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Register a new userTerahertz detection of in-situ switching between antiferromagnetic domains in the multiferroic Ba$_2$CoGe$_2$O$_7$
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Non-reciprocal directional dichroism assigns an optical diode-like property to non-centrosymmetric magnets, making them appealing for low-dissipation optical devices. However, the direct electric control of this phenomenon at constant temperatures is scarce. In Ba$_2$CoGe$_2$O$_7$, we demonstrate the isothermal electric switch between domains possessing opposite magnetoelectric susceptibilities. Combining THz spectroscopy and multiboson spin-wave analysis, we show that unbalancing the domain population realizes the non-reciprocal light absorption of spin excitations.
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We report on spherical neutron polarimetry and unpolarized neutron diffraction in zero magnetic field as well as flipping ratio and static magnetization measurements in high magnetic fields on the multiferroic square lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$. We found that in zero magnetic field the magnetic space group is $Cmm2$ with sublattice magnetization parallel to the [100] axis of this orthorhombic setting. The spin canting has been found to be smaller than $0.2^circ$ in the ground state. This assignment is in agreement with the field-induced changes of the magnetic domain structure below 40 mT as resolved by spherical neutron polarimetry. The magnitude of the ordered moment has been precisely determined. Above the magnetic ordering temperature short-range magnetic fluctuations are observed. Based on the high-field magnetization data, we refined the parameters of the recently proposed microscopic spin model describing the multiferroic phase of Ba$_2$CoGe$_2$O$_7$.
For a symmetry consistent theoretical description of the multiferroic phase of Ba$_2$CoGe$_2$O$_7$ a precise knowledge of its crystal structure is a prerequisite. In our previous synchrotron X-ray diffraction experiment on multiferroic Ba$_2$CoGe$_2$O$_7$ at room temperature we found forbidden reflections that favour the tetragonal-to-orthorhombic symmetry lowering of the titled compound. Here, we report the results of room-temperature single-crystal diffraction studies with both hot and cold neutrons to differentiate between the real symmetry lowering and multiple diffraction (the Renninger effect). A comparison of the experimental multiple diffraction patterns with simulated ones rules out the symmetry lowering. Thus, the structural model based on the tetragonal space group $Pbar{4}2_1m$ was selected to describe the Ba$_2$CoGe$_2$O$_7$ symmetry at room temperature. The precise structural parameters from neutron diffraction at 300K are presented and compared with the previous X-ray diffraction results.
We studied the novel multiferroic material Sr$_2$FeSi$_2$O$_7$, and found 3 absorption modes above the magnetic ordering transition temperature using time-domain terahertz spectroscopy. These absorption modes can be explained as the optical transitions between the spin-orbit coupling and crystal field split 3d$^6$ Fe$^{2+}$ ground state term in this material. Consideration of the compressed tetrahedral environment of the Fe$^{2+}$ site is crucial to understand the excitations. We point out, however, discrepancies between the single-site atomic picture and the experimental results.
Exotic spin-multipolar ordering in spin transition metal insulators has so far eluded unambiguous experimental observation. A less studied, but perhaps more feasible fingerprint of multipole character emerges in the excitation spectrum in the form of quadrupolar transitions. Such multipolar excitations are desirable as they can be manipulated with the use of light or electric field and can be captured by means of conventional experimental techniques. Here we study single crystals of multiferroic Sr$_2$CoGe$_2$O$_7$, and show that due to its nearly isotropic nature a purely quadrupolar bimagnon mode appears in the electron spin resonance (ESR) spectra. This non-magnetic spin-excitation couples to the electric field of the light and becomes observable for a specific experimental configuration, in full agreement with a theoretical analysis of the selection rules.
We measured the temperature dependences of the static magnetization and the spin excitation in a square-lattice multiferroics Ba$_2$MnGe$_2$O$_7$. An anisotropy gap of the observed low energy mode is scaled by electric polarization rather than a power of sublattice moment. Spin nematic interaction in effective spin Hamiltonian, which is equivalent to interaction of electric polarization, is responsible for the easy-axis anisotropy. The nontrivial behavior of the anisotropy gap can be rationalized as change of the hybridized $d$-$p$ orbital with temperature, leading to the temperature dependence of the spin nematic interaction.
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