Neutrons Sciences Directorate at ORNL

Capabilities of the FIE-TAX Instrument

The HB-1A triple axis spectrometer is an excellent instrument for measuring low-lying magnetic excitations in solids, and for measuring structural and magnetic order parameters in bulk materials as well as in nanostructured materials such as thin films and nanoparticles.  In the case of thin films the use of energy analysis is key capability which enables the desired signal to be separated from the massive background due to the substrate. 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,  multiferroics and giant magnetocaloric materials.

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

  • A neutron diffraction experiment on a delta-doped superlattice (SL) analog to La1-xSrxMnO3 (x=0.44, 0.47) performed at HB-1A clearly showed the development of the (0 0 0.5) antiferromagnetic (AF) peak indicative of an A-type AF structure. The delta doped SL was made by alternating unit cell layers of LaMnO3 (LMO1) and SrMnO3 (SMO1) on SrTiO3 substrates and inserting an additional LMO1 layer for every four LMO1/SMO1 bilayers. This sequence was repeated 9 times (9 supercells) to form a [(SMO1/ LMO1) ´ 4 ; LMO1] ´ 9 SL. From the full width at half maximum of the peak, a magnetic coherence length of 28:5 nm was determined, nearly the entire film thickness (30.9 nm). The temperature dependence of this peak indicates the disappearance of the AF order in the SL at 260K. This experiment illustrates the usefulness of HB-1A to study magnetic films on much thicker substrates (see T. S. Santos, B. J. Kirby, S. Kumar, S. J. May, J. A. Borchers, B. B. Maranville, J. Zarestky, S. G. E. te Velthuis, J. van den Brink, and A. Bhattacharya, “Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices”, Phys. Rev. Lett 107, 167202 (2011)).
  • A neutron scattering experiment at HB-1A on multiferroic BiFeO3 films (200 and 800 nm) epitaxially grown on SrTiO3 substrates revealed that these films exhibit a G-type antiferromagnetic structure modulated with cycloidal spiral magnetic ordering.  These measurements took 4 days of HB-1A beam time (see X. Ke, P. P. Zhang, S. H. Baek, J. Zarestky, W. Tian, and C. B. Eom, “Magnetic structure of epitaxial multiferroic BiFeO3 films with engineered ferroelectric domains,” Phys. Rev. B 82, 134448 (2010)).
  • A neutron diffraction experiment on Ba(Fe1-xCox)2As2 revealed that commensurate antiferromagnetic order gives way to incommensurate (IC) magnetic order in a narrow region of Co doping 0.056<x<0.06. The IC propagation vector t = QAFM + (0, e, 0) corresponds to a transverse splitting (e » 0.02-0.03) whose value is composition dependent. These findings are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors. This systematic study took 10 days of HB-1A beam time  (see D. . K. Pratt, M. G. Kim, A. Kreyssig, Y. B. Lee, G. S. Tucker, A. Thaler, W. Tian, J. L. Zarestky, S. L. Bud’ko, P. C. Canfield, B. N. Harmon, A. I. Goldman, and R. J. McQueeney, “Incommensurate Spin-Density Wave Order in Electron-Doped BaFe2As2 Superconductors”, Phys. Rev. Lett 106, 257001 (2011).
  • Neutron scattering experiments at HB-1A probed the influence of uniaxial strain on the magnetic and structural order parameters in the iron pnictide BaFe2As2.  The magnetic order parameter was measured for various applied strain in a small (3mm rod) single crystal of this material This experiment showed that modest strain fields along the in-plane orthorhombic b-axis remove structural twinning effects and induce significant changes in phase behavior. A 20g single crystal of  BaFe2As2 was used for this experiment, the measurements took 4 days of HB-1A beam time (see  Chetan Dhital, Z. Yamani, Wei Tian, J. Zaretsky, A. S. Sefat, Ziqiang Wang, R. J. Birgeneau, and Stephen D. Wilson, “Effect of uniaxial strain on the structural and magnetic phase transitions in BaFe2As2”, Phys. Rev. Lett. 108, 087001 (2012).
  • A neutron scattering experiment on a single crystal of the honey comb lattice Na2IrO3  (disk shape of 10 mm diameter and 0.1 mm thickness)  revealed that this system orders magnetically below 18.1(2) K with the Ir4+ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of 0.22(1)μB/Ir.  This configuration is at odds with the Neel or stripe states proposed in the Kitaev-Heisenberg model (see Feng Ye, Songxue Chi, Huibo Cao, Bryan Chakoumakos, Jaime A. Fernandez-Baca, Radu Custelcean, Tongfei Qi, O. B. Korneta, and G. Cao, “Direct evidence of a zigzag spin chain structure in the honeycomb lattice Na2IrO3”, Phys Rev. B 85, 180403 (R) (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. 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 Three single crystals (14 mg (x=0.9), 4 mg (x=1.2), and 33.7 mg (x=0.54)) were used for this experiment, the measurements took 6 days at HB1A beam time)  (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 diffraction” Phys. Rev. B 83, 060509(R) (2011)).