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Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr 2 O 4

DOI: 10.1103/PhysRevB.97.134430 DOI Help

Authors: S. Gao (Paul Scherrer Institute) , K. Guratinder (Paul Scherrer Institute) , U. Stuhr (Paul Scherrer Institute) , J. S. White (Paul Scherrer Institute) , Martin Mansson (Paul Scherrer Institute) , B. Roessli (Paul Scherrer Institute) , T. Fennell (Paul Scherrer Institute) , V. Tsurkan (University of Augsburg; Germany and Institute of Applied Physics, Academy of Sciences of Moldova) , A. Loidl (University of Augsburg) , M. Ciomaga Hatnean (University of Warwick) , G. Balakrishnan (University of Warwick) , S. Raymond (Univ. Grenoble Alpes) , Laurent Chapon (Institut Laue-Langevin) , V. O. Garlea (Oak Ridge National Laboratory) , A. T. Savici (Oak Ridge National Laboratory) , A. Cervellino (Swiss Light Source) , A. Bombardi (Diamond Light Source) , D. Chernyshov (ESRF) , Ch. Rüegg (University of Geneva) , J. T. Haraldsen (University of North Florida) , O. Zaharko (Paul Scherrer Institute)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 97

State: Published (Approved)
Published: April 2018

Abstract: 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 TN=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=( 1 2 1 2 0),k2=(10 1 2 ) appear. The ordered moment reaches 1.94(3) μB/Cr3+ for k1 and 2.08(3) μB/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. 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.

Journal Keywords: Antiferromagnetism; Frustrated magnetism; Spin waves; Antiferromagnets; Pyrochlore; Heisenberg model; Inelastic neutron scattering; X-ray diffraction

Subject Areas: Materials, Physics

Facility: Swiss Spallation Neutron Source