Diamagnetism to ferromagnetism in Sr-substituted epitaxial BaTiO_3 thin films

摘要

We report on the ferromagnetic-like behavior in otherwise diamagnetic BaTiO_3 (BTO) thin films upon doping with non-magnetic element Sr having the composition Ba_(0.4)Sr_(0.6)TiO_3 (BST). The epitaxial integration of BST (～800nm) thick films on Si (100) substrate was achieved using MgO (40 nm) and TiN (20 nm) as buffer layers to prepare BST/MgO/TiN/Si (100) heterostructure by pulsed laser deposition. The c-axis oriented and cube-on-cube epitaxial BST is formed on Si (100) as evidenced by the in-plane and out-of-plane X-ray diffraction. All the deposited films are relaxed through domain matching epitaxy paradigm as observed from X-ray diffraction pattern and A_1TO_3 mode (at 521.27 cm~(-1)) of Raman spectra. As-deposited BST thin films reveal ferromagnetic-like properties, which persist up to 400 K. The magnetization decreases two-fold upon oxygen annealing. In contrast, as-deposited un-doped BTO films show diamagnetism. Electron spin resonance measurements reveal no evidence of external magnetic impurities. XRD and X-ray photoelectron spectros-copy spectra show significant changes influenced by Sr doping in BTO. The ferromagnetic-like behavior in BST could be due to the trapped electron donors from oxygen vacancies resulting from Sr-doping.