Sign reversal of the Mn-O bond compressibility in La1.2Sr1.8Mn2O7 below T-C: Exchange striction in the ferromagnetic state

The crystal structure of the layered perovskite La1.2Sr1.8Mn2O7 has been studied under hydrostatic pressure up to similar to 6 kbar, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of MnO6 octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the FM state the Mn-O-Mn linkage between MnO2 planes expands under applied pressure, whereas they contract in the PI state. This counterintuitive behavior is interpreted in terms of exchange striction, which reflects the competition between super and double exchange. An increase of the Mn moment with applied pressure in the FM state is consistent with a positive dT(C)/dP, as well as a cant angle theta(0) between the magnetizations of neighboring MnO2 sheets that decreases with pressure.