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

Abstract

Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with a total average diameter around ∼27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co 3 O 4 crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias H E and coercive H C fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both phases. The EB effects persist up to temperatures above the ordering temperature of the oxide shell. Results of our atomistic Monte Carlo simulations of particles with the same size and composition as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal, giving rise to the observed maximum in H E and H C .

Maximizing Zero-Field-Cooled Exchange Bias in Crystallized Co@CoO Nanocluster Assembled Thin Film by Varying Film Thickness

“…As shown by the blue line, the core−shell Co@CoO nanoparticles are similar to previously reported cases. 21,22 Moreover, the coexistence of crystallized Co and CoO was also proved by XRD shown in Figure 2. With the increase of film thickness, the intensity of both Co and CoO increases, which means more and more crystalline phase is formed on the matrix.…”

Magnetic nanoparticles: From the nanostructure to the physical properties

“…At the same time, they lead to the emergence of enriched physico-chemical properties, which distinguish them from their bulk counterparts. Among different classes of nanomaterials, core/shell structures are fundamentally interesting because of carrying two different physico-chemical properties in one single particle at the nanoscale [6]. Though primarily core/shell structures were synthesized to protect and stabilize the metallic core [7], advances in materials fabrication and synthesis have made core/shell structures potential candidates for a myriad of new applications including targeted drug delivery [8], biomedical sensors [9], enhanced electronic properties [10] or EB effect [6].…”

“…Among different classes of nanomaterials, core/shell structures are fundamentally interesting because of carrying two different physico-chemical properties in one single particle at the nanoscale [6]. Though primarily core/shell structures were synthesized to protect and stabilize the metallic core [7], advances in materials fabrication and synthesis have made core/shell structures potential candidates for a myriad of new applications including targeted drug delivery [8], biomedical sensors [9], enhanced electronic properties [10] or EB effect [6]. If the core and shell are composed of two materials with different magnetic orders, the interfacial region may experience a structural modification due to differences in the crystalline structures of both regions as well as a competition between the different magnetic orders favored at the core and shell.…”

“…In this regard, tuning EB-related properties by controlling variation of size [27], the thickness of core and shell [11], interparticle interactions [28] and microscopic structure of the interface of any magnetically inhomogeneous system [20] might add significant value in several application-oriented phenomena. Attempts have been made to understand the underlying physics of the EB mechanism when the variation of shape [29], size [14], surface composition [30], core to shell diameter ratio come into play [6], in a core/shell nanostructure. Recently, we have reported correlating experimental findings and atomistic Monte Carlo (MC) simulations showing that the variation of core and shell thickness of Co-Co 3 O 4 nanostructure leads to systematic changes in the EB effect [6].…”

Surface Electronic States Induced High Terahertz Conductivity of Co3O4 Microhollow Structure

“…This significantly demands a better understanding of d-electron behavior, chemical bonding, and interfacial charge transfer properties of with Co 2+ (S = 3/2) and Co 3+ (S = 0) ions placed at the tetrahedral (A) and octahedral (B) sites, respectively, where the magnetic moments of Co 2+ ions order antiferromagnetically below Neél transition temperature (T N ). 17 Density functional theory calculations 18 suggest that the conduction band of Co 3 O 4 is mainly composed of Co 2+ t 2g minority spin orbitals and Co 3+ e g orbitals, whereas the valence band has contributions from filled d orbitals of both cobalt oxidation states and 2p orbitals of oxygen anion with some extent of hybridization between them. This indicates its approximate Mott−Hubbard type insulating nature with a small band gap of ∼0.8 eV.…”