Manh Duc Le

Chirality in magnetic systems gives rise to a wide range of exotic phenomena, yet its influence in S = 1 chains remains largely unexplored. Herein, we present a comprehensive experimental study of a chiral antiferromagnetic S = 1 chain, [ Ni ( pym ) ( H 2 O ) 4 ] SO 4 · H 2 O (pym = pyrimidine), where the Ni(II) octahedral orientation exhibits a fourfold chiral periodicity. Muon-spin rotation measurements indicate the onset of long-range magnetic order below T N = 1.82 ( 1 ) K . Neutron diffraction measurements reveal a chiral antiferromagnetic order driven by a chiral modulation of the easy-axis anisotropy direction, rather than the typical scenario of Dzyaloshinskii-Moriya interactions, geometrical frustration, or higher-order interactions. Inelastic neutron scattering measurements reveal dispersive spin-wave excitations well described by linear spin-wave theory, with Hamiltonian parameters J 0 = 6.81 ( 1 ) K (intrachain exchange), J 1 a ′ = − 0.09 ( 1 ) K (interchain exchange), and D = − 3.02 ( 1 ) K (easy-axis single-ion anisotropy). These parameters are further validated by Monte Carlo simulations of the magnetization. Additionally, the inelastic neutron scattering data reveal multiple dispersionless bands, suggesting the presence of further excitations beyond the scope of our linear spin-wave theory.

Chiral magnons, the quanta of handed spin waves, transport spin angular momentum without energy loss due to Joule heating. The recently discovered altermagnets were proposed to host chiral magnons arising from a nonrelativistic exchange mechanism, similar to that in ferromagnets but without net magnetization, offering a stray-field-free platform for efficient magnon spin-current manipulation. In this Letter, we directly observed chiral magnons in the altermagnetic prototype MnTe using polarized inelastic neutron scattering. Furthermore, the magnon chirality was found to be reversibly switched by magnetic-field control, establishing a robust foundation for functional altermagnetic magnonics.

In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Here we study the metal-organic material ( C 5 H 9 NH 3 ) 2 CuBr 4 (Cu-CPA), in which an explanation of the low-lying excitations depends crucially on a full understanding of both the spin and lattice subsystems. We report high-resolution neutron spectroscopy enabled by large, deuterated single crystals that reveal how both sectors are affected by the recently discovered structural phase transition. By measuring over several Brillouin zones, we disentangle the vibrational contribution to the spectrum in order to obtain an accurate estimate of the quasi-one-dimensional magnetic signal. The low-energy magnetic excitations are dominated by two gaps, Δ b = 0.41 meV and Δ a = 0.55 meV , which contribute with equal intensity ratios, confirming that Cu-CPA realizes a two-ladder spin Hamiltonian, and we deduce the magnetic interaction parameters of both ladders. The phonon spectrum contains highly localized modes at an anomalously low energy around 2 meV. For half of these modes, this characteristic frequency drops by approximately 5% as magnetic correlations become established with decreasing temperature, and we connect this apparent “elastomagnetic” behavior with the location and structure of the cyclopentylammonium rings.

In the search for unconventional magnetism, exotic quantum states are characterized by a lack of order and a broad spin excitation continuum approaching zero temperature. We study the two-dimensional triangular-lattice effective spin- 1 2 system CeMgAl ₁₁ O ₁₉ , which shows slight disorder but no magnetic ordering down to 100 millikelvin. Spin-wave analysis in the magnetic-field–polarized state determines the spin Hamiltonian featuring a mixed ferromagnetic-antiferromagnetic nearest-neighbor exchange interaction [ J z = −0.024(5) milli–electron volts, J ⊥ = 0.056(3) milli–electron volts]. This places the system near an exactly solvable point of the spin- 1 2 triangular-lattice XXZ model ( J z = − 1 2 J ⊥ ) with extensive ground-state degeneracy. In zero field, neutron spectroscopy reveals a prominent continuum; we show that this arises from an ensemble average of spin-wave spectra across the degenerate ground-state manifold. This demonstrates that the role of weak quenched disorder can be quantitatively constrained: It inhibits unique ground-state selection and stabilizes a local distribution within the degenerate manifold, yielding continuum-like spectra that necessitate a critical reevaluation of the experimental signatures of exotic quantum states.

Monoclinic compounds YbPt 5 B 2 and LuPt 5 B 2 [space group C 2 / m (No. 12), own structure type] have been studied by means of elastic and inelastic neutron scattering experiments. The heavy fermion system YbPt 5 B 2 orders antiferromagnetically below T N 1 = 7.8 K and exhibits an incommensurate magnetic structure with a temperature-dependent propagation vector, k ⃗ 1 = ( 0.194 , 0 , − 0.045 ; T = 6 K ) . Below T N 2 = 4.7 K , a transition into a commensurate structure, k ⃗ 2 = ( 0 , 0 , 0 ) , takes place. The transition at T N 2 turns out to be of first order, as obvious from commensurate and incommensurate contributions to the magnetic moments around T = T N 2 . Rather large Yb moments, m ( Yb ) > 3 μ B , yet reduced due to the Kondo effect, were obtained as a result of the splitting of the Yb 3 + eightfold-degenerate ground state into four doublets, owing to crystalline electric field (CEF) effects. Such unusually large moments in Yb systems can be explained only by an appropriate wave function ∑ i = − 7 / 2 i = 7 / 2 ± α i | i / 2 〉 , ∑ i α i 2 = 1 , constituting the CEF ground state doublet, which indeed was concluded from the present inelastic neutron data, considering the crystalline electric field theory. The overall CEF splitting is on the order of 44 meV, while the first excited level is located ≈ 25 meV above the ground state.

We have investigated the quasi-one-dimensional Ni-chain compound PbMn 2 Ni 6 Te 2 O 18 using theoretical DFT calculations, inelastic neutron scattering, and optical spectroscopy in order to understand the nature of magnetic exchange interactions. Our inelastic neutron scattering study at 5 K on a powder sample reveals two bands of magnetic excitations, the first near 8 meV and the second near 18 meV originating from the antiferromagnetic zone center near Q = 1 Å. On the other hand, at 100 K (which is above T N = 86 K) a broad diffuse scattering signal is observed indicating the presence of short range magnetic correlations. We have analyzed the magnetic excitations based on the Linear Spin Wave Theory (LSWT) and compared the experimentally estimated exchange parameters with the DFT calculations. Our analysis reveals that the value of the exchange parameter at the larger distance ( d = 3.654 Å ) J 3 = 4.21 ( 8 ) meV between Ni-Ni (from interchain) is the strongest amongst the allowed six exchange parameters, which suggests that this system is not really a quasi-one-dimensional and confirmed by the absence of a Haldane gap. We have also presented the electronic structure calculations. The spin-polarized partial density of states (DOS) projected onto the Mn-d and Ni-d orbitals reveals that the Ni − d x 2 − y 2 contribution is dominant below the Fermi level in the spin-up and spin-down channel, while a minimal contribution from spin-up Mn states in the occupied region, suggesting a nearly high-spin state. The estimated Néel temperature, based on experimental exchange parameters is found to be in close agreement with the experimental value. Published by the American Physical Society 2025