Charge ordering due to magnetic symmetry breaking.
It is argued that both transitions observed in 50% doped manganites, at the Néel temperature (T(N)) and the so-called charge ordering temperature (T(CO)), are magnetic. T(N) corresponds to the order-disorder transition, which takes place between ferromagnetic zigzag chains, while the coherent motion of spins within the chains is destroyed only around T(CO). The magnetic structure below T(CO) is highly anisotropic. It is dressed by the lattice distortion and leads to the huge anisotropy of the electronic structure, which explains stability of this state as well as the form of the charge-orbital pattern above T(N). The type of phase transition at T=T(N) is determined by lattice interactions.
Solovyev
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Tokura Spin Superstructure Project, ERATO Japan Science and Technology Corporation, National Institute of Advanced Industrial Science and Technology, Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan. igor@issp.u-tokyo.ac.jp