Field-Induced Hidden Order in Magnetoelectric TbPO₄

by Rasmus Toft-Petersen (European Spallation Source)

Monday 12 Jan 2026, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

In magnetic f-electron systems, unquenched orbital angular momentum can result in the ordering of multipolar moments, often termed “hidden order” [1]. Although challenging to observe experimentally, hidden order can give rise to novel phases, such as octupole liquids [2], and may even influence the emergence of superconductivity [3].

Ferroquadrupolar order is well established in rare earth orthovanadates, with TmVO₄ serving as a benchmark model system at ambient conditions. This study centers on the more intricate compound TbPO₄, notable for its exceptionally strong magnetoelectric coupling [4,5]. TbPO₄’s complex low-energy crystal field level scheme and pronounced magnetostriction lead to a rich phase diagram, and near-degeneracy of the magneto-elastic ground state is hypothesized to be the underlying cause of the soft mode behavior leading to the strong magnetoelectric effect.

Our recent investigations reveal a striking field-induced ferroquadrupolar phase, stemming from magnetoelastic interactions. Using synchrotron X-ray and neutron diffraction, heat capacity measurements, and mean-field modeling, we demonstrate that above a critical magnetic field, ferroquadrupolar order becomes the dominant order parameter, competing with field-induced magnetic polarization. This is evidenced by significant lattice distortion and suppression of dipolar antiferromagnetic order. Additionally, magnetoelastic hybrid modes, explored with the CAMEA spectrometer, show a multifaceted character and highlight the necessity of studying magnetic dynamics across different phases to fully understand such complex materials. I will be discussing our recent diffraction results and the magnetoelastic phase diagram, as well as the status of our ongoing efforts to understand the magnetoelastic dynamics.   

[1] P. Santini et al., Rev. Mod. Phys. 81, 807 (2009).
[2] R. Sibille et al., Nat. Phys. 16, 546 (2020).
[3] C. Pfleiderer, Rev. Mod. Phys. 81, 1551 (2009).
[4] S. Bluck & H. G. Kahle, J. Phys. 21, 5193 (1988).
[5] J. P. Rivera, Eur. Phys. J. B 71, 299–313 (2009).