Morphology influence on nanoscale magnetism of Co nanoparticles: Experimental and theoretical aspects of exchange bias

Abstract

Co-based nanostructures ranging from core/shell to hollow nanoparticles were prepared by varying the reaction time and the chemical environment during the thermal decomposition of Co 2 (CO) 8 . Both structural characterization and kinetic model simulation illustrate that the diffusivities of cobalt and oxygen determine the growth ratio and the final morphology of the nanoparticles. Exchange coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of field-cooled hysteresis loops that is proportional to the shell thickness, as verified by numerical studies. The increasing nanocomplexity, when passing from core/shell to hollow particles, also leads to the appearance of hysteresis above 300 K due to an enhancement of the surface anisotropy resulting from the additional spin-disordered surfaces.

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

“…Remarkably, although the Néel temperature of Co 3 O 4 is below 40 K, the loop shifts persist up to ∼250 K or above, demonstrating the robustness of the exchange coupling between core and shell and the persistence of EB effects up to almost room temperature. Previous reports on the EB effect in systems with coexisting Co/Co 3 O 4 phases indicated that a minor presence of CoO led to the persistence of EB much above its T N [24][25][26][27][28][29]. Analogously, persistence of EB above T N was also observed in Co/FeF 2 [56] and was explained by retention of short range magnetic ordering above T N .…”

“…With the increase of the oxide component at the shell, the loop shapes become more elongated with higher closure fields [26,39] and a high field linear component with increasing contribution. This high field susceptibility can be ascribed to the contribution of uncompensated spins in the antiferromagnetic shell and core/shell interface [43][44][45] as it dies off at temperatures higher than T N of Co 3 O 4 .…”

“…The behavior of the three samples is similar, showing irreversibility up to the highest measured temperature of 300 K, which indicates that the nanoparticles have blocking temperatures above this value due to their relatively large size. Note that both curves decrease monotonously below 300 K and do not display any peak characteristic of the Néel temperature of CoO, which should be in the range of 235-293 K depending on the particle size [26,39,40]. However, below ∼40 K, a weak anomaly marked by an upturn of the magnetization can be observed in both the ZFC and FC curves, which could be ascribed to the onset of the antiferromagnetic order [26] in crystallites forming the shell since Co 3 O 4 orders antiferromagnetically around 40 K [41].…”

“…Note that both curves decrease monotonously below 300 K and do not display any peak characteristic of the Néel temperature of CoO, which should be in the range of 235-293 K depending on the particle size [26,39,40]. However, below ∼40 K, a weak anomaly marked by an upturn of the magnetization can be observed in both the ZFC and FC curves, which could be ascribed to the onset of the antiferromagnetic order [26] in crystallites forming the shell since Co 3 O 4 orders antiferromagnetically around 40 K [41]. This is consistent with the significant reduction of the Néel temperature of Co 3 O 4 down to the range of 26-35 K (depending on the particle size) due to finite-size effects [34,42].…”

“…This process can be controlled under proper synthesis conditions to * oscar@ffn.ub.es; http://www.ffn.ub.es/oscar † sspsg2@iacs.res.in prevent further oxidation, leading to the formation of core/shell structures with the oxide phase (often an AF or ferrimagnetic material [22]) usually formed at the outer part of the structures. In the case of Co NP, most of the published studies [23] report the formation of the CoO phase on the shell, although in some cases the presence of the Co 3 O 4 has also been evidenced by structural [24,25] or magnetic characterization [26], The possibility of observing EB in Co based nanostructures in contact with Co 3 O 4 has been rather less investigated and the published studies focus on layered structures [27][28][29][30][31]. Synthesis of single phase CoO or Co 3 O 4 NP have been achieved by several authors, that have reported AF magnetic behavior with ordering temperatures reduced compared to the bulk values [32][33][34] and remanence values much higher than those for the bulk due to finite-size effects [26,35].…”