Spin waves across three-dimensional, close-packed nanoparticles

Abstract

Inelastic neutron scattering is utilized to directly measure inter-nanoparticle spin waves, or magnons, which arise from the magnetic coupling between 8.4 nm ferrite nanoparticles that are self-assembled into a close-packed lattice, yet are physically separated by oleic acid surfactant. The resulting dispersion curve yields a physically-reasonable, non-negative energy gap only when the effective Q is reduced by the inter-particle spacing. This Q renormalization strongly indicates that the dispersion is a collective excitation between the nanoparticles, rather than originating from within individual nanoparticles. Additionally, the observed magnons are dispersive, respond to an applied magnetic field, and display the expected temperature-dependent Bose population factor. The experimental results are well explained by a limited parameter model which treats the three-dimensional ordered, magnetic nanoparticles as dipolar-coupled superspins.

Using small-angle scattering to guide functional magnetic nanoparticle design

“…MIEZE can further play a crucial role in the investigation of magnons in MNP systems. Such collective magnetic excitations were predicted by Krawczyk and Puszkarski 366 and have rst been experimentally detected by Krycka et al 367 using the tripleaxis spectrometer BT7 at NIST. 368 Tartakovskaya et al 369 describe theoretically the dispersion of spin waves in two-and threedimensional MNP systems by calculations based on a linear Fig.…”

“…Additionally, within densely packed MNP assemblies, spin-waves may form due to a dipolar coupling between the precessing spins. 367 The expected time and length scales are accessible with MIEZE-SANS and should be investigated similar to studies performed on bulk ferromagnets. 370 Finally, we want to mention another small-angle scattering technique that may be useful for future MNP studies.…”

Using small-angle scattering to guide functional magnetic nanoparticle design

“…MIEZE can further play a crucial role in the investigation of magnons in MNP systems. Such collective magnetic excitations were predicted by Krawczyk and Puszkarski 366 and have first been experimentally detected by Krycka et al 367 , using the tripleaxis spectrometer BT7 (NIST 368 ). Tartakovskaya et al 369 describe theoretically the dispersion of spin waves in two-and three-dimensional MNP systems by calculations based on a linear combination of atomic orbitals (LCAO) model taking into account dipolar interactions.…”

Magnetic structure factor of correlated moments in small-angle neutron scattering

“…2(c). However, the upturn of the Fourier coefficient h 2 (q) of the randomperturbation field at small q indicates a non-zero effective magnetocrystalline anisotropy of the cluster (preferred local orientation), or, alternatively, collective thermal excitations 67,68 . Finally, it can be noted that M 2 z (q) and h 2 (q) are of the same magnitude, which demonstrates that the fluctuations in the longitudinal magnetization have comparable strength as the random-perturbing field of the crystallites, and hence the same influence on the magnetic energy of the system.…”