Surface effects on the magnetic properties of ultrafine cobalt particles

Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: Influence of the surface and of interparticle interactions

“…Our experimental results indicate then that there exists a rather large contribution of the surface of the particles to the net anisotropy. The enhancement of the magnetic anisotropy of nanometer sized metallic particles with respect to the bulk has been previously reported by several authors 8,9,24 . For Co particles with diameters varying between 4.4 and 1.8 nm, Chen and coworkers 9 obtained K ef f which increases from about 5 × 10 6 erg/cm 3 to about 3 × 10 7 erg/cm 3 .…”

“…This dependence is opposite to that observed for free Co clusters in beams 7 and also for Co particles of similar size supported in a solid matrix. 9,24 In those experiments, the measured magnetic moment per atom exceeded the bulk value and it was found to increase as the diameter of the particles decreases.…”

“…However, they are of the same order as the values that would have been obtained if we had used the temperature of the maximum of the ZFC susceptibility, as it was done by the authors. More recently, Respaud et al 24 studied the anisotropy of Co particles of 1.5 and 1.9 nm by fitting the whole ZFC and FC magnetization curves, a method that can be considered as equivalent to ours. They found K ef f ≃ 8.3 × 10 6 erg/cm 3 and K ef f ≃ 7.3 × 10 6 erg/cm 3 , respectively, in reasonably good agreement with our data.…”

Determination of the blocking temperature of magnetic nanoparticles: The good, the bad, and the ugly

“…Some authors maintain the IP criterion, 10 while others report the maximum ZFC magnetization temperature (MAX); 11,12 both criteria are still under discussion. [13][14][15] In an alternative approach, Micha et al 16 propose a method in which the T B distribution is obtained from the T derivative of the difference between ZFC and FC curves. An approximated theoretical justification for this method was presented by Mamiya et al 17 In this work, a SW model with thermal agitation is applied to obtain the temporal dependence of the magnetization M(t) of an ordered system of identical MNPs in a similar way to the previous works of Lu, 18 Usov,19 and Carrey.…”

Organometallic Nanoparticles of Metals or Metal Oxides

“…Interestingly, and somewhat surprisingly, these particles were demonstrated to display the same magnetic properties as particles of same size prepared in ultra high vacuum in the gas phase by time-of-flight experiments [61]. Thus the particles are superparamagnetic with blocking temperatures near 10 K and display an enhanced magnetization at saturation per cobalt atom compared to bulk cobalt [62]. Addition of O 2 , pyridine, isocyanides or CO leads to a dramatic decrease of the magnetic properties of the particles.…”