Germanium-based multiferroic compounds with melilite structure

  Ba2CoGe2O7 Copyright: © IfK Crystal and magnetic structure in Ba2CoGe2O7 at 4 K.

Magnetoelectric multiferroic compounds where both electric polarization and magnetic order coexist at the same temperature and are coupled to each other are of significant interest in the modern solid-state physics because of their potential applicability for the low-energy information storage media. The elucidation of the mechanisms of magnetoelectric coupling in different materials as well as finding of the new types of magnetoelectric multiferroics remains a challenging task from both theoretical and experimental point of view.

  Magnon modes in Ba2CoGe2O7 Copyright: © IfK Magnon modes evolution with applied magnetic field in Ba2CoGe2O7 at 4 K.

Electric polarization under applied magnetic fields has been reported for different barium based melilite type germanates Ba2XGe207 (X = Mn, Co, Cu) and Co åkermanite silicates A2CoSi207 (A = Ca, Sr). The peculiar behavior of the electric polarization with magnetic fields (both direction and magnitude dependent), the occurrence of such interesting magneto-optical phenomena like giant directional dichroism of terahertz light in resonance with magnetic excitations (electromagnons), optical magneto-chiral effect, quadrochroism generated by the optical magnetoelectric effect, as well as quantum fluctuations in dielectric parameters at high fields rules out some of the generally accepted models for type II multiferroics, namely spin-current or exchange-striction. Various new microscopic mechanisms have been proposed to describe strong magnetoelectric coupling in these materials. Most of them are bases on purely theoretic considerations. A precise knowledge of the magnetic order and the magnetic anysotropy at the unit-cell level and their evolution with applied magnetic and electric field would significantly help in the distinguishing between different theoretical models and enhance the understanding of magnetoelectric coupling in these interesting materials. Also, understanding the role of the specific magnetic atoms and magnetic interactions leading to different magnetic structures within the same crystal symmetry, and their impact on the emerging electric polarization are of fundamental interest.

 

Team:

Henrik Thoma, M. Sc. (IfK-JCNS PhD student)
Dr. Martin Meven (scientist)
Dr. Rajesh Dutta (postdoc)
Prof. Dr. Georg Roth (advisor of the thesis)

Publications:

  1. A. Sazonov, V. Hutanu, M. Meven, G. Roth, R. Georgii, T. Masuda and B. Nafradi, “Crystal structure of magnetoelectric Ba2MnGe2O7 at room and low temperatures by neutron diffraction”, Inorg. Chem. 57, 5089−5095 (2018).
  2. A. Sazonov, V. Hutanu, M. Meven, G. Roth, H. Murakawa, Y. Tokura, V. K. Guduru, L. C. J. M. Peters, U. Zeitler, L. F. Kiss, D. Szaller, B. Nafradi, and I. Kezsmarki, “Magnetic structure of the magnetoelectric material Ca2CoSi2O7” Phys. Rev. B 95, 174431 (2017).
  3. A. Sazonov, M. Meven, G. Roth, R. Georgii, I. Ke´zsma´rki, V. Kocsis, Y. Tokunaga, Y. Taguchi, Y. Tokura, & V. Hutanu, “Origin of forbidden reflections in multiferroic Ba2CoGe2O7 by neutron diffraction: symmetry lowering or Renninger effect?” J. Appl. Cryst. 49, 556-560 (2016).
  4. V. Hutanu, A. P. Sazonov, M. Meven, G. Roth, A. Gukasov, H. Murakawa, Y. Tokura, D. Szaller, S. Bordács, I. Kézsmárki, V. K. Guduru, L. C. J. M. Peters, U. Zeitler, J. Romhányi, and B. Náfrádi, “Evolution of two-dimensional antiferromagnetism with temperature and magnetic field in multiferroic Ba2CoGe2O7”, Phys. Rev. B 89, 064403 (2014).
  JCNS Copyright: © JCNS

Funding:

Jülich Centre for Neutron Science/Forschungszentrum Jülich,
Tasso Springer Fellowship Program