Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCo2_2F7_7

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f \sim 2.4$ K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found as well as a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective $g$-factors close to 2 are obtained for both modes in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic $g$-tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective $g$-factors as determined from ESR and the local $g$-tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores.Comment: 9 pages, 6 figure

Frustration enhanced by Kitaev exchange in a j~eff=12\boldsymbol{\tilde{j}_{\text{eff}}=\frac12} triangular antiferromagnet

Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}_{\text{eff}}=\frac12}$ moments in the triangular-lattice magnet Na$_2$BaCo(PO$_4$)$_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the $\tilde{j}_{\text{eff}}=\frac12$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as additional frustrating agent, altogether putting forward Na$_2$BaCo(PO$_4$)$_2$ as a promising Kitaev spin-liquid material.Comment: 6 pages, 4 figures (published version) + supplemental material (4 pages

A Size-Dependent Analysis of the Structural, Surface, Colloidal, and Thermal Properties of Ti_1-xB₂ (x=0.03-0.08) Nanoparticles

We report the size-dependent structural, surface, colloidal, and thermal properties of the well-known refractory material titanium diboride prepared at the nanoscale by a size-controlled inorganic molten-salt technique. A combined analysis of the powder X-ray diffraction data through a modified Williamson-Hall plot, a size-strain plot, and Rietveld fitting methods indicates that TiB2 nanocrystals with average crystallite sizes smaller than D approximate to 22 nm prefer to form defects at the titanium site rather than experience strain. The resulting composition is Ti1-xB2 [x = 0.03(1)-0.08(1)]. The size-induced defect formation is accompanied by anisotropic lattice contraction that decreases primarily the cell parameter c. Transmission electron microscopy revealed that the nanocrystals are embedded in an amorphous matrix. A comparison between the electron spin resonance spectra of bulk and nanosized samples indicated that only the nanosized samples yield observable signals. This signal is associated with unpaired electrons trapped in the boron-oxygen species that surround the Ti1-xB2 nanocrystals. These species provide stability to the Ti1-xB2 nanocrystals in aqueous dispersions, as evidenced by electrokinetic measurements. Upon heating under an Ar atmosphere, the boron-oxygen species evaporate from ca. 1000 degrees C, and the Ti vacancies in the interior nanocrystalline core vanish. TiB2 nanocrystals show excellent chemical stability against decomposition up to 1500 degrees C

Magnetic resonance spectroscopy on the spin-frustrated magnets YBaCo₃MO₇ (M = Al, Fe)

We present experimental results of combined electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements on single crystals of the Swedenborgite-type compounds YBaCo3MO7 (M=Al, Fe). The magnetic lattice of these materials can be described as stacks of strongly geometrically frustrated kagome layers built up of Co2+ (S = 3/2) ions. Due to the Co-M site intermixing, there are M-type defects in the kagome planes as well as magnetic Co ions at the interplanar positions. Previous investigations revealed large antiferromagnetic (AFM) Curie-Weiss temperatures in both compounds. Yet no AFM long-range order but a spin glass behavior at low temperatures has been observed. Using (27)AlNMRas well as Co2+ and Fe3+ ESR spin probes that are sensitive to local magnetic properties at different crystallographic sites, we have identified two magnetic subsystems in both compounds and have studied their distinct properties. In particular in the case of YBaCo3AlO7, we have observed a gradual development of the spin correlations in the two-dimensional (2D) kagome planes containing nonmagnetic Al defects and the establishment of a 3D frozen glasslike spin state of the interplanar Co ions at lower temperatures. We have found that despite a strong dependence of the ordering temperature in the kagome layers on the type of the M ion (magnetic or nonmagnetic), the final 3D static ground state sets in at rather similar temperatures. We argue that the peculiar spin dynamics and a disordered magnetic ground state of the studied compounds result from the interplay of strong magnetic frustration and intrinsic structural disorder arising due to the intersite mixing of Co and M atoms

Coherent spin dynamics of solitons in the organic spin chain compounds (oo-DMTTF)2X_2X (XX = Cl, Br)

14 pages, 8 figuresInternational audienceWe studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X =$ Br, Cl) by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [Foury-Leylekian et al., Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. In addition, contributions of triplon excitations to the ESR response below the transition temperature were observed which provides a spectroscopic estimate for the spin-gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi-oscillations as signatures of coherent spin dynamics. We discuss the results obtained from these complementary methods of ESR spectroscopy in terms of solitons localized at the defect sites within the chains. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and CPMG methods it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence

Anisotropic magnetic interactions and spin dynamics in the spin-chain compound Cu(py)₂Br₂

We compare theoretical results for electron spin resonance (ESR) properties of the Heisenberg-Ising Hamiltonian with ESR experiments on the quasi-one-dimensional magnet Cu(py)$_2$Br$_2$ (CPB). Our measurements were performed over a wide frequency and temperature range giving insight into spin dynamics, spin structure, and magnetic anisotropy of this compound. By analyzing the angular dependence of ESR parameters (resonance shift and linewidth) at room temperature we show that the two weakly coupled inequivalent spin chain types inside the compound are well described by Heisenberg-Ising chains with their magnetic anisotropy axes perpendicular to the chain direction and almost perpendicular to each other. We further determine the full $g$-tensor from these data. In addition, the angular dependence of the linewidth at high temperatures gives us access to the exponent of the algebraic decay of a dynamical correlation function of the isotropic Heisenberg chain. From the temperature dependence of static susceptibilities we extract the strength of the exchange coupling ($J/k_B = 52.0\,\text{K}$) and the anisotropy parameter ($\delta\approx -0.02$) of the model Hamiltonian. An independent compatible value of $\delta$ is obtained by comparing the exact prediction for the resonance shift at low temperatures with high-frequency ESR data recorded at $4\,\text{K}$. The spin structure in the ordered state implied by the two (almost) perpendicular anisotropy axes is in accordance with the propagation vector determined from neutron scattering experiments. In addition to undoped samples we study the impact of partial substitution of Br by Cl ions on spin dynamics. From the dependence of the ESR linewidth on doping level we infer an effective decoupling of the anisotropic component $J\delta$ from the isotropic exchange $J$ in these systems.Comment: 26 pages, 12 figures, section V.A.2 revised, appendices A and B shortene