$BiFeO_3$ (BFO) is a multiferroic material, which has ferroelectric and antiferromagnetic phases simultaneously at room temperature. Various order parameters in BFO such as ferroelectricity and ferromagnetism are coupled each other and they also interact with the crystal structure. The origin of each order parameter and the coupling have been investigated for understanding the basic principle underlying observed phenomena and ultimately realizing devices capable of controlling an order parameter by a non-conjugate field at room temperature.
Using pulsed laser deposition, BFO thin films with the tetragonal-like phase can be grown and stabilized on $LaAlO_3$ (LAO) substrates by misfit strain. Bismuth 6s lone pair electrons play a significant role in stabilizing the elongated tetragonal-like phase with a giant electric polarization. In this point of view, the reduction of the lone pair electrons of bismuth ions weakens the ferroelectric order, hence it greatly modifies the ferroelectric phase transition temperatures. Lanthanum A-site substitution in BFO is known to influence the monoclinic distortion angle and crystal symmetry, consequently changing the orientation of in-plane polarization.
In this study, 20% lanthanum-doped BFO (BLFO) thin films grown on LAO substrates are investigated for understanding the isovalent doping effects on the structural phase transitions of the BFO thin film system and the consequent emerging phenomena such as relaxor behaviors. The structural phase transitions are investigated by reciprocal space mapping with temperature changing and analyzed with consideration of the lanthanum substitution effect. The low-frequency dielectric relaxation observed at cryogenic temperatures is also analyzed by the modified Curie-Weiss law and the Vogel-Fulcher relation and it is interpreted based on the super-paraelectric theory.