BMe Research Grant
The materials I’m investigating are ferroelectric and ferromagnetic at the same time, therefore, these are termed in literature as multiferroics. Investigation and production of these materials are emergent topics of present-day solid-state physics. This is mainly due to the novel phenomena shown by multiferroics and the possibility for wide-range technological utilization.
In my research, I synthesize materials with strong magneto-electric and magneto-optical effects. Electric and magnetic excitations of these materials are investigated by means of dielectric and optical spectroscopy measurements to recognize the microscopic mechanisms behind magneto-electricity and magneto-optics.
Understanding the microscopic background will lead us to literally plan new materials, which can be applied in optical communication and data storage.
I do my research at the Department of Physics at BUTE in collaboration with the Center of Emergent Material Science programme at RIKEN Advanced Research Institute in Tokyo and the THz group of the National Institute of Chemical Physics and Biophysics in Tallinn. I investigate optical properties in the visible and near-infrared photon-energy range with a self-built broad-band magneto-optical spectrometer in Budapest.
Magneto-electric effect, where electric field can induce magnetization and magnetic field can cause polarization in materials, was first mentioned by Pierre Curie (1894) and Peter Debye (1926). Probably the most interesting manifestation of this phenomenon in optical properties is the directional dichroism , i.e. a difference in the index of refraction for counter-propagating light beams in insulating materials. A possible form of application of this effect is the development of the ‘light-rectifier’, an optical analogue for the semiconductor rectifiers.
Necessary condition for the directional dichroism is the breaking of both time-reversal, both inversion symmetries. Fulfilling only the first case results in the Faraday-effect or magneto-optical Kerr-effect, while in the second case the effect of optical activity can be observed. Directional anisotropy has been observed so far in two distinct cases: i) if the dielectric has polarization perpendicular to it’s magnetization ii) in chiral material with magnetization. Microscopical background of these phenomena are intensively researched topics to date [2-6].
Production of single crystals as simplified model systems for theoretical investigations is an expensive and hard task, only few groups in the world have the proper infrastructure and professional background. Cooperation with the Japanese laboratory is a crucial part in our field of research.
Purpose of my PhD research is to find, synthesize, and investigate materials showing high magneto-optical and magneto-electric effects by means of optical spectroscopy, to help understand the microscopic background of magneto-electricity and directional dichroism. Beside the expansion our theoretical knowledge it is also an important for finding ways for application.
Figure 1.: Steps of crystal growth: (a) starting materials before decarbonizing, (b) polycrystalline specimen in the floating-zone furnace, note the melt zone in the middle of the picture, (c) finished, large size single crystal before orientation and cut, (d) oriented cut of Sr2CoSi2O7 single crystal, (e) single crystals of NiCr2O4 grown by the flux deposition method.
Large scale samples are beneficial for optical measurements. For such a purpose many synthesizing techniques are available, here I will only introduce the one, I use most frequently (Figure 1).
As a first step of sample preparation, the polycrystalline form of the target material is produced from transition-metal oxides and carbonates with high temperature decarbonizing. For stabilizing the charge state of the metal-ion, controlled atmosphere (O2, N2, Ar, over-pressure) is needed. A rod formed from the polycrystalline material is melt to single crystalline with the following method. The polycrystalline rod is melt in the focal point of infrared lamps and poured on top of a small piece of starting crystal (seed). By moving the seed and feed rods together, the melt zone is pushed through the polycrystal resulting in the single crystal rod.
Before optical measurements the single crystal specimens are tested and oriented by powder X-ray scattering (2θ), Laue-photo, magnetization, dielectric polarization and conductivity measurements.
Figure 2.: Self-built, broad band (0.1eV - 4.5eV) magneto-optical spectrometer in the Department of Physics, BUTE. This setup is built up from two separate components.
The magneto-optical Kerr-effect (MOKE) spectrometer at the Department of Physics, BUTE is capable of high accuracy detection of optical anisotropies caused by magnetic changes in metals and semiconductors. The spectrometer applies polarization modulation technique , i.e. during the measurement it switches between the circular left and right polarization states with high frequency (f=50kHz). Measuring the f and 2f harmonics of the light intensity reflected by the sample as a function of photon energy, Kerr-rotation and ellipticity spectra are acquired. By means of magneto-optical and reflectivity spectra, in case of magnetic semiconductors, crystal field interaction, electron-electron interaction and spin-orbit interaction parameters, in case of metals the spin-polarized band structure can be derived. Magneto-optical spectra were interpreted in the framework of crystal field theory in case of FeCr2O4, CoCr2O4, NiCr2O4, CuCr2O4 magnetic semiconductors.
Further way of investigating magnetic order in insulating materials is by measuring low-energy absorption as a function of temperature and magnetic field, which was done with the setup in Tallinn.
According to our measurements, BiTeI, a non-magnetic polar semiconductor with outstanding large Rashba-effect shows huge Kerr-rotation in the infrared spectral range [A].
We have measured THz absorption on single crystals of Sr2CoSi2O7 grown by floating-zone method (Figure 3). This multiferroic material showed directional anisotropy in it’s spin wave excitations (magnons) under TN=7.5K; in B=12T the effect is almost 100%, i.e. spins by the help of absorbing photons can be excited in one propagation direction of the incoming light, while cannot be or barely can be excited for the opposite direction [C].
I have used also floating-zone technique for synthesizing CaBaCo4O7 pyroelectric ferrimagnet (TC=60K). Close to the magnetic ordering temperature, this material shows outstanding high magneto-electric effect. By means of indirect methods we have proved that the ferrimagnetic order can be stabilized with the help of electric field.
Including the aforementioned ones, I have synthesised the following multiferroic crystals: Sr2CoSi2O7, (Ca,Sr)2CoSi2O7, Ba2CoGe2O7, CaBaCo4O7, YBa(Co3,Al)O7, NiCr2O4, LiCoPO4, LiNiPO4. Investigation of these crystals is still ongoing.
Figure 3.: Magnon absorption spectra as a function of external magnetic field of Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 samples in different configurations [C].
These experiments may bring us closer to producing multiferroic materials which can be applied in future information technologies.
We have started new collaborations with several groups to investigate the grown crystals. We expect the results of this research to be published in high-level papers. So far I’m co-author of 6 papers, author of one.
[C] I. Kézsmárki, D. Szaller, S. Bordács, V. Kocsis, Y. Tokunaga, Y. Taguchi, H. Murakawa, Y. Tokura, H. Engelkamp, T. Room, and U. Nagel: One-way Transparency of Four-coloured Spin-wave Excitations in Multiferroic Materials. NATURE COMMUNICATIONS 5: pp. 3203-3211, Paper 3203, (2014)
[F] Á. Butykai,Á. Orbán,V. Kocsis,D. Szaller,S. Bordács,E. Tatrai-Szekeres,L. F. Kiss,A. Bóta,B. G. Vértessy,T. Zelles,I. Kézsmárki, Malaria pigment crystals as magnetic micro-rotors: key for high-sensitivity diagnosis. SCIENTIFIC REPORTS 3: p. 1431. Paper 1431. (2013)
 S. Bordács, I. Kézsmárki, D. Szaller, L. Demkó, N. Kida, H. Murakawa, Y. Onose, R. Shimano, T. Rõõm, U. Nagel, S. Miyahara, N. Furukawa, and Y. Tokura: Chirality of matter shows up via spin excitations, Nature Physics 8, 734–738 (2012)