Evolution of ferromagnetic order in LaMnO_3.05 single crystals:   Common origin of both pressure and self-doping effects

The pressure effect on magnetic order and anisotropy of low-doped LaMnO_3.05 single crystals with two competing ferromagnetic and antiferromagnetic insulating phases was studied by magnetization measurements. It was found that applied hydrostatic pressure leads to an increase of both Curie and Néel temperatures, with different rates: dT_C/dP=0.8 K/kbar and dT_N/dP=0.16 K/kbar. External pressure strongly suppresses the magnetic anisotropy, resulting in a decrease of the coercive field H_C with dH_C/dP=-0.1 kOe/kbar. The results obtained indicate that the applied pressure both enhances ferromagnetic interactions and enlarges the volume fraction of the ferromagnetic phase. The evolution of the ferromagnetic phase was explained taking into account the reduction of the Jahn-Teller distortion with increasing pressure. Changes in ferromagnetic characteristics T_C, H_C, and spontaneous magnetization M₀ result mainly from the shortening of the long Mn-O distance in the MnO6 octahedron. Moreover, we found a similarity in dependences of those parameters on the degree of the MnO₆ distortion for two different effects: hydrostatic pressure and the level of self-doping δ. Such a similarity strongly suggests that pressure-induced suppression of the Jahn-Teller distortion is responsible for the observed enhancement of ferromagnetism in the LaMnO_3.05 crystals.