Eric Ressouche

We use spherical neutron polarimetry to determine the ground state magnetic structure of Mn 3 Sn . We find that Mn 3 Sn adopts an inverse triangular structure with spins parallel to 〈 100 〉 (type III) rather than spins parallel to 〈 110 〉 (type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguishing between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains in this phase are decoupled from the magnetic field and can therefore not be controlled by any known method.

Combining macroscopic measurements, neutron scattering, and modeling, we identify in the GeFe_{2}O_{4} spinel a correlated paramagnetic state resulting from the predominance of third-neighbor antiferromagnetic interactions. These interactions materialize four interlaced families of triangular planes with 120° spins emerging from the underlying pyrochlore lattice. At lower temperatures, a phase transition occurs to a noncoplanar spin structure that is characterized by six propagation vectors. This unusual multi-q order is triggered by the presence of weaker interactions up to the sixth neighbors. The system is remarkably successful in coupling the different triangular planes while maintaining their two-dimensional 120° order. Our Letter highlights the hierarchy of interactions involved in GeFe_{2}O_{4}, which is singular among spinel compounds since first-neighbor interactions are only a small fraction of the dominant third-neighbor ones.

The discovery of multiple superconducting phases in UTe 2 boosted research on correlated-electron physics. This heavy-fermion paramagnet was rapidly identified as a reference compound to study the interplay between magnetism and unconventional superconductivity with multiple degrees of freedom. The proximity to a ferromagnetic quantum phase transition was initially proposed as a driving force to triplet-pairing superconductivity. However, we find here that long-range incommensurate antiferromagnetic order is established under pressure. The propagation vector k m = ( 0.07 , 0.33 , 1 ) of the antiferromagnetic phase is close to a wave vector where antiferromagnetic fluctuations have previously been observed at ambient pressure. These elements support that UTe 2 is a nearly antiferromagnet at ambient pressure. Our work appeals for theories modeling the evolution of the magnetic interactions and electronic properties, driving a correlated paramagnetic regime at ambient pressure to a long-range antiferromagnetic order under pressure. A deeper understanding of itinerant- f -electron magnetism in UTe 2 will be a key for describing its unconventional superconducting phases. Published by the American Physical Society 2025

The magnetic insulator Nd 3 BWO 9 has been previously proposed to realize the highly frustrated breathing kagome lattice model. We report a combination of single-crystal neutron scattering studies and numerical simulations that debunk this interpretation. We show that it is the interplane couplings that determine the physics. To explain the exotic magnetism, we derive a simple one-dimensional Ising model composed of twisted triangular spin tubes, i.e., triple braids of Ising spin chains with almost-orthogonal anisotropy frames and competing ferro-antiferromagnetic interactions. This model can account for the ground state, excitations, the numerous field-induced fractional magnetization plateau phases, and incommensurate magnetic correlations at elevated temperatures. Nd 3 BWO 9 constitutes a rare example where rich magnetic phenomena can be understood and simulated quantitatively in terms of a simple classical Hamiltonian.