Roman Antos

Magnetic Vortex Dynamics

We review the recent theoretical and experimental achievements on dynamics of spin vortices in patterned ferromagnetic elements. We first demonstrate the theoretical background of the research topic and briefly list the analytical and experimental approaches dealing with magnetic vortices. Then we report on the most remarkable studies devoted to steady state vortex excitations, switching processes, and coupled-vortex dynamic phenomena including the design of artificial crystals where the micromagnetic energy transfer takes place via the magnetic dipolar interaction among excited vortices. Finally we summarize the present state of the research with respect to novel prospects from both the fundamental and the application viewpoints.

Vortex motion in chilarity-controlled pair of magnetic disks

The authors investigate the influence of the vortex chirality on the magnetization processes of a magnetostatically coupled pair of magnetic disks. The magnetic vortices with opposite chiralities are realized by introducing asymmetry into the disks. The motion of the paired vortices are studied by measuring the magnetoresistance with a lock-in resistance bridge technique. The vortex annihilation process is found to depend on the moving directions of the magnetic vortices. The experimental results are well reproduced by the micromagnetic simulation.

Convergence properties of critical dimension measurements by spectroscopic ellipsometry on gratings made of various materials

Spectroscopic ellipsometry (SE) in the visible/near-UV spectral range is applied to monitor optical critical dimensions of quartz, Si, and Ta gratings, namely, the depth, linewidth, and period. To analyze the SE measurements, the rigorous coupled-wave theory is applied, whose implementation is described in detail, referred to as the Airy-like internal reflection series with the Fourier factorization rules taken into account. It is demonstrated that the Airy-like series implementation of the coupled-wave theory with the factorization rules provides fast convergence of both the simulated SE parameters and the extracted dimensions. The convergence properties are analyzed with respect to the maximum Fourier harmonics retained inside the periodic media and also with respect to the fineness of slicing imperfect Ta wires with paraboloidally curved edges.

Simulations of the dynamic switching of vortex chirality in magnetic nanodisks by a uniform field pulse

We present a possibility to switch the chirality of a spin vortex occurring in a magnetic nanodisk by applying a uniform in-plane field pulse, based on optimizing its strength and duration. The related spin-dynamical process, investigated by micromagnetic simulations, consists of several stages. After applying the field, the original vortex is expelled from the disk, after which two C-shaped states oscillate between each other. The essence of the method is based on turning the field off at a suitably chosen moment for which the orientation of the C-state will evolve into the nucleation of a vortex with the desirable chirality. This idea simply uses the information about the original chirality present inside the nanodisk during the dynamic process before losing it in saturation, and can thus be regarded as analogous to the recent studies on the polarity switching.

Fourier factorization with complex polarization bases in modeling optics of discontinuous bi-periodic structures.

The coupled wave theory dealing with optics of discontinuous two-dimensional (2D) periodic structures is reformulated by using Fourier factorization with complex polarization bases, which is a generalized implementation of the fast Fourier factorization rules. The modified approach yields considerably improved convergence properties, as shown on an example of a 2D quartz grating. The method can also be applied to the calculation of 2D photonic band structures or nonperiodic cylindrical devices, and can be generalized to elements with arbitrary cross-sections.

Evidence of native oxides on the capping and substrate of Permalloy gratings by magneto-optical spectroscopy in the zeroth- and first-diffraction orders

Magneto-optical Kerr effect (MOKE) spectroscopy in the zeroth- and first-diffraction orders at polar magnetization is applied to Permalloy wire gratings deposited on Si substrates and protected by Cr capping. The experimental MOKE data are compared with data simulated using the local modes method. The extensive simulations of the MOKE spectroscopic parameters exhibit significant sensitivity to t(Cr2O3) and t(SiO2), the thicknesses of native oxide layers developed on the capping and the substrate, respectively. The approach may be useful for monitoring the basic micromagnetic properties of small elements with nanometer-scale resolution, as well as for monitoring the deposition processes and aging of magnetic nanostructures in magnetic recording and magnetic random access memory technologies.

Optical and magneto-optical behavior of Cerium Yttrium Iron Garnet thin films at wavelengths of 200–1770 nm

Magneto-optical cerium-substituted yttrium iron garnet (Ce:YIG) thin films display Faraday and Kerr rotation (rotation of light polarisation upon transmission and reflection, respectively) as well as a nonreciprocal phase shift due to their non-zero off-diagonal permittivity tensor elements, and also possess low optical absorption in the near-infrared. These properties make Ce:YIG useful in providing nonreciprocal light propagation in integrated photonic circuits, which is essential for accomplishing energy-efficient photonic computation and data transport architectures. In this study, 80 nm-thick Ce:YIG films were grown on Gadolinium Gallium Garnet substrates with (100), (110) and (111) orientations using pulsed laser deposition. The films had bulk-like structural and magnetic quality. Faraday and Kerr spectroscopies along with spectroscopic ellipsometry were used to deduce the complete permittivity tensor of the films in the ultraviolet, visible and near-infrared spectral region, and the magneto-optical figure of merit as a function of wavelength was determined. The samples showed the highest IR Faraday rotation reported for thin films of Ce:YIG, which indicates the importance of this material in development of nonreciprocal photonic devices.

Specular spectroscopic ellipsometry for the critical dimension monitoring of gratings fabricated on a thick transparent plate

Specular-mode spectroscopic ellipsometry is applied to analyze the optical response of gratings fabricated on a thick transparent plate substrate. The principles of the optical response of the gratings are described by employing incoherent contributions due to backreflections in the finite transparent substrate medium. A special function identifies a “diminution effect” caused by deflecting the secondary contributions from the primary beam axis. Two different methods are used to measure the ellipsometric response, a liquid solution method with the backreflections eliminated and a method including the incoherent backreflections. The grating parameters deduced by fitting from the measurement using the first method are applied to simulate the ellipsometric response using the second method. The spectral dependencies yielded by both methods are compared with remarkable agreement between the simulations and the measurements, which suggests the high usability of the backreflection method in the metrological char...

Fourier factorization with complex polarization bases in the plane-wave expansion method applied to two-dimensional photonic crystals

We demonstrate an enhancement of the plane wave expansion method treating two-dimensional photonic crystals by applying Fourier factorization with generally elliptic polarization bases. By studying three examples of periodically arranged cylindrical elements, we compare our approach to the classical Ho method in which the permittivity function is simply expanded without changing coordinates, and to the normal vector method using a normal-tangential polarization transform. The compared calculations clearly show that our approach yields the best convergence properties owing to the complete continuity of our distribution of polarization bases. The presented methodology enables us to study more general systems such as periodic elements with an arbitrary cross-section or devices such as photonic crystal waveguides.

Evaluation of the quality of Permalloy gratings by diffracted magneto-optical spectroscopy.

Magneto-optical Kerr effect (MOKE) spectroscopy in the -1st diffraction order with p-polarized incidence is applied to study arrays of submicron Permalloy wires at polar magnetization. A theoretical approach combining two methods, the local modes method neglecting the edge effects of wires and the rigorous coupled wave analysis, is derived to evaluate the diffraction losses due to irregularities of the wire edges. A new parameter describing the quality of the edges is defined according to their contribution in the diffracted MOKE. The quality factor, evaluated for two different samples, is successfully compared with irregularities visible on atomic force microscopy pictures.