Nature of magnetic structure in a geometrically frustrated manganite spinel

Unlike the pyrochlore geometrically frustrated magnets, several normal spinels such as ZnCr$_{2}$O$_{4}$ and ZnV$_{2}$O$_{4}$ undergo magnetic long-range order as well as cubic to tetragonal structural transitions at low temperatures. By comparison, ZnMn$_{2}$O$_{4}$, although tetragonal at room temperature, does not undergo a magnetic transition in spite of the rather large Curie-Weiss temperature of $\sim $ -800K indicative of strong antiferromagnetic fluctuations. The absence of an antiferromagnetic transition is surprising considering the nature of the Mn$^{3+}$ ion that is Jahn-Teller active and its effect on the ground state degeneracy. With the orbital degree of freedom active and a crystal anisotropy created by the Jahn-Teller octahedral distortions the system is tetragonally distorted just as in the vanadates where the V ion is orbitally active, but no magnetic transition is observed. Using neutron scattering we investigated how the magnetic structure changes as a function of temperature. We observed the presence of incommensurate magnetic peaks close to nuclear Bragg peaks that exhibit a first order phase transition. The FWHM of the magnetic peaks is much wider than that of nuclear peaks that indicates that the magnetic order is short range. Considering the 1-dimensional Boner-Fisher like behavior of the bulk susceptibility, it is proposed that the spin structure is helical and one-dimensional.