Magneto-Optical and THz Spectroscopy

Petr Neugebauer Ph.D.
Research Group Leader

Career and collaboration

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Magneto-Optical and THz Spectroscopy

The aim of this research group led by doc. Petr Neugebauer is to establish the first modern THz magneto-optical spectroscopy group in central Europe, with focus both on method development and on applications in material- and bio-sciences. Possibilities to perform experiments on a variety of samples ranging from biomolecules, over coordinated metal centers to magnetic and solid-state materials, in a very broad spectral range from GHz frequencies to UV-VIS range, at cryogenic temperatures (1.8 K – 320 K) and high magnetic fields (16 T), make the group very attractive to many scientists worldwide. In the frequency range from 100 to 1000 GHz, we are designing and developing worldwide first broadband THz (High Field Electron Spin Resonance) spectrometer capable to capture spin dynamics (supported by Horizon 2020 Excellent Science, ERC Starting Grant, 714850 THz-FRaScan-ESR, THz Frequency Rapid Scan Electron Spin Resonance, with budget 2 MEUR) as well as developing unique Electron Paramagnetic Resonance Microscope (supported by Horizon 2020 Novel ideas for radically new technologies, FET-Open, 767227 PETER, Plasmon-Enhanced Terahertz Electron Paramagnetic Resonance, with budget 2.9 MEUR). The group holds the philosophy of the Central European Institute of Technology (CEITEC) – open access to its infrastructures. We will fill the gap and cover science which is not yet developed in the Central Europe. We take advantage of Brno as an innovative city and involve high-tech industry as well as many departments and faculties from its both two major universities (Brno University of Technology – BUT and Masaryk University – MU), such as the Faculty of Mechanical Engineering, the Faculty of Electrical Engineering and Communication, the Faculty of Science at MU, Josef Dadok National NMR, etc. We have already established collaboration with many of these departments in past years, also with support from DAAD-MSMT travel grant project. Furthermore, the group is strongly involved in the education of undergraduate and graduate students providing the Brno region with well-educated, skilled and motivated people, experienced in working in international teams.

Content of Research

THz spectroscopy development

The aim of our project is to set up and develop general-purpose state-of-the-art broadband Electron Paramagnetic Resonance spectrometer based on THz rapid frequency scans (THz-FRaScan-EPR). It will allow multi-frequency relaxation studies of a variety of samples ranging from bulk (crystal) materials, over powdered samples to air-sensitive samples in liquid solutions. It will operate at frequencies between 80 GHz to 1100 GHz, at temperatures in the range from 1.8 K to 320 K and at magnetic fields up to 16 T. The instrument will also be capable to perform measurements at zero magnetic field. A completely new concept of detection based on the rapid frequency sweeps will allow performing relaxation investigations at THz frequencies, which are currently either unexplored or undeveloped. Obtained results will stimulate the development of new materials and Magnetic Resonance Imaging (MRI) applications in hospitals by means of Dynamic Nuclear Polarization (DNP).

High Frequency Rapid Scan

The enhancement of the signal-to-noise ratio (SNR) in EPR/NMR spectroscopy is the most challenging problem in these fields. The development of Rapid Scan (RS) technique was historically connected mainly to this problem in NMR. RS EPR/NMR spectroscopy is a continuous wave (CW) technique, where excitation frequency or external magnetic field is swept continuously, but with a very high rate, which has to be equal or higher than the spin-spin relaxation rate. In this case, a spin system can handle much higher excitation power with no visible saturation than in the conventional slow sweep experiment. The problem with this method is the strong signal distortion in form of specific oscillations, so-called “wiggles” (see Fig. 1). Fortunately, it was shown that the slow sweep spectrum, free of any distortions, can be reconstructed by means of special numerical post-processing – “deconvolution of RS spectrum”. Another source of SNR improvement is the substantially shorter acquisition time per one sweep, which, in turn, increases considerably the efficiency of averaging. Later, it was demonstrated that RS spectra can be simulated using the modified system of Bloch equations. This allows the extraction of the relaxation times T1 and T2 from RS experimental data. 

Fig. 1 Simulated RS Spectrum.

With the development of high power microwave sources, the interest of many scientists shifted to the pulsed methods in EPR/NMR, due to the wide variety of obtaining data. However, the efforts of our scientific group push the limits of EPR spectroscopy to higher and higher frequencies in THz range. Unfortunately, the nowadays level of microwave sources at these frequencies, mostly in terms of output power, does not allow implementing pulsed techniques in the wide frequency range. For this reason, RS EPR spectroscopy is the only possible technique for the investigation of spin dynamics at THz frequencies. In this project, we will develop and implement a technique of fast frequency sweeps into our high-field/ high-frequency EPR spectrometer for the investigation of spin relaxation processes in a wide range of novel materials, such as molecular nanomagnets, graphene, polarizing agents for the dynamic nuclear polarization, etc.

Solid-State Physics and Modern Materials

The study of phenomena such as quantum spin-Hall effect, metal-insulator transitions, charge and spin transfer, magnetic anisotropy, spin-wave oscillations (magnons), quantum and thermodynamic phase transitions are fundamental to the understanding and characterization of modern materials with applications in a wide range of fields such as energy storage and conversion (batteries and solar cells) and quantum computation (data storage and transmission). 

We aim to investigate the above mentioned solid state phenomena using high-frequency ESR, a powerful technique that enables accessing the microscopic characteristics of the electronic structure, crystal symmetry, and spin dynamics. Its high sensitivity to measure spin relaxation times makes it suitable for a wide range of studies from detecting entangled states in the quantum bits to precisely detecting carrier mobility in low dimensional materials such as graphene, transition metal dichalcogenides (TMDs), and their layered derivatives. Moreover, it possesses a high spectral resolution to provide an exact determination of the enhanced splitting in the GHz-THz range between energy levels of spin systems, allowing the determination of magnetic anisotropy and other magnetic interactions in molecular magnets and spin clusters. Magnon modes observed in yttrium iron garnet (YIG), manganites, perovskites or multiferroics lies in the THz range, therefore, our study will also focus on the detection and control of magneto-optical properties in these materials.

Hybrid Materials and Spintronics

The main aim of this topic is to connect so-called two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs), both possessing unique physical and chemical properties, with single-molecule magnets by creating prototypical devices and concepts for applications in the emergent field of spin electronics (spintronics). In order to achieve construction of such devices, it is crucial to control the whole route of a material from a bulk with mostly known magnetic properties towards nanostructuring into hybrid materials, thin films, and sandwiches with greater application potential than their individual bulk counterparts. Therefore, emphasis is put on viability, feasibility and scalability of chosen preparative methods, as well as subsequent treatment and characterization of every stage and form, primarily by newly-built high-frequency and high magnetic field THz rapid-scan electron spin resonance (ESR) spectrometer. The further research includes the mechanism of the reaction between hybrid materials (2D materials and SMMs) and the fabrication of spintronic devices for determining the application potential of the hybrid materials in spintronics.

Coordination Chemistry

Chemical research of the Magneto-Optical and THz Spectroscopy group is aimed at synthesis and characterization of magnetically bistable coordination compounds, which exhibit: a) slow relaxation of magnetization of purely molecular origin, so-called single-molecule magnets (SMMs); b) spin crossover.
SMMs represent an interesting group of materials in which every molecule behaves as a molecular nanomagnet capable of storing binary coded information. Therefore, these materials are studied as candidates for construction of magnetic memories with a high density of storage.

Transition metal complexes with 3d4 – 3d7 valence-shell configurations might exhibit spin transition upon external perturbation such as temperature and pressure change or light irradiation. The phenomenon is called spin crossover (SCO) and it is often accompanied by dramatic color change, which bears the potential for practical applications in display devices. Furthermore, the extremely large sensitivity of the SCO molecules to their environment makes them to be promising candidates for preparation of molecular switches and sensors.

The CEITEC Nano Research Infrastructure provides complex equipment, expertise and methods for nanotechnology and advanced materials R&D. The CEITEC Nano facilities for nanofabrication, nanocharacterization, structural analysis and X-ray tomography enable to carry out complete fabrication of nanostructures and nanodevices and their characterization down to the sub-nanometre level in an entirely clean environment.