Size-dependent magnetization fluctuations in NiO nanoparticles

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 non-monotonic and oscillatory dependence on R, the size of the particles, with the amplitude of the fluctuations varying linearly with R. 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 by assuming that a net magnetic moment from core spins polarizes spins near the surface over a length scale ∆. Applying a variational approach we find that the core interaction strength is modified for nontrivial values of ∆. The surface roughness scale ∆ is also showing size dependent fluctuations, with an envelope decay ∆ ∼ R−1/3. The net magnetic moment values calculated with the surface roughness effect are close to the experimental values for different sizes.