Directional Dichroism and Spin-Wave Excitations in the Multiferroic Material BCGO
Nijmegen scientists studied spin excitations in the magnetically ordered phase of noncentrosymmetric Ba2CoGe2O7 in magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra they found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. They show that a multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active. Furthermore, this material shows a large directional dichroism, i.e. a difference in absorption for counterpropagating far-infrared light beams.
Multiferroics are materials that exhibit more than one ferroic order parameter (ferromagnetism, ferroelectricity, ferroelasticity) simultaneously. In the field of multiferroics, symmetry plays an important role; space inversion, for instance, will reverse the direction of the electrical polarization but magnetization (an axial vector) will remain the same. On the other hand, time reversal will reverse the direction of the magnetization but will leave electrical polarization intact. Consequently, magnetoelectric matarials require simultaneous violation of space and time inversion symmetry. In the case of Ba2CoGe2O7, a slightly canted antiferromagentic ordering breaks time reversal symmetry, whereas Co-O hybridization is responsible for a net polarization. In this material, as in most multiferroics, the ferromagnetic and ferroelectric order parameters are coupled. Depending on the direction of the magnetic field (but not the sign), a left- or right-handed chirality is induced, which reveals itself through the magneto-chiral effect: a difference in absorption for light propagating parallel or antiparallel to the magnetic field.
Magnetic field dependence of the far-infrared absorption spectra in Ba2CoGe2O7 for a representative set of crystal orientations and light polarizations. The grey lines show the field dependence of the modes obtained in this multiboson spin-wave approach.
This work is published in: K. Penc, J. Romhányi, T. Rõõm, U. Nagel, Á. Antal, T. Fehér, A. Jánossy, H. Engelkamp, H. Murakawa, Y. Tokura, D. Szaller, S. Bordács and I. Kézsmárki, "Spin-Stretching Modes in Anisotropic Magnets: Spin-Wave Excitations in the Multiferroic Ba2CoGe2O7", Phys. Rev. Lett. 2012, 108, 257203.