arXiv subjects

Oscar Fabelo

Publications and source records attributed to Oscar Fabelo.

17 recordsLinked to original sources

arXiv2026

Real-space manifestation of ferroic multipoles in altermagnetic MnF$_2$

Altermagnets are unconventional spin split magnets arising from the zero spin-orbit coupled limit. They host a magnetic multipolar order parameter yet direct real-space observation of these multipoles has remained elusive. Here we use polarized-neutron diffraction to reconstruct the three-dimensional magnetization density of the prototypical altermagnet MnF$_2$. By exploiting symmetry-selective magnetic reflections, we separate the dominant spherical Mn$^{2+}$ contribution from the much weaker anisotropic Mn magnetization and the covalent spin polarization of the fluorine ligands. The reconstructed spin density reveals a finite fluorine ion moment together with an anisotropic Mn magnetization consistent with the symmetry-allowed altermagnetic rank-5 magnetic multipole $O_{52}$(magnetic triacontadipole). These results provide direct real-space evidence of ferroic multipolar order in an altermagnet and establish polarized-neutron diffraction as a powerful probe of hidden magnetic multipoles in quantum materials.

cond-mat.str-el↗

arXiv2026

Higher-order Weyl nodes driven by helical magnetic order in EuAgAs

Magnetic topological semimetals provide a fertile ground for exploring how long-range magnetic order can alter electronic band structures and generate novel quasiparticles such as Weyl fermions. Here, we investigate the coupled magnetic and electronic structure of single-crystalline EuAgAs, a hexagonal pnictide whose magnetic ground state has remained elusive. Using neutron diffraction and resonant elastic X-ray scattering, we identify an unusual magnetic ordering sequence with two successive phase transitions at $T_\mathrm{N1} = 12$ K and $T_\mathrm{N2} = 8$ K. We observe two slightly different magnetic propagation vectors, one associated with $T_\mathrm{N1}$ and the other with $T_\mathrm{N2}$. Spherical neutron polarimetry reveals that the magnetic structure is a transverse helix aligned along the $c$ axis with a period that is approximately twice the $c$ lattice parameter. First-principles calculations for the helical phase predict subtle band folding effects and the emergence of effective higher-order Weyl nodes. These topological features appear near the calculated Fermi energy $E_{\mathrm{F}}$ which, however, lies above the position of $E_{\mathrm{F}}$ obtained from angle-resolved photoemission spectroscopy so could not be probed in this study.

cond-mat.str-el↗

arXiv2026

The effect of pressure in the crystal and magnetic structure of FeWO4

The temperature dependence of the structural and magnetic properties of wolframite-type FeWO4 were studied in situ by high pressure neutron diffraction. Neutron diffraction measurements were performed at the XtremeD instrument at the Institut Laue Langevin up to a maximum pressure of 8.7(4) GPa and a minimum temperature of 30.0(5) K. The diffraction data were analyzed via Rietveld refinements. We found that despite of producing a contraction of 5% of the volume, the maximum pressure applied in this study does not modify the Shubnikov space group below magnetic order. However, the orientation of magnetic moments and the N\'eel temperature, are slightly modified with the pressure, which is expected according to the preexistent understanding of magnetism in wolframites. We also determined a pressure-volume equation of state of FeWO4 at 300 K, which is compared with previous X-ray diffraction studies and density-functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2024

Influence of Magnetic Anisotropy on the Ground State of [CH$_3$NH$_3$]Fe(HCOO)$_3$: Insights into the Improper Modulated Magnetic Structure

The hybrid perovskites [CH$_3$NH$_3$]Co$_x$Ni$_{x-1}$(HCOO)$_3$ with $x$ = 0, 0.25, 0.5, 0.75 and 1.0 possess multiple phase transitions including incommensurate structures. [CH$_3$NH$_3$]Ni(HCOO)$_3$ has also been found to have a proper magnetic incommensurate structure in its ground state. We have carried out a detailed structural characterization of the isomorphous [CH$_3$NH$_3$]Fe(HCOO)$_3$ (1) to investigate whether it also has incommensurate structural and magnetic modulations. We confirm that 1 crystallizes in the $Pnma$ space group at room temperature (RT) with a perovskite structure. Upon cooling, at about 170 K, the occurrence of new satellite reflections in the diffraction pattern show a phase transition to a modulated structure, which could be refined in the $Pnma(00\gamma)0s0$ super space group with $q_1~=~0.1662(2)c^\ast$. On further cooling to 75 K the satellite reflections become closer to the main reflections, indicating a new phase transition that keeps the super space group invariant but changes the modulation wave vector, $q_2~=~0.1425(2)c^\ast$. The structure then does not change structural phase down to base temperature (2 K). Magnetic susceptibility measurements collected under field-cooled and zero-field-cooled reveal a 3D antiferromagnetic order below 17 K. The overlapping in temperature between structural modulation and long-range magnetic order presents a unique opportunity to study magneto-structural coupling. Our results point to an improper modulated structure where interestingly the spins oriented strictly antiferromagnetic are perpendicular to those of previously reported compounds. In the present work, a combination of magnetometry measurements, single crystal and powder neutron diffraction and density functional theory calculations have been used to accurately determine and understand the sequence of nuclear and magnetic phases present in compound 1.

cond-mat.mtrl-sci↗

arXiv2024

Crystal structure and absence of magnetic order in single crystalline RuO$_2$

RuO$_2$ was considered for a long time to be a paramagnetic metal with an ideal rutile-type structure down to low temperatures, but recent studies on single-crystals claimed evidence for antiferromagnetic order and some symmetry breaking in the crystal structure. We have grown single-crystals of RuO2 by vapor transport using either O$_2$ or TeCl$_4$ as transport medium. These crystals exhibit metallic behavior following a $T^2$ low-temperature relation and a small paramagnetic susceptibility that can be attributed to Pauli paramagnetism. Neither the conductance nor the susceptibility measurements yield any evidence for a magnetic or a structural transition between 300K and $\sim$4 K. Comprehensive single-crystal diffraction studies with neutron and X-ray radiation reveal the rutile structure to persist until 2K in our crystals, and show nearly perfect stoichiometry. Previous observations of symmetry forbidden reflections can be attributed to multiple diffraction. Polarized single-crystal neutron diffraction experiments at 1.6K exclude the proposed antiferromagnetic structures with ordered moments larger than 0.01 Bohr magnetons.

cond-mat.mtrl-sci↗

arXiv2024

Tuning structural modulation and magnetic properties in metal-organic coordination polymers [CH$_3$NH$_3$]Co$_x$Ni$_{1-x}$(HCOO)$_3$

Three solid solutions of [CH$_3$NH$_3$]Co$_x$Ni$_{1-x}$(HCOO)$_3$, with $x$ = 0.25 (1), 0.50 (2) and 0.75 (3), were synthesized and their nuclear structures and magnetic properties were characterized using single crystal neutron diffraction and magnetization measurements. At room temperature, all three compounds crystallize in the Pnma orthorhombic space group, akin to the cobalt and nickel end series members. Upon cooling, each compound undergoes distinct series of structural transitions to modulated structures. Compound 1 exhibits a phase transition to a modulated structure analogous to the pure nickel compound, while compound 3 maintains the behaviour observed in the pure cobalt compound, although in both cases the temperatures at which the phase transitions occur differ slightly from the pure phases. Monochromatic neutron diffraction measurements showed that the structural evolution of 2 diverges from that of either parent compound, with the competing hydrogen bond interactions which drive the modulation throughout the series producing a unique sequence of phases. It involves two modulated phases below 96(3) K and 59(3) K, with different q vectors, similar to the pure cobalt compound (with modulated phases below 128 K and 96 K), however it maintains the modulated phase below magnetic order (at 22.5(7) K), resembling the pure nickel compound (which present magnetic order below 34 K), resulting in an improper modulated magnetic structure. Despite these large scale structural changes, magnetometry data reveal that the bulk magnetic properties of these solid solutions form a linear continuum between the end members. Notably, doping of the metal site in these solid solutions allows for tuning of bulk magnetic properties, including magnetic ordering temperature, transition temperatures, and the nature of nuclear phase transitions, through adjustment of metal ratios.

cond-mat.str-el↗

arXiv2024

Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH$_2$)$_3$]Mn(NCS)$_3$

We report the magnetic structure and properties of a thiocyanate-based honeycomb magnet [Na(OH$_2$)$_3$]Mn(NCS)$_3$ which crystallises in the unusual low-symmetry trigonal space group $P\overline{3}$. Magnetic measurements on powder samples show this material is an antiferromagnet (ordering temperature ($T_\mathrm{N,mag} = 18.1(6)\,$)K) and can be described by nearest neighbour antiferromagnetic interactions $J=-11.07(4)\,$K. A method for growing neutron-diffraction sized single crystals (\textgreater10 mm$^3$) is demonstrated. Low temperature neutron single crystal diffraction shows that the compound adopts the collinear antiferromagnetic structure with $T_\mathrm{N,neut}= 18.94(7)\,$K, magnetic space group $P \bar{3}'$. Low temperature second-harmonic generation (SHG) measurements provide no evidence of breaking of the centre of symmetry.

cond-mat.str-el↗

arXiv2023

High pressure behaviour of the magnetic van der Waals molecular framework Ni(NCS)$_2$

Two-dimensional materials offer a unique range of magnetic, electronic and mechanical properties which can be controlled by external stimuli. Pressure is a particularly important stimulus, as it can be achieved readily and can produce large responses, especially in low-dimensional materials. In this paper we explore the pressure-dependence of the structural and magnetic properties of a two-dimensional van der Waals (vdW) molecular framework antiferromagnet with ferromagnetic layers, Ni(NCS)$_2$, up to 8.4 kbar. Through a combination of X-ray and neutron diffraction analysis, we find that Ni(NCS)$_2$ is significantly more compressible than comparable vdW metal halides, and its response is anisotropic not only out of the plane, but also within the layers. Using bulk magnetisation and neutron diffraction data, we show that the ambient layered antiferromagnetic phase is maintained up to the largest investigated pressure, but with an enhanced Néel temperature, $T_\mathrm{N}$, ($ΔT_\mathrm{N} / T_\mathrm{N} = +19$ %) and a large pressure sensitivity ($Q = \frac{1}{T_\mathrm{N}} \frac{\mathrm{d}T_\mathrm{N}}{\mathrm{d}P} = +2.3$ % kbar$^{-1}$), one of the larger values of magnetic pressure responsiveness for a vdW material. Density functional theory calculations suggest that this is due to increasing three-dimensionality. These results provide some of the first insights into the pressure response of molecular framework vdW magnets and suggest investigation of other molecular framework vdW magnets might uncover contenders for future pressure-switchable devices.

cond-mat.mtrl-sci↗

arXiv2023

Weyl metallic state induced by helical magnetic order

In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro- or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that Weyl nodes can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that EuCuAs develops a planar helical structure below $T_\textrm{N}$ = 14.5 K which induces Weyl nodes along the $Γ$--A high symmetry line in the Brillouin zone.

cond-mat.str-el↗

arXiv2023

Non-collinear magnetism in the post-perovskite thiocyanate frameworks CsM(NCS)$_3$

AMX$_3$ compounds are structurally diverse, a notable example being the post-perovskite structure which adopts a two-dimensional framework with corner- and edge-sharing octahedra. Few molecular post-perovskites are known and of these, none have reported magnetic structures. Here we report the synthesis, structure and magnetic properties of molecular post-perovskites: CsNi(NCS)$_3$, a thiocyanate framework, and two new isostructural analogues CsCo(NCS)$_3$ and CsMn(NCS)$_3$. Magnetisation measurements show that all three compounds undergo magnetic order. CsNi(NCS)$_3$ (Curie temperature, $T_\mathrm{C} = 8.5(1)\;$K) and CsCo(NCS)$_3$ ($T_\mathrm{C} = 6.7(1)\;$K) order as weak ferromagnets. On the other hand, CsMn(NCS)$_3$ orders as an antiferromagnet (Néel temperature, $T_\mathrm{N}=16.8(8)\;$K). Neutron diffraction data of CsNi(NCS)$_3$ and CsMn(NCS)$_3$, show that both are non-collinear magnets. These results suggest molecular frameworks are fruitful ground for realising the spin textures required for the next generation of information technology.

cond-mat.str-el↗

arXiv2023

Low-dimensional metal-organic magnets as a route towards the S=2 Haldane phase

Metal-organic magnets (MOMs), modular magnetic materials where metal atoms are connected by organic linkers, are promising candidates for next-generation quantum technologies. MOMs readily form low-dimensional structures, and so are ideal systems to realise physical examples of key quantum models, including the Haldane phase, where a topological excitation gap occurs in integer-spin antiferromagnetic (AFM) chains. Thus far the Haldane phase has only been identified for $S=1$, with $S \geq 2$ still unrealised because the larger spin imposes more stringent requirements on the magnetic interactions. Here, we report the structure and magnetic properties of CrCl$_2$(pym) (pym=pyrimidine), a new quasi-1D $S=2$ AFM MOM. We show, using X-ray and neutron diffraction, bulk property measurements, density-functional theory calculations and inelastic neutron spectroscopy (INS) that CrCl$_2$(pym) consists of AFM CrCl$_2$ spin chains ($J_1=-1.13(4)\;$meV) which are weakly ferromagnetically coupled through bridging pym ($J_2=0.10(2)\;$meV), with easy-axis anisotropy ($D=-0.15(3)\;$meV). We find that although small compared to $J_1$, these additional interactions are sufficient to prevent observation of the Haldane phase in this material. Nevertheless, the proximity to the Haldane phase together with the modularity of MOMs suggests that layered Cr(II) MOMs are a promising family to search for the elusive $S=2$ Haldane phase.

cond-mat.str-el↗

arXiv2021

Magnetic properties of a highly ordered single crystal of the layered perovskite YBaCuFe$_{0.95}$Mn$_{0.05}$O$_5$

The layered perovskite YBaCuFeO5 (YBCFO) is able to adopt chiral magnetic order up to unexpectedly high temperatures, paving the way to strong magnetoelectric coupling at room temperature. In this perovskite A-site cations are fully ordered whereas the occupancy of the B-sites strongly depend on the preparation process. Though the structure is not geometrically frustrated, the presence of partial Fe3+/Cu2+ disorder at the B-sites produces magnetic frustration. In an effort to increase the spin-orbit coupling in the system, we have synthesized and studied YBaCuFe0.95Mn0.05O5 in single crystal form, where the highly symmetric Fe3+ ions (3d5) are partially substituted with Jahn-Teller active 3d4 Mn3+ ions. We report the structural and magnetic properties of a highly ordered single crystal of this layered perovskite, which are presented in comparison with a polycrystalline specimen (three times more disordered). Single-crystal neutron diffraction measurements reveal two collinear magnetic phases and the lack of incommensurate spiral order. The magnetic phases and transitions found in the crystal grown by the traveling solvent floating zone (TSFZ) method are fully described and analyzed in the light of its high level of Fe/Cu cationic order (~90%).

cond-mat.mtrl-sci↗

arXiv2021

Relating spin-polarized STM imaging and inelastic neutron scattering in the van-der-Waals ferromagnet Fe3GeTe2

Van-der-Waals (vdW) ferromagnets have enabled the development of heterostructures assembled from exfoliated monolayers with spintronics functionalities, making it important to understand and ultimately tune their magnetic properties at the microscopic level. Information about the magnetic properties of these systems comes so far largely from macroscopic techniques, with little being known about the microscopic magnetic properties. Here, we combine spin-polarized scanning tunneling microscopy and quasi-particle interference imaging with neutron scattering to establish the magnetic and electronic properties of the metallic vdW ferromagnet Fe3GeTe2. By imaging domain walls at the atomic scale, we can relate the domain wall width to the exchange interaction and magnetic anisotropy extracted from the magnon dispersion as measured in inelastic neutron scattering, with excellent agreement between the two techniques. From comparison with Density Functional Theory calculations we can assign the quasi-particle interference to be dominated by spin-majority bands. We find a dimensional dichotomy of the bands at the Fermi energy: bands of minority character are predominantly two-dimensional in character, whereas the bands of majority character are three-dimensional. We expect that this will enable new design principles for spintronics devices.

cond-mat.mtrl-sci↗

arXiv2018

Switching of the Chiral Magnetic Domains in the Hybrid Multiferroic (ND4)2[FeCl5(D2O)]

Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of (ND4)2[FeCl5(D2O)]. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, determining the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K.

cond-mat.mtrl-sci↗

arXiv2017

Effect of lattice distortion on U magnetic moments in $\mathrm{U_{4}Ru_{7}Ge_{6}}$ studied by polarized neutron diffraction

Small spontaneous lattice distortion of a cubic ferromagnet at temperatures below $T_{\mathrm{C}}$ is expected, but usually a neglected phenomenon. This study is focused on such effect on the example of $\mathrm{U_{4}Ru_{7}Ge_{6}}$. We propose the lattice distortion from the cubic space group to a lower symmetry as a result of our DFT calculations. The strong spin-orbit coupling on the U site plays the most essential role, that should lower the symmetry of the system and give rise to the two different U sites (U1 and U2). We bring convincing experimental evidence for this splitting resulting from our polarized neutron diffraction experiment. Proper treatment of the flipping ratios collected in the ordered state and in the paramagnetic state reveals dramatically different magnetic moments on the U1 and U2 sites. We have reached good agreement of the results from the MAXENT method and from direct fitting of the data. The ratio of the orbital and spin component on the U2 site is lower than for the free $\mathrm{U^{3+}}$ or $\mathrm{U^{4+}}$ ion and points to a strong hybridization between the U $5f$ and Ru $4d$ wave functions. The same ratio on the U1 site is unexpectedly low. This would mean that its orbital component is almost quenched. We have observed the absence of a magnetic moment on the Ru1 site, but a rather large induced moment on the Ru2 site. Very similar results were obtained also in the paramagnetic state in the regime induced by the magnetic field. It points to the intimate coupling of the magnetic ordering and structural distortion as it is possible to split the U site in to two different polarizing paramagnetic U moments above $T_{\mathrm{C}}$ by a strong magnetic field. We propose that the difference between the magnetic moment on the U sites is caused by the change of local point symmetry of the sites that is tightly bound to the role of the strong spin-orbit interaction.

cond-mat.str-el↗

arXiv2016

Magnetic-Field-Induced Change of Magneto-Electric Coupling in the in Molecular Multiferroic (ND4)2[FeCl5(D2O)]

Our results describe an unprecedented example of change in the mechanism of magnetically-induced electric polarization from spin current to spin-dependent p-d hybridization model. We have followed the evolution of the magnetic structures of (ND4)2[FeCl5 D2O] compound using single crystal neutron diffraction under external magnetic field. The spin arrangements change from incommensurate cycloidal to commensurate distorted-cycloidal and finally to quasi-collinear. The determination of the magnetic structures allows us to explain the observed electric polarization in the different ferroelectric phases. Two different magneto-electric coupling mechanisms are at play: the spin-current mechanism for external magnetic field below 5 T, and the spin dependent p-d hybridization mechanism for magnetic field above this value, being this compound the first example reported presenting this sequence of magneto-electric coupling mechanisms.

cond-mat.mtrl-sci↗

arXiv2015

Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.

cond-mat.mtrl-sci↗