arXiv subjects

J. Gainza

Publications and source records attributed to J. Gainza.

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

Successive magnetic transitions and multiferroicity in layered honeycomb BiCrTeO$_{6}$

Low-dimensional magnetic systems based on honeycomb lattices provide a promising platform for exploring exotic quantum phenomena that emerge from the intricate interplay of competing spin, orbital, lattice, and dipolar degrees of freedom. Here, we present a comprehensive study of the layered honeycomb lattice antiferromagnet BiCrTeO$_6$ using magnetization, specific heat, muon spin--relaxation ($\mu$SR) spectroscopy, dielectric, pyrocurrent, and high-resolution synchrotron X-ray diffraction (SXRD) measurements. Our results reveal an array of intriguing and strongly correlated phenomena, including two successive antiferromagnetic transitions at $T_{\rm N1}\approx16$ K and $T_{\rm N2}\approx11$ K, a pronounced magnetodielectric coupling effect, and ferroelectric order at $T_{\rm N2}$. Consequently, this compound emerges as a new spin-driven multiferroic system. The SXRD analysis reveals a magnetoelastic-coupling-induced structural phase transition at $T_{\rm N2}$, characterized by a symmetry lowering from P$\bar{3}$1c (163) to P31c (159), which likely triggers the onset of ferroelectricity. In addition to its low-temperature multiferroic behavior, the system exhibits dielectric relaxor characteristics at higher temperatures within the paramagnetic region ($T<50$ K), which is intrinsically linked to the antisite disorder of Cr and Te atoms.

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↗