The La0.7Ca0.3−xBaxMnO3 (x=0; 0.03; 0.06…0.3) and La0.7Ca0.3−xSrxMnO3 (x=0.105, 0.12, 0.135, 0.15, 0.165, and 0.18) lanthanum manganites were synthesized and studied. The Curie temperature TC of Ba range showed a sharp change near the concentration structural orthorhombic–rhombohedral phase transition. The TC values for that range are under the “classical” ferromagnetic–paramagnetic line. The TC temperatures of Sr range of samples are independent on x value at the vicinity of structural transition in the rhombohedral phase. Temperature dependence of magnetization of the La0.7Ca0.165Sr0.135MnO3 composition looks like a two-step curve which is caused by the closeness of ferromagnetic ordering and structural phase transition temperatures.
We report a study of the structure and pressure effects on magnetic and transport properties of a lanthanum manganite, La0.7Ca0.3−xSrxMnO3 (0⩽x⩽0.3, Δx=0.03). The pressure coefficient of the Curie temperature (Tc), dTc/dP, is shown to depend on x. The temperature Tc increases approximately linearly under applied pressure as dTc/dP≈14 K/GPa in the orthorhombic (Pbnm) phase and as dTc/dP≈7.5 K/GPa in the rhombohedral (R3̄c). The value of dTc/dP shows a minimum (≈3.5 K/GPa) and the temperature of the resistance maximum, Tp(x), shows a change of slope at x=0.15, corresponding to a concentration structural phase transition. Differences between the values of dTc/dP and the slopes of Tp vs x in Pbnm and R3̄c phases are explained by the different effect of external pressure on the Mn–O bond length and the Mn–O–Mn bond angle, and by the different internal pressure effect (depending on the value of x) in those phases, respectively.
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