PhD School of Physical Science

Department of Physics/Institute of Physics

Supervisor: Dr. István Kézsmárki

Exotic Magneto-Optical Effects in Solids

Introducing the research area

Brief introduction of the research place

Solid State Physics Laboratory at BUTE investigates the transport, magnetic and optical properties of novel materials often under extreme conditions, such as low temperatures, strong magnetic fields and high pressures. Currently the two main research fields are the physics of nanoscale devices and the optical properties of correlated electron materials [1–7, A, B, C].

History and context of the research

Matter when placed in a magnetic field rotates the polarization of light and makes it elliptical. Chiral materials show similar circular dichroism as discovered in the XIX^th century. The former and latter phenomena are called magneto-optical Faraday-effect and natural optical activity, respectively.

The counterpart of the Faraday effect in reflection, the magneto-optical Kerr-effect (MOKE) is widely used for the detection of surface magnetization. Magnetic structures, such as magnetic domains, vortices in superconductors [8] etc., can be mapped with micrometer resolution. Owing to the optical detection technology, dynamics of magnetization can be followed in time with picosecond resolution [9], offering a way for generating optically excited magnetic resonance [10], or a real-time investigation method of spin diffusion in semiconductors [11].

Diffusion of spin-polarized carriers excited by circularly polarized LASER beam in a semiconductor [11].

Absorption of chiral molecules, when placed into a magnetic field, depends on the direction of light propagation but independent of its polarization [13].

Faraday-effect is the consequence of broken time reversal symmetry, while natural optical activity is due to the lack of mirror symmetry. If both symmetries are broken, an interesting new phenomenon, the so-called directional dichroism appears. Absorption of light becomes different for the counter-propagating beams and more importantly, this property can be manipulated by magnetic field [12,13,14]. In contrast to the cases already realized in optical devices, such directional light-switch does work for unpolarized beams, as well.

Aim of the research

Crystals showing large magneto-optical effects are widely used in optical isolators (Faraday rotators, optical circulators), mode-conversion waveguides and optical data storage. Although directional dichroism phenomenon would offer a large variety of applications, from intelligent solar-coating of windows to telecommunication systems, the effect is hardly observable for usual materials. Therefore, the search for novel compounds with large magneto-optical effects has great technological importance, while understanding the microscopic mechanism behind these exotic effects is a milestone for basic research.

Magneto-optical spectroscopy, when performed over a broad photon-energy range from infrared (E>1 meV) to ultraviolet (E<6 eV), is an excellent method for the characterization of complex spin structures emerging in correlated electron systems and for determining fundamental physical parameters like magnetic exchange interaction, relativistic spin-orbit coupling, crystal-field splittings or other properties of electronic states.

The goal of my PhD work is the investigation of materials showing interesting magneto-optical effects and tailoring of novel compounds with unique properties for optical applications and information technology. The study of such compounds over an extended range of magnetic field and temperature often provides fundamental information about the complex magnetic states and gives a clue for the well-controlled synthesis of new systems.

Methods

High-sensitivity detection of light polarization

High-sensitivity detection of light polarization with a precision of 0.001° in optical rotation and ellipticity can be achieved by polarization modulation techniques. In our magneto-optical setup a photo-elastic modulator (PEM) switches back and forth between the two circularly polarized states with 50 kHz frequency. Correspondingly, the circular birefringence and dichroism of the sample can be detected with Lock-in amplifiers by selecting the appropriate harmonics from the detected light intensity.

Time-domain terahertz spectroscopy Recent development of terahertz spectrometers, operating in the energy range between the microwaves and infrared, attracted much interest, since non-ionizing long-wavelength radiation is advantageous in medical applications, material sciences and telecommunication as well [15].

The time-domain terahertz spectrometer used in my experiments was built by Ryo Shimano and Noriaki Kida at the University of Tokyo. Two kind of high-brightness terahertz emitter was applied as light source: (i) a microscopic antenna evaporated on GaAs and (ii) a non-linear ZnTe crystal were excited by a femtosecond Ti:Sapphire LASER. Both detectors operate on the same principles. A great advantage of this method, besides its high brilliance, is phase-sensitive detection, which allows us to measure the whole complex optical response. Schematic view of a time-domain terahertz spectrometer.

Results 

Dilute magnetic semiconductors Doping semiconductors with magnetic ions without phase-segregation is a basic requirement for the integration of magnetic data storage and semiconductor-based information technology. I performed magneto-optical Kerr-effect measurements on several magnetic semiconductors – (In,Mn)Sb, (In,Mn)As and (Ga,Mn)As – in high magnetic fields. The results clarify the mechanism of magnetic interaction between Mn ions and the typical magnetic cluster size in these systems [D].

Magneto-optical Kerr-effect of In0.972Mn0.028Sb thin film in high magnetic field [D].

Ferrimagnetic spinels The spinel crystal CoCr2O4 is a ferrimagnet with several magnetic sublattices. Lack of inversion symmetry for Co2+ ions makes the transition between their d orbitals optically allowed. For these excitations we have found the largest, θ=12°, magneto-optical Kerr rotation observed so far in insulating magnets. We extended our research to other compounds of the same family (substituting Co with Cu and Fe) which also exhibit large magneto-optical signal at low temperatures. As chalcogenide spinels are already ferromagnetic at room temperature, they have a potential for various technology applications if a similarly high magneto-optical effect is achieved [E].

CoCr2O4 ferrimagnetic spinel shows giant magneto-optical Kerr rotation and ellipticity for the transition between the Co d orbitals.

Determination of spin-polarized band structure in itinerant ferromagnets is challenging from both experimental and theoretical side and would help in the synthesis of completely spin-polarized conductors. For several metals we could deduce the accurate spin-polarized electronic structure from simultaneous measurements of absorption and magneto-optical spectra [E, F, G], as demonstrated for CuCr₂Se₄. Interaction of localized Cr³⁺ spins and itinerant charge carriers leads to an almost completely spin-polarized Fermi surface in this material [E].

Expected impact and further research 

I participated in the development of a broadband magneto-optical spectrometer that is an effective tool for studying new magnetic compounds. My results show that magneto-optical spectroscopy provides fundamental information about complex magnetic orders in solids and the underlying interactions. I am going to extend the investigation of electromagnon excitation in such multiferroic materials which exhibit exotic magneto-optical effects (directional dichroism and field-induced chirality) in the microwave and terahertz region even at room temperature.

I am a coauthor of 9 articles, 7 of them are related to optical spectroscopy and published in high-impact journals such as Physical Review Letters and Applied Physics Letters.

Publications, references, links

Publications

[A]    I. Kézsmárki I, S. Bordacs: An alternative of spectroscopic ellipsometry: The double-reference method
Applied Physics Letters 92, 131104 (2008)

[B]    I. Kézsmárki, R. Gaál, C. C. Homes, B. Sipos, H. Berger, S. Bordács, G. Mihály, L. Forró: High-pressure infrared spectroscopy: Tuning of the low-energy excitations in correlated electron systems

Physical Review B 76, 205114 (2007)

[C]    S. Bordacs, D. Varjas, I. Kézsmárki, G. Mihaly, L. Baldassarre, A. Abouelsayed, C. A. Kuntscher, K. Ohgushi, Y. Tokura: Magnetic-Order-Induced Crystal Symmetry Lowering in ACr₂O₄ Ferrimagnetic Spinels

Physical Review Letters 103, 077205 (2009)

[D]    G. Mihály, M. Csontos, S. Bordács, I. Kézsmárki, T. Wojtowicz, X. Liu, B. Jankó and J. K. Furdyna: Anomalous Hall Effect in the (In,Mn)Sb Dilute Magnetic Semiconductor Physical Review Letters 100, 107201 (2008)

[E]    S. Bordács, I. Kézsmárki, K. Ohgushi, Y. Tokura: Experimental band structure of the nearly half-metallic CuCr₂Se₄: An optical and magneto-optical study New Journal of Physics 12, 053039 (2010)

[F]    N. Hosaka, H. Yamada, Y. Shimada, J. Fujioka, S. Bordács, I. Kézsmárki, M. Kawasaki and Y. Tokura: Magneto-optical characterization of the ferromagnetic-paramagnetic phase boundary in the composition-spread epitaxial film of Sr_1-xCa_xRuO₃ Applied Physical Express 1, 113001 (2008)

[G]    S. Iguchi, S. Kumakura, Y. Onose, S. Bordács, I. Kézsmárki, N. Nagaosa and Y. Tokura: Optical probe for anomalous Hall resonance in ferromagnets with spin chirality Physical Review Letters 103, 267206 (2009)

Links

Szilárdtestfizika Laboratórium, Fizika Tanszék, BME

Tokura & Onose Laboratory, Department of Applied Physics, University of Tokyo

References

[1]    Kézsmárki István: Magneto-optikai spektroszkópia a modern szilárdtestkutatásban.: – avagy a látható mágnesség (Magneto-optical spectroscopy in contemporary solid-state physics – magnetism made visible, published in Hungarian), Magyar Tudomány 2009/7 (2009)

[2]    I. Kezsmarki, Y. Tomioka, S. Miyasaka, L. Demko, Y. Okimoto, Y. Tokura: Optical phase diagram of perovskite colossal magnetoresistance manganites near half doping Physical Review B 77, 075117 (2008)

[3]    I. Kezsmarki, G. Mihaly, R. Gaal, N. Barisic, A. Akrap, H. Berger, L. Forro, C. C. Homes, L. Mihaly: Separation of orbital contributions to the optical conductivity of BaVS3 Physical Review Letters 96, 186402 (2006)

[4]    I. Kezsmarki, Y. Shimizu, G. Mihaly, Y. Tokura, K. Kanoda, G. Saito: Depressed charge gap in the triangular-lattice Mott insulator kappa-(ET)₂Cu₂(CN)₃ Physical Review B 74, 201101 (2006)

[5]    I. Kezsmarki, G. Mihaly, R. Gaal, N. Barisic, H. Berger, L. Forro, C. C. Homes, L. Mihaly: Pressure-induced suppression of the spin-gapped insulator phase in BaVS3: An infrared optical study

Physical Review B 71, 193103 (2005)

[6]    I. Kezsmarki, S. Onoda, Y. Taguchi, T. Ogasawara, M. Matsubara, S. Iguchi, N. Hanasaki, N. Nagaosa, Y. Tokura: Magneto-optical effect induced by spin chirality of the itinerant ferromagnet Nd₂Mo₂O₇
Physical Review B 72, 094427 (2005)

[7]    I. Kezsmarki, N. Hanasaki, D. Hashimoto, S. Iguchi, Y. Taguchi, S. Miyasaka, Y. Tokura: Charge dynamics near the electron-correlation induced metal-insulator transition in pyrochlore-type molybdates
Physical Review Letters 93, 266401 (2004)