Effect of grain size and Gd-Co substitutions on multiferroic BiFeO3 thin films

Abstract

Multiferroics has attracted attention of researchers worldwide due to its fundamental and technological applications. Amongst multiferroics BiFeO3 (BFO) is the only known material to possess ferroelectric Curie temperature (TC=1143K) and antiferromagnetic Ne`el temperature (TN=643 K) well above room temperature. BFO crystallizes in a rhombohedrally distorted perovskite structure with R3c space group at room temperature. This is analogous to the hexagonal setting that contains six formula units of BFO in it with lattice constants ah = 5.587 Å, ch = 13.867 Å. The antiferromagnetic ordering of BFO is G-type having a spiral spin modulation with an incommensurate long-wavelength period of ~ 62 nm. Due to its antiferromagnetic nature; bulk BFO shows ineffectual magnetism that limits its device applications. On the other hand low resistivity of BFO leads to large leakage current limiting its ferroelectric applications. To overcome these limitations, attention has been diverted towards synthesis of thin films. Till date various techniques have been used to prepare and optimize conditions for preparation of BFO thin films. Considering the great potential of large magneto-electric effect in thin films, efforts have been made by researchers in the production of quality thin films of BFO. The magnetization and ferroelectric behaviour in BFO can be tailored by altering the grain size by adopting different methodology which includes varying annealing temperature and the film thickness. Besides, low level substitution of rare earth metal ions at Bi3+ site or transition metal ions at Fe3+ site also affects the magnetic and electrical properties. However, the recent approaches in the preparation of high quality polycrystalline thin films restrict the formation of grains with uniform size. It is therefore necessary to investigate the effect of grain size distribution to further reveal the intricate magnetic and electrical behaviour of BFO thin films. The present work focuses on the grain size distribution dependent magnetic and electrical properties of sol-gel driven BFO thin films. Further, the effect of substitution of Gd ion at Bi site and Co ion at Fe site has been studied. Also, the effect of simultaneous substitution of Gd and-Co has been investigated.