Ferromagnetism and metallicity in Sm_0.2Ca_0.8Mn_1-xRu_xO₃ (x=0-0.08): Interplay between Ru doping and hydrostatic pressure

Magnetic properties of Sm_0.2Ca_0.8Mn_1-xRu_xO₃ (x=0, 0.04, and 0.08) are investigated in the temperature range 4.2-240 K, external magnetic fields up to 15 kOe and hydrostatic pressure up to 12 kbar. Transport properties of Sm_0.2Ca_0.8Mn_0.92Ru_0.08 O₃ are also investigated in the temperature range 77-280 K and under pressure up to 10 kbar. It is found that for a pristine phase (x=0), hydrostatic pressure increases the ferromagnetic domains embedded in an antiferromagnetic matrix, and slightly decreases the Néel temperature. The effect of pressure on the magnetic interactions depends upon Ru doping. For Sm_0.2Ca_0.8Mn_0.96Ru_0.04O an applied pressure decreases both Curie and Néel temperatures, but enhances the ferromagnetic fraction. In the case of Sm_0.2Ca_0.8Mn_0.92Ru_0.08 O₃ an applied pressure suppresses the ferromagnetic interactions and surprisingly increases the resistivity. To the best of our knowledge this is the first time that such an effect was observed in perovskites. All of the above observations are discussed in the context of phase separation.