Flux-Mediated Doping of Bi into (La,Sr)MnO₃ Films Grown on NdGdO₃ (110) Substrates

Bi-doped (La,Sr)MnO3 [(Bi0.18La0.56Sr0.30) Mn0.96O3: BLSMO, tBLSMO = 63 nm] epitaxial films were grown on NaGdO3 (NGO) substrates by a flux-mediated doping approach. That is, the growth of a (La,Sr)MnO3 (LSMO) film under the coexistence of a Bi-based liquid flux leads to successful Bi doping into the LSMO film, though Bi is a highly volatile element. The composition analysis revealed that Sr preferentially existed in the film over the flux and that Sr has a larger distribution equilibrium coefficient (K) than those of Mn and La, which is one of the evidence that Bi has substituted for the La site. The obtained BLSMO films had a tetragonal distorted structure owing to a compressive strain from the NGO substrate, and the tetragonality and the crystallinity were enhanced by increasing the amount of Bi. The tetragonal distortion of the BLSMO films induced a perpendicular magnetic anisotropy. Both the Bi substitution at the La site and the ratio of Mn2+/Mn3+ increased from the film body to the surface, a magnetic moment was decreased to 0.6 μB/Mn. Although the magnetic moment was low, the Curie temperature of the BLSMO film was above room temperature (RT), and the magnetic entropy change |ΔSM| of 2.4 mJ/(cm3·K) was observed near RT. The BLSMO film is favored for magnetocaloric effect applications as well as spin-transfer-torque devices such as microwave oscillators.