Brockhouse Institute for Materials Research

Hamilton, Canada

Brockhouse Institute for Materials Research

Hamilton, Canada

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Gaudet J.,McMaster University | Ross K.A.,Johns Hopkins University | Ross K.A.,U.S. National Institute of Standards and Technology | Ross K.A.,Colorado State University | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

The ground state of the quantum spin ice candidate magnet Yb2Ti2O7 is known to be sensitive to weak disorder at the ∼1% level which occurs in single crystals grown from the melt. Powders produced by solid state synthesis tend to be stoichiometric and display large and sharp heat capacity anomalies at relatively high temperatures, TC∼0.26 K. We have carried out neutron elastic and inelastic measurements on well characterized and equilibrated stoichiometric powder samples of Yb2Ti2O7 which show resolution-limited Bragg peaks to appear at low temperatures, but whose onset correlates with temperatures much higher than TC. The corresponding magnetic structure is best described as an icelike splayed ferromagnet. The spin dynamics in Yb2Ti2O7 are shown to be gapless on an energy scale <0.09 meV at all temperatures and organized into a continuum of scattering with vestiges of highly overdamped ferromagnetic spin waves present. These excitations differ greatly from conventional spin waves predicted for Yb2Ti2O7's mean field ordered state, but appear robust to weak disorder as they are largely consistent with those displayed by nonstoichiometric crushed single crystals and single crystals, as well as by powder samples of Yb2Ti2O7's sister quantum magnet Yb2Sn2O7. © 2016 American Physical Society.


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.


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.


Hallas A.M.,McMaster University | Gaudet J.,McMaster University | Butch N.P.,U.S. National Institute of Standards and Technology | Tachibana M.,Japan National Institute of Materials Science | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

The ytterbium pyrochlore magnets, Yb2B2O7 (B=Sn,Ti,Ge) are well described by Seff=1/2 quantum spins decorating a network of corner-sharing tetrahedra and interacting via anisotropic exchange. Structurally, only the nonmagnetic B-site cation, and hence primarily the lattice parameter, changes across the series. Nonetheless, a range of magnetic behaviors is observed: the low-temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has a ferromagnetic character, while Yb2Ge2O7 displays an antiferromagnetically ordered Néel state at low temperatures. While the static properties of the ytterbium pyrochlores are distinct, inelastic neutron scattering measurements reveal a common character to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless continuum of spin excitations, resembling overdamped ferromagnetic spin waves at low Q. Furthermore, the specific heat of the series also follows a common form, with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly at TC or TN. The novel spin dynamics we report correlate strongly with the broad specific heat anomaly only, remaining unchanged across the sharp anomaly. This result suggests that the primary order parameter in the ytterbium pyrochlores associated with the sharp anomaly is "hidden" and not simple magnetic dipole order. © 2016 American Physical Society.


Clark L.,McMaster University | Nilsen G.J.,Laue Langevin Institute | Kermarrec E.,McMaster University | Ehlers G.,Oak Ridge National Laboratory | And 7 more authors.
Physical Review Letters | Year: 2014

We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: S=1 oxide Lu2Mo2O7 and S=12 oxynitride Lu2Mo2O5N2. Lu2Mo2O7 undergoes a transition to an unconventional spin glass ground state at Tf∼16K. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of Δ∼0.05meV or Δ/|θ|∼0.004, in units of its antiferromagnetic Weiss constant θ∼-121K. © 2014 American Physical Society.


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.


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


Fritsch K.,McMaster University | Fritsch K.,Helmholtz Center Berlin | Ehlers G.,Oak Ridge National Laboratory | Rule K.C.,Helmholtz Center Berlin | And 8 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

The application of a magnetic field transverse to the easy axis, Ising direction in the quasi-two-dimensional kagome staircase magnet, Co3V2O8, induces three quantum phase transitions at low temperatures, ultimately producing a novel high field polarized state, with two distinct sublattices. New time-of-flight neutron scattering techniques, accompanied by large angular access, high magnetic field infrastructure allow the mapping of a sequence of ferromagnetic and incommensurate phases and their accompanying spin excitations. At least one of the transitions to incommensurate phases at μ0Hc1∼6.25 T and μ0Hc2∼7 T is discontinuous, while the final quantum critical point at μ0Hc3∼13 T is continuous. © 2015 American Physical Society.


Gaudet J.,McMaster University | Maharaj D.D.,McMaster University | Sala G.,McMaster University | Kermarrec E.,McMaster University | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

Time-of-flight neutron spectroscopy has been used to determine the crystalline electric field (CEF) Hamiltonian, eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in the candidate quantum spin ice pyrochlore magnet Yb2Ti2O7. The precise ground state (GS) of this exotic, geometrically frustrated magnet is known to be sensitive to weak disorder associated with the growth of single crystals from the melt. Such materials display weak "stuffing," wherein a small proportion, ≈2%, of the nonmagnetic Ti4+ sites are occupied by excess Yb3+. We have carried out neutron spectroscopic measurements on a stoichiometric powder sample of Yb2Ti2O7, as well as a crushed single crystal with weak stuffing and an approximate composition of Yb2+xTi2-xO7+y with x=0.046. All samples display three CEF transitions out of the GS, and the GS doublet itself is identified as primarily composed of mJ=±1/2, as expected. However, stuffing at low temperatures in Yb2+xTi2-xO7+y induces a similar finite CEF lifetime as is induced in stoichiometric Yb2Ti2O7 by elevated temperature. We conclude that an extended strain field exists about each local "stuffed" site, which produces a distribution of random CEF environments in the lightly stuffed Yb2+xTi2-xO7+y, in addition to producing a small fraction of Yb ions in defective environments with grossly different CEF eigenvalues and eigenvectors. © 2015 American Physical Society.


Gaudet J.,McMaster University | Hallas A.M.,McMaster University | Maharaj D.D.,McMaster University | Buhariwalla C.R.C.,McMaster University | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

Below TN=1.1 K, the XY pyrochlore Er2Ti2O7 orders into a k=0 noncollinear, antiferromagnetic structure referred to as the ψ2 state. The magnetic order in Er2Ti2O7 is known to obey conventional three-dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the ψ2 structure should be unstable to the formation of a related ψ3 magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er2-xYxTi2O7 with x=0 (pure), 0.2 (10%Y) and 0.4 (20% Y), where magnetic Er3+ is substituted by nonmagnetic Y3+. We find that the ψ2 ground state of pure Er2Ti2O7 is significantly affected by magnetic dilution. The characteristic domain selection associated with the ψ2 state, and the corresponding energy gap separating ψ2 from ψ3, vanish for Y3+ substitutions between 10% Y and 20% Y, far removed from the three-dimensional percolation threshold of ∼60% Y. The resulting ground state for Er2Ti2O7 with magnetic dilutions from 20% Y up to the percolation threshold is naturally interpreted as a frozen mosaic of ψ2 and ψ3 domains. © 2016 American Physical Society.

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