Origin and pressure dependence of ferromagnetism in A2Mn2O7 pyrochlores (A=Y, In, Lu, and Tl)

Abstract

Nonconventional mechanisms have been recently invoked in order to explain the ferromagnetic ground state of $A_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ pyrochlores $(A=\mathrm{Y},$ In, Lu, and Tl) and the puzzling decrease of their Curie temperatures with applied pressure. Here we show, using a perturbation expansion in the Mn-O hopping term, that both features can be understood within the superexchange model, provided that the intra-atomic oxygen interactions are properly taken into account. An additional coupling between the Mn ions mediated by the $\mathrm{In}\mathrm{}\left(5s\right)/\mathrm{Tl}\mathrm{}\left(6s\right)\mathrm{}$ bands yields the higher $T_C$’s of these two compounds, this mechanism enhancing their ferromagnetism for higher pressures.