Stephane Raymond

The ground state and Hamiltonian of the honeycomb lattice material BaCo 2 ( AsO 4 ) 2 hosting magnetic Co 2 + have been debated for decades. The recent proposal for anisotropic bond-dependent interactions in such honeycomb cobaltates has raised the prospect of revisiting its Hamiltonian in the context of Kitaev physics. To test this hypothesis, we have combined magnetization, ac susceptibility, and neutron scattering measurements on a BaCo 2 ( AsO 4 ) 2 single crystal, together with advanced modeling. Our experimental results highlight a collinear magnetic ground state with intrinsic disorder associated with an average incommensurate propagation vector. Monte Carlo simulations and linear spin wave calculations were performed to obtain a spin model compatible with this unusual ground state, the dispersion of magnetic excitations, and a magnetization plateau under magnetic field. We thus show that bond-dependent anisotropic interactions, including Kitaev-like interactions, are necessary to account for the puzzling properties of this long-explored material, and are hence a general ingredient in the cobaltates.

Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr 3 is an excellent realization of the S = 1 / 2 antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic experiments with both unpolarized and polarized neutrons at all applied magnetic fields up to saturation. We observe extensive excitation continua, which cause strong renormalization and the decay of single magnons at higher fields, while coherent features include field-induced “shadows” of the single magnons and the spectacular emergence of a rotonlike excitation. To guide and interpret our experiments, we performed systematic calculations by the method of cylinder matrix-product states that provide quantitative agreement with the neutron scattering data and a qualitative benchmark for the spectral signatures of strong quantum fluctuations even in the absence of magnetic frustration.

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

An inelastic neutron scattering technique is employed to investigate the paramagnetic spin dynamics in a single-crystalline sample of the magnetocaloric compound MnFe 4 Si 3 . In the investigated temperature range, 1.033 × T C to 1.5 × T C , where T C is the Curie temperature, the spin fluctuations are well described by the ferromagnetic Heisenberg model predictions. Apart from the Heisenberg exchange, additional pseudodipolar interactions manifest through a finite long-wavelength relaxation rate that vanishes at the transition temperature ( T C = 305 K). Based on the characteristic extent of spin fluctuations in wave-vector and energy space, we determine that the nature of magnetism in MnFe 4 Si 3 is localized above room temperature. This contrasts with the most celebrated Mn- and Fe-based magnetocaloric materials that are considered as itinerant magnets. The field dependence of the paramagnetic spectra shows a strong suppression of the quasielastic excitations, while a field-induced spin-wave mode appears at finite-energy transfers for a magnetic field of 2 T. This modification of the spectra suggests a decrease of magnetic entropy with applied magnetic field that finds echo in the magnetocaloric properties of the system. Published by the American Physical Society 2025

The antiferromagnetic structure of Yb_{3}Ga_{5}O_{12} is identified by neutron diffraction experiments below the previously known transition at T_{λ}=54 mK. The magnetic propagation vector is found to be k=(1/2,1/2,0), an unusual wave vector in the garnet structure. The associated complex magnetic structure highlights the role of exchange interactions in a nearly isotropic system dominated by dipolar interactions and finds echoes with exotic structures theoretically proposed. Reduced values of the ordered moments may indicate significant quantum fluctuations in this effective spin-1/2 geometrically frustrated magnet.

Encoding information in antiferromagnetic (AFM) domains is a promising solution for the ever growing demand in magnetic storage capacity. What fundamentally enables ultrahigh density AFM-based spintronics is the absence of unintentional crosstalk between different domain states due to vanishing stray fields [1]. However, the absence of macroscopic magnetization is detrimental to the manipulation and detection of AFM domains. Disentangling the merits and disadvantages of small stray fields seemed so far unattainable. In this work, we report evidence for a new AFM domain selection mechanism based on the anisotropy in the susceptibility not induced by Zeeman energy terms, but by the relative orientation of the external magnetic field to the two perpendicularly oriented k-domains only. As a result, the charge transport response is controlled by the rotation of the magnetic field. In particular, a pronounced new anisotropic magnetoresistance effect is found in the AFM phase of bulk materials Nd(1-x)Ce(x) CoIn(5), due to differences in transport scattering rates for currents applied parallel and perpendicular to the spin-density wave modulation. Our results and the domain switching theory [2] indicate that this constitutes a new universal effect across multiband materials and thus provide a novel mechanism to control and detect AFM domains opening new perspectives for AFM sprintronics.