Novel spin structures in Mn and Co chromite

The ferrimagnetic or conical spiral (FS), a variational approximation to the ground state of the classical Heisenberg model with competing AB and BB interactions in cubic spinels$^1$, is in qualitative to quantitative agreement with neutron diffraction (ND) results for MnCr$_2$O$_4$ and CoCr$_2 $O$_4$.$^2 $ This despite its local instability for the experimental parameter values.$^1$ It also was used to interpret related NMR studies.$^3$ Understanding recent ND experiments$^4$ on both materials again was generally based on the FS. However, these measurements, done on single crystals, and with highly improved resolution, uncovered a subtle but important modification of the FS: while the fundamental Bragg peaks (originating from the spin components along the cone axes) are typical, the satellite peaks (coming from the transverse or spiral components) are broadened.$^4$ Efforts to understand this based on the classical spin model will be discussed. \newline 1. D. H. Lyons et al., Phys. Rev. \textbf{126}, 540 (1962). \newline 2. J. Hastings et al., Phys. Rev. \textbf{126}, 556 (1962). N. Menyuk et al., J. de Physique \textbf{25}, 528 (1964). \newline 3. A. J. Heeger et al., J. Phys. Chem Solids \textbf{29}, 1085 (1968); T. Tsuda et al., Solid State Comm. \textbf{9}, 2207 (1971). \newline 4. K. Tomiyasu et al., Phys. Rev. B \textbf{70}, 214434 (2004).