Simulation of ferromagnetic resonance spectra of linear chains of magnetite nanocrystals

Abstract

Ensembles of linear chains of stable single domain magnetite crystals, as found in magnetotactic bacteria, exhibit a distinctly asymmetric ferromagnetic resonance (FMR) signal, with a pronounced high-field minimum and two or three low-field maxima in the derivative spectrum. To identify the microscopic origin of these traits, we have simulated FMR spectra of dilute suspensions of linear chains oriented randomly in space by modeling the chain as a Stoner−Wohlfarth-type rotation ellipsoid whose long axis coincides with an easy [111] axis of the cubic magnetocrystalline anisotropy system. The validity of the model is examined by comparing the results with explicit calculations of the interactions among the particles in the chain. The single ellipsoid model reproduces the experimentally observed FMR traits and can be related to the explicit chain model by adjusting the contribution to the uniaxial anisotropy along the chain axis to account for the magnetostatic interactions. Finally, we provide a practical approximation for simulating and fitting the FMR spectra of one-dimensional assemblies.

S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils

“…Using these results, an FMR signal at S-band can be simulated by considering the relative intensity at each field orientation. Details about the simulation are reported in the study of Charilaou et al [37]. Figure 1b presents such S-band spectrum with distinctly separated features, i.e.…”

“…In another simulation, the effect of uniaxial anisotropy field B uni on the spectral response is tested using the same model [37]. As shown in figure 2, B uni generated by the rsif.royalsocietypublishing.org J R Soc Interface 10: 20120790 chain configuration of magnetic particles determines the width of the FMR signal of the ensemble, i.e.…”

“…contributions must also be taken into consideration when interpreting these spectra, because with decreasing temperature the superparamagnetic magnetosomes become blocked and contribute to the total anisotropy and also to the total magnetization of the sample [11,13,37]. Below 120 K, the disappearance of a clear low-field absorption feature and the high-field shoulder is mainly affected by the Verwey transition (T V ).…”

“…Depending on the intrinsic anisotropy fields in the system and the strength of the external field, the magnetization vector will dwell in an equilibrium orientation. The equilibrium angle of the magnetization for a certain FMR signal can be calculated by minimizing the free energy density and solving the resonance equation following an iterative procedure [37]. Figure 1c shows an example of this calculation considering a system with uniaxial anisotropy field B uni ¼ 80 mT and a cubic anisotropy field of B cub ¼ 2 23.5 mT, values typical for magnetite chains of Magnetospirillum gryphiswaldense [37].…”

“…The equilibrium angle of the magnetization for a certain FMR signal can be calculated by minimizing the free energy density and solving the resonance equation following an iterative procedure [37]. Figure 1c shows an example of this calculation considering a system with uniaxial anisotropy field B uni ¼ 80 mT and a cubic anisotropy field of B cub ¼ 2 23.5 mT, values typical for magnetite chains of Magnetospirillum gryphiswaldense [37]. Figure 1c shows the equilibrium polar angle of the magnetization u as a function of the magnetic field angle u B .…”