Signatures of a Spin-1/2 Cooperative Paramagnet in the Diluted Triangular Lattice of Y_{2}CuTiO_{6}.

We present a combination of thermodynamic and dynamic experimental signatures of a disorder driven dynamic cooperative paramagnet in a 50% site diluted triangular lattice spin-1/2 system: Y_{2}CuTiO_{6}. Magnetic ordering and spin freezing are absent down to 50 mK, far below the Curie-Weiss scale (-θ_{CW}) of ∼134 K. We observe scaling collapses of the magnetic field and temperature dependent magnetic heat capacity and magnetization data, respectively, in conformity with expectations from the random singlet physics. Our experiments establish the suppression of any freezing scale, if at all present, by more than 3 orders of magnitude, opening a plethora of interesting possibilities such as disorder stabilized long range quantum entangled ground states.

Gapless Quantum Spin Liquid in the Triangular System Sr_{3}CuSb_{2}O_{9}.

We report gapless quantum spin liquid behavior in the layered triangular Sr_{3}CuSb_{2}O_{9} system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well separated by Sb planes. Muon spin relaxation measurements show that the S=1/2 moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature θ_{CW}≃-143 K as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat (C_{m}) below 5 K reveals a C_{m}=γT+αT^{2} behavior. The significant T^{2} contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-T specific heat data, we estimate the dominant exchange scale to be ∼36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations (∼45 K) as well as high-temperature susceptibility analysis (∼70 K). The linear specific heat coefficient is about 18 mJ/mol K^{2} which is somewhat larger than for typical Fermi liquids.

LiZn2V3O8: a new geometrically frustrated cluster spin-glass.

We have investigated the structural and magnetic properties of a new cubic spinel LiZn2V3O8 (LZVO) through x-ray diffraction, dc and ac susceptibility, magnetic relaxation, aging, memory effect, heat capacity and 7Li nuclear magnetic resonance (NMR) measurements. A Curie-Weiss fit of the dc susceptibility [Formula: see text]([Formula: see text]) yields a Curie-Weiss temperature [Formula: see text] K. This suggests strong antiferromagnetic (AFM) interactions among the magnetic vanadium ions. The dc and ac susceptibility data indicate the spin-glass behavior below a freezing temperature T f  [Formula: see text] 3 K. The frequency dependence of the T f  is characterized by the Vogel-Fulcher law and critical dynamic scaling behavior or power law. From both fitting, we obtained the value of the characteristic angular frequency [Formula: see text] [Formula: see text] 3.56 [Formula: see text] 106 Hz, the dynamic exponent [Formula: see text] [Formula: see text] 2.65, and the critical time constant [Formula: see text] [Formula: see text] 1.82 [Formula: see text] 10-6 s, which falls in the conventional range for typical cluster spin-glass (CSG) systems. The value of relative shift in freezing temperature [Formula: see text] [Formula: see text] 0.039 supports a CSG ground states. We also found aging phenomena and memory effects in LZVO. The asymmetric response of the magnetic relaxation below T f  supports the hierarchical model. Heat capacity data show no long-range or short-range ordering down to 2 K. Only about 25% magnetic entropy change [Formula: see text] signifies the presence of strong frustration in the system. The 7Li NMR spectra show a shift and broadening with decreasing temperature. The spin-lattice and spin-spin relaxation rates show anomalies due to spin freezing around 3 K as the bulk magnetization.

Long-range antiferromagnetic order and possible field induced spin-flop transition in BiMnVO5.

We report the bulk magnetic characterization of a dimeric chain material, BiMnVO5, by means of magnetic susceptibility, magnetization and heat capacity measurements. Our results provide compelling evidence of an antiferromagnetic (AFM) transition at (T N) ~ 11.5 K. Moreover, the magnetic entropy change in zero field saturates to 14.6 J mol-1 K-1 which is close to the total spin entropy of Mn2+. The development of long-range magnetic order in this chain material demonstrates the interplay of strong intra-chain and inter-chain interactions between the dimers, in addition to the intra-dimer interaction. Low-temperature (T < T N) heat capacity data indicate the presence of a gap (Δ/k B ≈ 5 K) in the spin excitations. Furthermore, the isothermal magnetization below T N shows an anomaly in the slope between 30 and 40 kOe which is suggestive of a spin-flop transition. Such a low-field spin-flop transition and gapped spin wave excitations may be attributed to the presence of (weak) magnetic anisotropy in this material. We attempt to construct a phase diagram in the magnetic field-temperature plane by extracting data from in-field heat capacity and isothermal magnetization measurements.

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.

Observation of S = 1/2 quasi-1D magnetic and magneto-dielectric behavior in a cubic SrCuTe2O6.

We investigate the magnetic, thermal, and dielectric properties of SrCuTe2O6, which is isostructural to PbCuTe2O6, a recently found, Cu-based 3D frustrated magnet with a corner-sharing triangular spin network having dominant first and second nearest neighbor (nn) couplings (Koteswararao et al 2014 Phys. Rev. B 90 035141). Although SrCuTe2O6 has a structurally similar spin network, the magnetic data exhibit the characteristic features of a typical quasi-1D magnet, which mainly resulted from the magnetically dominant third nn coupling, uniform chains. The magnetic properties of this system are studied via magnetization (M), heat capacity (C p ), dielectric constant ([Formula: see text]), and measurements along with ab initio band structure calculations. The magnetic susceptibility [Formula: see text] data show a broad maximum at 32 K and the system orders at low temperatures [Formula: see text] K and [Formula: see text] K, respectively. The analysis of the [Formula: see text] data gives an intra-chain coupling, [Formula: see text], to be about ≈ - 42 K with non-negligible frustrated inter-chain couplings ([Formula: see text] and [Formula: see text]). The hopping parameters obtained from the LDA band structure calculations also suggest the presence of coupled uniform chains. The observation of simultaneous anomalies in [Formula: see text] at [Formula: see text] and [Formula: see text] suggests the presence of a magneto-dielectric effect in SrCuTe2O6. A magnetic phase diagram is also built based on the M, C p , and [Formula: see text] results.

Cluster spin glass behavior in geometrically frustrated Zn3V3O8.

We report the bulk magnetic properties of a yet unexplored vanadium-based multi-valenced spinel system, Zn3V3O8. A Curie-Weiss fit of our dc magnetic susceptibility χ(T) data in the temperature region of 140-300 K yields a Curie constant C = 0.75 cm(3)K mole(-1) V(-1), θCW = -370 K. We have observed a splitting between the zero field cooled (ZFC) and field cooled (FC) susceptibility curves below a temperature Tirr of about 6.3 K. The value of the 'frustration parameter' (|θcw|/T(N) ~ 100) suggests that the system is strongly frustrated. From the ac susceptibility measurements we find a logarithmic variation of freezing temperature (Tf) with frequency ν attesting to the formation of a spin glass below Tf. However, the value of the characteristic frequency obtained from the Vogel-Fulcher fit suggests that the ground state is closer to a cluster glass rather than a conventional spin glass. We explored further consequences of the spin glass behavior and observed aging phenomena and memory effect (both in ZFC and FC). We found that a positive temperature cycle erases the memory, as predicted by the hierarchical model. From the heat capacity CP data, a hump-like anomaly was observed in CP/T at about 3.75 K. Below this temperature the magnetic heat capacity shows a nearly linear dependence with T which is consistent with the formation of a spin glass state below Tf in Zn(3)V(3)O(8).

Li2ZnV3O8: a vanadium-based geometrically frustrated spinel system.

We report the synthesis and characterization of Li2ZnV3O8, which is a new Zn-doped LiV2O4 system containing only tetravalent vanadium. A Curie-Weiss susceptibility with a Curie-Weiss temperature of θCW ≈ -214 K suggests the presence of strong antiferromagnetic correlations in this system. We have observed a splitting between the zero-field cooled (ZFC) and field cooled (FC) susceptibility curves below 6 K. A peak is present in the ZFC curve around 3.5 K, suggestive of spin-freezing. Similarly, a broad hump is also seen in the inferred magnetic heat capacity around 9 K. The consequent entropy change is only about 8% of the value expected for an ordered S = 1/2 system. This reduction indicates the continued presence of large disorder in the system in spite of the large θCW, which might result from strong geometric frustration in the system. We did not find any temperature T dependence in our (7)Li nuclear magnetic resonance shift down to 6 K (an abrupt change in the shift does take place below 6 K), though considerable T-dependence has been found in the literature for LiV2O4--undoped or with other Zn/Ti contents. Consistent with the above observation, the (7)Li nuclear spin-lattice relaxation rate 1/T1 is relatively small and nearly T-independent, except for a small increase close to the freezing temperature which, once again, is small compared to undoped or 10% Zn or 20% Ti-doped LiV2O4.

PbCu3TeO7: an S = 1/2 staircase kagome lattice with significant intra-plane and inter-plane couplings.

We have synthesized polycrystalline and single-crystal samples of PbCu3TeO7 and studied its properties via magnetic susceptibility, χ(T), and heat-capacity, Cp(T), measurements and also electronic structure calculations. Whereas the crystal structure is suggestive of the presence of a quasi-2D network of Cu(2+) (S = 1/2) buckled staircase kagome layers, the χ(T) data show magnetic anisotropy and three magnetic anomalies at temperatures TN1 âˆ¼ 36 K, TN2 âˆ¼ 25 K, and TN3 âˆ¼ 17 K. The χ(T) data follow the Curie-Weiss law above 200 K and a Curie-Weiss temperature θCW âˆ¼- 150 K is obtained. The data deviate from the simple Curie-Weiss law below 200 K, which is well above TN1, suggesting the presence of competing magnetic interactions. The magnetic anomaly at TN3 appears to be of first order from magnetization measurements, although our Cp(T) results do not display any anomaly at TN3. The hopping integrals obtained from our electronic structure calculations suggest the presence of significant intra-kagome (next-nearest neighbor and diagonal) and inter-kagome couplings. These couplings take the PbCu3TeO7 system away from a disordered ground state and lead to long-range order, in contrast to what might be expected for an ideal (isotropic) 2D kagome system.

Magnetic behavior of Ba3Cu3Sc4O12.

The chain-like system Ba(3)Cu(3)Sc(4)O(12) has potentially interesting magnetic properties due to the presence of Cu(2+) and a structure-suggested low dimensionality. We present magnetization M versus magnetic field H and temperature T, T- and H-dependent heat-capacity C(p), (45)Sc nuclear magnetic resonance (NMR), muon spin rotation (µSR), neutron diffraction measurements and electronic structure calculations for Ba(3)Cu(3)Sc(4)O(12). The onset of magnetic long-range antiferromagnetic (AF) order at T(N) ∼ 16 K is consistently evidenced from the whole gamut of our data. A significant sensitivity of T(N) to the applied magnetic field H (T(N) ∼ 0 K for H = 70 kOe) is also reported. Coupled with a ferromagnetic Curie-Weiss temperature (θ(CW) ∼ 65 K) in the susceptibility (from a 100 to 300 K fit), it is indicative of competing ferromagnetic and antiferromagnetic interactions. These indications are corroborated by our density functional theory based electronic structure calculations, where we find the presence of significant ferromagnetic couplings between some copper ions whereas AF couplings were present between some others. Our experimental data, backed by our theoretical calculations, rule out the one-dimensional magnetic behavior suggested by the structure and the observed long-range order is due to the presence of non-negligible magnetic interactions between adjacent as well as next-nearest chains.

(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.

Doping effects in the coupled, two-leg spin ladder BiCu(2)PO(6).

We report preparation, x-ray diffraction, magnetic susceptibility χ(T) and heat capacity C(p)(T) measurements on undoped samples as well as samples with Zn-doped (S = 0) at the Cu site BiCu(2(1-x))Zn(2x)PO(6), Ni-doped (S = 1) at the Cu site BiCu(2(1-y))Ni(2y)PO(6), and Ca doped (holes) at the Bi site Bi(1-z)Ca(z)Cu(2)PO(6) in the coupled two-leg spin ladder system BiCu(2)PO(6). While, Zn shows complete solid solubility, Ni could be doped to about 20% and Ca to about 15%. Magnetization and heat capacity data in the undoped compound point towards the existence of frustration effects. In all the samples, the χ(T) at low temperature increases with doping content. The Zn-induced susceptibility is smaller than that due to effective S = 1/2 moments, possibly due to frustrating next-nearest-neighbor interactions along the leg. For Zn content x>0.01, χ(T) deviates from the Curie law at low temperatures. The magnetic specific heat data C(m)(T) for the Zn-doped samples show weak anomalies at low temperature, in agreement with the χ(T) behavior. The anomalies are suggestive of spin freezing at low- T. In contrast, prominent effects are observed in χ(T) and C(m)(T) on Ni-doped samples. The zero-field-cooled (ZFC) and field-cooled (FC) χ(T) data are different from each other at low temperature, unlike that for Zn-doped samples, clearly indicating a transition to a spin-glass-like phase. No anomalies were found in Ca- or Pb-doped samples.

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.

Impurity-induced magnetic order in low-dimensional spin-gapped materials.

We have studied the effect of nonmagnetic Zn impurities in the coupled spin ladder Bi(Cu_{1-x}Zn_{x})_{2}PO_{6} using ;{31}P NMR, muon spin resonance (microSR), and quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane, or spin-Peierls chains. This allows us to propose a simple common framework to explain the generic low-temperature impurity-induced freezings observed in low-dimensional spin-gapped materials.

Electronic structure of Na(2)CuP(2)O(7): a nearly 2D Heisenberg antiferromagnetic system.

We have employed first principles calculations to study the electronic structure and the implications for the magnetic properties of Na(2)CuP(2)O(7). Using the self-consistent tight-binding linearized muffin-tin orbital method and the Nth-order muffin-tin orbital downfolding method, we have calculated the various intrachain and interchain hopping parameters between the magnetic Cu(2+) ions. Our calculations for Na(2)CuP(2)O(7) reveal dominant intrachain hopping, but in contrast to the typical quantum spin chains the interchain hoppings are not negligible. The Wannier function plot of the Cu d(x(2)-y(2)) orbitals shows that the exchange interactions are primarily mediated by the oxygens and the distance between the oxygens in Na(2)CuP(2)O(7) is favorable for both intrachain and interchain interactions, suggesting the inapplicability of the one-dimensional Heisenberg model for this system, in agreement with recent experimental results.

(31)P NMR study of Na(2)CuP(2)O(7): an S = 1/2 two dimensional Heisenberg antiferromagnetic system.

The magnetic properties of Na(2)CuP(2)O(7) were investigated by means of (31)P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the (31)P NMR shift, the spin-lattice [Formula: see text], and spin-spin (1/T(2)) relaxation rate data as a function of temperature T. The temperature dependence of the NMR shift K(T) is well described by the S = 1/2 square lattice Heisenberg antiferromagnetic model with an intraplanar exchange of [Formula: see text] K and a hyperfine coupling A = 3533 ± 185 Oe /µ(B). The (31)P NMR spectrum was found to broaden abruptly below T∼10 K, signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data down to 1.8 K. The heat capacity appears to have a weak maximum around 10 K. With decrease in temperature, the spin-lattice relaxation rate 1/T(1) decreases monotonically and appears to agree well with the high temperature series expansion expression for a S = 1/2 2D square lattice.