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Electromagnetic and spin wave interactions in magnetic nanostructure-based metamaterials and devices

Final Report Summary - EMSWIM (Electromagnetic and spin wave interactions in magnetic nanostructure-based metamaterials and devices)

Project context and objectives

Periodically arranged nanoscale elements (nanostructures) are very important from many viewpoints including optics, electronics, micromagnetism and spintronics, crystal, surface and transport physics, artificial metamaterials, biophysics, data storage and manipulation, nanolithography, etc. Concurrently, theoretical and experimental research of their optical response can strongly help in all these areas of science and technology. This project was thus devoted to studying the periodic nanostructures from the viewpoint of their optical, magneto-optical (MO) and spin-excitation response, the latter two of which related to the use of magnetic materials.

In particular, the research work within the project consisted of: I. Theory (developing and computer-implementing a fast and precise theoretical model); II. Experiment (optical, MO, micromagnetic, and complementary-technique measurements); III. Applications (proposing, simulating and designing nanostructure-based devices); IV. Research of related physical phenomena and looking for new directions.

Project results

I. Work and results in theory:

Optics of periodic nanostructures were treated in two mathematically similar viewpoints: first, coupled wave theory was used to simulate the optical and MO response of 1D, 2D, and 3D nanostructures; second, the plane-wave expansion method was used to simulate modes in 2D photonic and MO crystals.

The involvement of MO anisotropy enabled the use of MO spectroscopy, which is highly sensitive to various phenomena in ultrathin films and nanostructures, and allowed the investigation of hybridisation of modes in magneto-photonic crystals, where two different polarisations, normally independent in isotropic crystals, become mutually crossed.

Discovery of the method of Fourier factorisation with complex polarisation bases enabled the enhancement of optical theory of periodic nanostructures. Moreover, the Fourier factorisation was found important not only for discontinuous structures as it has claimed so far, but for any inhomogeneous systems treated in the Fourier space (e.g. for continuous holographic gratings).

Several mathematical tricks were implemented in order to effectively simulate complex structures such as relief gratings with a native oxide overlayer or aperiodic structures such as waveguides.

An analogous numerical model, with restricted functionality for the present, was implemented in C++ to acquire a convenient form for commercial application.

Micromagnetic theory was adopted to simulate dynamic micromagnetic excitations in micromagnetic nanostructures such as magnetic vortices.

II. Work and results in experiment:

Various grating samples including aluminium sine-like gratings and periodic ferromagnetic wires were studied by means of spectroscopic ellipsometry, MO spectroscopy and complementary methods such as atomic force microscopy. The above theoretical models were used to analyse the geometrical and material quality of the samples.

The aluminium sine-like gratings were monitored to find out the precise grating profile - not precisely sinusoidal - including their shape and geometrical dimensions (period and depth).

Periodic nickel and permalloy nanowires were monitored by means of MO spectroscopy to reveal the limits of the method. One of the most interesting results is the possibility to evaluate the line edge roughness of lithographically-patterned wires.

MO spectra of thin films of promising magnetic materials (La2/3Sr1/3MnO3, NiFe/Cu/Co, CoFe2O4, etc.) were measured and analysed as references for possible future applications.

Additional measurements of micromagnetic dynamic and switching behaviour of magnetic vortices and thin films were carried out in collaborating laboratories.

III. Work and results related to applications:

A major application result of the project is optical and/or MO scatterometry, which is the monitoring of grating parameters (geometrical profile and materials) by measuring spectroscopic ellipsometry and/or MO spectroscopy and analysing it by a theoretical model. By comparing the modelled and measured data, the precise parameters can be determined. The method is also usable for in situ measurements when the optical apparatus is part of a lithographer and is hence interesting for commercial employment.

The generalised model is capable of proposing and analysing novel artificial metamaterials and devices based on periodic nanostructures such as better tuned moth-eye anti-reflective surfaces, wire-grid polarisers, grating phase plates, mode isolators, chromatic, spatial and other optical filters, waveguides, fibers and microcavities. It was successfully applied to simulate some of these devices. Moreover, a possibility of controlling the propagation of modes through magneto-photonic waveguides by external magnetic field was shown.

The discovered switching behaviour of vortex chirality in magnetic circular nanodiscs is important for promising applications of magnetic vortices in data storage and manipulation, e.g. vortex random access memory (VRAM). Before that, only vortex polarity, second of the two binary states of vortices, was switchable and it was even claimed that to switch the chirality requires a geometric asymmetry. The researcher has proven the opposite.

IV. Work on related physics and future prospects:

The spin-dynamic behaviour of magnetic vortices under short pulses of external magnetic field was investigated in detail, from which interesting physical phenomena were observed. This also enabled the discovery of the method of vortex chirality switching as mentioned above. Other related phenomena discovered during the investigation include anomalous trajectories of vortex cores under specially adjusted field pulses which are highly sensitive to other solid state physics features.

Additional mathematical tricks are planned for the model: adaptive spatial resolution for higher numerical effectivity, perfectly matched layers instead of periodic boundary conditions in order to better simulate aperiodic systems such as waveguide bends and junctions, and the Chandezon method for non-rectangular reliefs.

Measurements and analyses of various lithographic patterns and magneto-photonic crystals are planned as long term collaboration with several laboratories in the Czech Republic, Japan, France, and the USA.

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