Pressure Dependence of Ferromagnetic Curie Temperature and Colossal Magnetoresistance Effect in pyrochlore manganites.

Negative pressure dependence of ferromagnetic Curie temperature Tc is established by the susceptibility measurements on a series of isostructural pyrochlores A2Mn2O7, (A=Tl, In, Y, Lu). At temperatures well below and above Tc the pronounced magnetothermohistory effects are revealed, indicating the presence of competing antiferromagnetic interaction. The results contradict not only the double exchange model, which dictates positive pressure shift of Tc, but also the simple superexchange scenario where the ferromagnetic character of the nearest-neighbor interaction allows no deviations from the standard ferromagnetic behaviour. We propose a model where a long-range ferromagnetic ordering originates from next-near-neighbor ferromagnetic interactions which compete with the antiferromagnetic nearest-neighbor superexchange, greatly reduced by the frustrated topology of pyrochlores. We also report a dramatic increase under pressure of the colossal magnetoresistance effect.