Electric and Magnetic Properties of ALD-Grown BiFeO3 Films

The magnetization and electric polarization in thin bismuth ferrite films (BFO) films have been under extensive study for high technological potential of single-phase multiferroic materials. Surpassing the antiferromagnetic nature and weak magneto-electric coupling of bulk BFO has required highly specialized substrates and epitaxial growth methods so far. Polycrystalline single-phase multiferroic BFO (50-500 nm thick) films were grown by atomic layer deposition (ALD) on technologically simple Pt/SiO2/Si substrates. The BFO films were found to exhibit strong saturating ferromagnetism and coercivity at temperatures ranging from cryogenic to room temperature even with 500 nm thick layers, a property which cannot be obtained with thick epitaxial films or bulk BFO. The magnetization mechanism was associated with magnetic domain wall dynamics and collapsing of the helimagnetic spin modulation. The electric properties were found to be strongly dependent on the film thickness. The film crystallization, composition, and chemical state have been analyzed by various techniques. The magnetic and ferroelectric properties were determined by using a SQUID magnetometer and a ferroelectricity tester. The results of the work indicate clearly that the ALD technique offers an efficient way for synthesis of polycrystalline BFO films and for tailoring their electromagnetic properties.