Ferromagnetic state of La1-xBaxCoO3 under applied pressure: Factors controlling the sign reversal of pressure effect in cobaltites

Pressure effect on magnetic properties of polycrystalline La1-xBaxCoO3 (0.1 <= x <= 0.5) was studied by magnetization measurements in the temperature range 5-240 K, magnetic field up to 16 kOe, and under hydrostatic pressure up to 10.5 kbar. For low-doped cobaltites (x = 0.1,0.15), it was found that applied pressure enhances the ferromagnetic (FM) cluster phase, resulting in an increase of both spontaneous FM moment and transition temperature of clusters T-C(cl), while the freezing temperature T-f lowers under pressure appreciably (dT(f)/dP approximate to -1.4 K/kbar), suggesting the weakening of interaction between clusters. For higher-doped La1-xBaxCoO3 (0.2 <= x <= 0.5), having developed long-range FM order, the systematic increase of T-C under pressure was observed with coefficient dT(C)/dP linearly increasing with doping up to 1.8 K/kbar at x = 0.5, the highest value reported for cobaltites. The positive value of the dT(C)/dP coefficient found for the Ba compound in a whole range of studied doping is in strong contrast to that found in Ca and Sr cobaltites, where the dT(C)/dP was found to change a sign from a negative to a positive one with increasing doping. It is shown that the sign reversal of the dT(C)/dP coefficient for La1-xMxCoO3 (M = Ca,Sr,Ba) cobaltites can be caused by the hole doping and also, independently, by the lattice expansion only induced by an increase of the dopant ion size. The complex pressure effect on ferromagnetic transition T-C in cobaltites is well described in terms of competing e(g)-electron bandwidth W and crystal-field splitting energy, basing on the known pressure variations of the steric factors. For La1-xBaxCoO3 with x = 0.3,0.5, having a high enough concentration of intermediate spin Co states, we observed a Jahn-Teller-like magnetic transition signified by a distinct hysteresis in field-cooled magnetization below T-C. This transition was found to disappear under moderate applied pressure, evidenced for La0.7Ba0.3CoO3 by abrupt changes in the spontaneous magnetization and coercive field at 5 K.