arXiv subjects

C. I. Hiley

Publications and source records attributed to C. I. Hiley.

3 recordsLinked to original sources

arXiv2026

Hidden Magnetic Octupolar Order driven by Spin-Orbit coupling in KFeF$_3$

Phase transitions are typically driven by symmetry-breaking structural distortions that lift electronic degeneracies, yet in some cases, these transitions may be driven by a hidden order without conventional structural signatures. Here, we demonstrate that the 3$d$ perovskite, KFeF$_3$, hosts such a hidden instability. Using a combination of high-resolution powder diffraction, magnetometry, symmetry-based analysis and first principles calculations, we reveal that while the 90 K cubic to rhombohedral transition arises from conventional magnetostriction accompanying antiferromagnetic order, a second transition at 40 K lowers the symmetry to monoclinic without any detectable Jahn-Teller distortion or translational symmetry-breaking. Symmetry-based Landau analysis supports a hidden zone centered magnetic octupole order parameter, whose improper coupling accounts for the weak ferromagnetism and pronounced symmetry-breaking strain. Density functional calculations show that spin-orbit coupling suppresses the competing Jahn-Teller instability, and thereby favors higher rank magnetic multipolar degrees of freedom. These findings establish KFeF$_3$ as a model system in which the competing energy scales between spin-orbit coupling and orbital degeneracy result in the emergence of hidden, multipolar order. We show that the control of such magnetic, multipolar order could, in principle, provide a route to induce altermagnetism.

cond-mat.str-el↗

arXiv2025

Signatures of Orbital Order and Disorder in Fluoro-Perovskites with $t_{2g}$ Electronic Degeneracies

A detailed high-resolution, variable temperature powder diffraction study of the fluoro-perovskites NaFeF$_3$ and NaCoF$_3$ is performed to probe their orbital ordering transitions. Through analysis of the symmetry adapted macrostrains and atomic distortions, we show that NaFeF$_3$ undergoes a C-type orbital order transition associated with the $t_{2g}^4$ states of Fe$^{2+}$. Counter-intuitively, the phase transition leading to the orbital order appears second order-like, which contradicts the thermodynamic requirements for electronic and isosymmeric phase transitions, implying that there must be an associated hidden symmetry breaking. On the other hand, for NaCoF$_3$, consideration of the symmetry adapted strains allows us to confidently rule out the occurrence of any long-range orbital orders down to 4 K. Since NaCoF$_3$ is an insulator with quenched orbital angular momentum at this temperature, our findings point towards a novel kind of orbital disorder associated to the $t_{2g}^5$ electronic degeneracy.

cond-mat.str-el↗

arXiv2015

Antiferromagnetism at T > 500 K in the Layered Hexagonal Ruthenate SrRu2O6

We report an experimental and computational study of magnetic and electronic properties of the layered Ru(V) oxide SrRu2O6 (hexagonal, P-3 1m), which shows antiferromagnetic order with a Néel temperature of 563(2) K, among the highest for 4d oxides. Magnetic order occurs both within edge-shared octahedral sheets and between layers and is accompanied by anisotropic thermal expansivity that implies strong magnetoelastic coupling of Ru(V) centers. Electrical transport measurements using focused ion beam induced deposited contacts on a micron-scale crystallite as a function of temperature show p-type semiconductivity. The calculated electronic structure using hybrid density functional theory successfully accounts for the experimentally observed magnetic and electronic structure and Monte Carlo simulations reveals how strong intralayer as well as weaker interlayer interactions are a defining feature of the high temperature magnetic order in the material.

cond-mat.str-el↗