arXiv subjects

R. Kolay

Publications and source records attributed to R. Kolay.

8 recordsLinked to original sources

arXiv2026

Magnetic and crystal electric field excitations in a spin-orbit coupled frustrated hyperkagome magnet Nd$_3$Li$_3$W$_2$O$_{12}$

Rare-earth based garnets provide a viable platform for studying the frustrated driven magnetic properties of the hyperkagome lattices. Herein, we report a comprehensive study of the magnetic properties and crystal electric field (CEF) scheme of a new Nd$^{3+}$ based hyperkagome antiferromagnet, Nd$_3$Li$_3$W$_2$O$_{12}$ belonging to the garnet family via magnetization, heat capacity, and inelastic neutron scattering (INS) measurements. Magnetization measurement reveals a dominant antiferromagnetic interaction with a low temperature Curie-Weiss temperature $\theta_{\rm CW}^{\rm LT} \simeq -0.2$ K. Two broad maxima are observed in the magnetic heat capacity data under magnetic fields, implying multilevel Schottky anomalies due to the effect of CEF and display a two-step magnetic entropy release. No magnetic long-range order is observed down to 0.1 K. The CEF excitations of the Nd$^{3+}$ ($J=9/2$) ion with $D_2$ point group symmetry, probed via INS experiments, show non-dispersive excitations characterizing the transitions among the CEF energy levels. The simultaneous fit of the INS spectra at different temperatures enabled the mapping of the CEF Hamiltonian and the energy eigenvalues of the Kramers' doublets. The simulation using the obtained CEF parameters reproduces the experimental magnetic susceptibility, magnetic isotherms, and magnetic heat capacity data. The thermodynamic properties and INS-derived crystal-field scheme confirm a Kramers' doublet ground state with an effective spin $J_{\rm eff} = 1/2$ at low temperatures.

cond-mat.mtrl-sci↗

arXiv2026

Spin-orbit-entangled frustrated magnetism in fcc Ba$_2$(Yb,Nd)NbO$_6$ double perovskites

The search for candidate Kitaev materials has largely focused on 4$d$ and 5$d$ transition-metal compounds with various lattice geometries. In contrast, investigations of rare-earth 4$f$ systems have thus far been restricted mainly to honeycomb and triangular lattices. In this work, we investigate the rare-earth-based double perovskites Ba$_2$YbNbO$_6$ and Ba$_2$NdNbO$_6$, which crystallize in a face-centered cubic structure. Magnetization and heat-capacity measurements establish isolated ${j_{\rm eff}} = 1/2$ Kramers doublet ground states arising from strong spin-orbit coupling (SOC) and crystal electric-field effects, which are further supported by density-functional theory calculations. Millikelvin-temperature thermodynamic measurements reveal long-range magnetic order with moderate frustration in both compounds. The emergence of magnetic order may be understood within an order-by-disorder scenario, as theoretically proposed for rare-earth fcc lattices with finite Kitaev interactions. Our results thus identify Ba$_2$YbNbO$_6$ and Ba$_2$NdNbO$_6$ as promising rare-earth spin-orbit-entangled magnets and motivate further experimental and theoretical investigations aimed at determining the complete exchange tensor to elucidate the microscopic origin of the underlying magnetic interactions.

cond-mat.str-el↗

arXiv2026

Signatures of unconventional magnetism in the layered metallic ferromagnet LaCrSb$_3$ from ferromagnetic resonance spectroscopy

LaCrSb$_{3}$ is a metallic ferromagnet with a layered crystal structure demonstrating intriguing electronic and magnetic properties, such as large anomalous Hall effect, strong canting of the spin lattice, and a peculiar spin-reorientation transition. Here, we report the results of the temperature-dependent x-ray diffraction, static magnetization, and in particular electron spin resonance (ESR) and ferromagnetic resonance (FMR) experiments carried out over a wide range of frequencies, magnetic fields, and temperatures. Though x-ray data reveals no structural transition down to 15 K, a strong magneto-elastic coupling is detected across the ferromagnetic transition at $T_{\rm C} \simeq 126$ K. ESR results indicate a presence of the quasi-static short-range correlations extending far above $T_{\rm C}$, which is a typical fingerprint of the low-dimensional magnetism. The frequency-field diagram of the FMR modes mapped below $T_{\rm C}$ strongly suggests presence of two magnetic sublattices in LaCrSb$_{3}$. A quantitative understanding of the FMR excitations was achieved within a phenomenological model of interacting orthogonal ferro- and antiferromagnetic sublattices which was earlier proposed to explain unusually strong spin canting observed by neutron diffraction [E.~Granado et al., Phys. Rev. Lett. 89, 107204 (2002)]. The FMR results corroborate this scenario and call for the development of the underlying microscopic model of unconventional magnetism in LaCrSb$_{3}$.

cond-mat.str-el↗

arXiv2025

Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$

The three-dimensional (3D) magnet KBaFe$_2$(PO$_4$)$_3$ hosts a double-trillium lattice of Fe$^{3+}$ (spin, $S=5/2$) ions offering a prototypical platform to study the frustration induced effects in 3D. Through magnetization, specific heat, $^{31}$P nuclear magnetic resonance (NMR), and muon spin relaxation ($\mu$SR) experiments, supported by first principles calculations, we uncover an unconventional ground state. Despite strong antiferromagnetic interactions with a large Curie-Weiss temperature $\theta_{\rm CW} = -70(2)$ K, no magnetic long-range order is observed down to 30 mK. Below $T^{\ast}\simeq 3.5$ K, the NMR linewidth becomes nearly field-independent and the spin-spin relaxation rate $1/T_2$ saturates, accompanied by an inhomogeneous distribution of transverse nuclear magnetization $M_{xy}$. The latter indicates the emergence of short-range dynamical correlations, which was further corroborated by a robust and field-insensitive broad maximum in specific heat. In $\mu$SR, we detect neither a static internal field nor spin freezing; instead the relaxation remains dynamic and is best described by two coexisting dynamic relaxation channels: a dominant fast (sporadic) channel and a slower Markovian component. Their differing weights and fluctuation rates suggest microscopic inhomogeneity in spin dynamics. Altogether, KBaFe$_2$(PO$_4$)$_3$ exemplifies a rare high-spin stochiometric 3D antiferromagnet that evades ordering and instead fosters a mosaic of spin dynamics driven by strong geometric frustration intrinsic to the trillium lattice.

cond-mat.mtrl-sci↗

arXiv2025

Spin fluctuations, absence of magnetic order, and crystal electric field studies in the Yb$^{3+}$-based triangular lattice antiferromagnet Rb$_3$Yb(VO$_4$)$_2$

We report a comprehensive experimental investigation of the structural, thermodynamic, static, and dynamic properties of a triangular lattice antiferromagnet Rb$_3$Yb(VO$_4$)$_2$. Through the analysis of magnetic susceptibility, magnetization, and specific heat, complemented by crystal electric field (CEF) calculations, we confirm the Kramers' doublet with effective spin $J_{\rm{eff}}=1/2$ ground state. Magnetic susceptibility and isothermal magnetization analysis reveal a weak antiferromagnetic interaction among the $J_{\rm{eff}}=1/2$ spins, characterized by a small Curie-Weiss temperature ($\theta_{\text{CW}}^{\text{LT}}\simeq-0.26$ K) or a reduced exchange coupling ($J/k_{\rm B} \simeq 0.18$ K). The $^{51}$V NMR spectra and spin-lattice relaxation rate ($1/T_1$) show no evidence of magnetic long-range-order down to 1.6 K but reflect strong influence of CEF excitations in the intermediate temperatures. At low temperatures, $1/T_1(T)$ shows pronounced frequency dependence and $1/T_1$ vs field in different temperatures follows the scaling behaviour, highlighting the role of paramagnetic fluctuations. The CEF calculations using the point charge approximation divulge a large energy gap ($\sim 18.61$ meV) between the lowest and second lowest energy doublets, further establishing Kramers' doublet as the ground state. Our calculations also reproduce the experimental magnetization and specific heat data and indicate an in-plane magnetic anisotropy. These findings position Rb$_3$Yb(VO$_4$)$_2$ as an ideal and disorder-free candidate to explore intrinsic quantum fluctuations and possible quantum spin-liquid physics in a Yb$^{3+}$-based triangular lattice antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic and crystal electric field studies of two Yb$^{3+}$-based triangular lattice antiferromagnets

We present the low-temperature magnetic properties of two Yb$^{3+}$-based triangular lattice compounds NaSrYb(BO$_3$)$_2$ and K$_3$YbSi$_2$O$_7$ via thermodynamic measurements followed by crystal electric field (CEF) calculations. Magnetization and specific heat data as well as the CEF energy levels confirm that the ground state is characterized by the low-lying Kramers' doublet of Yb$^{3+}$ with effective spin-1/2 ($J_{\rm eff} = 1/2$). A small Curie-Weiss temperature and scaling of magnetic isotherms corroborate very weak magnetic correlations among $J_{\rm eff} = 1/2$ spins. The crystal field parameters are calculated using the point charge model and the CEF Hamiltonian is determined for both the compounds. The simulation using the eigenvalues of the CEF Hamiltonian reproduces the experimental susceptibility, magnetic isotherm, and magnetic specific heat data very well. The large separation between the ground state and first excited state doublets implies that the ground state is a Kramers' doublet with $J_{\rm eff} = 1/2$ at low temperatures, endorsing the experimental findings.

cond-mat.mtrl-sci↗

arXiv2024

Ground-state properties of the double trillium lattice antiferromagnet KBaCr$_2$(PO$_4$)$_3$

Trillium lattices formed by corner-shared triangular units are the platform for magnetic frustration in three dimensions. Herein, we report structural and magnetic properties of the Cr-based double trillium lattice material KBaCr$_2$(PO$_4$)$_3$ studied by x-ray diffraction, magnetization, heat capacity, thermal conductivity, and $^{31}$P nuclear magnetic resonance (NMR) measurements complemented by density-functional band-structure calculations. Heat capacity and $^{31}$P NMR measurements reveal the magnetic transition at $T_{\rm N1} \simeq 13.5$ K in zero field followed by another transition at $T_{\rm N2} \simeq 7$ K in weak applied fields. The NMR sublattice magnetization confirms that the transition at $T_{\rm N1}$ is 3D in nature. The $^{31}$P spin-lattice relaxation rate in the ordered state follows the $T^3$ behavior indicative of the two-magnon Raman process. The spin lattice of KBaCr$_2$(PO$_4$)$_3$ comprises two crystallographically nonequivalent ferromagnetic sublattices that are coupled antiferromagnetically, thus eliminating frustration in this trillium network.

cond-mat.mtrl-sci↗

arXiv2024

Cluster-glass behaviour and large magnetocaloric effect in frustrated hyperkagome ferromagnet Li$_2$MgMn$_3$O$_8$

A detailed study of the structural and magnetic properties of the spin-$3/2$ hyperkagome lattice compound Li$_2$MgMn$_3$O$_8$ is reported. This material shows ferromagnetic response below $T_{\rm C} \simeq 20.6$ K, the temperature almost three times lower than the Curie-Weiss temperature $\theta_{\rm CW} \simeq 56.6$ K. Density-functional band-structure calculations suggest that this reduction in $T_{\rm C}$ may be caused by long-range antiferromagnetic couplings that frustrate nearest-neighbor ferromagnetic couplings on the hyperkagome lattice. Large magnetocaloric effect is observed around the $T_{\rm C}$ with a maximum value of isothermal entropy change $\Delta S_{\rm m}\simeq 20$ J/kg-K and a maximum relative cooling power of $RCP\simeq 840$ J/kg for the 7 T magnetic field change. Critical analysis of the magnetization data and scaling analysis of the magnetocaloric effect suggest the 3D Heisenberg/XY universality class of the transition. The DC and AC magnetization measurements further reveal glassy nature of the ferromagnetic transition. A detailed study of the non-equilibrium dynamics via magnetic relaxation and memory effect measurements demonstrates that the system evolves through a large number of intermediate metastable states and manifests significant memory effect in the cluster-glass state.

cond-mat.mtrl-sci↗