Spin dynamics in weakly and strongly interacting NiO nanoparticles

Abstract

The spin dynamics of plate-shaped nanoparticles of NiO has been studied by inelastic neutron scattering and Mössbauer spectroscopy. A value of the in-plane anisotropy energy constant significantly larger than the bulk value has been measured. The temperature and field dependence of the energy of the antiferromagnetic resonance mode associated with this in-plane anisotropy has been studied. Both Mössbauer spectroscopy and neutron scattering data show that the magnetic fluctuations are strongly affected by the strength of interparticle interactions.

Spin reorientation inα-Fe2O3nanoparticles induced by interparticle exchange interactions inFrandsen1, Lefmann2, Lebech3 et al. 2011Phys. Rev. B

“…The observation of strong exchange interactions in dried nanoparticle samples suggests that the drying actually brings the particles closely together and that the exact particle arrangement may include some kind of preferred orientation. [8][9][10][11][12][13][14][15]17 Oriented attachment has also been found in transmission electron microscopy studies of larger α-Fe 2 O 3 particles with different morphologies. 19 In this work, we have studied the influence of interactions between nanoparticles of NiO and α-Fe 2 O 3 on the magnetic structure of α-Fe 2 O 3.…”

“…In α-Fe 2 O 3 nanoparticles, the out-of plane anisotropy constant is on the order of 10 4 -10 5 Jm −3 . 21 In 8-nm hematite particles, this corresponds to an anisotropy energy on the order of 200-2000 K. Using the bulk value for the out-of-plane anisotropy constant of NiO (K 1 ∼ = 4.3 × 10 5 Jm −3 ), 33 one finds that for the NiO nanoparticles, the anisotropy energy K 1 V 1 is around 9000 K. 13 Mössbauer studies of samples of strongly interacting pure α-Fe 2 O 3 and pure NiO particles yielded interaction energies J eff M p M q for ensembles of particles on the order of 600 K 17 and 360 K, 13 respectively. Similarly, studies of the α-Fe 2 O 3 /NiO composite 18 indicate interaction energies on the same order of magnitude.…”

“…In samples of antiferromagnetic nanoparticles in close proximity, magnetic interactions can also have a significant influence on the magnetic properties. 7 Antiferromagnetic particles have small magnetic dipole moments and therefore dipole interactions are negligible; yet, Mössbauer studies of antiferromagnetic nanoparticles of hematite (α-Fe 2 O 3 ), [8][9][10][11] NiO, [12][13][14] and ferrihydrite 15 have shown that interparticle interactions between particles, prepared by drying aqueous suspensions, can result in a substantial suppression of the superparamagnetic relaxation. This has been explained by exchange interactions between surface atoms of neighboring particles.…”

Polarized neutron powder diffraction studies of antiferromagnetic order in bulk and nanoparticle NiO

“…The magnetic properties of platelet-shaped NiO nanoparticles have been investigated with magnetization techniques [27][28][29][30][31][33][34][35][36], neutron scattering [17,37], and Mössbauer spectroscopy [17,29,31], revealing that the particles may undergo superparamagnetic relaxation and that they may carry a net magnetic moment. The net magnetic moment in antiferromagnetic nanoparticles is often attributed to uncompensated spins on the surface (or in the interior) of the particles and studies have found the magnetic moment of the particles to be roughly proportional to N 1/3 (N being the number of atomic spins in the particle), consistent with an uncompensated magnetic moment from random occupation of surface sites [31,38].…”

Dynamic rotor mode in antiferromagnetic nanoparticles

“…This displaces the neutron scattering signal from (quasi-)elastic to inelastic, in accordance with experimental observation. We speculate that the rotor mode may be present in other nanoparticles with a strong planar anisotropy, e.g., NiO, although the temperature effect in this system was found to be less pronounced [14].…”