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Kermarrec E.,McMaster University | Marjerrison C.A.,McMaster University | Thompson C.M.,McMaster University | Maharaj D.D.,McMaster University | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite Ba2YOsO6. The Fm3¯m space group is found both at 290 K and 3.5 K with cell constants a0=8.3541(4) Å and 8.3435(4) Å, respectively. Os5+ (5d3) ions occupy a nondistorted, geometrically frustrated face-centered-cubic (fcc) lattice. A Curie-Weiss temperature θ∼-700 K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below TN∼69 K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) μB on Os5+. The ordered moment is much reduced from either the expected spin-only value of ∼3 μB or the value appropriate to 4d3 Ru5+ in isostructural Ba2YRuO6 of 2.2(1) μB, suggesting a role for spin-orbit coupling (SOC). Triple-axis neutron scattering measurements of the order parameter suggest an additional first-order transition at T=67.45 K, and the existence of a second-ordered state. Time-of-flight inelastic neutron results reveal a large spin gap Δ∼17 meV, unexpected for an orbitally quenched, d3 electronic configuration. We discuss this in the context of the ∼5 meV spin gap observed in the related Ru5+,4d3 cubic double perovskite Ba2YRuO6, and attribute the ∼3 times larger gap to stronger SOC present in this heavier, 5d, osmate system. © 2015 American Physical Society. Source


Fritsch K.,McMaster University | Kermarrec E.,McMaster University | Ross K.A.,Johns Hopkins University | Ross K.A.,U.S. National Institute of Standards and Technology | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We present a parametric study of the diffuse magnetic scattering at (12,12,12) positions in reciprocal space, ascribed to a frozen antiferromagnetic spin ice state in single-crystalline Tb2Ti2O7. Our high-resolution neutron scattering measurements show that the elastic (-0.02 meV Source


Fritsch K.,McMaster University | Ross K.A.,McMaster University | Ross K.A.,Johns Hopkins University | Ross K.A.,U.S. National Institute of Standards and Technology | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We present high-resolution single-crystal time-of-flight neutron scattering measurements on the candidate quantum spin liquid pyrochlore Tb 2Ti2O7 at low temperature and in a magnetic field. At ∼70 mK and in zero field, Tb2Ti2O 7 reveals diffuse magnetic elastic scattering at (12,12,12) positions in reciprocal space, consistent with short-range correlated regions based on a two-in, two-out spin ice configuration on a doubled conventional unit cell. This elastic scattering is separated from very low-energy magnetic inelastic scattering by an energy gap of ∼0.06-0.08 meV. The elastic signal disappears under the application of small magnetic fields and upon elevating temperature. Pinch-point-like elastic diffuse scattering is observed near (1,1,1) and (0,0,2) in zero field at ∼70 mK, in agreement with Fennell, supporting the quantum spin ice interpretation of Tb2Ti2O7. ©2013 American Physical Society. Source


Fritsch K.,McMaster University | Yamani Z.,Chalk River Laboratories | Chang S.,U.S. National Institute of Standards and Technology | Qiu Y.,U.S. National Institute of Standards and Technology | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Co 3V 2O 8 is an orthorhombic magnet in which S=3/2 magnetic moments reside on two crystallographically inequivalent Co2 + sites, which decorate a stacked, buckled version of the two-dimensional kagome lattice, the stacked kagome staircase. The magnetic interactions between the Co2 + moments in this structure lead to a complex magnetic phase diagram at low temperature, wherein it exhibits a series of five transitions below 11K that ultimately culminate in a ferromagnetic ground state below T∼6.2 K. Here we report magnetization measurements on single- and polycrystalline samples of (Co 1-xMg x) 3V 2O 8 for x<0.23, as well as elastic and inelastic neutron scattering measurements on single crystals of magnetically dilute (Co 1-xMg x) 3V 2O 8 for x=0.029 and x=0.194, in which nonmagnetic Mg2 + ions substitute for magnetic Co2 +. We find that a dilution of 2.9% leads to a suppression of the ferromagnetic transition temperature by ∼15% while a dilution level of 19.4% is sufficient to destroy ferromagnetic long-range order in this material down to a temperature of at least 1.5 K. The magnetic excitation spectrum is characterized by two spin wave branches in the ordered phase for (Co 1-xMg x) 3V 2O 8 (x=0.029), similar to that of the pure x=0 material, and by broad diffuse scattering at temperatures below 10 K in (Co 1-xMg x) 3V 2O 8 (x=0.194). Such a strong dependence of the transition temperatures on long-range order in the presence of quenched nonmagnetic impurities is consistent with two-dimensional physics driving the transitions. We further provide a simple percolation model that semiquantitatively explains the inability of this system to establish long-range magnetic order at the unusually low dilution levels which we observe in our experiments. ©2012 American Physical Society. Source


Hallas A.M.,McMaster University | Gaudet J.,McMaster University | Wilson M.N.,McMaster University | Munsie T.J.,McMaster University | And 13 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

We report neutron scattering and muon spin relaxation measurements (μSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=72Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to mJ=±12 with local XY anisotropy, consistent with an Seff=12 description for the Yb moments. μSR measurements reveal the presence of an ordering transition at TN=0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k=(000)Γ5 ground state with an ordered magnetic moment of 0.3(1)μB per Yb ion. We compare and contrast this phenomenology with the low-temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical Seff=12 XY pyrochlore magnets. © 2016 American Physical Society. Source

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