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. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.