Elsa Lhotel: Magnetic Fragmentation in Pyrochlore Iridates and Ruthenates

CNRS Grenoble

Date: Thursday, March 13th, 2025

Location: Elings Hall, room 1601

Time: 12:30 pm - 1:30 pm

Magnetic Fragmentation in Pyrochlore Iridates and Ruthenates

Abstract: Among the original magnetic states which emerge from frustrated magnetic systems, spin ice has aroused a strong interest because its macroscopically degenerate ground state belongs the class of Coulomb phases: its organising principle is dictated by a local constraint called the ice-rule, which can be interpreted as a divergence free condition of an emergent gauge field. Magnetic excitations in spin ice, called magnetic monopoles, correspond to a violation of the local ice-rule and introduce a non-zero divergence in the emergent field. Recently, theoreticians introduced the concept of magnetic moment fragmentation, whereby the local magnetic moment field fragments into the sum of two parts, a divergence full and a divergence free part. A prominent example of fragmentation occurs in spin ice, if the monopoles organise as a crystal of alternating magnetic charges. In that case, the fragmentation leads to the superposition of an ordered configuration and of an emergent Coulomb phase. Experimentally, the fragmentation manifests itself via the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. In this talk, I will show how this magnetic fragmented state can be stabilized in pyrochlore iridates R2Ir2O7, with R=Ho, Dy. Then I will present a novel type of fragmented state that we recently discovered in in Ho2Ru2O7. Those phases could be identified thanks to powder neutron diffraction as well as thermodynamic measurements (specific heat, magnetization), down to very low temperature (50mK).

Bio: Elsa Lhotel has been a CNRS researcher at Institut Néel in France since 2007. She works on the magnetic properties of materials at very low temperature. She initially focused on molecular magnetism and is now mainly interested in frustrated magnetism, especially spin liquids, spin ices and fragmented states and their magnetocaloric properties. Her approach combines macroscopic (magnetometry, specific heat) and neutron scattering measurements.