  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.
