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

Abstract

In spinels ACr 2 O 4 (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 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. Complex partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k 1 =( 1 2 1 2 0), k 2 =(1 0 1 2 ) appear. The ordered moment reaches 1.94(3) µ B /Cr 3+ for k 1 and 2.08(3)µ B /Cr 3+ for k 2 , if equal amount of the k 1 and k 2 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.3

Thermodynamics and spin mapping of quantum excitations in a Heisenberg spin heptamer

“…While the comparison is similar to that of the classical Monte Carlo simulations used in Ref. [41] and [42], this model explains all transitions, including the proper energy spacing between excitations as well as the inelastic neutron scattering intensity, in a complete and self-contained model of the spin 3/2 heptamer.…”

“…These structures can be compared directly to the excitations observed in Ref. [41] and [42]. pentamer).…”

“…Furthermore, we model the spin excitations of MgCr 2 O 4 and show that the excitations observed can be described by quantum spin heptamer. This analysis was first introduced in combination with the experimental measurements and analysis presented by Gao et al 42 . However, here, we show the full calculations and present the complete, closed-form representations for the energy eigenstates, thermodynamic properties, and the inelastic neutron scattering structure factors.…”

Thermodynamics of the pyrochlore-lattice quantum Heisenberg antiferromagnet

“…show a magneto-structural transition at low temperatures) [38][39][40], or FeF 3 (for which besides the nearestneighbor antiferromagnetic Heisenberg interaction, also biquadratic and Dzyaloshinskii-Moriya interactions are present) [41].…”

Magnetic clustering, half-moons, and shadow pinch points as signals of a proximate Coulomb phase in frustrated Heisenberg magnets

“…With the continuous nature of magnetic vector, one can expect a variety of stable textures beyond hexamer clustering. From this viewpoint, it is interesting to look at a class of spinel oxides with 3d magnetic ions, AB 2 O 4 (A = Zn, Hg, Mg, B = Cr, Fe) [21][22][23][24][25][26][27][28][29] . These compounds have weak magnetic anisotropy with small spinorbit interaction of 3d ions, and the classical Heisenberg model with farther-neighbor interactions is expected to give a good starting point of analysis 29 .…”