Abstract
We have carried out extensive measurements on novel Fe3O4–γ-Fe2O3 core–shell nanoparticles of nearly similar core diameter (8 nm) and of various shell thicknesses of 1 nm (sample S1), 3 nm (sample S2), and 5 nm (sample S3). The structure and morphology of the samples were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The direct current (DC) magnetic measurements were carried out using a superconducting quantum interference device (SQUID). Exchange bias and coercivity were investigated at several temperatures where the applied field was varied between 3 and −3 T. Several key results are obtained, such as: (a) the complete absence of exchange bias effect in sample S3; (b) the occurrence of nonconventional exchange bias effect in samples S2 and S1; (c) the sign-change of exchange bias field in sample S2; (d) the monotonic increase of coercivity with temperature above 100 K in all samples; (e) the existence of a critical temperature (100 K) at which the coercivity is minimum; (f) the surprising suppression of coercivity upon field-cooling; and (g) the observation of coercivity at all temperatures, even at 300 K. The results are discussed and attributed to the existence of spin glass clusters at the core–shell interface.
Highlights
Magnetic nanoparticles (MNPs) are of great interest due to their energy and bio-medical applications [1,2,3,4,5,6,7,8,9], e.g., data storage systems
We have investigated the role of temperature and shell thickness on the coercivity and exchange bias of Fe3O4–γ-Fe2O3 nanoparticles with nearly the same core size but with three different shell thicknesses
The core was fixed at 8 nm in our core–shell nanoparticles, the shell thickness was varied, which results in varied stress values on core leading to different interfacial defects and, different quantities of spin-glass clusters
Summary
Magnetic nanoparticles (MNPs) are of great interest due to their energy and bio-medical applications [1,2,3,4,5,6,7,8,9], e.g., data storage systems. A large number of studies reported and discussed the exchange bias in bilayer and multilayer thin films [16,17], nanoparticles with core–shell structures [18,19,20], and particles dispersed in matrix [21]. It was suggested that surface effects in magnetic nanoparticles induce exchange bias [22] Due to their small size, a large portion of atoms in nanoparticles are surface atoms [23]. Many studies reported the exchange bias effect in core–shell FM–AFM nanoparticles. Most of these studies investigated the conventional exchange bias which occurs when TC of the FM core is larger than Néel temperature (TN) of the AFM shell. Very few studies were conducted on FIM–FIM core–shell nanoparticles [32,33]
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