Sign Reversal of the Mn-O Bond Compressibility in L${\mathrm{a}}_{1.2}$S${\mathrm{r}}_{1.8}$M${\mathrm{n}}_2$${\mathrm{O}}_7$ below $T_C$: Exchange Striction in the Ferromagnetic State

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 $\sim 6\mathrm{kbar}$, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of ${\mathrm{MnO}}_6$ octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the FM state the Mn-O-Mn linkage between ${\mathrm{MnO}}_2$ 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 ${\mathrm{dT}}_C/dP$, as well as a cant angle ${\theta }_0$ between the magnetizations of neighboring ${\mathrm{MnO}}_2$ sheets that decreases with pressure.