Novel Ni(II)-chromia based core-shell nanoparticles have room-temperature ferromagnetic properties

Significance 

Core-shell nanoparticles (CSNs) have been seen to offer a high degree of tunability of magnetic and other physiochemical properties through the adjustment of core and shell size, morphology, chemical content, and structure at the atomic scale, owing to their composite nature. Recent publications have highlighted that room-temperature ferro- or ferrimagnetism, along with the exchange bias effect, is an essential element of bimagnetic core-shell nanoparticles for many future applications, including in magnetic media storage, magnetic devices and biomedical therapy and diagnostics. One way of achieving room-temperature ferromagnetism/ferrimagnetism and sufficient exchange bias effect for therapeutic and device applications may be through development of novel bimagnetic CSNs that incorporate metastable phases. Additionally, α-chromia has well known magnetoelectric properties that hold considerable promise for spintronic device applications. Unfortunately, despite vigorous research activity in the field, the number of reports of synthesis of bimagnetic core-shell nanoparticles having room-temperature ferromagnetic/ferrimagnetic properties is quite small.

Recently, Missouri State University researchers led by Dr. Robert Mayanovic synthesized bimagnetic core–shell nanoparticles containing a first-of-its-kind Ni(II)-chromia nanophase shell and a well-defined, epitaxial core–shell interface. In addition, they carried out in depth experimental characterization on the corundum-structured Ni(II)-chromia nanophase metastable compound constituting the shell of the CSNs. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.

The research method used by the scientists entailed explicit imitation of mineral zonation that occur naturally under hydrothermal conditions, which enabled them to grow an epitaxial highly structurally ordered nanophase encapsulating α-chromia nanocrystalline core to form the desired inverted CSNs. This entailed a two-step process where the first step involved synthesis of the chromia nanoparticles following the Farzaneh et al procedure and the second step involved the use of their hydrothermal nanophase epitaxy (HNE) method. They then undertook X-ray diffraction analysis, transmission electron microscopy imaging and energy-dispersive X-ray spectroscopic analyses of the nanoparticles. Lastly, the researchers engaged in first principles calculations where they used the local spin density approximation method implemented in Quantum Espresso to investigate the spin-spin interactions and electronic structure properties of Ni(II)-chromia.

The authors observed that the magnetic measurements undertaken revealed a substantial coercivity of the nanoparticles and a significant exchange bias effect between the antiferromagnetic chromia core and the ferromagnetic Ni(II)-chromia shell at low temperatures. Additionally, the ferromagnetism and a weak exchange bias effect were found to persist to room temperature in the core–shell nanoparticles of ~57 nm average size. The first principles Density Functional Theory calculations confirmed that the novel corundum-structured Ni(II)-chromia phase had an equilibrium cluster-localized ferromagnetic spin configuration.

In a nutshell, the study presented the successful synthesis of the novel corundum-structured Ni(II)-chromia inverted core–shell nanoparticles using the HNE method. Generally, the calculations undertaken showed that the Ni(II)-chromia is a Mott–Hubbard insulator. Altogether, the structural selectivity exhibited by the HNE technique employed in their study should be useful for synthesis of target-specific, metastable nanomaterials having enhanced properties.

Reference

D. Hossain, R. A. Mayanovic, S. Dey, R. Sakidja, M. Benamara. Room-temperature ferromagnetism in Ni(II)-chromia based core–shell nanoparticles experiment and first principles calculations. Physical Chemistry Chemical Physics, 2018, volume 20, page 10396.

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