arXiv subjects

Tetiana Haidamak

Publications and source records attributed to Tetiana Haidamak.

7 recordsLinked to original sources

arXiv2026

Spin-orbit-entangled $J_{\rm eff}=\frac{1}{2}$ magnetism and unconventional spin freezing in the bond-disordered pyrochlore antiferromagnet NaCdCo$_2$F$_7$

Bond disorder in frustrated pyrochlore antiferromagnets can give rise to fundamentally different quantum ground states depending on the nature of the local magnetic moments. Here, we show that the bond-disordered $J_{\rm eff}=\frac{1}{2}$ pyrochlore antiferromagnet NaCdCo$_2$F$_7$ realizes an unconventional spin-glass-like state with continued dynamics, in stark contrast to its isostructural $S=\frac{1}{2}$ NaCdCu$_2$F$_7$ counterpart. High-field magnetization and Co $L_{2,3}$-edge XAS/XMCD establish spin-orbit-entangled $J_{\rm eff}=\frac{1}{2}$ Co$^{2+}$ moments with a substantial unquenched orbital contribution, consistent with local $XY$ anisotropy seen in the isostructural Na$A''$Co$_2$F$_7$ ($A''$ = Ca, Sr) analogues. $μ$SR and $^{23}$Na NMR measurements reveal progressive slowing of spin fluctuations below $\sim10$ K, culminating in a partially frozen state with persistent low-temperature dynamics that deviates from a canonical spin glass. Comparison with the isostructural bond-disordered pyrochlore NaCdCu$_2$F$_7$, which realizes a random-singlet state, reveals a fundamentally different response of spin-orbit-entangled Co$^{2+}$ moments to bond disorder. These results identify spin-orbit coupling as a key ingredient governing the fate of bond-disordered frustrated pyrochlore magnets.

cond-mat.str-el↗

arXiv2026

Thermodynamic Evidence of Tetracritical Topology in the $H$-$T$ Phase Diagram of UTe$_2$ for $H \parallel b$

We report ultrasound velocity measurements on an ultraclean UTe$_2$ single crystal with $T_c>2$~K for $H \parallel b$, performed up to 18~T and down to 0.33~K. The measurements provide the missing bulk thermodynamic evidence for an additional high-field phase boundary near $μ_0H\sim14$--15~T. A distinct, non-hysteretic anomaly in the longitudinal $C_{33}$ mode, together with a weaker response in $C_{44}$, no resolvable anomaly in $C_{55}$, and a coincident kink in transverse magnetostriction, reveals a symmetry-selective coupling to lattice strain. This mode selectivity places constraints on the symmetry of the field-induced superconducting component. The phase line remains nearly field-constant near 14~T and meets three other phase boundaries at a tetracritical point near 13.5~T and 1.25~K. The results complete the local tetracritical topology of the $H$--$T$ phase diagram and support field-induced multicomponent superconductivity in UTe$_2$.

cond-mat.supr-con↗

arXiv2026

Random singlet physics in the $S = \frac{1}{2}$ pyrochlore antiferromagnet NaCdCu$_2$F$_7$

We report a random singlet ground state in the $S=\frac{1}{2}$ Heisenberg pyrochlore antiferromagnet NaCdCu$_2$F$_7$. Cationic Na$^+$/Cd$^{2+}$ disorder on the pyrochlore $A$ site generates a broad distribution of Cu$^{2+}$--F$^-$--Cu$^{2+}$ exchange couplings, introducing intrinsic magnetic bond disorder. Despite strong antiferromagnetic interactions ($θ_{\mathrm{CW}}=-72$~K), no magnetic order or global spin freezing is observed in DC and AC susceptibility, specific heat or $^{23}$Na nuclear magnetic resonance to 120 mK, with muon spin relaxation experiments confirming persistent spin dynamics to 58 mK. $T$-linear specific heat, a Curie-like susceptibility tail, and power-law scaling with data collapse in $χ(T)$, $M(H)$, $C_{\mathrm{mag}}/T$, $^{23}$Na $(1/T_1T)$ and the muon spin polarization $P(t)$ reveal a disorder-driven network of random singlets and orphan spins. Scaling across multiple bulk and local probes is consistent with a broad distribution of exchange energies, $P[\mathcal{J}] \sim \mathcal{J}^{-α}$. This behavior contrasts with previously-studied Na$A''B_2$F$_7$ pyrochlore fluorides, where magnetic bond disorder precipitates spin-glass freezing, underscoring the crucial role of strong $S=\frac{1}{2}$ quantum fluctuations in NaCdCu$_2$F$_7$.

cond-mat.str-el↗

arXiv2025

Magnetoelasticity - magnetic structure interrelation - tetragonal MnPt system study

Magnetic materials represent an essential ingredient for the contemporary industry. Apart from common material parameters such as magnetocrystalline anisotropy, coercivity, or saturation magnetization, magnetoelastic behavior is vital for applications serving in various devices, e.g., in acoustic actuators, transducers, or sensors providing a desirable fast response and high efficiency with respect to applied magnetic field. Magnetoelastic properties have been studied for ferromagnetic 3d elements, or especially in high symmetry systems containing rare-earth elements to achieve higher values. Since, unlike for rare earth Laves phases, in the transition metals or alloys, these effects are very weak. Here, in contrast, we analyze the magnetoelastic behavior of antiferromagnetic tetragonal system MnPt, explaining the experimentally measured data based on the theoretical calculations and discussing the influence of the magnetic structure. Particularly, we inspect the origin of magnetocrystalline anisotropy energy, as well as the size and source of the isotropic and anisotropic parts of magnetoelastic (magnetostriction) coefficients.

cond-mat.mtrl-sci↗

arXiv2024

Dramatic elastic response at the critical end point in UTe$_2$

The first-order transition line in the \textit{H-T} phase diagram of itinerant electron metamagnets terminates at the critical end point-analogous to the critical point on the gas-liquid condensation line in the \textit{p-T} phase diagram. To unravel the impact of critical magnetic fluctuations on the crystal lattice of a metamagnet at the critical end point, we performed an ultrasonic study of the itinerant electron metamagnet UTe$_2$ across varying temperatures and magnetic fields. At temperatures exceeding 9 K, a distinct V-shaped anomaly emerges, precisely centered at the critical field of the metamagnetic transition in the isothermal field dependence of elastic constants. This anomaly arises from lattice instability, triggered by critical magnetic fluctuations via strong magnetoelastic interactions. Remarkably, this effect is maximized precisely at the critical-end-point temperature. Comparative measurements of another itinerant metamagnet, UCoAl, reveal intriguing commonalities. Despite significant differences in the paramagnetic ground state, lattice symmetry, and the expected metamagnetic transition process between UTe$_2$ and UCoAl, both exhibit similar anomalies in elastic properties near the critical end point. These shared aspects may hold universality for other itinerant electron metamagnets.

cond-mat.str-el↗

arXiv2023

Robust intralayer antiferromagnetism and tricriticality in a van der Waals compound: VBr3 case

We studied magnetic states and phase transitions in the van der Waals antiferromagnet VBr3 by specific heat and magnetization measurements of single crystals in high magnetic fields and by ab initio density functional theory calculations focused on exchange interactions. The magnetization behavior resembles Ising antiferromagnets with magnetic moments kept in the out-of-plane direction by strong uniaxial magnetocrystalline anisotropy. The out-of-plane magnetic field induces a spin-flip metamagnetic transition, which is of first-order type at low temperatures while at higher temperatures the transition becomes continuous. The first-order and continuous transition segments in the field-temperature phase diagram meet at a tricritical point at = 12 K. The magnetization response to the in-plane field manifests a continuous spin-flop transition, which at 2 K terminates at a field mu0Hc = 27 T that can serve as an estimate of the anisotropy field in VBr3. The magnetization curves above the metamagnetic transition saturate at the same value of magnetic moment musat = 1.2 muB/f.u., which is much smaller than the spin-only (S = 1) moment of the V3+ ion. The reduced moment can be explained by the existence of a significant orbital magnetic moment antiparallel to the spin. The orbital moment is a key ingredient of a mechanism responsible for the observed large anisotropy. The exact energy evaluation of possible magnetic orders unambiguously shows that the magnetic ground state of VBr3 is the intralayer zigzag antiferromagnetic order that renders the antiferromagnetic ground state significantly more stable against the spin-flip transition than the other options. The calculations also predict that a minimal distortion of the Br ion sublattice causes a radical change of the orbital occupation in the ground state, connected with the formation of the orbital moment and the stability of magnetic order.

cond-mat.mtrl-sci↗

arXiv2022

Tricritical fluctuations and elastic properties of the Ising antiferromagnet UIrSi$_3$

Elastic constants, thermal expansion, magnetostriction and heat capacity measurements were performed with and without applied magnetic field on a single crystal of UIrSi$_3$. The elastic properties were interpreted within the theory of the strain-exchange effect. The exchange-striction model together with the Ising model for the behavior of magnetic localized 5$f$ electrons and itinerant electrons of uranium correctly reproduces the main features of our magneto-acoustic experiments in UIrSi$_3$. Data on thermal expansion and magnetostriction confirm the conclusion that the dominant contribution of the measured temperature and field change in the speed of sound comes from the change in the elastic modulus itself. Based on the analysis of heat capacity measurements in the magnetic field, we explain the significant anomalies at the second-order branch of the phase transition boundary as a manifestation of the tricritical fluctuations, dominating the region below the tricritical point. The outcome confirms the 3D Ising model, at zero and low magnetic fields, with a crossover to the mean-field tricritical behavior at fields close to the tricritical point, where tricritical fluctuations dominate the temperature evolution of the given property.

cond-mat.str-el↗