Michalis Charilaou

Simulation of ferromagnetic resonance spectra of linear chains of magnetite nanocrystals

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 ap...

Non-linear alignment dynamics in suspensions of platelets under rotating magnetic fields

Under rotating magnetic fields, micron-sized platelets suspended in a fluid and decorated with magnetic nanoparticles are found to assume two orientational states. This behavior is very attractive for the development of unusual reinforcement architectures in synthetic composites. However, it is highly dependent on the frequency of the magnetic field and the rheological properties of the fluid. At low frequencies or fluid viscosities, the magnetized platelets continuously rotate in the fluid. At high frequencies and fluid viscosities, a non-linear response is observed in which the platelets align parallel to the plane of the rotating field. In this study we offer a theoretical description and experimental verification of this phenomenon, which can be used to build composites with fully aligned platelet reinforcement.

Evolution of magnetic anisotropy and thermal stability during nanocrystal-chain growth

We compare measurements and simulations of ferromagnetic resonance spectra of magnetite nanocrystal-chains at different growth-stages. By fitting the spectra, we extracted the cubic magnetocrystalline anisotropy field and the uniaxial dipole field at each stage. During the growth of the nanoparticle-chain assembly, the magnetocrystalline anisotropy grows linearly with increasing particle diameter. Above a threshold average diameter of D ≈ 23 nm, a dipole field is generated, which then increases with particle size and the ensemble becomes thermally stable. These findings demonstrate the anisotropy evolution on going from nano to mesoscopic scales and the dominance of dipole fields over crystalline fields in one-dimensional assemblies.

Magnetic anisotropy of non-interacting collinear nanocrystal-chains

The magnetic anisotropy of linear chains of spherical magnetite nanocrystals was investigated by means of angle-resolved ferromagnetic resonance spectroscopy, in order to determine the different anisotropy contributions. The linear assembly of nanocrystals generates an interaction-induced uniaxial anisotropy, which is nearly an order of magnitude stronger than the intrinsic magnetocrystalline anisotropy of magnetite, and can only exist in magnetic nano-chains, where the easy axes of the nanocrystals are collinear.

Magneto-electronic coupling in modulated defect-structures of natural Fe1-xS

We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe7S8, a major magnetic remanence carrier in the Earths crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T < 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magneto-resistance.

S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils.

We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic properties of magnetite particles in chains of cultured intact magnetotactic bacteria (MTB) between 300 and 15 K with those of sediment samples of Holocene age in order to infer the presence of magnetofossils and their preservation in a geological time frame. The spectrum of intact MTB at 300 K exhibits distinct uniaxial anisotropy because of the chain alignment of the cellular magnetite particles and their easy axes. This anisotropy becomes less pronounced upon cooling and below the Verwey transition (TV) it is nearly vanished mainly owing to the change of direction of the easy axes. In a natural sample, magnetofossils were detected by uniaxial anisotropy traits similar to those obtained from cultured MTB above TV. Our comparative study emphasizes that indispensable information can be obtained from S-band FMR spectra, which offers even a better resolution than X-band FMR for discovering magnetofossils, and this in turn can contribute towards strengthening our relatively sparse database for deciphering the microbial ecology during the Earths history.

Skyrmion oscillations in magnetic nanorods with chiral interactions

We report that in cylindrical nanorods skyrmion chains, i.e., three-dimensional spin textures, emerge dynamically, as revealed by micromagnetic simulations. The skyrmion-chain state occurs when the diameter of the rod is larger than the helical pitch length of the material, and the number of skyrmions on the chain is proportional to the length of the nanorod. This finding provides a deeper understanding of the interplay between geometry and skyrmionic symmetry, and shows how spatial confinement can stabilize spontaneous topological spin textures.

Ordered defects in Fe1−xS generate additional magnetic anisotropy symmetries

Non-stoichiometric monoclinic 4C pyrrhotite (Fe7S8), a ferrimagnetic monosulfide that has been intensively used as a remanence carrier to infer the magnetization of the Earths crust and extraterrestrial materials, exhibits a characteristic low-temperature transition accompanied by complex modifications in anisotropy and magnetization. We demonstrate that the magnetic rotational symmetry of the 4C pyrrhotite is critically affected by the order of the defective Fe-sites, and this in turn is a key to decipher the physics behind the low-temperature transition. Our torque experiments and numerical simulations show an emergent four-fold rotational symmetry in the c-plane of the 4C pyrrhotite at T < 30 K. This symmetry breaking associated with the transition is caused by the competitive interaction of two inherently hexagonal systems generated by two groups of Fe-sites with different local anisotropy fields that stem from the vacancy arrangement in the 4C stacking sequence, and it is triggered by changes in the...

Torque analysis of incoherent spin rotation in the presence of ordered defects

Defects in magnetic systems often have a drastic impact on the magnetic state, particularly the anisotropy. In this letter, we show that magnetic torque analysis with appropriate phenomenological theoretical treatment can be used to analyze in detail the magnetic state in complex heterogeneous materials. We base our study on the Fe7S8 omission structure, which exhibits a non-trivial temperature dependence of magnetic anisotropy, involving incoherent out-of-plane spin rotation due to an elaborate defect distribution.

Ferromagnetic resonance of biogenic nanoparticle-chains

A robust method for the quantitative analysis of magnetic anisotropy in linear chains of magnetic nanocrystals, based on ferromagnetic resonance spectroscopy and a phenomenological theory, is presented. By fitting experimental resonance spectra with model calculations, we can extract the anisotropy contributions in assemblies of nanoparticle chains, as found in both cultured and natural magnetotactic bacteria, with high precision and in absolute units. This method enables the quantification of nano-scale anisotropy phenomena from a single bulk measurement and could be the key for the further development of particle magnetism and the optimization of diverse applications ranging from geo-exploration to biomedicine, where magnetic nanoparticles are key materials.A robust method for the quantitative analysis of magnetic anisotropy in linear chains of magnetic nanocrystals, based on ferromagnetic resonance spectroscopy and a phenomenological theory, is presented. By fitting experimental resonance spectra with model calculations, we can extract the anisotropy contributions in assemblies of nanoparticle chains, as found in both cultured and natural magnetotactic bacteria, with high precision and in absolute units. This method enables the quantification of nano-scale anisotropy phenomena from a single bulk measurement and could be the key for the further development of particle magnetism and the optimization of diverse applications ranging from geo-exploration to biomedicine, where magnetic nanoparticles are key materials.