Persistent spin dynamics intrinsic to amplitude-modulated long-range magnetic order

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to 53(3) mK (T/T_N ~ 1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T -> 0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence.Comment: 5 pages + supplementar

Spin-stripe phase in a frustrated zigzag spin-1/2 chain

Motifs of periodic modulations are encountered in a variety of natural systems, where at least two rival states are present. In strongly correlated electron systems such behaviour has typically been associated with competition between short- and long-range interactions, e.g., between exchange and dipole-dipole interactions in the case of ferromagnetic thin films. Here we show that spin-stripe textures may develop also in antiferromagnets, where long-range dipole-dipole magnetic interactions are absent. A comprehensive analysis of magnetic susceptibility, high-field magnetization, specific heat, and neutron diffraction measurements unveils $\beta$-TeVO$_4$ as a nearly perfect realization of a frustrated (zigzag) ferromagnetic spin-1/2 chain. Strikingly, a narrow spin stripe phase develops at elevated magnetic fields due to weak frustrated short-range interchain exchange interactions possibly assisted by the symmetry allowed electric polarization. This concept provides an alternative route for the stripe formation in strongly correlated electron systems and may help understanding other widespread, yet still elusive, stripe-related phenomena.Comment: accapted in Nature Communication

Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the non-ferroelectric phase preceding the low-temperature multiferroic state in FeTe2O5Br. Refnement of the neutron diffraction data and simulations of 79,81Br NQR spectra reveal that the incommensurate magnetic ordering in the non-ferroelectric state comprises amplitude-modulated magnetic moments, similarly as in the multiferroic state. The two ordered states differ in the orientation of the magnetic moments and phase shifts between modulation waves. Surprisingly, all symmetry restrictions for the electric polarization are absent in both states. The different ferroelectric responses of the two states are thus argued to arise from the differences in the phase shifts between certain modulation waves, which cancel out in the non-ferrolectric state.Comment: 9 pages, 8 figures including appendix, published in PR

Magnetic ground state and 2D behavior in pseudo-Kagome layered system Cu3Bi(SeO3)2O2Br

Anisotropic magnetic properties of a layered kagome-like system Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic susceptibility measurements as well as powder and single-crystal neutron diffraction. At T_N = 27.4 K the system develops an alternating antiferromagnetic order of (ab) layers, which individually exhibit canted ferrimagnetic moment arrangement, resulting from the competing ferro- and antiferro-magnetic intralayer exchange interactions. A magnetic field B_C ~ 0.8 T applied along the c axis (perpendicular to the layers) triggers a metamagnetic transition, when every second layer flips, i.e., resulting in a ferrimagnetic structure. Significantly higher fields are required to rotate the ferromagnetic component towards the b axis (~7 T) or towards the a axis (~15 T). The estimates of the exchange coupling constants and features indicative of an XY character of this quasi-2D system are presented.Comment: 7 pages, 6 figures, final versio

Negative-vector-chirality 120∘ spin structure in the defect- and distortion-free quantum kagome antiferromagnet YCu3(OH)6Cl3

The magnetic ground state of the ideal quantum kagome antiferromagnet (QKA) has been a longstanding puzzle, mainly because perturbations to the nearest-neighbor isotropic Heisenberg Hamiltonian can lead to various fundamentally different ground states. Here we investigate a recently synthesized QKA representative YCu3(OH)6Cl3, where perturbations commonly present in real materials, like lattice distortion and intersite ion mixing, are absent. Nevertheless, this compound enters a long-range magnetically ordered state below TN = 15 K. Our powder neutron diffraction experiment reveals that its magnetic structure corresponds to a coplanar 120 state with negative vector spin chirality. The ordered magnetic moments are suppressed to 0.42(2)μB, which is consistent with the previously detected spin dynamics persisting to the lowest experimentally accessible temperatures. This indicates either a coexistence of magnetic order and disorder or the presence of strong quantum fluctuations in the ground state of YCu3(OH)6Cl3

Thermal effects versus spin nematicity in a frustrated spin-1/2 chain

The spin-nematic phase is an intriguing state of matter that lacks usual long-range dipolar order, yet it exhibits higher multipolar order. This makes its detection extremely difficult and controversial. Recently, nuclear magnetic resonance (NMR) has been proposed as one of the most suitable techniques to confirm its existence. We report a $^{17}$O NMR observation of the reduction of the local magnetization in the polarized state of the frustrated spin-1/2 chain $\beta$-TeVO$_4$, which was previously proposed to be a fingerprint of the spin-nematic behavior. However, our detailed study shows that the detected missing fraction of the magnetization, probed by NMR frequency shift, is thermally activated, thus undermining the presence of the spin-nematic phase in the investigated compound. This highlights the importance of careful considerations of temperature-dependent NMR shift that has been overlooked in previous studies of spin nematicity.Comment: accepted for publication in PRB(R), with supplementar

Magnetic properties of triangular lattice antiferromagnets Ba3RB9O18 (R = Yb, Er)

Frustration, spin correlations and interplay between competing degrees of freedom are some of the key ingredients that underlie exotic states with fractional excitations in quantum materials. Rare-earth based two dimensional magnetic lattice wherein crystal electric field, spin-orbit coupling, anisotropy and electron correlation between rare-earth moments offer a new paradigm in this context. Herein, we present crystal structure, magnetic susceptibility and specific heat accompanied by crystal electric field calculations on the polycrystalline sample of Ba3RB9O18 (R = Yb, Er) in which R3+ ions form a perfect triangular lattice without anti-site disorder. The localized R3+ spins show neither long-range order nor spin-glass state down to 1.9 K in Ba3RB9O18. Magnetization data reveal a pseudospin Jeff = 1/2 ( Yb3+) in the Kramers doublet state and a weak antiferromagnetic interaction between Jeff = 1/2 moments in the Yb variant. On the other hand, the effective moment {\mu}eff = 8.8 {\mu}B was obtained from the Curie-Weiss fit of the low-temperature susceptibility data of Er variant suggests the admixture of higher crystal electric field states with the ground state. The Curie-Weiss fit of low-temperature susceptibility data for Er system unveils the presence of a relatively strong antiferromagnetic interaction between Er3+ moments compared to its Yb3+ analog. Ba3ErB9O18 does not show long-range magnetic ordering down to 500 mK. Furthermore, our crystal electric field calculations based on magnetization data of Ba3ErB9O18 suggest the presence of a small gap between the ground and first excited Kramers doublets. The broad maximum around 4 K in magnetic specific heat in zero-field is attributed to the thermal population of the first CEF excited state in Ba3ErB9O18, which is consistent with our CEF calculations