Effect of shell thickness on the exchange bias blocking temperature and coercivity in Co-CoO core-shell nanoparticles

Abstract

The exchange bias blocking temperature distribution of naturally oxidized Co-CoO core-shell nanoparticles exhibits two distinct signatures. These are associated with the existence of two magnetic entities which are responsible for the temperature dependence of an exchange bias field. One is from the CoO grains which undergo thermally activated magnetization reversal. The other is from the disordered spins at the Co-CoO interface which exhibits spin-glass-like behavior. We investigated the oxide shell thickness dependence of the exchange bias effect. For particles with a 3 nm thick CoO shell, the predominant contribution to the temperature dependence of exchange bias is the interfacial spin-glass layer. On increasing the shell thickness to 4 nm, the contribution from the spin-glass layer decreases, while upholding the antiferromagnetic grain contribution. For samples with a 4 nm CoO shell, the exchange bias training was minimal. On the other hand, 3 nm samples exhibited both the training effect and a peak in coercivity at an intermediate set temperature Ta. This is explained using a magnetic core-shell model including disordered spins at the interface.

Investigating Exchange Bias and Coercivity in Fe3O4–γ-Fe2O3 Core–Shell Nanoparticles of Fixed Core Diameter and Variable Shell Thicknesses

“…For core–shell nanoparticles with good core–shell contact, less (or no) concentrations of interfacial defects might occur with the absence of interfacial SGCs, leading to a different behavior of exchange bias field and coercivity with temperature and field cooling. It is worth mentioning that the amount of the interfacial SGCs can be experimentally determined using X-ray magnetic circular dichroism (XMCD) [28,46,47]. Another method that was applied to layered FM–AFM structures uses the so-called blocking temperature distribution from magnetic hysteresis loops at several conditions [34,35].…”

“…In the case of core–shell nanoparticles, the interface atoms have a different environment than those in the core of the particle. The exchange bias is suggested to be influenced by several effects, such as the area of the core–shell interface [26], the roughness of the core–shell interface [27], the thickness of the shell [28], and the competition between the magnetostatic anisotropy and the exchange coupling [27]. The magnetic properties of Fe 3 O 4 –γ-Fe 2 O 3 core–shell nanoparticles are not well-investigated.…”

Core Size and Interface Impact on the Exchange Bias of Cobalt/Cobalt Oxide Nanostructures

“…Magnetic nanoparticles (NPs) are attracting growing attention in both basic research and industry [1][2][3][4][5][6]. In particular, the study of magnetic nanostructures comprising two or more different magnetic materials has achieved considerable progress [1,2,[7][8][9][10].…”

“…One of the key parameters governing EB is the FM/AFM volume ratio [1,2,18], as the interfacial nature of the effects imposes a dependence of the EB field with the reciprocal of the FM volume [1]. Exchange bias has been extensively studied in thin films, where the FM and AFM layer thicknesses, and even the interface quality, can be readily controlled [1,2,5,7,11,12,24]. In bilayer films, the EB field has been shown to be proportional to the AFM thickness up to a critical value, above which H E remains constant or decreases slightly [1,2,7,25,26].…”

“…Das et al discussed that in order to optimize EB properties, it is necessary to have irregular or defective interfaces in core/shell structures between Co and CoO [32]. Some studies have estimated a minimum AFM shell thickness required to stabilize the FM core moment and, thus, enhance blocking temperatures [5,33,34]. Feygenson et al found that the EB field does not depend monotonically on the CoO shell thickness, with the highest H E value corresponding to particles with Co core and CoO shell of similar volume [27].…”