Dynamic rotor mode in antiferromagnetic nanoparticles

Abstract

We present experimental, numerical, and theoretical evidence for an unusual mode of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering experiments on 8-nm particles of hematite display a loss of diffraction intensity with temperature, the intensity vanishing around 150 K. However, the signal from inelastic neutron scattering remains above that temperature, indicating a magnetic system in constant motion. In addition, the precession frequency of the inelastic magnetic signal shows an increase above 100 K. Numerical Langevin simulations of spin dynamics reproduce all measured neutron data and reveal that thermally activated spin canting gives rise to an unusual type of coherent magnetic precession mode. This "rotor" mode can be seen as a high-temperature version of superparamagnetism and is driven by exchange interactions between the two magnetic sublattices. The frequency of the rotor mode behaves in fair agreement with a simple analytical model, based on a high-temperature approximation of the generally accepted Hamiltonian of the system. The extracted model parameters, such as the magnetic interaction and the axial anisotropy, are in excellent agreement with results from Mössbauer spectroscopy.

Spin waves across three-dimensional, close-packed nanoparticles

“…Inelastic magnetic neutron scattering can easily penetrate deep within materials, including 3D ordered arrays and powders, yet is an intensity-limited technique, and its usage in nanoparticle systems has been infrequent. For example, inelastic neutron scattering has been successfully applied to measure magnetocrystalline anisotropy within the nanoparticles [19,20] and to observe collective antiferromagnetic (AFM) spin precession within individual nanoparticles [20][21][22]. Here we use inelastic neutron scattering to demonstrate that magnetic spin waves form Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.…”

Confined Magnons

“…3,4 This raises the question whether magnons in larger "mesoscopic" systems such as magnetic nanoparticles and fewmonolayer thin films can be fully described with the magnon picture. Comparison between theory and experiment in magnetic structures such as nanoparticles [5][6][7] is quite difficult due to a variety of finite size effects including large surface to volume ratio, lower symmetry, and size/shape distribution. 8,9 As a result, there is a notable gap in our understanding of magnetic excitations in nanostructures and related confined magnetic systems.…”