Influence of annealing on the absorption of Bi-based iron garnet films

Abstract

Annealing of LPE‐grown iron garnet films in reducing atmosphere changes their optical absorption and their electrical conductivity. In Bi‐based films where the dominant impurity, lead reaches 0.35 mole%, both the conductivity and the absorption below 2.4 eV decrease after annealing.Annealing experiments were performed on films approximated as (Bi.6Pb.06Tm22.34)(Fe4Ga1)12. The maximum observed decrease in absorption was of the order of 40%, while the conductivity decreased by 2−3 orders of magnitude. The decrease in the absorption is independent of the accompanying redistribution of Ga ions and its final value depends mostly on the Pb content and on the annealing conditions. Experimental results lead to a model where oxygen vacancies diffusing into the crystal(D∠10−8 cm2 sec −1 at 870°C) replace Fe4+ as charge‐compensators for the Pb2+ impurity.The hig Faraday Rotation observed in Bi‐based iron garnet films 1,2 is usually accompanied by an absorption higher than the absorption of pure Y3Fe4O12. This is a study of the origin of this dopant induced absorption.Below 2.4 eV the absorption spectrum of iron garnets is strongly affected by dopants.3 The most commonly invoked example is the increase in the absorption which is observed when a nontrivalent cation‐dopant, such as Ge4+ or Ca2+, enters the garnet substitutionally. Although recent studies 4 have demonstrated the presence of Fe2+ in Y3Fe5−xSixO12 (YIG:Si), little is known about acceptor levels in garnets or about the effect of the doping level on the defect distribution in LPE films. The second mechanism through which a given dopant might influence the abosrption of its host‐garnet depends on interatomic interactions. The evidence of such influence the absorption of its host‐garnet depends on interatomic interactions. The evidence of such influence is contained in the spectral studies of YIG:Ga, Sc, and it can be inferred from the work on Bi‐Induced FR in Bi‐doped iron garnets.6,7In this paper we discuss the dopant‐induced absorption in LPE films of (BiTm)3(FeGa)5O12 grown from PbO flux. Specifically we distinguish between the effects related to the valcence of Pb impurity and a possible enhancement of electronic transitions of yIG, due to strong interatomic interactions with Bi or Pb.In an elegant experiment Le craw et. al.8 have shown that microscopic redistribution of Ga between the octohedral (a) %and the tetrahedral (d) sites in (YEu)3(FeGa)5OPd12 is facilitated when these films are annealed in a reducing S9‐environment. The increased diffusion rate of Ga was ascribed to oxygen vacancies, introduced into the crystal during annealing. We have decided to use this technique to alter the stoichiometry of our films and to look for the resulting changes in their absorption and in theirr electrical conductivity.Annealing of LPE‐grown iron garnet films in reducing atmosphere changes their optical absorption and their electrical conductivity. In Bi‐based films where the dominant impurity, lead reaches 0.35 mole%, both the conductivity and the absorption below 2.4 eV decrease after annealing.Annealing experiments were performed on films approximated as (Bi.6Pb.06Tm22.34)(Fe4Ga1)12. The maximum observed decrease in absorption was of the order of 40%, while the conductivity decreased by 2−3 orders of magnitude. The decrease in the absorption is independent of the accompanying redistribution of Ga ions and its final value depends mostly on the Pb content and on the annealing conditions. Experimental results lead to a model where oxygen vacancies diffusing into the crystal(D∠10−8 cm2 sec −1 at 870°C) replace Fe4+ as charge‐compensators for the Pb2+ impurity.The hig Faraday Rotation observed in Bi‐based iron garnet films 1,2 is usually accompanied by an absorption higher than the absorption of pure Y3Fe4O12. This is a s...