Implications of Room Temperature Oxidation on Crystal Structure and Exchange Bias Effect in Co/CoO Nanoparticles

Abstract

We describe here how the exchange bias effect in Co/CoO nanoparticles depends on the size focusing and temperature treatment of precursor Co nanoparticles before oxidation at ambient conditions. By appealing to magnetization, microscopy, neutron, and synchrotron X-ray measurements, we found that as-synthesized Co nanoparticles readily oxidize in air only after 20 days. The highest exchange bias field of 814 Oe is observed at T = 2 K. When the same nanoparticles are centrifuged and annealed at 70 °C under vacuum prior to oxidation, the exchange bias field is increased to 2570 Oe. Annealing of Co nanoparticles in vacuum improves their crystallinity and prevents complete oxidation, so that Co-core/CoO-shell structure is preserved even after 120 days. The crystal structure of CoO shell in both samples is different from its bulk counterpart. Implications of such distorted CoO shells on exchange bias are discussed. Coating of Co nanoparticles with amorphous silica shell makes them resistant to oxidation but ultimately modifies the crystal structure of both Co core and SiO 2 shell.

Asymmetric Hysteresis for Probing Dzyaloshinskii–Moriya Interaction

“…Hence, we can conclude that the measured asymmetric hysteresis loops are purely originating from the presence of a DMI-induced tilt of the local magnetization, making these shifted loops a unique and straightforward fingerprint for the existence of the DMI. Moreover, we like to emphasize that the possibility of an oxidation-induced exchange bias effect to explain our data is excluded: it has been experimentally demonstrated that, at the interface between Co/AlOx, over-oxidized Co may lead to an antiferromagnetic CoO and exhibit asymmetric hysteresis loops 49 . Although the shift in hysteresis loop from the exchange bias could occur in our system, it should be irrespective of the in-plane field and the shape of the microstructure, i.e., this would show up as a shifted hysteresis for Hx = 0 and for all geometries including square patterns.…”

Interface Modeling Leading to Giant Exchange Bias from the CoO/CoFe2O4 Quantum Dot Heterostructure

“…It has been shown that it is possible to tune the EB by tuning the core size and shell thickness by controlled synthesis giving more insights into this field. , Moreover, using metal nanoparticles as a seed, various core–shell heterostructures have been reported such as Co/CoO, , Ni/NiO, Mn/Mn 3 O 4 , or metal oxide with different oxidation states such as Mn 3 O 4 /MnO . Systems using different metal oxides such as Fe 3 O 4 /Co, Fe 3 O 4 /FeO, Fe 3 O 4 /CoO, Fe 2 O 3 /CoO, MnO/Mn 3 O 4 , Cr 2 O 3 /CrO 2 , BiFeO 3 /CoFe 2 O 4 , and even Fe/Cr have also been reported.…”

“…It has been shown that it is possible to tune the EB by tuning the core size and shell thickness by controlled synthesis giving more insights into this field. 21,31 Moreover, using metal nanoparticles as a seed, various core−shell heterostructures have been reported such as Co/CoO, 12,32 Ni/NiO, 33 40 and even Fe/Cr 41 have also been reported. Additionally, transition-metal-based oxides with various core−shell morphologies show tunability in magnetic response also in accordance with their internal structure.…”

Enhancement of Magnetization through Interface Exchange Interactions of Confined NiO Nanoparticles within the Mesopores of CoFe2O4

“…3,4,7 A majority of the AFM phases are oxides of FM transition metals, and the disordered oxide shell is due to uncontrolled oxidation of the FM core. 8,9 In addition to that, nanocomposites have been designed with randomly distributed counterparts to demonstrate the EB effect. 10−13 However, nanostructures suffer from disordered AFM/FM interfaces which result in a large fraction of uncompensated spins at the interface, thus reducing the EB and hence lowering the induced anisotropy as compared to thin films and multilayers.…”

“…Most of the NPs demonstrating EB involve systems with a FM core and AFM shell, where T C > T N according to the Meiklejohn and Bean paradigm. ,, A majority of the AFM phases are oxides of FM transition metals, and the disordered oxide shell is due to uncontrolled oxidation of the FM core. , In addition to that, nanocomposites have been designed with randomly distributed counterparts to demonstrate the EB effect. − However, nanostructures suffer from disordered AFM/FM interfaces which result in a large fraction of uncompensated spins at the interface, thus reducing the EB and hence lowering the induced anisotropy as compared to thin films and multilayers. ,− In the past decade, there have been efforts to create inverted core–shell NPs where the AFM core was shelled with FM or FiM materials. ,,− The basic idea was to develop biphasic materials where the EB will show nonmonotonic dependence on the core diameter. These materials also exhibit large H C and spontaneous FM/FiM order.…”