions expected from particles smaller than the critical size d_C associated with the collapse of the hysteresis loop. Kinetic effects were also evidenced thanks to isothermal stages. Reversibility of the FORC curves at the vicinity of the reversal temperature was controlled. Thanks to the reversibility property we could determine the reversible contributions to the total response of all samples and derive the corresponding d_C values. Consistent results were obtained by accounting for an anhysteretic contribution from the large particles, leading to an accurate determination d_C ～ 45-50 nm, much better than the width of the size distributions.