Abstract
Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to their different potential application and due to the tunability of their magnetic properties. The antiferromagnetic nature of NiO and high Néel temperature (523 K) makes this material well suited for inverse core-shell nanoparticle applications. Our primary objective in this project has been to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixCo1-xO (FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixCo1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixCo1-xO CSNs have the rock salt cubic crystal structure. SEM-EDS data indicates the presence of Co in the CSNs. Magnetic measurements show that the CSNs exhibit AFM/FiM characteristics with a small coercivity field of 30 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a magnetization axis shift which is attributed to the exchange bias effect between the AFM NiO core and an FiM NixCo1-xO shell of the CSNs. Our ab initio based calculations of the NixCo1-xO rock salt structure confirm a weak FiM character and a charge transfer insulator property of the compound.
Highlights
Bimagnetic core-shell nanoparticles (CSNs) are of considerable interest due to their potential applications in magnetic spin valves, spintronics, magnetic random access memory, hyperthermia, MRI imaging, drug delivery and in other areas.[1,2,3,4] The magnetic properties of bimagnetic CSNs are tuned by adjustment of overall size, core vs shell size, core vs shell composition, and morphology
There has been particular attention paid to the exchange bias effect in bimagnetic CSNs, whereby a magnetic interaction between the core and shell has a direct bearing on the overall magnetic properties of the nanostructures.[5,6]
We report on the synthesis and characterization of inverted NiO@NixCo1-xO CSNs, where the core-shell nanostructure was made using our hydrothermal nanophase epitaxy method.[7,8,9]
Summary
Bimagnetic core-shell nanoparticles (CSNs) are of considerable interest due to their potential applications in magnetic spin valves, spintronics, magnetic random access memory, hyperthermia, MRI imaging, drug delivery and in other areas.[1,2,3,4] The magnetic properties of bimagnetic CSNs (without being embedded in a surrounding matrix) are tuned by adjustment of overall size, core vs shell size, core vs shell composition, and morphology. Core-shell nanoparticles are typically fabricated in a conventional configuration, having a ferromagnetic (FM) core and an antiferromagnetic (AFM) or ferrimagnetic (FiM) shell, or in an inverted configuration, having an AFM core and a FM or FiM shell Due to their highly ordered AFM cores, the inverted bimagnetic CSNs have highly tunable coercivities, blocking temperatures, and other magnetic properties that are highly promising for device and medicinal applications. There has been particular attention paid to the exchange bias effect in bimagnetic CSNs, whereby a magnetic interaction between the core and shell has a direct bearing on the overall magnetic properties of the nanostructures.[5,6] By suitable tuning of the CSNs in terms of composition, size and morphology characteristics alluded to above, the exchange bias properties can potentially be exploited for various magnetic device and medicinal applications
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