SIZE-DEPENDENT MAGNETIZATION FLUCTUATIONS IN NiO NANOPARTICLES

Abstract

The finite size and surface roughness effects on the magnetization of NiO nanoparticles is investigated. A large magnetic moment arises for an antiferromagnetic nanoparticle due to these effects. The magnetic moment without the surface roughness has a nonmonotonic and oscillatory dependence on R, the size of the particles, with the amplitude of the fluctuations varying linearly with R. The geometry of the particle also matters a lot in the calculation of the net magnetic moment. An oblate spheroid shape particle shows an increase in net magnetic moment by increasing oblateness of the particle. However, the magnetic moment values thus calculated are very small compared to the experimental values for various sizes, indicating that the bulk antiferromagnetic structure may not hold near the surface. We incorporate the surface roughness in two different ways; an ordered surface with surface spins inside a surface roughness shell aligned due to an internal field, and a disordered surface with randomly oriented spins inside surface roughness shell. Taking a variational approach we find that the core interaction strength is modified for nontrivial values of ∆ which is a signature of multi-sublattice ordering for nanoparticles. The surface roughness scale ∆ is also showing size dependent fluctuations, with an envelope decay ∆ ∼ R −1/5 . The net magnetic moment values calculated using spheroidal shape and ordered surface are close to the experimental values for different sizes.

Comparison of Nanoscaled and Bulk NiO Structural and Environmental Characteristics by XRD, XAFS, and XPS

“…NiO nanoparticles have been characterized by a variety of methods, − and they have been shown to possess interesting magnetic, catalytic, and sensing properties. Because of the robust nature of the NiO rocksalt lattice, the particles typically retain the bulk symmetry and stoichiometry over a wide range of sizes and dopant concentrations.…”

NiO–Ti nanocomposites for contact electrification and energy harvesting: experimental and DFT+U studies

“…It has also been reported that, as the crystallite size reduces to the nano-scale, there is an enhancement of the ferromagnetic nature due to the exchange interaction between the uncompensated magnetic surface spins and the antiferromagnetic core spins. 54 The non-zero magnetic moment in a nano-complex is also due to oxygen vacancies. So, first-principle calculations were performed by taking away one O atom from the NiO–Ti complex.…”