Origin of the large ferroelectric polarization enhancement under high pressure for multiferroic DyMnO3 studied by polarized and unpolarized neutron diffraction
Physical Review B American Physical Society 102:8 (2020) 85131
Abstract:
The multiferroic perovskite rare earth manganites RMnO3 (R=Dy, Tb, Gd) are known as multiferroics exhibiting pressure-induced gigantic ferroelectric polarization. In this study, we have investigated the magnetic orderings in the pressure-induced phases for DyMnO3, by neutron diffraction and spherical neutron polarimetry (SNP) experiments up to 8.0 GPa. The magnetic ordering for Mn spins changes from the incommensurate bc-cycloid to the commensurate collinear E-type structure with kMn=0,12,0 above 4.0 GPa, which is concomitant with the appearance of a giant ferroelectric polarization. The magnetic ordering for the Dy spins has been determined to be a noncollinear spin structure with a and b spin components and kDy=(0,12,0) for the low- and high-pressure phases. The magnetic field along the a axis, Ha, affects the Dy ordering, which is seen in the changes in the k vector from kDy=(0,12,0) in Ha≤3T to kDy=(0,0,0) in Ha≥3T. Considering the lattice distortion generated by the determined magnetic orderings through the exchange striction mechanism, we conclude that the exchange striction for rare earth and Mn bonds, which is added to the uniform polarization generated by the E-type Mn ordering, is strongly related to the significant magnetic field enhancement of ferroelectric polarization in the high-pressure phase of the rare earth manganites.Low-temperature thermal transport measurements of oxygen-annealed Yb2Ti2O7
Physical Review B American Physical Society 102:1 (2020) 14434
Abstract:
Low-temperature thermal conductivity measurements have been conducted on an oxygen-annealed single crystal of Yb2Ti2O7 from 60 mK to 50 K and in magnetic fields up to 8 T applied in the [111] crystallographic direction. The temperature dependence of the conductivity in zero field shows a significant peak in thermal conductivity at T∼13 K and a sharp anomaly at Tc∼0.2 K suggesting that the sample's behavior is representative of the high-purity limit, with low levels of disorder. The magnetic field dependence of the thermal conductivity close to Tc reveals a reentrant magnetic phase for a field in the [111] direction. With this information, analysis of the very low magnetic field behavior of the thermal conductivity suggests the presence of significant fluctuations close to the phase line.Resonant x-ray scattering study of diffuse magnetic scattering from the topological semimetals EuCd2As2 and EuCd2Sb2
Physical Review B American Physical Society 102 (2020) 14408
Abstract:
We have investigated the magnetic correlations in the candidate Weyl semimetals EuCd2Pn2 (Pn=As, Sb) by resonant elastic x-ray scattering at the Eu2+ M5 edge. The temperature and field dependence of the diffuse scattering of EuCd2As2 provide direct evidence that the Eu moments exhibit slow ferromagnetic (FM) correlations well above the Néel temperature. By contrast, the diffuse scattering in the paramagnetic phase of isostructural EuCd2Sb2 is at least an order of magnitude weaker. The FM correlations present in the paramagnetic phase of EuCd2As2 could create short-lived Weyl nodes.Polarizing an antiferromagnet by optical engineering of the crystal field
Nature Physics Nature Research 16 (2020) 937-941
Abstract:
Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.Approaching the quantum critical point in a highly correlated all-in-all-out antiferromagnet
Physical Review B American Physical Society 101:22 (2020) 220404