Study of multiferroic and optical properties of pure and divalent doped BiFeO 3

Abstract

This thesis constitutes a series of studies on the effects of divalent dopants (A) in place of Bi in the BiFeO 3 (BFO) multiferroic systems. In addition the effect of film thickness on the multiferroic properties of BFO has also been investigated. The studies include a complete set of structural, electronic, magnetic, optical and dielectric studies on the Bi 1- x Sr x FeO 3-δ (0 ≤ x ≤ 0.45) and Bi 0.75 A 0.25 FeO 3 (A = Pb, Ba) systems. The studies have been carried out with the knowledge that substitution of trivalent Bi with the respective divalent ions will induce changes in the crystalline as well as electronic structures which in turn would affect the multiferroic properties of the system. Accordingly we find that in contrast to the weak ferromagnetism of the parent material each of the x=0.25 doped systems exhibits ferromagnetism at room temperature. A strong correlation is observed between the magnetic and structural properties, e.g. in the case of Sr doped BFO there is a strong enhancement of ferromagnetism between x=0.15 and x=0.25 that coincides with a structural transition from rhombohedral to cubic phase (R3c → Pm-3m) at x=0.25. Magnetic moment was observed to increase with increase in the radius of A-site ion being largest for the Ba doped system. Further insight into the origins of the ferromagnetism enhancement was obtained through Mössbauer measurements which showed that despite being doped with a divalent ion, the Fe ions in BFO retained their trivalent state. We find that A 2+ ion substitution at Bi 3+ site results in oxygen deficiency and consequently in a tetrahedral coordination for some of the Fe−O ions. The role of oxygen vacancies in promoting ferromagnetism was evident from the decrease in ferromagnetic moment in oxygen annealed samples. Thus the ferromagnetism in these systems is correlated with the structural transformation at x=0.25 which tends to destroy/weaken the parent antiferromagnetic cycloidal spin structure while induced changes in the superexchange interactions, due to size differences and oxygen vacancy effects, also contribute to the observed differences. We find a systematic but anomalous decrease in the magnetic coercivity at low temperatures which is explained in terms of an effective magnetic anisotropy that includes the effects of magnetoelectric coupling. The dielectric response for these doped systems depends in general on the structural details as well as the presence of oxygen vacancies. The Sr x=0.45 composition shows a marked anomaly in ε(T) which is explained in terms of relaxation effects originating presumably in nanoscale polar inhomogeneities. The lone pair substituent Pb leads to the largest dielectric | ixconstant, enhanced magnetization, and large effects on the low-temperature magnetic hysteresis. The optical response of multiferroic Bi 1-x Sr x FeO 3 (0 ≤ x ≤ 0.45) samples is studied in the spectral range from 1 eV to 4 eV by means of UV/Vis spectrometry. Optical response in the studied spectral range was dominated by two charge transfer transitions and two doubly degenerate d-d transitions for all samples. The d-d transitions weakened as the Sr content was increased which is attributed to the transformation of the crystal structure from rhombohedral to cubic. A red shift in the d-d transition energies was observed with increasing Sr concentration which is related to an increase in the crystal field strength. The direct band gap was found to decrease whereas the indirect band gap decreased as the Sr content is increased from x = 0 to 0.45. The ultimate source of the decrease in the energy band gap values and the observed red shift in the d-d transition energies is ascribed to the chemical pressure induced reduction in unit cell volume. For Ba and Pb doped samples the differences in these optical properties are also consistent with the associated changes in the unit cell volume. BFO thin films with thickness increasing from 40 to 480 nm were successfully grown on LaNiO 3 buffered Pt/Ti/SiO 2 /Si(100) substrate and the effects of thickness evolution on magnetic and ferroelectric properties have been investigated. All the BFO films exhibited weak ferromagnetic response at room temperature and saturation magnetization decreases with increase in film thickness. Well saturated ferroelectric hysteresis loops were obtained for thicker films; however, the leakage current dominated the ferroelectric properties in thinner films. The mechanisms for the evolution of ferromagnetic and ferroelectric characteristics in these films have been discussed.