Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet ${\mathrm{MgCr}}_2{\mathrm{O}}_4$

Journal Article · Fri Apr 27 00:00:00 EDT 2018 · Physical Review B

DOI:https://doi.org/10.1103/PhysRevB.97.134430· OSTI ID:1468011

Gao, S. ^[1]; Guratinder, Kaur ^[1]; Stuhr, Uwe ^[1]; White, Jonathon S. ^[1]; Mansson, M. ^[1]; Roessli, Bertrand ^[1]; Fennell, Tom ^[1]; Tsurkan, V. ^[2]; Loidl, Alois ^[3]; Hatnean, Monica Ciomaga ^[4]; Balakrishnan, Geetha ^[4]; Raymond, S ^[5]; Chapon, L.C. ^[6]; Garlea, Vasile O.  ^[7]; Savici, Andrei T.  ^[7]; Cervellino, A. ^[8]; Bombardi, A. ^[9]; Chernyshov, D. ^[10]; Ruegg, Ch. ^[11]; Haraldsen, J. T. ^[12] ; Zaharko, Oksana ^[13]

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1. Paul Scherrer Inst., Villigen (Switzerland). Lab. for Neutron Scattering and Imaging
2. Univ. of Augsburg, Augsburg (Germany). Experimental Physics V, Center for Electronic Correlations and Magnetism; Academy of Sciences of Moldova, Chisinau (Republic of Moldova). Inst. of Applied Physics
3. Univ. of Augsburg, Augsburg (Germany). Experimental Physics V, Center for Electronic Correlations and Magnetism
4. Univ. of Warwick, Coventry (United Kingdom). Dept. of Physics
5. Univ. Grenoble Alpes, Grenoble (France)
6. Inst. Laue-Langevin (ILL), Grenoble (France)
7. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Neutron Scattering Division
8. Paul Scherrer Inst. (PSI), Villigen (Switzerland). Swiss Light Source
9. Science and Technology Facilities Council (STFC), Harwell Campus, Oxford (United Kingdom). Diamond Light Source, Ltd.
10. European Synchrotron Radiation Facility (ESRF), Grenoble (France). Swiss-Norwegian Beamlines
11. Paul Scherrer Inst. (PSI), Villigen (Switzerland). Lab. for Neutron Scattering and Imaging; Univ. of Geneva (Switzerland). Dept. of Quantum Matter Physics
12. Univ. of North Florida, Jacksonville, FL (United States). Dept. of Physics
13. Paul Scherrer Inst. (PSI), Villigen (Switzerland). Lab. for Neutron Scattering and Imaging

In spinels ACr₂O₄(A=Mg,Zn), realization of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at T_N=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et al. [Pow. Diff. 17, 230 (2002)], while other features depend on sample or cooling protocol. A complex, partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k₁=($$\frac{1}{2}$$ $$\frac{1}{2}$$ 0),k₂=(1 0 $$\frac{1}{2}$$) appear. The ordered moment reaches 1.94(3) μ_B/Cr³⁺ for k₁ and 2.08(3) μ_B/Cr³⁺ for k₂, if equal amount of the k₁ and k₂ phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c component. We use inelastic neutron scattering to investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. Finally, the distinctive fingerprint of clusterlike excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1468011
• Alternate ID(s): OSTI ID: 1434980
• Journal Information: Physical Review B, Vol. 97, Issue 13; ISSN 2469-9950
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

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