H. A. Dabkowska

Two-dimensional kagome correlations and field induced order in the ferromagnetic XY pyrochlore Yb2Ti2O7.

Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two-dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields. Rods of scattering along 111 directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of approximately 0.5 T applied along the [110] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet.

Revisiting static and dynamic spin-ice correlations in Ho₂Ti₂O₇ with neutron scattering

J.P. Clancy, J.P.C. Ruff, S.R. Dunsiger, Y. Zhao, H.A. Dabkowska, J.S. Gardner, 3 Y. Qiu, 4 J.R.D. Copley, T. Jenkins, and B.D. Gaulin 5 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1 NIST Center for Neutron Research, NIST, Gaithersburg, Maryland 20899-8102, USA Indiana University, 2401 Milo B. Sampson Lane, Bloomington, Indiana 47408, USA Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA Canadian Institute for Advanced Research, 180 Dundas Street W., Toronto, Ontario, Canada M5G 1Z8

High-resolution study of spin excitations in the singlet ground state of SrCu2(BO3)2.

High-resolution, inelastic neutron scattering measurements on SrCu2(BO3)2, a realization of the Shastry-Sutherland model for two-dimensional Heisenberg antiferromagnets, reveal the dispersion of the three single triplet excitations continuously across the (H,0) direction within its tetragonal basal plane. These measurements also show distinct Q dependencies for the single and multiple triplet excitations, and that these excitations are largely dispersionless perpendicular to this plane. The temperature dependence of the intensities of these excitations is well described as the complement of the dc susceptibility of SrCu2(BO3)2.

Magnetic Excitations from the Exotic Ground State of the Quantum FCC Antiferromagnet Ba2YMoO6

J. P. Carlo, 2 J. P. Clancy, T. Aharen, Z. Yamani, J. P. C. Ruff, J. Wagman, G. J. Van Gastel, H. M. L. Noad, G. E. Granroth, J. E. Greedan, 5 H. A. Dabkowska, and B. D. Gaulin 5, 6 Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1 Canada Canadian Neutron Beam Centre, National Research Council, Chalk River, ON K0J 1J0 Canada Department of Chemistry, McMaster University, Hamilton, ON L8S 4M1 Canada Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA Brockhouse Institute for Materials Research, McMaster University, Hamilton, ON L8S 4M1 Canada Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8 Canada (Dated: January 21, 2013)

Magnetic flux jumps in textured Bi 2 Sr 2 CaCu 2 O 8 + δ

Magnetic flux jumps in textured Bi 2 Sr 2 CaCu 2 O 8 + δ have been studied by means of magnetization measurements in the temperature range between 1.95 K and T c , in an external magnetic field up to 9 T. Flux jumps were found in the temperature range 1.95-6 K. with the external magnetic field parallel to the c axis of the investigated sample. The effect of sample history on magnetic flux jumping was studied and it was found to be well accounted for by the available theoretical models. The magnetic-field sweep rate strongly influences the flux jumping and this effect was interpreted in terms of the influence of both flux creep and the thermal environment of the sample. Strong flux creep was found in the temperature and magnetic-field range where flux jumps occur suggesting a relationship between the two. The heat exchange conditions between the sample and the experimental environment also influence the flux jumping behavior. Both these effects stabilize the sample against flux instabilities, and this stabilizing effect increases with decreasing magnetic-field sweep rate. Demagnetizing effects are also shown to have a significant influence on flux jumping.

Field-Induced Order and Spin Waves in the Pyrochlore Antiferromagnet Tb~2Ti~2O~7

High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.

Dynamics of the magnetic susceptibility deep in the Coulomb phase of the dipolar spin ice material Ho2Ti2O7

Low temperature measurements of the ac magnetic susceptibility along the [110] direction of a single crystal of the dipolar spin ice material Ho2Ti2O7, in zero static field, are presented. While behavior that is qualitatively consistent with previous work on Ho2Ti2O7 and the related material Dy2Ti2O7 is observed, this work extends measurements to appreciably lower temperatures and frequencies. In the freezing regime, below 1 K, the dynamics are found to be temperature activated, thus well described by an Arrhenius law with an activation energy close to 6J_eff, a result that is not easily explained with the current model of magnetic monopole excitations in dipolar spin ice. The form and temperature dependence of the ac susceptibility spectra are found to be nontrivial and distinct from standard glassy relaxation. Particular attention has been paid to correcting for the demagnetization effect, which is quite significant in these materials and has important, even qualitative, effects on the susceptibility spectra.

Continuous and discontinuous quantum phase transitions in a model two-dimensional magnet.

The Shasty–Sutherland model, which consists of a set of spin 1/2 dimers on a 2D square lattice, is simple and soluble but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry–Sutherland material, SrCu2(BO3)2, in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution X-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure-dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.

Antiferromagnetic Spin Ice Correlations at (1/2,1/2,1/2) in the Ground State of the Pyrochlore Magnet Tb

We present high-resolution single crystal time-of-flight neutron scattering measurements on the candidate quantum spin liquid pyrochlore Tb2Ti2O7 at low temperature and in a magnetic field. At ~70 mK and in zero field, Tb2Ti2O7 reveals diffuse magnetic elastic scattering at (1/2,1/2,1/2) 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 et al. (Ref. 1), supporting the quantum spin ice interpretation of Tb2Ti2O7.

_2

Neutron scattering measurements on powder