Strongly frustrated magnetism of the metallic pyrochlore oxide Pr2Ir2O7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of /T*/ = 20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below /T*/. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.

Single crystals of YbAlB 4 were grown in excess Al flux. Plate-and needle-shaped crystals were found. The plates are found to be -YbAlB 4 , which crystallizes with the ThMoB 4 structure type in space group Cmmm (No. 65), Z ) 4, with lattice parameters of a ) 7.3080(4), b ) 9.3150(5), and c ) 3.4980(2) Å. The needle-shaped crystals were identified as the first form of YbAlB 4 which crystallizes with the YCrB 4 structure type in space group Pbam (No. 55), Z ) 4, with lattice parameters of a ) 5.9220(2), b ) 11.4730(3), and c ) 3.5060(5) Å. While both compounds have heavy fermion ground states with Isinglike magnetic anisotropy, the electronic specific heat coefficients (γ) differ. The -phase has a γ value near 300 mJ mol, more than twice that of the R-phase, γ ) 130 mJ mol. A comparison of the structures and physical properties of both polymorphs is presented.

Solution combustion synthesis (SCS) is shown to be versatile for the rapid-one-pot synthesis of three compounds and four polymorphs in the Cu−V−O ternary family: α-CuV 2 O 6 , αand β-Cu 2 V 2 O 7 , and γ-Cu 3 V 2 O 8. These compounds feature copper/vanadium stoichiometric ratios ranging from 1:1 to 3:1; their structural, electronic, optoelectronic, and photoelectrochemical attributes were comprehensively characterized by a combination of theoretical and experimental techniques. The main contribution of the present study is the demonstration that a range of stoichiometries in this compound family can be derived simply by tuning the precursor mole ratio in the SCS procedure. The Cu−V−O family of samples, derived by SCS, is shown to exemplify the strong effect of compound stoichiometry on the optoelectronic and photoelectrochemical properties. Overall, α-CuV 2 O 6 showed the best performance, rooted in the direct nature of the optical transition in this material. Finally, SCS is very timeefficient and the various compositions can be obtained in a matter of minutes, as opposed to hours or even days in classical solution-based or ceramic synthesis routes.

Our single crystal study reveals that the single-layer S=2 triangular Heisenberg antiferromagnet FeGa2S4 forms a frozen spin-disordered state, similar to the S=1 isostructural magnet NiGa2S4. In this state, the magnetic specific heat C{M} is not only insensitive to the field, but shows a T2 dependence that scales to C{M} of NiGa2S4, suggesting the same underlying mechanism of the 2D coherent behavior. In contrast, the bilayer system Fe2Ga2S5 exhibits a 3D antiferromagnetic order.

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Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order 1 . It is generally accepted that both frustration and disorder are essential ingredients in all spin glasses, so it was surprising that PrAu 2 Si 2 , a stoichiometric compound with a well-ordered crystal structure, was reported 2 to show spin-glass freezing. Here, we report on inelastic neutron scattering measurements of crystal-field excitations, which show that PrAu 2 Si 2 has a singlet ground state and that the exchange coupling is very close to the critical value to induce magnetic order. We propose that spin-glass freezing results from dynamic fluctuations of the crystal-field levels that destabilize the induced moments and frustrate the development of long-range magnetic correlations. This novel mechanism for producing a frustrated ground state could provide a method of testing the concept of 'avoided criticality' in glassy systems.Frustration arises from competing interactions that favour incompatible ground states 1,3 . For example, in rare-earth intermetallic compounds, the magnetic moments of the f electrons on each rare-earth site interact with neighbouring moments through Ruderman-Kittel-Kasuya-Yosida exchange interactions that oscillate in sign with increasing separation. Except for specific classes of geometrically frustrated lattices, well-ordered crystal structures produce either ferromagnetic or antiferromagnetic order, depending on the energy minimization of the interactions between all the neighbouring moments. However, when there is disorder, in either the site occupations or the exchange interactions, the additional randomness can prevent a unique ordered ground state. Instead, these systems may form spin glasses, which possess a multitude of possible disordered ground states, into one of which the system freezes below the glass transition temperature, T g (ref. 1). In the thermodynamic limit, spin glasses show broken ergodicity, preventing significant fluctuations to any of the other degenerate spin configurations.In recent years, two stoichiometric intermetallic compounds have shown evidence of spin-glass order, URh 2 Ge 2 (ref. 4) and PrAu 2 Si 2 (ref. 2), both nominally with the same tetragonal (ThCr 2 Si 2 -type) crystal structure. In both samples, classic spin-glass behaviour was observed (T g = 11 K and 3 K, respectively), with a frequency-dependent peak in the Figure 1 Crystal-field transition in PrAu 2 Si 2 . Inelastic neutron scattering from PrAu 2 Si 2 measured at 1.5 K (red symbols) with error bars derived from the square root of the raw data counts. The solid line is a fit to the singlet-doublet crystal-field transition at ∆ = 0.7 meV (dashed line) and an elastic peak from nuclear incoherent scattering (dotted line). The shaded area represents double scattering from the transition at 2∆. The inset illustrates the mechanism for induced-moment formation, in which interionic exchange coupling, J ex , admixes the excited magnetic doublet into the singlet grou...

Critical points that can be suppressed to zero temperature are interesting because quantum fluctuations have been shown to dramatically alter electron gas properties. Here, the metal formed by Co doping the paramagnetic insulator FeS2, Fe1-xCoxS2 is demonstrated to order ferromagnetically at x > xc = 0.01+/-0.005, where we observe unusual transport, magnetic, and thermodynamic properties. We show that this magnetic semiconductor undergoes a percolative magnetic transition with distinct similarities to the Griffiths phase, including singular behavior at xc and zero temperature.