Narrow Results

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 Δ which is a signature of multi-sublattice ordering for nanoparticles. The surface roughness scale Δ is also showing size-dependent fluctuations, with an envelope decay Δ~R^-1/5. The net magnetic moment values calculated using spheroidal shape and ordered surface are close to the experimental values for different sizes.

In an earlier work, it was suggested that the “magnetic sense” in birds may be mediated by the blue light receptor protein, cryptochrome, which is known to be localized in the retinas of migratory birds. The present contribution discusses an alternative mechanism of avian magnetoreception based on the interaction of two iron minerals (magnetite and maghemite), recently found in subcellular compartments within the sensory dendrites of the upper beak of several bird species. The analysis of forces acting between the iron particles shows that the orientation of the external geomagnetic field can significantly change the probability of the mechanosensitive ion channels opening and closing. Our theoretical analysis shows that the suggested magnetoreceptor system might be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.