arXiv subjects

P. Manuel

Publications and source records attributed to P. Manuel.

At least 19 recordsLinked to original sources

arXiv2026

Field-tuned incommensurate fan phase in an Ising-like triangular antiferromagnet

Using single-crystal neutron diffraction we report the observation of a field-tuned incommensurate phase in a field range just below magnetization saturation in the Ising-like triangular-lattice antiferromagnet Na$_2$BaCo(PO$_4$)$_2$. This phase occurs for magnetic fields applied along the Ising axis and exhibits an in-plane incommensurate propagation vector that moves along the hexagonal Brillouin zone boundary upon varying field, challenging the simple $XXZ$ antiferromagnetic description of the system. Within a semiclassical framework, we successfully identify this pre-saturation phase as a coplanar fan phase and study the role of dipole-dipole interactions and bond-dependent exchange anisotropy in its stabilization mechanism. We additionally present a minimal exchange model including interlayer couplings, which reproduces the complex magnetic phase diagram in field including the observed field-dependence of the incommensurate propagation vector in the fan phase.

cond-mat.str-el↗

arXiv2026

Coexistence of static order and spin dynamics in an S = 5/2 frustrated triangular antiferromagnet

Frustrated triangular-lattice antiferromagnets in the classical high-spin limit provide a paradigmatic setting in which the interplay of competing exchange interactions, anisotropy, and collective degrees of freedom can lead to unconventional low-energy excitations, anomalous criticality, and persistent dynamical responses. Here, we present comprehensive thermodynamic, $\mu$SR, and neutron diffraction experiments, along with first-principles calculations, on a triangular-lattice antiferromagnet, MnSnB$_2$O$_6$, where Mn$^{2+}$ ($S=5/2$) moments form a nearly perfect 2D triangular network without any anti-site disorder. The Curie-Weiss fit to the magnetic susceptibility yields a moderate Curie-Weiss temperature of $-12$ K, indicating dominant antiferromagnetic interactions between Mn$^{2+}$ moments, which is supported by first-principles calculations. Specific-heat measurements reveal the onset of long-range magnetic order at $T_{\rm N}\approx 1$ K, which is ascribed to intraplane exchange interactions. The specific heat exhibits pronounced short-range correlations above $T_{\rm N}$ and an unconventional power-law behavior, $C\propto T^{1.37}$, deep in the ordered state, suggesting the presence of non-trivial low-energy excitations. Zero-field $\mu$SR experiments down to 50~mK confirm the presence of magnetic ordering below $T_{\rm N}$, in agreement with thermodynamic and neutron diffraction experiments. The $\mu$SR measurements detect persistent spin dynamics coexisting with static magnetic order. The temperature evolution of the order parameter down to 50~mK from neutron diffraction suggests that the ordered state is consistent with a 3D Ising-like antiferromagnet. This family of archetypal frustrated magnets offers a promising venue for the experimental realization of emergent phenomena governed by competing exchange interactions and exotic low-energy excitations.

cond-mat.str-el↗

arXiv2026

Vortex order in magnetic frustrated GeNi$_2$O$_4$ and GeCo$_2$O$_4$ spinels

In the search for new spin textures based on singular magnetic objects like Bloch-points or vortices, spinel compounds emerge as an interesting playground due to the interplay between magnetic anisotropy and complex interactions that extend well beyond first neighbors on a pyrochlore lattice. Based on an exploration of the exchange interaction phase diagrams of members of the Ge$B_2$O$_4$ family with $B$=Co and Ni, we show, using simultaneous modeling of inelastic neutron scattering measurements and single-crystal neutron diffraction data, that a 2-$k$ magnetic structure may be stabilized in these compounds. This leads to a short period spin vortex crystal, a variant induced by the magnetic anisotropy of the 3-$k$ Bloch-point structure predicted for isotropic spins. Our study rationalizes the formation of these multi-$k$ spin textures in frustrated antiferromagnets, as well as their anisotropy-dependent evolution.

cond-mat.str-el↗

arXiv2026

Melting upon cooling in a quantum magnet

Heating enhances thermal fluctuations and typically leads to melting of solids, but in exceptional cases, heating can also cause liquids to solidify. The paradigm of this counterintuitive phenomenon is solidification of liquid $^3$He upon increasing temperature, known as the Pomeranchuk effect. Here we show that such inverse melting also appears in quantum magnetism. We find that, on cooling, the Ising-like triangular-lattice antiferromagnet erbium heptatantalate first develops a three-sublattice long-range magnetic order -- analogous to a solid -- which then, unexpectedly, melts at even lower temperatures into a short-range correlated spin-stripe state -- analogous to a liquid. We propose that such an unprecedented ``spin Pomeranchuk effect" can generically arise from strong competition between spin-spin interactions in frustrated magnets, and provides a novel avenue to transformations between exotic magnetic phases.

cond-mat.str-el↗

arXiv2025

Spin-density wave of ferrimagnetic building blocks masking the ferromagnetic quantum-critical point in NbFe2

In the metallic magnet NbFe2, the low temperature threshold of ferromagnetism can be investigated by varying the Fe concentration within a narrow homogeneity range. NbFe2 is one of a number of compounds where modulated order is found to mask the ferromagnetic quantum critical point. However, here we report the rare case where the masking modulated magnetic order has been fully refined. Spherical neutron polarimetry and high-intensity single-crystal neutron diffraction reveal the first case of a longitudinal spin-density wave masking the ferromagnetic quantum critical point. The spin-density wave is characterised by a large-wavelength incommensurate modulation of its low average moment. It is formed from ferrimagnetic building blocks with antiparallel ferromagnetic sheets. The existence of ferromagnetic sheets and cancellation of the magnetisation only over mesoscopic length scales show local similarity between the spin-density wave and the ferromagnetic parent phase and indicate the spin-density wave's unconventional nature as emerging from underlying ferromagnetic quantum criticality.

cond-mat.str-el↗

arXiv2025

$\mathbb{Z}_2$ Vortex Crystal Candidate in the Triangular $S=1/2$ Quantum Antiferromagnet

The prospect of merging the paradigms of geometric frustration on a triangular lattice and bond anisotropies in the strong spin-orbit coupling limit holds tremendous promise in the ongoing hunt for exotic quantum materials. Here we identify a new candidate system to realize such physics, the organic quantum antiferromagnet (CD$_3$ND$_3$)$_2$NaRuCl$_6$. We report a combination of thermodynamic, magneto-elastic and neutron scattering experiments on single-crystals to determine the phase diagram in axial magnetic fields $\mathbf{H \parallel c}$ and propose a minimal model Hamiltonian. (CD$_3$ND$_3$)$_2$NaRuCl$_6$ displays an ideal triangular arrangement of Ru$^{3+}$ ions adopting the spin-orbital entangled $j_{\rm eff} = 1/2$ state. It hosts residual magnetic order below $T_{\rm N} = 0.23$ K and a highly unusual $H-T$ phase diagram including three different incommensurate states. Spin-waves in the high-field polarized regime are well described by a Heisenberg-like triangular lattice Hamiltonian with a potential sub-leading bond dependent anisotropy term $J_{\pm\pm}$. We discuss possible candidate magnetic structures in the various observed phases and propose two mechanisms that could explain the field-dependent incommensurability, requiring either a small ferromagnetic Kitaev term or a tiny magneto-elastic $J-J'$ isosceles distortion driven by pseudospin-lattice coupling. We argue that the multi-$\mathbf{q}$ ground state in zero magnetic field is a prime candidate for hosting the $\mathbb{Z}_2$ vortex crystal proposed on the triangular Heisenberg-Kitaev model. (CD$_3$ND$_3$)$_2$NaRuCl$_6$ is the first member in an extended family of quantum triangular lattice magnets, providing a new playground to study the interplay of geometric frustration and spin-orbit effects.

cond-mat.str-el↗

arXiv2025

Single-ion anisotropy driven chiral magnetic order in a spin-1 antiferromagnetic chain

Chirality in magnetic systems gives rise to a wide range of exotic phenomena, yet its influence in $S=1$ chains remains largely unexplored. Here, we present a comprehensive experimental study of a chiral antiferromagnetic (AFM) $S=1$ chain, [Ni(pym)(H$_{2}$O)$_{4}$]SO$_{4} \cdot$ H$_{2}$O (pym = pyrimidine), where the Ni(II) octahedral orientation exhibits a four-fold chiral periodicity. Muon spin rotation measurements indicate the onset of long-range magnetic order below $T_{\rm N} = 1.82(2)\,\mathrm{K}$. Neutron diffraction measurements reveal a chiral AFM 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 (INS) measurements reveal dispersive spin-wave excitations well described by linear spin-wave theory, with Hamiltonian parameters $J_{0} = 6.81(1)\,\mathrm{K}$ (intrachain exchange), $J'_{1\rm a} = -0.091(1)\,\mathrm{K}$ (interchain exchange), and $D = -3.02(1)\,\mathrm{K}$ (easy-axis single-ion anisotropy). These parameters are further validated by Monte Carlo simulations of the magnetisation. Additionally, the INS data reveal multiple dispersionless bands, suggesting the presence of further excitations beyond the scope of our linear spin-wave theory.

cond-mat.str-el↗

arXiv2025

Enhanced two-dimensional ferromagnetism in van der Waals $\beta$-UTe$_3$ monolayers

The discovery of local-moment magnetism in van der Waals (vdW) semiconductors down to the single-layer limit has led to a paradigm shift in the understanding of two-dimensional (2D) magnets and unleashed their potential for applications in microelectronic and optoelectronic devices. The incorporation of strong electronic and magnetic correlations in 2D vdW metals remains a sought-after platform not only to enable control of emergent quantum phases, such as superconductivity, but also to achieve more theoretically tractable microscopic models of complex materials. To date, however, there is limited success in the discovery of such metallic vdW platforms, and $f$-electron monolayers remain out of reach. Here we demonstrate that the actinide $\beta$-UTe$_3$ can be exfoliated to the monolayer limit. A sizable electronic specific heat coefficient provides the hallmark of strong correlations. Remarkably, $\beta$-UTe$_3$ remains ferromagnetic in the half-unit-cell limit with an enhanced ordering temperature of 35 K, a factor of two larger than its bulk counterpart. Our work establishes $\beta$-UTe$_3$ as a novel materials platform for investigating and modeling correlated behavior in the monolayer limit and opens numerous avenues for quantum control with, e.g., strain engineering.

cond-mat.str-el↗

arXiv2025

Magnetic properties of the zigzag ladder compound SrTb2O4

We report on the properties of SrTb2O4, a frustrated zigzag ladder antiferromagnet, studied by single crystal neutron diffraction (with polarised neutrons in zero field and unpolarised neutrons in an applied magnetic field), as well as by neutron spectroscopy on a polycrystalline sample. The neutron scattering results are supported by single crystal magnetisation and heat capacity measurements. In zero field, neutron diffraction data show no transition to a magnetically ordered state down to the lowest experimentally available temperature of 35 mK, and the material remains magnetically disordered down to this temperature. Polarised neutron diffraction measurements reveal the presence of a diffuse scattering signal suggesting only very weak spin-spin correlations in the ground state. For H // c (the easy magnetisation direction), we followed the magnetisation process using neutron diffraction measurements and observed the appearance of field-induced magnetic Bragg peaks with integer h and k indices in the (hk0) scattering plane. No magnetic peaks with a non-zero propagation vector were detected. The observed in-field data fit well to a simple two-sublattice model with magnetic moments aligned along the field direction but being significantly different in magnitude for the two inequivalent Tb3+ sites in the unit cell. Overall, the collected data point to a nonmagnetic ground state in SrTb2O4 despite the presence of strong interactions.

cond-mat.str-el↗

arXiv2025

Incommensurate quantum magnet based on 4f-electron in a zigzag spin-1/2 chain of YbCuS$_2$

We performed high-resolution powder neutron diffraction experiments and discovered an elliptic helical incommensurate magnetic structure in the semiconducting rare-earth magnet YbCuS2, featuring effective spin-1/2 Yb$^{3+}$ ions that form a zigzag chain. Upon cooling the sample to 0.2 K, we observed very weak magnetic peaks indexed with an incommensurate propagation vector k = [0, 0.305, 0] along the zigzag chain. The magnitude of the magnetic moment is at least one-third smaller than the expected value for the Yb$^{3+}$ Kramers doublet ground state. In an applied magnetic field, up-up-down magnetic order was observed at 7.5 T, characterized by diffraction peaks indexed with k = [0, 1/3, 0] and substantial uniform magnetic components. These observations agree well with theoretical calculations based on the density matrix renormalization group for a zigzag spin-1/2 model with isotropic Heisenberg interactions and off-diagonal symmetric $\Gamma$-type exchange interactions derived from material parameters. The theory elucidates the quantum mechanical nature of the incommensurate magnetism as remnant off-diagonal spin correlations in a nematic dimer-singlet state.

cond-mat.str-el↗

arXiv2025

A Magnon Band Analysis of GdRu2Si2 in the Field-Polarized State

Understanding the formation of skyrmions in centrosymmetric materials is a problem of fundamental and technological interest. GdRu2Si2 is one such candidate material which has been shown to host a variety of multi-Q magnetic structures, including in zero-field. Here, inelastic neutron scattering is used to measure the spin excitations in the field-polarized phase of GdRu2Si2. Linear spin wave theory and a method of interaction invariant path analysis are used to derive a Hamiltonian accounting for the observed spectra, and comparisons to \textit{ab initio} calculations are made. No evidence for anisotropic or higher order-exchange terms beyond bilinear Heisenberg exchange is found. This is discussed in the context of the multi-Q states existing at lower fields, for which these types of terms have previously been conceived to be significant in the formation of multi-Q ground states.

cond-mat.str-el↗

arXiv2024

Magnetic field tuned superconducting and normal phase magnetism in CeCo$_{0.5}$Rh$_{0.5}$In$_{5}$

By tuning superconductivity with an applied magnetic field, we use neutrons to compare the magnetic ordered phases in superconducting and normal states of CeCo$_{0.5}$Rh$_{0.5}$In$_{5}$. At zero field, CeCo$_{0.5}$Rh$_{0.5}$In$_{5}$ displays both superconductivity ($T_{c}$=1.3 K) and spatially long-ranged commensurate $\uparrow\downarrow\uparrow\downarrow$ antiferromagnetism ($T_{N}$=2.5 K, $\vec{Q}_{0}=({1\over 2}, {1\over 2}, {1\over 2})$). Neutron spectroscopy fails to measure propagating magnetic excitations with only temporally overdamped fluctuations observable. On applying a magnetic field we find anisotropic behavior in the static magnetism. When the field is along the crystallographic $c$-axis, no change in the static magnetic response is observable. However when the field is oriented within the $a-b$ plane, an increase in $T_{N}$ and change in the critical response are measured. At low temperatures in the superconducting phase, the elastic magnetic intensity increases linearly ($\propto |H|$) with small $a-b$ oriented fields. However, this trend is interrupted at intermediate fields where commensurate block $\uparrow\uparrow\downarrow\downarrow$ magnetism with propagation vector $\vec{Q}=({1\over 2}, {1\over 2}, {1\over 4})$ forms. For large applied fields in the [1 $\overline{1}$ 0] direction which completely suppresses superconductivity, weakly incommensurate magnetic order along $L$ is observed to replace the commensurate response present in the superconducting and vortex phases. We suggest field-induced incommensurate static magnetism, present in the normal state of superconducting and antiferromagnetic CeCo$_{0.5}$Rh$_{0.5}$In$_{5}$ for $a-b$ plane oriented magnetic fields. We speculate that these field dependent properties are tied to the field induced anisotropy associated with the local Ce$^{3+}$ crystal field environment of the tetragonal `115' structure.

cond-mat.str-el↗

arXiv2024

Ferromagnetic fragmented state in the pyrochlore Ho$_2$Ru$_2$O$_7$

The consecutive magnetic ordering of the Ho and Ru ions in the pyrochlore Ho2Ru2O7 and their interplay are investigated by neutron scattering, magnetic and specific heat measurements. The Ru moments order at 95 K into a $\Gamma_5$ easy-plane antiferromagnetic state. At 1.55 K the Ho moments order into an unusual $\Gamma_9$ ferromagnetic state with extensive ground state entropy and structured spin dynamics. It is shown how the internal fields with $\Gamma_5$ and $\Gamma_9$ geometry allow for two symmetry breaking transitions. The lower temperature ordering is driven by ruthenium mediated interactions between holmium moments as spin ice correlations develop. The unsaturated order is compatible with a fragmented ferromagnetic state equivalent to pyrochlore kagome ice.

cond-mat.str-el↗

arXiv2024

Magnetic properties of a staggered $S=1$ chain Ni(pym)(H$_{2}$O)$_{2}$(NO$_{3}$)$_{2}$ with an alternating single-ion anisotropy direction

Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states which are sensitive to the single-ion-anisotropy (SIA) energy ($D$), and intrachain ($J_{0}$) and interchain ($J'_{i}$) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of Ni(pyrimidine)(H$_{2}$O)$_{2}$(NO$_{3}$)$_{2}$, a chain compound where the tilting of Ni octahedra leads to a 2-fold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at $T_{\text{N}}=2.3$\,K and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find $J_{0} = 5.107(7)$\,K, $D = 2.79(1)$\,K, $J'_{2}=0.18(3)$\,K and a rhombic anisotropy energy $E=0.19(9)$\,K. Mean-field modelling reveals that the ground state structure hosts spin canting of $\phi\approx6.5^{\circ}$, which is not detectable above the noise floor of the elastic neutron diffraction data. Monte-Carlo simulation of the powder-averaged magnetization, $M(H)$, is then used to confirm these Hamiltonian parameters, while single-crystal $M(H)$ simulations provide insight into features observed in the data.

cond-mat.str-el↗

arXiv2024

Spin-liquid-like spin dynamics in the frustrated antiferromagnet TbBO3

The synergistic interplay between spin correlations, spin-orbit coupling, and competing exchange interactions provides a promising route to realize exotic quantum states with nontrivial excitations in rare-earth based frustrated magnets. Here, by using thermodynamic and local-probe measurements down to 16 mK, we demonstrate the exotic magnetism and spin dynamics in the distorted triangular lattice TbBO3. Thermodynamic experiments reveal the presence of dominant antiferromagnetic exchange and subdominant dipolar interactions. Despite sizable antiferromagnetic exchange interactions between the Tb3+ moments, muon-spin relaxation experiment does not detect any signatures of long-range magnetic order or spin-freezing down to 16 mK, corroborating the specific heat and ac magnetic susceptibility down to 45 mK that suggests a persistent spin dynamics in this frustrated triangular lattice. The scaling of muon relaxation rate as a function of the characteristic energy scale for several spin-liquid candidates, including TbBO3, demonstrates that a common underlying mechanism is at play. The persistent dynamics in this frustrated triangular lattice antiferromagnet is reminiscent of a universal spin-liquid-like spin fluctuations, here attributed to dominant two dimensional (2D) antiferromagnetic short-range spin correlations, confirmed by the presence of a broad magnetic diffuse scattering in the elastic and low-energy inelastic neutron scattering channels at Q ~ 1.03 Ang**$^{-1}$** at low temperatures. Our results demonstrate that non-Kramers ion based triangular lattice hosts spin-liquid-like dynamics of local moments arising from the admixture of excited crystal electric field states into the ground state and intertwining of frustration and spin-orbit interaction.

cond-mat.str-el↗

arXiv2024

Pseudo-easy-axis anisotropy in antiferromagnetic $S=1$ diamond-lattice systems Ni$X_{2}$(pym)$_{2}$

We investigate the magnetic properties of $S=1$ antiferromagnetic diamond lattice, Ni$X_{2}$(pyrimidine)$_{2}$ ($X$ = Cl, Br), hosting a single-ion anisotropy (SIA) orientation which alternates between neighbouring sites. Through neutron diffraction measurements of the $X$ = Cl compound, the ordered state spins are found to align collinearly along a pseudo-easy-axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy-axis is also present for $X$ = Br. We estimate the Hamiltonian parameters by combining analytical calculations and Monte-Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy-axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy-axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields.

cond-mat.str-el↗

arXiv2024

Superorders and acoustic modes folding in BiFeO$_3$/LaFeO$_3$ superlattices

Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone which gives rise to additional electronic bands and phonon modes. This has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices, for example. Here, we show that multiferroic BiFeO$_3$/LaFeO$_3$ superlattices are more than just periodic chemical stacking. Using transmission electron microscopy, X-ray diffraction and first-principles calculations, we demonstrate the existence of a new order of FeO$_6$ octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e. a superorder. The effect of this new structural order on the lattice dynamics is studied with ultrafast optical pump-probe experiments. While a mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural description. Our work shows that multiferroic BiFeO$_3$/LaFeO$_3$ superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

cond-mat.mtrl-sci↗

arXiv2024

Strong impact of low-level substitution of Mn by Fe on the magnetoelectric coupling in $TbMnO_{3}$

The correlation between static magnetoelectric coupling and magnetic structure was investigated in $TbMn_{0.98}Fe_{0.02}O_{3}$ with magnetic field up to 8 T and down to 2 K. Single-crystal neutron diffraction experiments reveal a substantial increase in the temperature dependence of the incommensurate modulation wave vector of the antiferromagnetic phase as the magnetic field strength increases. Magnetic field-dependent pyroelectric current measurements revealed significantly higher magnetoelectric coupling at magnetic fields below 4 T than in pure TbMnO3. This is due to the higher sensitivity of the incommensurably modulated cycloid structure to weak magnetic fields. Detailed analysis of our data confirmed that the ferroelectric polarization is induced by inverse Dzyaloshinskii-Moriya interaction for magnetic field strength up to 4 T, but at higher fields a departure from theoretical predictions is ascertained, giving evidence for an additional, as yet misunderstood, contribution to magnetoelectric coupling. It shows that a small 2% substitution of Mn3+ by Fe3+ has a strong impact on the magnetic structure, promoting the destabilization of the incommensurably modulated magnetic cycloidal structure of $TbMnO_{3}$ in a magnetic field above 5 T. We demonstrate that the magnetoelectric coupling magnitude can be tuned through suitable substitutional elements, even at low level, inducing local lattice distortions with different electronic and magnetic properties.

cond-mat.mtrl-sci↗