Work function changes in the double layered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$

We have investigated the behavior of the work function of ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_2{\mathrm{O}}_7$ as a function of temperature by means of photoemission. We found a decrease of $55\pm 10\mathrm{meV}$ in going from 60 K to just above the Curie temperature (125 K) of the sample. Above $T_C$ the work function appears to be roughly constant. Our results are exactly opposite to the work function changes calculated from the double-exchange model [N. Furukawa, J. Phys. Soc. Jpn. 66, 2528 (1997)] but are consistent with other data. The disagreement with double exchange can be explained using a general thermodynamic relation valid for second order transitions [D. van der Marel and G. Rietveld, Phys. Rev. Lett. 69, 2575 (1992); D. van der Marel, D. I. Khomskii, and G. M. Eliashberg, Phys. Rev. B 50, 16 594 (1994)] and including the extra processes involved in the manganites besides double-exchange interaction.