Identifying defects in multiferroic nanocrystalline BaTiO3 by positron annihilation techniques

Room temperature ferromagnetism in nanoparticles of otherwise nonmagnetic materials has been attributed to point defects at the surface of the nanoparticles. Here, we have employed positron annihilation spectroscopy to identify the nature of defects in multiferroic BaTiO3 nanocrystalline materials with varying average particle sizes. Ratio curve analysis of the Doppler broadening profile to a reference profile suggests that the defect is an oxygen vacancy. The decrease of intensity of the intermediate lifetime component with increasing particle size indicates a decrease of surface defect concentration. The large defect concentration in nanocrystalline BaTiO3 can explain the observed room temperature ferromagnetism.