The Ising triangular-lattice antiferromagnet neodymium heptatantalate as a quantum spin liquid candidate.

Disordered magnetic states known as spin liquids are of paramount importance in both fundamental and applied science. A classical state of this kind was predicted for the Ising antiferromagnetic triangular model, while additional non-commuting exchange terms were proposed to induce its quantum version-a quantum spin liquid. However, these predictions have not yet been confirmed experimentally. Here, we report evidence for such a state in the triangular-lattice antiferromagnet NdTa7O19. We determine its magnetic ground state, which is characterized by effective spin-1/2 degrees of freedom with Ising-like nearest-neighbour correlations and gives rise to spin excitations persisting down to the lowest accessible temperature of 40 mK. Our study demonstrates the key role of strong spin-orbit coupling in stabilizing spin liquids that result from magnetic anisotropy and highlights the large family of rare-earth (RE) heptatantalates RETa7O19 as a framework for realization of these states, which represent a promising platform for quantum applications.

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba[Formula: see text]MnTeO[Formula: see text].

Frustrated magnets based on oxide double perovskites offer a viable ground wherein competing magnetic interactions, macroscopic ground state degeneracy and complex interplay between emergent degrees of freedom can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. By local-probe muon spin relaxation ([Formula: see text]SR) and complementary thermodynamic measurements accompanied by first-principles calculations, we here demonstrate novel electronic structure and magnetic phases of Ba[Formula: see text]MnTeO[Formula: see text], where Mn[Formula: see text] ions with S = 5/2 spins constitute a perfect triangular lattice. Magnetization results evidence the presence of strong antiferromagnetic interactions between Mn[Formula: see text] spins and a phase transition at [Formula: see text] = 20 K. Below [Formula: see text], the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K, which is due to magnetic anisotropy. [Formula: see text]SR reveals the presence of static internal fields in the ordered state and short-range spin correlations high above [Formula: see text]. It further unveils critical slowing-down of spin dynamics at [Formula: see text] and the persistence of spin dynamics even in the magnetically ordered state. Theoretical studies infer that Heisenberg interactions govern the inter- and intra-layer spin-frustration in this compound. Our results establish that the combined effect of a weak third-nearest-neighbour ferromagnetic inter-layer interaction (owing to double-exchange) and intra-layer interactions stabilizes a three-dimensional magnetic ordering in this frustrated magnet.

Probing the Griffiths like phase, unconventional dual glassy states, giant exchange bias effects and its correlation with its electronic structure in Pr2-x Sr x CoMnO6.

Crystal, electronic structure, dc and ac magnetization properties of the hole substituted (Sr2+) and partially B-site disordered double perovskite Pr2-x Sr x CoMnO6 system have been investigated. The XRD pattern analysis showed a systematic decrease in the lattice parameters owing to the enhanced oxidation states of the Co/Mn ions. The electronic structure study by XPS measurements suggested the presence of mixed valence states of the B-site ions (Co2+ /Co3+ and Mn3+ /Mn4+) with significant enhancement of the average oxidation states due to hole doping. The mere absence of electronic states near the Fermi level in the valence band (VB) spectra for both pure (x = 0.0) and Sr doped (x = 0.5) systems indicated the insulating nature of the samples. Sr substitution is observed to increase the spectral weight near the Fermi level suggesting for an enhanced conductivity of the hole doped system. The dc magnetization data divulged a Griffiths like phase above the long-range ordering temperature. A typical re-entrant spin glass like phase driven by the inherent anti-site disorder (ASD) has been recognized by ac susceptibility study for both the pure and doped systems. Most interestingly, the emergence of a new cluster glass like phase (immediately below the magnetic ordering temperature and above the spin-glass transition temperature) solely driven by the Sr substitution has been unravelled by ac magnetization dynamics study. Observation of these dual glassy states in a single system is scarce and hence placed the present system amongst the rare materials. The isothermal magnetization measurements further probed the exhibition of the giant exchange bias effect originated from the interfacial exchange interactions due to existence of low temperature antiferromagnetic clusters embedded in the glassy matrix.

Symmetry Reduction in the Quantum Kagome Antiferromagnet Herbertsmithite.

Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the threefold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state.

Absence of low energy magnetic spin-fluctuations in isovalently and aliovalently doped LaCo2B2 superconducting compounds.

Magnetization, resistivity and (11)B, (59)Co NMR measurements have been performed on the Pauli paramagnet [Formula: see text], and the superconductors [Formula: see text] ([Formula: see text] K) and [Formula: see text] ([Formula: see text] K). The site selective NMR experiment reveals the multiband nature of the Fermi surface in these systems. The temperature independent Knight shift and 1/T 1 T clearly indicate the absence of correlated low energy magnetic spin-fluctuations in the normal state, which is in contrast to other Fe-based pnictides. The density of states (DOS) of Co 3d electrons has been enhanced in superconducting [Formula: see text] and [Formula: see text] with respect to the non superconducting reference compound [Formula: see text]. The occurrence of superconductivity is related to the DOS enhancement.

Spin Liquid State in the 3D Frustrated Antiferromagnet PbCuTe_{2}O_{6}: NMR and Muon Spin Relaxation Studies.

PbCuTe_{2}O_{6} is a rare example of a spin liquid candidate featuring a three-dimensional magnetic lattice. Strong geometric frustration arises from the dominant antiferromagnetic interaction that generates a hyperkagome network of Cu^{2+} ions although additional interactions enhance the magnetic lattice connectivity. Through a combination of magnetization measurements and local probe investigations by NMR and muon spin relaxation down to 20 mK, we provide robust evidence for the absence of magnetic freezing in the ground state. The local spin susceptibility probed by the NMR shift hardly deviates from the macroscopic one down to 1 K pointing to a homogeneous magnetic system with a low defect concentration. The saturation of the NMR shift and the sublinear power law temperature (T) evolution of the 1/T_{1} NMR relaxation rate at low T point to a nonsinglet ground state favoring a gapless fermionic description of the magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics is witnessed, which may signal a reconstruction of spinon Fermi surface. Nonetheless, the compound remains in a fluctuating spin liquid state down to the lowest temperature of the present investigation.

Contiguous 3d and 4f magnetism: strongly correlated 3d electrons in YbFe2Al10.

We present magnetization, specific heat, and (27)Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard-x-ray photoemission spectroscopy study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb(3+) carries a full and stable moment, and Fe carries a moment of about 3.1 µB. The enhanced value of the Sommerfeld-Wilson ratio and the dynamic scaling of the spin-lattice relaxation rate divided by T[(27)(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. (27)(1/T1T) simultaneously tracks the valence fluctuations from the 4f Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature; the latter evolve out of an Yb 4f admixed conduction band.

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR.

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.

(75)As NMR study of antiferromagnetic fluctuations in Ba(Fe(1-x)Ru(x))(2)As(2).

The evolution of (75)As NMR parameters with composition and temperature was probed in the Ba(Fe(1-x)Ru(x))(2)As(2) system where Fe is replaced by isovalent Ru. While the Ru end member was found to be a conventional Fermi liquid, the composition (x = 0.5) corresponding to the highest T(c) (20 K) in this system shows an upturn in the (75)As [Formula: see text] below about 80 K, evidencing the presence of antiferromagnetic (AFM) fluctuations. These results are similar to those obtained in another system with isovalent substitution, BaFe(2)(As(1-x)P(x))(2) (Nakai et al 2010 Phys. Rev. Lett. 105 107003) and point to a possible role of AFM fluctuations in driving superconductivity.

Magnetic susceptibility and heat capacity of a novel antiferromagnet: LiNi2P3O10 and the effect of doping.

We report the synthesis, x-ray diffraction, magnetic susceptibility and specific heat measurements on polycrystalline samples of undoped LiNi(2)P(3)O(10) and samples with non-magnetic impurity (Zn(2+), S = 0) and magnetic impurity (Cu(2+), S = 1/2) at the Ni site. The magnetic susceptibility data show a broad maximum at around 10 K and a small anomaly at about 7 K in the undoped sample. There is a λ-like anomaly in the specific heat at 7 K, possibly due to the onset of antiferromagnetic ordering in the system. The magnetic entropy change at the ordering temperature is close to the value corresponding to Rln(2S + 1) expected for an S = 1 system. The temperature corresponding to the broad maximum and the ordering temperature both decrease on Zn and Cu substitutions and also in applied magnetic fields.