Magneto-optical spectroscopy laboratory

 

Budapest University of Technology and Economics

Faculty of Natural Sciences

Institute of Physics

Optical spectroscopy on complex magnetic materials gains increasing research potential due to the fundamental and detailed information recently obtained about the character of the magnetic ordering in many systems. Beyond the bulk magnetization, the method often allows the determination of the magnetization for the different components or sublattices. Futhermore, basic physical parameters such as the exchange and spin-orbit couplings can be also deduced from the magneto-optical (MO) spectra. The broadband MO spectrometer developed in our laboratory offers a solid experimental background to study novel and technologically important magnetic materials on a broad length-scale, down to low temperatures and in high magnetic fields. Beyond the research of inorganic magnets, we also investigate biological systems by polarized optical and magneto-optical spectroscopy. Our main topics in this field are the secondary structure analysis of proteins by circular dichrosim spectroscopy and the magneto-optical diagnosis of malaria.

 

Most of the MO measurement systems used in our laboratory have been – partly or fully – developed by our group. Based on these facilities we can detect reflectivity, transmission, and magneto-optical parameters – including magneto-optical Kerr effect (MOKE), Faraday effect, magnetic and natural circular dichroism – over the spectral range of 0.1-7eV (170-12000nm in wavelength). The high-sensitivity magneto-optical and natural circular dichroism experiments are developed with the application of a polarization modulation technique using fast photodetectors, photoelastic modulators and lock-in amplifiers.

 

·     Low-temperature MO measurement unit

 

This facility is composed of infrared and visible laser diodes, polarizing optical elements and a photoeleastic modulator. Experiments can be performed in the temperature range of T=2-300K using a continuous-flow liquid helium optical cryostat. The precision of the detection for the MO parameters is typically ≤0.001°. Magnetic field of B=0.4T in maximum is applied by NdFeB permanent magnets.

 

 

·        High-field MO measurement system

 

Concerning the building blocks this system is similar to the low-temperature MO unit. The substantial difference is that it can be integrated with a high-field, B=14 T, supercondacting magnet. Using a home-designed optical insert the sample is also loaded into the liquid helium cryostat and placed to center position within the superconducting magnet. The whole light path is realized using optical elements. The temperature of the samples can be varied over the range of T=2-300 K.

 

 

 

·        Broadband MO spectrometer

 

The MO spectrometer developed in our laboratory with a uniquely broad spectral range (0.1-7 eV) is built on the basis of two optical spectrometer. For lower photon energies (0.1-0.9 eV) we adapted the polarization modulation technique for our custom-designed Varian-670 two-channel Fourier transform infrared spectrometer.  For higher-energy (0.7-7 eV) studies a Cornerstone ¼ monochromator is applied with three interchangeable holographic gratings. In order to cover such a broad spectral region we use various types of detectors such as MCT, InSb, InGaAs, Si photodiodes and a photomultiplier. A resolution of ~0.001° for the magneto-optical parameters is achieved over the whole range of photon energy specified above. This MO spectrometer also allows low-temperature experiments in magnetic field of B=0.4 T in maximum.

 

 

 

·     Complex form of magnetic ordering studied by MO spectroscopy
 

MO spectroscopy can often reveal much detailed information about the magnetic structure and the character of the electrons responsible for the magnetism as compared with bulk magnetization experiments, especially in case of magnets with various magnetic components or sublattices. For some materials case we also investigated the magnetic phase diagram using composition, temperature and pressure as control parameters.

 

·         Mo spectroscopy on itinerant magnets
 

Determination of spin-polarized band structure in itinerant ferromagnets is challenging both from experimental and theoretical side and would help the synthesis of completely spin-polarized conductors. For several metals we could deduce the accurate spin-polarized electronic structure from the simultaneous measurements of absorption and magneto-optical spectra which give complementary information about the nature of the electronic states.

·        Semiconductors with giant MO activity
 

This class of materials is widely used in optical data transfer and magneto-optical data storage. Due to the widespread semiconductor based information technology, the magneto-optical elements (optical isolators, magneto-optical waveguides, etc.) are also made of semiconductors. Well-known representatives are the II-VI-type magnetic semiconductors and yttrium iron garnets. We are going to broaden the range of applicable materials by the systematic investigation of III-V-type and other Co-, Fe-based magnetic semiconductors.

MO spectroscopy is also a good candidate to detect pathological mutations and defects in blood proteins due to their iron content. As a representative example, we study the so-called malaria pigment which is a part of the hemoglobin indigestable and toxic for the parasites and thus it is transformed to a form of insolvable crystals. These are rod-like crystallites with a typical length of one micron containing high-spin paramagnetic iron. We found that high-sensitivity optical detection of malaria pigment can be achieved owing to its magnetically induced linear dichroism.

 

 

·      Analysis of the secondary structure of proteins by circular dichroism spectroscopy
 

UV circular dichroism spectroscopy is a widespread method for the study of the secondary structure of proteins. This technique does not require protein samples in crystallographic form (like usual diffraction based methods) and can also be performed on samples with very limited quantity. The broadband MO spectrometer is well-suited for such investigations, as well. Furthermore, we plan to extend our experimental capability to cover the vibrational circular dichroism of proteins recently discovered in the mid-infrared region.