Charge ordering and phase competition in the layered perovskite LaSr2Mn2O7

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy at temperatures as high as 340 K, and with decreasing temperature they become static, forming a charge-ordered (CO) phase below TCO=210 K. In the static regime, superlattice reflections are observed by neutron and x-ray diffraction with a propagation vector (1/4, -1/4, 0). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than that in the three-dimensional perovskite manganites. Below TN=170 K, type-A antiferromagnetism (AF) develops and competes with the charge ordering, causing it to eventually melt below T*=100 K. High-resolution diffraction measurements suggest that the CO and AF states do not coincide within the same region of material, but rather coexist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases.NRC publication: N