Capabilities of the PTAX Instrument

The HB-1 triple axis spectrometer is an excellent instrument for measuring magnetic excitations in solids, and for measuring structural and magnetic order parameters in bulk materials. This instrument is most beneficial to the condensed matter and materials science communities. Due to its versatility and easy access this instrument can be used for parametric studies using a variety of ancillary sample environments to provide a complete control of thermodynamic variables such as temperature, magnetic field, and pressure. During the last few years most of the demand for this instrument has been focused in studies of unconventional superconductors, quantum magnets, thermoelectrics, and multiferroics.

Examples of typical experiments carried out at HB-1 over the last few years are listed below:

  • The full spin-wave dispersion (up to 70 meV) of multiferroic BiFeO3 was measured at HB-1. A ). 8cc single crystal was used and the measurements took 6 days of beam time. (see M. Matsuda, R. S. Fishman, T. Hong, C. H. Lee, T. Ushiyama, Y. Yanagisawa, Y. Tomioka,T. Ito, "Magnetic dispersion and anisotropy in multiferroic BiFeO3", Physical Review Letters, 109, 6, 067205, (2012).)
  • A polarized neutron diffraction experiment at HB-1 showed that the appearance of commensurate magnetic order in a 4.6g of polycrystalline specimen of NaOsO3 coincides with the onset of the metal to insulator transition in this 5d compound. (see S. Calder, V. O. Garlea, D. F. McMorrow, M. D. Lumsden, M. B. Stone, J. C. Lang, J.-W. Kim, J. A. Schlueter, Y. G. Shi, K. Yamaura, Y. S. Sun, Y. Tsujimoto, A. D. Christianson, “Magnetically driven metal-insulator transition in NaOsO3”, Phys. Rev. Lett. 108, 257209 (2012)).
  • An inelastic neutron scattering experiment at HB-1 on insulating Rb0.89Fe1.58Se2 showed the clear existence of a spin gap below 8 meV. (see Miaoyin Wang , Chen Fang, Dao-Xin Yao, GuoTai Tan, Leland W. Harriger, Yu Song, Tucker Netherton, Chenglin Zhang, Meng Wang, Matthew B. Stone, Wei Tian, Jiangping Hu, and Pengcheng Dai “Spin waves and magnetic exchange interactions in insulating Rb0.89Fe1.58Se2”, Nat. Commun. 2, 580, DOI: 10.1038/ncomms1573).
  • A powder diffraction experiment on the S=3/2 triangular lattice antiferromagnet Ag2CrO2 revealed the appearance of a long-range partially disordered 5-sublattice phase which persists at low temperatures. The appearance of this phase is accompanied by a structural distortion (observed at the HB-2A high-resolution powder diffractometer) indicating that the Neel order is stabilized by spin-lattice coupling. This experiment was performed on 2.5g of Ag2CrO2 powder and the measurements took 2 days of HB-1 beam time. (see M. Matsuda, C. de la Cruz, H. Yoshida, M. Isobe, and R. S. Fishman, “Partially Disordered State and Spin-Lattice Coupling in an S=3/2 Triangular Lattice Antiferromagnet Ag2CrO2”. Phys. Rev. B 85, 144407 (2012)).
  • The superconducting system La2−xBaxCuO4 is known to show a minimum in the transition temperature Tc at x = 1/8 where maximal stripe order is pinned by the anisotropy within the CuO2 planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For x = 0.095, where Tc reaches its maximum value of 32 K, a structural transition is roughly coincident to a phase that is close to LTT. An inelastic neutron scattering experiment at HB-1 on a 11g single crystal of La2−xBaxCuO4 (x=0.095) showed that an observed kink in the critical scattering associated with the structural transitions correlates with an anomaly in the thermal conductivity k, where k is enhanced upon cooling. This finding supports the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO2 layers, which interrupts the growth of the superconducting order. (see Jinsheng Wen, Zhijun Xu, Guangyong Xu,Qing Jie, M. H¨ucker, A. Zheludev, Wei Tian, B. L. Winn, J. L. Zarestky, D. K. Singh, Tao Hong, Qiang Li, Genda Gu, and J. M. Tranquada, “Probing the connections between superconductivity, stripe order, and structure in La1.905Ba0.095Cu1−yZnyO4”, Phys. Rev. B 85, 134512 (2012)).
  • Doping most transition metals on the Fe site of BaFe2As2 yields superconductivity. One rare exception to this occurs in the case of Cr-doping where the spin density wave (SDW) transition is suppressed (as with other transition metal dopants) but no superconductivity is observed. Single crystal neutron diffraction measurements were performed to explore the magnetic and structural transitions of this system for Cr concentrations ranging from x=0 to x=0.47. Crystals ranging from 4 to 150mg were utilized for these measurements. The resulting phase diagram indicates long-range magnetic order for all samples. The addition of Cr stabilizes magnetism and favors a new magnetic ground state (consistent with G-type AFM order) at higher concentrations. These results showed that the presence of strong magnetism throughout the phase diagram provides a natural explanation for the absence of superconductivity (see K. Marty, A. D. Christianson, C. H. Wang, M. Matsuda, H. Cao, L. H. VanBebber, J. L. Zarestky, D. J. Singh, A. S. Sefat, and M. D. Lumsden, “Competing magnetic ground states in nonsuperconducting Ba(Fe1−xCrx)2As2 as seen via neutron diffractionPhys. Rev.B 83, 060509(R) (2011)).
  • An inelastic neutron scattering experiment at HB-1 determined the spin-wave spectrum in the antiferromagnetic bilayer ruthenate Ca3Ru2O7. This material behaves as a quasi-two-dimensional ferromagnetic bilayer system with weak interbilayer antiferromagnetic coupling, and its magnetic dispersion can be well described with a nearest-neighbor Heisenberg model. This study found that the intrabilayer exchange interaction along the out-of-plane c-axis is much stronger than the in-plane intralayer interaction, in sharp contrast to the previously reported bilayer manganite system. This study also observed finite lifetime effects of the magnetic excitations which are presumably attributable to the existence of a small number of itinerant charge carriers within the planes. Three co aligned crystals with a total mass of 3 g were used for these measurements (see X. Ke, Tao Hong, J. Peng, S. E. Nagler, G. E. Granroth, M. D. Lumsden, and Z. Q. Mao, “Spin-wave excitation in the antiferromagnetic bilayer ruthenate Ca3Ru2O7”, Phys. Rev. B 84, 014422 (2011)) (3 single crystals coaligned: total mass 3g).