arXiv subjects

Yurii Skourski

Publications and source records attributed to Yurii Skourski.

At least 19 recordsLinked to original sources

arXiv2026

Magnetization processes and spin dynamics across field-induced phase transitions in the quasi-two-dimensional quantum magnet Cu$_2$(OH)$_3$Br

We present magnetic-field-dependent evolution of magnetization and spin dynamics in the quasi-two-dimensional spin-$1/2$ magnet $\mathrm{Cu_2(OH)_3Br}$, consisting of alternately coupled ferromagnetic Cu1 and antiferromagnetic Cu2 spin chains. Terahertz spectroscopy reveals a pronounced field-direction dependence of the low-energy magnetic excitation spectrum. For magnetic fields applied perpendicular to the spin chains, $B\parallel a$ and $B\parallel c^*$, the spectra undergo abrupt reconstructions at the spin-flop transitions identified independently by high-field magnetization measurements. For $B\parallel b$, by contrast, no spin-flop occurs; instead, the excitation spectrum evolves continuously with field and exhibits a strong terahertz radiation polarization dependence as the ferromagnetic Cu1 subsystem becomes progressively polarized. At higher fields, the complex low-field spectrum is replaced by a reduced set of broad excitations, consistent with a weakening of the coupling between the ferromagnetic and antiferromagnetic chain subsystems. Complementary Raman spectroscopy resolves magnon and spinon excitations alongside several phonon modes and traces the characteristic temperature- and magnetic-field-dependent evolution of the magnetic excitations. The combined spectroscopic and magnetization results map out how a magnetic field reorganizes the coupled ferromagnetic and antiferromagnetic subsystems in $\mathrm{Cu_2(OH)_3Br}$ across field-induced phase transitions.

cond-mat.str-el↗

arXiv2026

Metamagnetism in UTe2: the roles of itinerancy and localization

The metamagnetic transition in UTe$_2$ plays a key role in stabilizing two enigmatic field-induced superconducting phases. One of these phases (SC2) is truncated by the transition, lying directly below it, while the other (SC3) sits predominantly above it and appears to be stabilized because of it. While numerous pulsed field studies have examined this transition, comparatively few steady field experiments have investigated it. Here we report a suite of measurements of metamgnetism in UTe$_2$, at ambient pressure by torque magnetometry and extraction magnetometry techniques, and of the magnetoconductance under pressure. Our steady field measurements resolve a complex sub-structure within the transition, with separate features that possess different temperature evolutions, pointing to distinct contributions from itinerant and localized moments. The itinerant contribution might relate to a possible spin-density wave state. We theoretically model the evolution of Kondo and RKKY interactions and propose that the SC2 state is stabilized under pressure due to the collapse of magnetic anisotropy, leading to an enhancement of longitudinal spin fluctuations along the hard $b$ axis, which are pair-forming in the $p$-wave channel.

cond-mat.str-el↗

arXiv2026

Reentrant behavior and possible $2/3$ magnetization plateau on the double-trillium langbeinite K$_2$Ni$_2$(SO$_4$)$_3$

K$_2$Ni$_2$(SO$_4$)$_3$ is a member of the langbeinite family, consisting of two intertwined $S=1$ trillium lattices, out of which one is strongly coupled (strong-TL) and the other is weakly coupled (weak-TL). Further inter-trillium interactions give rise to a highly-frustrated Heisenberg Hamiltonian. Despite ordering at low temperatures, K$_2$Ni$_2$(SO$_4$)$_3$ lies close in parameter space to a spin-liquid region that surrounds the tetratrillium limit, where each triangle belonging to strong-TL turns into a tetrahedron by connecting to a single spin from weak-TL. Here, we compare the experimentally determined magnetization process using pulsed magnetic fields up to $40$ T with classical Monte Carlo calculations, uncovering a series of phase transitions at both low and intermediate fields. Furthermore, we reveal a signature of a $2/3$ magnetization plateau consisting of a $1/3$ phase on strong-TL and a fully polarized phase on weak-TL. Although in the classical limit no plateau is expected, we find a very prominent dome structure reflecting the tendency of the system to stabilize this particular spin configuration. The presence of a dome leads to a reentrant phenomenon in which the system recovers the Hamiltonian symmetries when increasing the magnetic field. Finally, we show that this plateau-like phase is also present in the classical Heisenberg model on the single trillium and tetratrillium lattices, indicating its possible presence in the large family of double-trillium langbeinite compounds. Our findings motivate future studies on the presence of the plateau phase in the quantum limit of both trillium and double-trillium materials within the langbeinite family.

cond-mat.str-el↗

arXiv2026

Resolving growth-induced off-stoichiometry in AgCrSe$_2$ single crystals

The layered delafossite-like antiferromagnet AgCrSe$_2$ is a superionic conductor at high temperatures and has been reported to exhibit anomalous Hall behavior and Kondo physics at low temperatures. These extraordinary transport properties have been established almost exclusively on single crystals grown by chemical vapor transport, raising questions about the role of growth-induced off-stoichiometry. Using elemental analysis, single-crystal X-ray diffraction, and magnetization measurements, we show that such crystals are indeed systematically off-stoichiometric, with a general composition of Ag$_{1-x}$Cr(Se$_{2-y}$Cl$_y$) ($x \approx y \approx 0.08$) arising from the use of CrCl$_3$ as a transport agent. This off-stoichiometry manifests in altered magnetic properties, most notably a suppressed N\'{e}el temperature of 46\,K compared to 58\,K in stoichiometric polycrystalline samples prepared by solid-state synthesis. By optimizing an Ag/Se self-flux growth method, we obtained large single crystals of AgCrSe$_2$ that recover the magnetic transition temperature and saturation field of stoichiometric powder samples. These results establish self-flux growth as a route to high-quality stoichiometric AgCrSe$_2$ single crystals and provide a reliable platform for reassessing whether the reported anomalous transport phenomena are intrinsic or arise from off-stoichiometry.

cond-mat.mtrl-sci↗

arXiv2026

3D bulk-resolved $g$-wave altermagnetic order parameter in CrSb

Electronic phases of matter, such as magnetism and superconductivity, are defined and distinguished by their order parameters quantifying the spontaneous symmetry breaking underlying each phase. Simple cases include the uniform magnetisation of ferromagnets and isotropic gap function of conventional superconductors. Unconventional superconductors often have a nodal gap function, where the gap changes sign at nodes on the Fermi surface. This concept of unconventional/nodal order parameter symmetry has recently been extended to numerous magnetic systems, including altermagnets, in which up- and down-spin species are non-degenerate around the Fermi surface. Here we demonstrate that magnetic quantum oscillation measurements can provide a high resolution, bulk-sensitive, 3D mapping of the order parameter in an unconventional magnet. By rotating a magnetic field through high- and low-symmetry directions of the CrSb Brillouin zone, we show that this material's altermagnetic band structure leads to a reduction of symmetry for each spin-split Fermi sheet away from nodal orientations. In momentum space, the exchange splitting between up and down spins follows the profile of the $\mathcal{Y}_{4}^{-3}=zy(3x^2-y^2)$ real spherical harmonic - analogous to a $g$-orbital of the hydrogen atom. While notoriously difficult to resolve in unconventional superconductors, our work demonstrates that the order parameter symmetry of unconventional magnets can be precisely mapped via quantum-oscillatory quasiparticle spectroscopy, establishing CrSb as a prototypical $g$-wave metallic altermagnet.

cond-mat.mtrl-sci↗

arXiv2025

Single-ion anisotropy-stabilized short-period helimagnetism in frustrated chiral Co$_5$TeO$_8$

Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii-Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here we report that a cubic chiral insulator Co$_5$TeO$_8$ provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7-10 nm embedded in a complex phase diagram spanning eight distinct phases. Capacitance anomalies throughout the phase diagram indicate magneto-electric coupling, pointing to the possibility of future $E$-field control of these textures. The temperature- and field-dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, $ab\,initio$ calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co$_5$TeO$_8$ thus offers an experimental realization of sub-10 nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators.

cond-mat.str-el↗

arXiv2025

Field-induced magnetic phases in the Kitaev candidate Na$_3$Co$_2$SbO$_6$

We report a rich anisotropic magnetic phase diagram of Na$_3$Co$_2$SbO$_6$, a previously proposed cobaltate Kitaev candidate, based on field- and temperature-dependent magnetization, specific heat, and magnetocaloric effect studies. At low temperatures, our experiments uncover a low-lying $j_{\textrm{eff}} = \frac{1}{2}$ state with an antiferromagnetic ground state and pronounced in-plane versus out-of-plane anisotropy. The experimentally identified magnetic phases are theoretically characterized through classical Monte Carlo simulations within an extended Kitaev-Heisenberg model with additional ring exchange interactions. The resulting phase diagram reveals a variety of exotic field-induced magnetic phases, including double-$\textbf{q}$, $\frac{1}{3}$-AFM, zigzag, and vortex phases.

cond-mat.str-el↗

arXiv2025

Direct observation of nanoscale pinning centers in Ce(Co0.8Cu0.2)5.4 permanent magnets

Permanent magnets containing rare earth elements are essential components for the electrification of society. Ce(Co1-xCux)5 permanent magnets are a model system known for their substantial coercivity, yet the underlying mechanism remains unclear. Here, we investigate Ce(Co0.8Cu0.2)5.4 magnets with a coercivity of ~1 T. Using transmission electron microscopy (TEM) and atom probe tomography (APT), we identify a nanoscale cellular structure formed by spinodal decomposition. Cu-poor cylindrical cells (~5-10 nm in diameter, ~20 nm long) have a disordered CeCo5-type structure and a composition Ce(Co0.9Cu0.1)5.3. Cu-rich cell boundaries are ~ 5 nm thick and exhibit a modified CeCo5 structure, with Cu ordered on the Co sites and a composition Ce(Co0.7Cu0.3)5.0. Micromagnetic simulations demonstrate that the intrinsic Cu concentration gradients up to 12 at.% Cu/nm lead to a spatial variation in magnetocrystalline anisotropy and domain wall energy, resulting in effective pinning and high coercivity. Compared to Sm2Co17-type magnets, Ce(Co0.8Cu0.2)5.4 displays a finer-scale variation of conventional pinning with lower structural and chemical contrast in its underlying nanostructure. The identification of nanoscale chemical segregation in nearly single-phase Ce(Co0.8Cu0.2)5.4 magnets provides a microstructural basis for the long-standing phenomenon of "giant intrinsic magnetic hardness" in systems such as SmCo5-xMx, highlighting avenues for designing rare-earth-lean permanent magnets via controlled nanoscale segregation.

cond-mat.mtrl-sci↗

arXiv2025

Quantum spin dynamics of the honeycomb magnet K$_2$Co$_2$TeO$_6$ in high magnetic fields

We present terahertz spectroscopic measurements of quantum spin dynamics in the honeycomb magnet K$_2$Co$_2$TeO$_6$ as a function of temperature, polarization and in an external magnetic field applied in the honeycomb plane. Magnetic excitations are resolved below the magnetic ordering temperature of $T_\text{N}$ = 12 K. In the applied magnetic field, we reveal characteristic field dependence not only for the magnetic excitations observed at zero field, but also a rich set of modes emerging in finite fields. The observed magnetic excitations exhibit clear dependence on the terahertz polarization, and characteristic features at field-induced phase transitions consistent with our high-field magnetization data. We cannot evidently resolve a continuumlike feature, even when the long-range magnetic order is presumably suppressed in the strong magnetic field, indicating that a Kitaev-type interaction, if existing, is subleading in this compound.

cond-mat.str-el↗

arXiv2025

Fluctuation-driven topological Hall effect in room-temperature itinerant helimagnet Fe3Ga4

The topological Hall effect (THE) is a hallmark of a non-trivial geometric spin arrangement in a magnetic metal, originating from a finite scalar spin chirality (SSC). The associated Berry phase is often a consequence of non-coplanar magnetic structures identified by multiple k-vectors. For single-k magnetic structures however with zero SSC, the emergence of a finite topological Hall signal presents a conceptual challenge. Here, we report that a fluctuation-driven mechanism involving chiral magnons is responsible for the observed THE in a low-symmetry compound, monoclinic Fe3Ga4. Through neutron scattering experiments, we discovered several nontrivial magnetic phases in this system. In our focus is the helical spiral phase at room temperature, which transforms into a transverse conical state in applied magnetic field, supporting a significant THE signal up to and above room temperature. Our work offers a fresh perspective in the search for novel materials with intertwined topological magnetic and transport properties.

cond-mat.str-el↗

arXiv2024

Magnetization process of a quasi-two-dimensional quantum magnet: Two-step symmetry restoration and dimensional reduction

We report on a comprehensive thermodynamic study of a quasi-two-dimensional (quasi-2D) quantum magnet Cu$_2$(OH)$_3$Br which in the 2D layer can be viewed as strongly coupled alternating antiferromagnetic and ferromagnetic chains. In an applied magnetic field transverse to the ordered spins below $T_N=9.3$ K, a field-induced phase transition from the 3D ordered to a disordered phase occurs at $B_c=16.3$ T for the lowest temperature, which is featured by an onset of a one-half plateau-like magnetization. By performing quantum Monte Carlo simulations of the relevant 2D model, we find that the plateau-like magnetization corresponds to a partial symmetry restoration and the full polarization in the ferromagnetic chains. Our numerical simulations also show that the magnetization saturation occurs with full symmetry restoration at a much higher field of $B_s \simeq 95$ T, corresponding to a 1D quantum phase transition in the antiferromagnetic chains. We argue that the experimentally observed field-induced phase transition at $B_c$ follows from the partial symmetry restoration and the concomitant dimensional reduction.

cond-mat.str-el↗

arXiv2023

Tunable room temperature nonlinear Hall effect from the surfaces of elementary bismuth thin films

The nonlinear Hall effect (NLHE) with time-reversal symmetry constitutes the appearance of a transverse voltage quadratic in the applied electric field. It is a second-order electronic transport phenomenon that induces frequency doubling and occurs in non-centrosymmetric crystals with large Berry curvature -- an emergent magnetic field encoding the geometric properties of electronic wavefunctions. The design of (opto)electronic devices based on the NLHE is however hindered by the fact that this nonlinear effect typically appears at low temperatures and in complex compounds characterized by Dirac or Weyl electrons. Here, we show a strong room temperature NLHE in the centrosymmetric elemental material bismuth synthesized in the form of technologically relevant polycrystalline thin films. The ($1\,1\,1$) surface electrons of this material are equipped with a Berry curvature triple that activates side jumps and skew scatterings generating nonlinear transverse currents. We also report a boost of the zero field nonlinear transverse voltage in arc-shaped bismuth stripes due to an extrinsic geometric classical counterpart of the NLHE. This electrical frequency doubling in curved geometries is then extended to optical second harmonic generation in the terahertz (THz) spectral range. The strong nonlinear electrodynamical responses of the surface states are further demonstrated by a concomitant highly efficient THz third harmonic generation which we achieve in a broad range of frequencies in Bi and Bi-based heterostructures. Combined with the possibility of growth on CMOS-compatible and mechanically flexible substrates, these results highlight the potential of Bi thin films for THz (opto)electronic applications.

cond-mat.mes-hall↗

arXiv2023

Role of alkaline metal in the rare-earth triangular antiferromagnet KYbO$_2$

We report crystal structure and magnetic behavior of the triangular antiferromagnet KYbO$_2$, the A-site substituted version of the quantum spin liquid candidate NaYbO$_2$. The replacement of Na by K introduces an anisotropic tensile strain with 1.6% in-plane and 12.1% out-of-plane lattice expansion. Compared to NaYbO$_2$, both Curie-Weiss temperature and saturation field are reduced by about 20% as the result of the increased Yb--O--Yb angles, whereas the $g$-tensor of Yb$^{3+}$ becomes isotropic with $g=3.08(3)$. Field-dependent magnetization shows the plateau at 1/2 of the saturated value and suggests the formation of the up-up-up-down field-induced order in the triangular AYbO$_2$ oxides (A = alkali metal), in contrast to the isostructural selenides that exhibit the 1/3 plateau and the up-up-down field-induced order.

cond-mat.str-el↗

arXiv2022

Au4Mn, a localized ferromagnet with strong spin-orbit coupling, long-range ferromagnetic exchange and high Curie temperature

Metallic Mn-based alloys with a nearest-neighbor Mn-Mn distance greater than 0.4 nm exhibit large, well-localized magnetic moments. Here we investigate the magnetism of tetragonal Au4Mn with a Curie temperature of 385 K, where manganese has a spin moment of 4.1 muB and its orbital moment is quenched. Since 80% of the atoms are gold, the spin orbit interaction is strong and Au4Mn exhibits uniaxial magnetocrystalline anisotropy with surface maze domains at room temperature. The magnetic hardness parameter of 1.0 is sufficient to maintain the magnetization along the c-axis for a sample of any shape. Au also reduces the spin moment of Mn through 5d-3d orbital hybridization. An induced moment of 0.05 muB was found on Au under a pulsed field of 40 T. Density functional theory calculations indicate that the Mn-Mn exchange is mediated by spin-polarized gold 5d and 6p electrons. The distance-dependence shows that it is ferromagnetic or zero for the first ten shells of Mn neighbors out to 1.041 nm (64 atoms), and very weak and oscillatory thereafter.

cond-mat.mtrl-sci↗

arXiv2022

Coupled frustrated ferromagnetic and antiferromagnetic quantum spin chains in the quasi-one-dimensional mineral antlerite, Cu$_3$SO$_4$(OH)$_4$

Magnetic frustration, the competition among exchange interactions, often leads to novel magnetic ground states with unique physical properties which can hinge on details of interactions that are otherwise difficult to observe. Such states are particularly interesting when it is possible to tune the balance among the interactions to access multiple types of magnetic order. We present antlerite, Cu$_3$SO$_4$(OH)$_4$, as a potential platform for tuning frustration. Contrary to previous reports, the low-temperature magnetic state of its three-leg zigzag ladders is a quasi-one-dimensional analog of the magnetic state recently proposed to exhibit spinon-magnon mixing in botallackite. Density functional theory calculations indicate that antlerite's magnetic ground state is exquisitely sensitive to fine details of the atomic positions, with each chain independently on the cusp of a phase transition, indicating an excellent potential for tunability.

cond-mat.str-el↗

arXiv2022

Ce and Dy substitutions in Nd$_{2}$Fe$_{14}$B: site-specific magnetic anisotropy from first-principles

A first-principles approach combining density functional and dynamical mean-field theories in conjunction with a quasi-atomic approximation for the strongly localized 4$f$ shell is applied to Nd$_{2}$Fe$_{14}$B-based hard magnets in order to evaluate crystal-field and exchange-field parameters at rare-earth sites and their corresponding single-ion contribution to the magnetic anisotropy. In pure Nd$_2$Fe$_{14}$B, our calculations reproduce the easy-cone to easy axis transition; theoretical magnetization curves agree quantitatively with experiment. Our study reveals that the rare-earth single-ion anisotropy in the "2-14-1" structure is strongly site-dependent, with the $g$ rare-earth site exhibiting a larger value. In particular, we predict that increased $f$ and $g$-site occupancy of $R=$ Ce and Dy, respectively, leads to an increase of the magnetic anisotropy of the corresponding (Nd,$R$)$_{2}$Fe$_{14}$B substituted compounds.

cond-mat.str-el↗

arXiv2022

Optically-triggered strain-driven Néel vector manipulation in a metallic antiferromagnet

The absence of stray fields, their insensitivity to external magnetic fields, and ultrafast dynamics make antiferromagnets promising candidates for active elements in spintronic devices. Here, we demonstrate manipulation of the Néel vector in the metallic collinear antiferromagnet Mn$_2$Au by combining strain and femtosecond laser excitation. Applying tensile strain along either of the two in-plane easy axes and locally exciting the sample by a train of femtosecond pulses, we align the Néel vector along the direction controlled by the applied strain. The dependence on the laser fluence and strain suggests the alignment is a result of optically-triggered depinning of 90$^{\mathrm{o}}$ domain walls and their sliding in the direction of the free energy gradient, governed by the magneto-elastic coupling. The resulting, switchable, state is stable at room temperature and insensitive to magnetic fields. Such an approach may provide ways to realize robust high-density memory device with switching timescales in the picosecond range.

cond-mat.mtrl-sci↗

arXiv2022

Multi-Center Magnon Excitations Open the Entire Brillouin Zone to Terahertz Magnetometry of Quantum Magnets

Due to the small photon momentum, optical spectroscopy commonly probes magnetic excitations only at the center of the Brillouin zone; however, there are ways to override this restriction. In the case of the distorted kagome quantum magnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, under scrutiny here, the magnon density of states can be accessed over the entire Brillouin zone through three-center magnon excitations. This mechanism is aided by the three different magnetic sublattices and strong short-range correlations in the distorted kagome lattice. The results of THz time-domain experiments agree remarkably well with linear spin-wave theory. Relaxing the conventional zone-center constraint of photons gives a new aspect to probe magnetism in matter.

cond-mat.str-el↗