In spinels $A{\mathrm{Cr}}_2{\mathrm{O}}_4(A=\text{Mg},\text{Zn})$, 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 ${\mathbf{k}}_{\mathbf{1}}=\left(\frac{1}{2}\frac{1}{2}0\right),{\mathbf{k}}_{\mathbf{2}}=\left(10\frac{1}{2}\right)$ appear. The ordered moment reaches 1.94(3) ${\mu }_B/{\mathrm{Cr}}^{3+}$ for ${\mathbf{k}}_{\mathbf{1}}$ and 2.08(3) ${\mu }_B/{\mathrm{Cr}}^{3+}$ for ${\mathbf{k}}_{\mathbf{2}}$, if equal amount of the ${\mathbf{k}}_{\mathbf{1}}$ and ${\mathbf{k}}_{\mathbf{2}}$ 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.

• Received 1 December 2017
• Revised 5 March 2018

DOI:https://doi.org/10.1103/PhysRevB.97.134430