No abstract

Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a spin-freezing transition and magnetic irreversibilities below a temperature T f similar to what is observed in randomly frustrated spin glasses. We present results of DC nonlinear magnetization measurements on Y2Mo2O7 that provide strong evidence that there is an underlying thermodynamic phase transition at T f , which is characterized by critical exponents γ ≈ 2.8 and β ≈ 0.8. These values are typical of those found in random spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is immeasurably small. The past five years have seen a resurgence of significant interest devoted to the systematic study of geometrically frustrated antiferromagnets [1][2][3][4]. Geometric frustration arises in materials containing antiferromagneticallycoupled magnetic moments which reside on geometrical units, such as triangles and tetrahedra, that inhibit the formation of a collinear magnetically-ordered state. The main motivation for the current interest in these systems stems from suggestions that (i) they may display critical phenomena belonging to a "new" chiral universality class different from the universality classes of collinear magnets [2,4], or (ii) the increased propensity of frustrated antiferromagnets for quantum zero-temperature spin fluctuations compared to collinear antiferromagnets might be sufficient to destroy Néel order and drive these systems into novel non-classical quantum disordered ground states [2,3].Systems of classical Heisenberg spins residing on lattices of corner-sharing triangles or tetrahedra and antiferromagnetically coupled via nearest-neighbor exchange constitute particularly interesting cases of highly frustrated antiferromagnets (see Fig. 1). Here, theory [5,6] and numerical work [7], show that these systems do not order and remain in a "collective paramagnetic state" [5] down to zero temperature. Since, even for classical spins, these systems have such a small tendency to order, they are excellent candidates to display exotic quantum disordered ground states [2,3,8]. However, and perhaps most interestingly, experiments show that some nominally perfect (i.e. disorder-free) [9] pyrochlore antiferromagnets [10] exhibit a spin-freezing transition at some temperature T f , below which they develop magnetic irreversibilities (see Fig. 1) and long-time magnetic relaxation similar to what is found in conventional randomly frustrated spin glasses such as CuMn, EuSrS, and CdMnTe [11].

Two B-site ordered double perovskites, La 2 LiReO 6 and Ba 2 YReO 6 , with S = 1 were investigated as geometrically frustrated antiferromagnets, using x-ray and neutron diffraction, superconducting quantum interference device magnetometry, heat capacity, muon spin relaxation ͑SR͒, and 89 Y magic-angle spinning ͑MAS͒ NMR. La 2 LiReO 6 has a monoclinic structure ͑P2 1 / n͒ with cell parameters at room temperature; a = 5.58262͑22͒ Å, b = 5.67582͑20͒ Å, c = 7.88586͑27͒ Å, and ␤ = 90.240͑4͒°. A zero-field cooled/field cooled ͑ZFC/FC͒ divergence at 50 K was observed in the susceptibility. The ZFC susceptibility is zero below ϳ5 K for polycrystalline samples, suggesting a cooperative singlet ground state but weak moments are induced by cooling in very small fields ϳ1 mT. No evidence of long-range ordering is evident in heat capacity, neutrondiffraction, or SR data. The ZF spin dynamics from SR are anomalous and can be fitted to a stretched exponential rather than the Kubo-Toyabe form expected for random frozen spins but the muon spins are decoupled in longitudinal fields ͑LF͒, consistent with spin freezing of the fraction of spins relaxing within the muon time scale. The internal fields sensed by the muons are anomalously small, consistent with an electronic spin-singlet state. Ba 2 YReO 6 is found to be cubic ͑Fm3m͒ with cell parameter a = 8.36278͑2͒ Å at 300 K with no change in symmetry at 3.8 K, at variance with the Jahn-Teller theorem for a t 2g 2 configuration for Re 5+ . 89 Y MAS NMR shows a single peak indicating that Y/Re site disorder is at most 0.5%. The susceptibility shows two broad peaks around 50 and 25 K but no evidence for long-range order from heat capacity, neutron diffraction, or SR. The ZF SR result shows a two-component ground state with both slow and fast relaxations and decoupling results in a 1 kG LF, indicating spin freezing. These results are in sharp contrast to the long-range AF order found in the S =3/ 2 isostructural materials, La 2 LiRuO 6 and Ba 2 YRuO 6 , indicating that the reduction to S = 1 plays a major role in ground state determination.