Sign Reversal of the Mn-O Bond Compressibility in La1.2Sr1.8Mn2O7belowTC: Exchange Striction in the Ferromagnetic State

摘要: The crystal structure of the layered perovskite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ has been studied under hydrostatic pressure up to $\ensuremath{\sim}6\mathrm{kbar}$, in paramagnetic and ferromagnetic states, with neutron powder diffraction. compressibility Mn-O apical bonds double layer ${\mathrm{MnO}}_{6}$ octahedra changes sign from insulator (PI) metal (FM) state; FM state Mn-O-Mn linkage between ${\mathrm{MnO}}_{2}$ planes expands applied pressure, whereas they contract PI state. This counterintuitive behavior is interpreted terms exchange striction, which reflects competition super exchange. An increase Mn moment consistent a positive ${\mathrm{dT}}_{C}/dP$, as well cant angle ${\ensuremath{\theta}}_{0}$ magnetizations neighboring sheets that decreases pressure.