Structural, thermodynamic, and local probe investigations of a honeycomb material Ag3_{3}LiMn2_{2}O6_{6}

The system Ag[Li$_{1/3}$Mn$_{2/3}$]O$_{2}$ belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group $C\,2/m$ and the magnetic Mn$^{4+}$($S=3/2$) ions form a honeycomb network in the $ab$-plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature $\theta_{CW}\sim-51$ K) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48\,K and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. The (ESR) line broadening on approaching the ordering temperature $T_{{\rm N}}$ could be described in terms of a Berezinski-Kosterlitz-Thouless (BKT) scenario with $T_{{\rm KT}}=40(1)$ K. $^{7}$Li NMR line-shift probed as a function of temperature tracks the static susceptibility (K$_{iso}$) of magnetically coupled Mn$^{4+}$ ions. The $^{7}$Li spin-lattice relaxation rate (1/$T$$_{1}$) exhibits a sharp decrease below about 50 K. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system Ag$_{3}$LiMn$_{2}$O$_{6}$.Our ab-initio electronic structure calculations suggest that in the $ab$-plane, the nearest neighbor (NN) exchange interaction is strong and AFM, while the next NN and the third NN exchange interactions are FM and AFM respectively. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment.Comment: 11 pages, 13 figures, accepted in Phys Rev

Unconventional magnetism in the 4d4^{4} based (S=1S=1) honeycomb system Ag3_{3}LiRu2_{2}O6_{6}

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$ (4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-Weiss temperature ($\theta_{CW}$ = -57 K) suggests antiferromagnetic interactions among Ru$^{4+}$ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. $^{7}$Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S = 1$ description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our $\mu$SR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.Comment: 10 pages, 11 figures. accepted in Phys. Rev.

Universal fluctuating regime in triangular chromate antiferromagnets

We report x-ray diffraction, magnetic susceptibility, heat capacity, $^{1}$H nuclear magnetic resonance (NMR), and muon spin relaxation ($\mu$SR) measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\alpha$-HCrO$_{2}$ (trigonal, space group: $R\bar{3}m$). This compound undergoes a clear magnetic transition at $T_{\rm N} \simeq 22.5$~K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat. Local probes (NMR and $\mu$SR) reveal a broad regime with slow fluctuations down to $0.7\,T_{\rm N}$, this temperature corresponding to the maximum in the $\mu$SR relaxation rate and in the NMR wipe-out. From the comparison with NaCrO$_{2}$ and $\alpha$-KCrO$_{2}$, the fluctuating regime and slow dynamics below $T_{\rm N}$ appear to be hallmarks of the TLHAF with $ABC$ stacking that leads to a frustration of interlayer couplings between the triangular planes. This interlayer frustration is a powerful lever to generate spin states with persistent dynamics and may bear implications to spin-liquid candidates with the triangular geometry.Comment: 14 pages, 11 figures, 2 table

Field evolution of low-energy excitations in the hyperhoneycomb magnet β\beta-Li2_2IrO3_3

$^7$Li nuclear magnetic resonance (NMR) and terahertz (THz) spectroscopies are used to probe magnetic excitations and their field dependence in the hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$. Spin-lattice relaxation rate ($1/T_1$) measured down to 100\,mK indicates gapless nature of the excitations at low fields (below $H_c\simeq 2.8$\,T), in contrast to the gapped magnon excitations found in the honeycomb Kitaev magnet $\alpha$-RuCl$_3$ at zero applied magnetic field. At higher temperatures in $\beta$-Li$_2$IrO$_3$, $1/T_1$ passes through a broad maximum without any clear anomaly at the N\'eel temperature $T_N\simeq 38$\,K, suggesting the abundance of low-energy excitations that are indeed observed as two peaks in the THz spectra, both correspond to zone-center magnon excitations. At higher fields (above $H_c$), an excitation gap opens, and a re-distribution of the THz spectral weight is observed without any indication of an excitation continuum, in contrast to $\alpha$-RuCl$_3$ where an excitation continuum was reported.Comment: 6 pages, 4 figure

Structural and magnetic properties of a new cubic spinel LiRhMnO4

We report the structural and magnetic properties of a new system LiRhMnO$_{4}$ (LRMO) through x-ray diffraction, bulk magnetization, heat capacity and $^{7}$Li nuclear magnetic resonance (NMR) measurements. LRMO crystallizes in the cubic space group $\mathit{Fd}$$\bar{3}$$\mathit{m}$. From the DC susceptibility data, we obtained the Curie-Weiss temperature $\mathrm{\theta}_{\mathrm{CW}}$ = -26 K and Curie constant $\mathit{C}$ = 1.79 Kcm$^{3}$/mol suggesting antiferromagnetic correlations among the magnetic Mn$^{4+}$ ions with an effective spin $\mathit{S}$ = $\frac{3}{2}$. At $\mathit{H}$ = 50 Oe, the field cooled and zero-field cooled magnetizations bifurcate at a freezing temperature, $T_{f}$ = 4.45 K, which yields the frustration parameter $\mathit{f=\frac{\mid\theta_{CW}\mid}{T_{f}}}>$5. AC susceptibility, shows a cusp-like peak at around $T_{f}$, with the peak position shifting as a function of the driving frequency, confirming a spin-glass-like transition in LRMO. LRMO also shows typical spin-glass characteristics such as memory effect, aging effect and relaxation. In the heat capacity, there is no sharp anomaly down to 2 K indicative of long-range ordering. The field sweep $^{7}$Li NMR spectra show broadening with decreasing temperature without any spectral line shift. The $^{7}$Li NMR spin-lattice and spin-spin relaxation rates also show anomalies due to spin freezing near $T_{f}$.Comment: 9 pages, 16 figures, 2 table

Anisotropic temperature-field phase diagram of single crystalline β−Li2IrO3: magnetization, specific heat, and Li7 NMR study

Detailed magnetization, specific heat, and $^7$Li nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$ are reported. At high temperatures, {\cred anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions}, in agreement with the combination of a ferromagnetic Kitaev interaction ($K$) and a negative off-diagonal anisotropy ($\Gamma$) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along $a$ or $c$ have only a weak effect on the system and reduce the N\'eel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along $b$ causes a drastic reduction in the $T_N$ that vanishes around $H_c=2.8$ T giving way to a crossover toward a quantum paramagnetic state. $^7$Li NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature.Comment: 10 pages, 9 figures; published versio

Universal fluctuating regime in triangular chromate antiferromagnets

We report x-ray diffraction, magnetic susceptibility, heat capacity, H-1 nuclear magnetic resonance (NMR), and muon spin relaxation (mu SR) measurements, as well as density-functional band-structure calculations for the frustrated S = 3/2 triangular lattice Heisenberg antiferromagnet (TLHAF) alpha-HCrO2 (trigonal, space group: R (3) over barm). This compound undergoes a clear magnetic transition at T-N less than or similar to 22.5 K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat capacity. Local probes (NMR and mu SR) reveal a broad regime with slow fluctuations down to 0.7 T-N, this temperature corresponding to the maximum in the mu SR relaxation rate and in the NMR wipe-out. From the comparison with NaCrO2 and alpha-KCrO2, the fluctuating regime and slow dynamics below T-N appear to be hallmarks of the TLHAF with ABC stacking. We discuss the role of interlayer frustration, which may have impacted recent spin-liquid candidates with triangular geometry.This article is published as Somesh, K., Y. Furukawa, Gediminas Simutis, F. Bert, Markus Prinz-Zwick, Norbert Büttgen, Andrej Zorko, Alexander A. Tsirlin, P. Mendels, and R. Nath. "Universal fluctuating regime in triangular chromate antiferromagnets." Physical Review B 104, no. 10 (2021): 104422. DOI: 10.1103/PhysRevB.104.104422. Copyright 2021 American Physical Society DOE Contract Number(s): AC02-07CH11358; CRG/2019/000960; 107745057; ANR-18-CE30-0022; P2EZP2-178604; ANR-10-LABX-0039-PALM; P1-0125; BI-US/18-20-064; J1-2461; N1-0148; DMR-1644779. Posted with permission