Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2O4

Abstract

In spinels ACr2O4 (A=Mg, Zn) realisation 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. 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=(1 0 1/2) appear. The ordered moment reaches 1.94(3) muB/Cr3+ for k1 and 2.08(3) muB/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. By inelastic neutron scattering we 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 cluster-like excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

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• Magnetic excitations of the classical spin liquid MgCr2O4
• Magnetic clustering, half-moons, and shadow pinch points as signals of a proximate Coulomb phase in frustrated Heisenberg magnets
• Adapting Planck's route to investigate the thermodynamics of the spin-half pyrochlore Heisenberg antiferromagnet
• Thermodynamics of the pyrochlore-lattice quantum Heisenberg antiferromagnet
• Neutron Scattering Studies of the Breathing Pyrochlore Antiferromagnet LiGaCr${}_4$O${}_8$
• Hierarchical excitations from correlated spin tetrahedra on the breathing pyrochlore lattice
• Line-Graph Approach to Spiral Spin Liquids
• Thermodynamics and spin mapping of quantum excitations in a Heisenberg spin heptamer
• On the complexity of spinels: Magnetic, electronic, and polar ground states
• Pulling order back from the brink of disorder: Observation of a nodal line spin liquid and fluctuation stabilized order in K${}_2$IrCl${}_6$
• Three-dimensional checkerboard spin structure on a breathing pyrochlore lattice
• Dynamic Spin-Lattice Coupling and Statistical Interpretation for the Molecular-Like Excitations in Frustrated Pyrochlores
• Effects of exchange distortions in the magnetic Kagome lattice
• Thermally-induced mimicry of quantum cluster excitations and implications for the magnetic transition in FePSe${}_3$