The temperature dependence of resistivity $\rho \mathrm{}\left(T\right)\mathrm{}$ and of the thermoelectric power $\alpha \mathrm{}\left(T\right)\mathrm{}$ of a single crystal of the ferromagnetic Ruddlesden-Popper compound ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7\mathrm{}(x=0.32\mathrm{})\mathrm{}$ have been measured under different hydrostatic pressures. Pressure induces segregation into antiferromagnetic electron-rich and ferromagnetic electron-poor ${\mathrm{Mn}}_2{\mathrm{O}}_7$ layers. With increasing pressure, the ferromagnetic majority phase exhibits contraction of the apical (Mn-O) bond and spin reorientation from the c axis to the basal plane at a critical pressure $P_c(6\mathrm{}<~P_c<~7\mathrm{kbar}).\mathrm{}$ The phase segregation appears to reflect a first-order inversion of the exchange coupling between ferromagnetic ${\mathrm{Mn}}_2{\mathrm{O}}_7$ layers.

• Received 16 November 1999

DOI:https://doi.org/10.1103/PhysRevB.61.R9217