Functional Oxides Materials
Multiferroic materials and Cosmology
In physics beyond the Standard model the permanent electric dipole moment (EDM) of an electron plays an important role: it violates the time-reversal symmetry, which, according to the CPT theorem, leads to charge-parity symmetry violation. This symmetry breaking could be the source of the pronounced asymmetry between matter and antimatter in the visible Universe. While the Standard Model predicts an electronic EDM so tiny (10–40 e.cm.) that attempts at measuring it would be hopeless, newer particle theories predict EDMs of 8 to 12 orders of magnitude larger. So far, however, no EDM has been shown to exist. The current record in the sensitivity of the EDM measurement is 10–27 e.cm. A measurement on the material we propose, Eu0.5Ba0.5TiO3, could push this limit even lower, to 10–28 e.cm, which is already enough to confirm or disprove a number of particle theories.
The demands on a material used in such a sensitive experiment are, of course, stringent. We have therefore firstly designed it in the computer, i.e. we determined its properties from first-principles calculations. We have shown computationally that perovskite-structure europium barium titanate should exhibit the required large and pressure-dependent ferroelectric polarization, large local magnetic moments and absence of magnetic ordering at liquid-helium temperature. The material was subsequently synthesized and the first EDM measurements are being currently conducted.
K.Z. Rushchanskii, S. Kamba, V. Goian, P. Vaněk, M. Savinov, J. Prokleška, D. Nuzhnyy, K. Knížek, F. Laufek, S. Eckel, S. K. Lamoreaux, A. O. Sushkov, M. Ležaić, and N.A. Spaldin
"A multiferroic material to search for the permanent electric dipole moment of the electron", Nature Materials, 9, 649–654 (2010), DOI: 10.1038/nmat2799
The paper was highlighted in News & Views section of Nature Materials:
Dmitry Budker, "Magnetoelectrics: The Universe in a solid design", Nature Materials, 9, 608–609 (2010). DOI: doi:10.1038/nmat2809
National Geographic, article "Universe's Existence May Be Explained by New Material" by John Roach,
and Press-Release of FZ Juelich: http://www.fz-juelich.de/SharedDocs/Pressemitteilungen/UK/DE/2010/PM784.html
Properties of EuTiO3
Europium titanate, EuTiO3, was the first ternary compound of divalent Eu2+ to be identified. It is a G-type antiferromagnet (AFM) with the low magnetic ordering temperature of 5.3 K, consistent with the highly localized 4f electrons on the Eu2+ ions. Until recently, its structural ground state was thought to be the ideal cubic perovskite structure, with Pm-3m symmetry. EuTiO3 is insulating with a high dielectric constant 400 at low temperature, indicating quantum paraelectric behavior and proximity to ferroelectric instability. Strong interactions between the magnetic and dielectric properties have been recently reported. It has been suggested that EuTiO3 is the prototype for studying quantum paraelectric behavior in magnetic systems.
We performed a systematic first-principles study of the structural and vibrational properties of perovskite-structure EuTiO3. Our calculated phonon spectrum of the high symmetry cubic structural prototype shows strong M- and R-point instabilities, indicating a tendency to symmetry-lowering structural deformations composed of rotations and tilts of the oxygen octahedra. Subsequent explicit study of 14 different octahedral tilt-patterns showed that the I4/mcm, Imma and R-3c structures, all with antiferrodistortive rotations of the octahedra, have significantly lower total energy than the prototype Pm-3m structure. We discussed the dynamical stability of these structures, and the influence of the antiferrodistortive structural distortions on the vibrational, optical and magnetic properties of EuTiO3, in the context of recent unexplained experimental observations.
These results are published in
K.Z. Rushchanskii, N.A. Spaldin, and M. Ležaić, "First-principles prediction of oxygen octahedral rotations in perovskite-structure EuTiO3", Phys. Rev. B 85, 104109 (2012), DOI: 10.1103/PhysRevB.85.104109
S. Kamba1, V. Goian, M. Orlita, D. Nuzhnyy, J. H. Lee, D.G. Schlom, K.Z. Rushchanskii, M. Ležaić, T. Birol, C.J. Fennie, P. Gemeiner, B. Dkhil, V. Bovtun, M. Kempa, J. Hlinka, and J. Petzelt, "Magnetodielectric effect and phonon properties of compressively strained EuTiO3 thin films deposited on (001)(LaAlO3)0.29-(SrAl1/2Ta1/2O3) 0.71", Phys. Rev. B 85, 094435 (2012), DOI: 10.1103/PhysRevB.85.094435
Studying multiferroicity in GaFeO3 material
Magnetoelectric materials, presenting a coupling between their magnetic and electric properties, alow the control of the magnetization of a material throuth an electric field. This opens perspectives towerds a new type of non-volatile memory with high endurance, low access time and low power consumption. Unfortunately, materials with a magnetoelectric coupling that is large enough for industrial applcation are scarce. Moreover, those presenting this effect at room temperature and having at the same time non-zero magnetization are evern scarcer. In the GALium Iron room temperature MagnetoElectric Oxides (GALIMEO) project a study of the possibilities in terms of magnetoelectricity of Ga2-xFexO3 (GFO) was proposed.
This material is still poorely investigated, however it is pyroelecttric, ferrimagnetic and its magnetoelectric coupling is among the strongest observed for single-phase materials. The goal of the project is to perform a complete experimental and theoretical study of magnetoelectricity in GFO. The theoretical part consist in first-principles calculations aiming at determining the origin of the magnetoelectric effect in GFO, with a view to find ways to increase it.
The image shows theoretical equation of state for several high-pressure phases of GFO.
Oxide interfaces with polar discontinuities
In the last years, oxide interfaces have attracted considerable attention due to the emerging novel properties, which do not exist in the corresponding parent bulk compounds. For example, joining the two band insulators LaAlO3 and SrTiO3 on the (001) faces can induce a wealth of new properties ranging from conductivity, to magnetism, and even to superconductivity. On the other hand, intermixing at the interface or the formation of a ferroelectric-like polarization in the insulators are mechanisms that may contribute to prevent the divergence of the electric potential. Recent experiments reported intermixing for the related DyScO3/SrTiO3 interface. Our simulations by means of density functional theory investigated the two mentioned scenarios for avoiding the polar catastrophe, either by forming a mixed layer at the interface or, in a sharp interface, by formation of a polarization in the SrTiO3 substrate, finding a clear preference of the former mechanism with respect to the latter. The calculated ground state configuration was confirmed by experimental observations: Dy and Sr atoms (grey and golden spheres in the Figure on the right, respectively) order at the boundary between the insulators, also Ti and Sc (blue and green, respectively) mix in the interface layer.
Magnetoelectric coupling in Ferroelectric-Metal interfaces.
Multiferroics are materials which exhibit more that one ferroic order parameter. They can be made of a single phase, where multiple ferroic order parameters co-exist simultaneously, or of composites, where different ferroic order parameters are combined in separate phases. Due to the limited number of known single phase multiferroics, most of which present multiple ordering only at low temperatures, engineering of composite junctions based on interfaces of magnetic and ferroelectric compounds are therefore of great scientific interest but are also promising due to their potential applications.
Cobalt-platinum alloys are known as compounds with a strong potential for applications in magnetic data storage, due to the strong exchange interactions and strong spin-orbit coupling (and, as a consequence, a large magnetocrystalline anisotropy energy).
The image shows structure used in ab initio calculations of the magneto-electric coupling in cobalt-platinum alloys interfaced with BaTiO3 ferroelectric.
Phonon-paramagnon coupling in Hexagonal-YMnO3 multiferroics.
Hexagonal Yttrium manganese oxide (h-YMnO3) is known from the 1960th to have ferroelectric (FE) and antiferromagnetic (AFM) transitions within a single compound. Recent renewed interest in compounds combining several order parameters made it one of the model materials for investigation of the physics observed in multiferroics. In contrast to its orthorhombic phase, where ferroelectricity is induced by spin waves and is therefore very weak and observable only below T=30K, hexagonal phase is FE already below T=914K. However, magnetic ordering occurs only at TN=80K, where transition to non-collinear AFM state is observed.
Due to frustration of the magnetic spins localized on Mn atoms, a lot of interesting physics was observed in these materials. Non-collinear AFM ground state has the same symmetry as crystalline lattice, therefore the linear magnetoelectric coupling is forbidden in these materials. However, experimentally several interesting effects were observed: optical second harmonic generation, reversing the AFM order parameter by switching ferroelectric polarization, etc. Discovery of three magnon branches below TN using inelastic neutron scattering opens new frontiers for investigation of spin-phonon interaction.Neutron scattering experiments pointed out that short-range correlations between spins at Mn atoms exist also in paramagnetic state. This observation motivated detailed investigation of the possible magnon-phonon interaction in the paramagnetic state of this material. Additional absorption areas in low-energy region were found in low-energy regions which could be associated with multiphonon and phonon-paramagnon interaction. Related to this it is important to have information not only about long-wave phonons but also to know their dispersion in the whole Brilloin zone, because of short-range interaction of spins above Neel temperature, which is pronounced at the Brilloin zone edges.
The image shows the dispersion of the phonons in polar phase of h-YMnO3, calculated with density functional perturbation theory.
For more details please refer to the papers
C. Kadlec, V. Goian, K.Z. Rushchanskii, P. Kužel, M. Ležaić, K. Kohn, R. V. Pisarev, and S. Kamba, "Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3", Phyr. Rev. B, 84, 174120 (2011), DOI: 10.1103/PhysRevB.84.174120
- K.Z. Rushchanskii, and M. Ležaić, "Ab Initio Phonon Structure of h-YMnO3 in Low-Symmetry Ferroelectric Phase", Ferroelectrics, Volume: 426, Pages: 90-96 (2012), DOI: 10.1080/00150193.2012.671116