Flux-mediated doping of Bi into (La,Sr)MnO₃ films grown on NdGdO₃ (110) substrates

Bi-doped (La,Sr)MnO₃ [(Bi_0.18La_0.56Sr_0.30) Mn_0.96O₃: BLSMO, t_BLSMO = 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 μ_B/Mn. Although the magnetic moment was low, the Curie temperature of the BLSMO film was above room temperature (RT), and the magnetic entropy change |ΔS_M| 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.