Dr Dharmalingam Prabhakaran

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.

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.

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.

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.

Abstract:

Continuous quantum phase transition involving all-in-all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in Sm2Ir2O7 using direct resonant X-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure Pc=6.30GPa, while the lattice symmetry remains in the cubic Fd-3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R2Ir2O7 reveals that the approach to the AIAO quantum phase transition is characterized by contrasting evolutions of the pyrochlore lattice constant a and the trigonal distortion surrounding individual Ir moments, which affects the 5d bandwidth and the Ising anisotropy, respectively. We posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low pressure scale of the AIAO quantum phase transition in Sm2Ir2O7 identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl semimetal state.