Nanostructured Materials under Ion and Microwave Radiation

This thesis discusses how ion radiation and microwaves interact with nanoscale-structured materials. In the case of ion radiation, the experiments show that ion processing, either with low-energy ions in reactive ion etching or with higher energy ions in focused ion beams, produces inelastic strain in polycrystalline thin metallic films. This results in the bending of thin strips of metallic films, which cannot be explained by elastic models. The concept of ion-induced plastic strain implies the insertion of adatoms into grain boundaries within the metal matrix. In ion etching processes, thin strips of metallic films with different widths were released from the substrate at different times. Therefore, the rate of atomic flow into grain boundaries is different for different strips. The larger curvatures in narrower strips are the result of a faster rate of adatom insertion into the grain boundaries. With a high-energy focused ion beam, plastic strain can be created locally, allowing the fabrication of non-trivial three-dimensional structures at nanometer scales. In the case of microwave radiation, the materials studied include cobalt nanoparticles and carbon nanotubes. The magnetic resonance and absorption in cobalt nanoparticles are observed in various magnetizing fields at frequencies between 0.5 and 18 GHz, by using a wideband method. The obtained experimental results show that the energy absorption is associated with the ferromagnetic resonance of cobalt nanoparticles. The results include measurements of blocking temperature and saturation magnetization with SQUID magnetometry. The absorption spectra are analyzed theoretically by combining Kittel's theory for uniaxial spherical particles, the Landau-Lifshitz-Gilbert equation and effective medium models. At zero magnetizing field, the observed resonance occurs at higher frequencies compared to the non-interacting particle model. The shift of resonance is suggested to be caused by the clustering of particles. Transmission electron microscopic images demonstrate that indeed particles aggregate in the forms of clusters, superlattices, and chains. The absorption properties of yarns of carbon nanotubes are also presented in the thesis

Reference-Plane Invariant Method for Measuring Electromagnetic Parameters of Materials

This paper presents a simple and effective wideband method for the determination of material properties, such as the complex index of refraction and the complex permittivity and permeability. The method is explicit (non-iterative) and reference-plane invariant: it uses a certain combination of scattering parameters in conjunction with group-velocity data. This technique can be used to characterize both dielectric and magnetic materials. The proposed method is verified experimentally within a frequency range between 2 to 18 GHz on polytetrafluoroethylene and polyvinylchloride samples. A comprehensive error and stability analysis reveals that, similar to other methods based on transmission/reflection measurement, the uncertainties are larger at low frequencies and at the Fabry-Perot resonances.Comment: 12 pages, 21 figure

Ferromagnetic resonance in ϵ\epsilon-Co magnetic composites

We investigate the electromagnetic properties of assemblies of nanoscale $\epsilon$-cobalt crystals with size range between 5 nm to 35 nm, embedded in a polystyrene (PS) matrix, at microwave (1-12 GHz) frequencies. We investigate the samples by transmission electron microscopy (TEM) imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles inside clusters can be treated as non-interacting, and the peak position can be predicted from Kittel's ferromagnetic resonance theory for non-interacting uniaxial spherical particles combined with the Landau-Lifshitz-Gilbert (LLG) equation. In contrast, at low magnetic fields this magnetic order breaks down and the resonant frequency in zero magnetic field reaches a saturation value reflecting the interparticle interactions as resulting from aggregation. Our results show that the electromagnetic properties of these composite materials can be tuned by external magnetic fields and by changes in the aggregation structure.Comment: 14 pages, 13 figure

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

Skowronski W, Czapkiewicz M, Frankowski M, et al. Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions. Physical Review B. 2013;87(9): 094419.Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier (0.75 nm < t(MgO) < 1.05 nm) is studied using the spin-torque diode effect. The application of an rf current into nanosized MTJs generates a dc mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin-transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different dc bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed. DOI: 10.1103/PhysRevB.87.09441