Chapter 7: Optical and Magnetic Properties of Fe and Cr
7.1. Introduction
Diluted magnetic semiconductors (DMSs), where transition metal (TM) atoms are introduced into the cationic sites of the semiconducting host lattice have been at the forefront of research interest due to the existence of both semiconducting and ferromagnetic properties. These
materials could be applicable for the possibility of use in future spintronic and magneto-optic devices,1-3 where both spin and charge degrees of freedom can be manipulated in comparison to the conventional electronic devices where only electronic charges are considered.
Therefore, intensive attention has been focused on DMS such as TM doped Z nO,4, 5 SnO2,6 , 7 In2O38 and TiO29 due to realization of room temperature ferromagnetism (RTFM) in these systems. A mong various oxide based DMS, TiO2 has drawn extensive research interest because it is an excellent photo-catalyst for water splitting, high solubility limit of dopant magnetic ions and it possesses good optical transmission in the visible and near infrared region mak ing it a suitable candidate for magneto-optic devices as well. However, in spite of several studies reported on TiO2-based DMS, there is no clear agreement about the nature and origin of the observed ferromagnetism (FM) in the diluted magnetic oxide doped with few percentages of 3d cations. It is still controversial, whether the FM is indeed intrinsic which is mediated by carriers or defects inside the host semiconductor and/ or related to the purely extrinsic origin due to formation of ferromagnetic secondary phases or metallic cluster. It was suggested that the magnetic properties of DMS materials are critically dependent on fabrication, growth procedure, doping agents and processing conditions.
Different synthesis methods may result in different defect concentrations, structures and surface morphologies, affecting the magnetic properties. Compared to other methods of preparation, solvothermal/ hydrothermal method is very simple, low cost, and one can easily tune the surface as well as bulk defects by controlling the growth temperature, reaction duration, and doping concentrations etc. In particular, solvothermal/ hydrothermal method is an effective method of doping, because the dopant ion precursor distributes uniformly in a high pressure reaction chamber under continuous stirring throughout the synthesis process, depending upon the solvent used, pH concentration and the solubility limit of doping agent in the mixed precursor solution.
Interestingly, RTFM have been observed in a wide range of undoped oxides such as TiO2,10 -12 HfO2,13 In2O3,10 SnO214 and Z nO.15 These reports help to address the controversies about the issues related to the role of defects in the ferromagnetic ordering. Some of the outstanding reports revealed no evidence of ferromagnetic ordering in Fe doped TiO2
(Fe:TiO2) systems.16 , 17 U sing density functional theory (DFT) calculation, Chen et al.18 proposed that plays the important role in determining the FM in Fe doped rutile TiO2.
There are several experimental reports supported the intrinsic nature of FM in Fe:TiO2
DMS.12, 19-21 However, B alcells et al.16 did not observed FM in their Fe:TiO2 nanoparticles and suggested that FM in Fe:TiO2 system is an extrinsic effect due to either ferromagnetic secondary phases or any other impurity phases. Moreover, K im et al.22 reported that FM in their samples is related to the formation of secondary Fe2O3 phase suggesting extrinsic origin of FM. U nlik e many other metals (i.e., Fe, Co, N i), Cr itself is antiferromagnetic and their clusters/compounds (except nanocrystalline CrO2) do not contribute to ferromagnetism.
Thus, it would not induce an extrinsic ferromagnetism even if Cr clustering occurs in the Cr doped TiO2 (Cr:TiO2) nanostructures. Moreover, the pure phase ferromagnetic CrO2 is difficult to synthesize23 because it is metastable at atmospheric pressure.
A t present, most of the reported FM in undoped and doped TiO2 systems has been for thin films24-26 and nanoparticles27 while their undoped bulk counterparts are diamagnetic or paramagnetic. This implies that the spatial dimensionality might play an important role in the ferromagnetic ordering. Compared to thin films and nanoparticles, exploitation of 1D TiO2
nanostructures such as nanowires, nanorods (N Rs) and nanoribbons (N Rbs) with high surface area mak e it easier to engineer high availability of defect sites for trapping electrons and may favor the ferromagnetic ordering, thus mak ing them an ideal candidate for the realization of intrinsic enhanced RTFM. Moreover, 1D nanostructures are favored compared to nanoparticles in terms of electron transport, storage and information processing that can enhance the performance of spintronic devices at the nanoscale for practical applications.
A lthough lots of reports on the RTFM in undoped and TM (Fe, Cr) doped TiO2 thin films and nanoparticles are available in the literature,19, 24, 25, 28, 29 there is little information about undoped and doped 1D nanostructures.21, 3 0 , 3 1 In our recent report3 2 (which is discussed in Chapter 5), we observed RTFM in undoped TiO2 N Rbs and the FM was enhanced after vacuum annealing which suggested that defects are the source of magnetism in undoped TiO2. The earlier results motivated us to examine the individual and combined effect of TM doping and oxygen vacancy defects and clarify the controversial issues related to the long- range ferromagnetic ordering in TM doped TiO2 systems. Moreover, a clear demonstration by correlating the magnetism between dopant concentration and defects in the host semiconducting oxide lattice is crucial and it needs thorough investigations. Our present attempt in this study is to enhance the magnetic moments by Fe and Cr doping into the TiO2
matrix and explore a better understanding about the origin of observed FM in doped TiO2 1D systems.
Here, we grow Fe and Cr doped TiO2 1D nanostructures by solvothermal/
hydrothermal method and study the effect of doping concentrations and calcination temperatures on the optical and magnetic properties. We investigate the origin of RTFM in the Fe and Cr doped TiO2 nanostructures. The high surface area and higher concentration of surface defects, such as oxygen vacancies, expected in these nanostructures could ultimately lead to enhanced ferromagnetic ordering and strong FM even at room temperature. In Chapter 5, it was demonstrated that a large concentration of oxygen vacancies with high thermal stability indeed results in RTFM in undoped TiO2 systems. Moreover, when TiO2 is doped with low concentration of Fe and Cr, its properties are modified. For example, additional oxygen vacancy defects are expected due to the substitution of by and in TiO2 lattice and thus, formation of - and - defect complexes resulting in enhanced FM in Fe and Cr doped TiO2 nanostructures, respectively is expected compared to undoped TiO2. However, we observed that higher concentration of Fe doping leads to decrease in FM that may be due to the - antiferromagnetic ordering in the absence of in between two nearby Fe atoms. Similar behavior is observed for higher concentration of Cr doping. These results highlight that TM ion in the presence/ absence of in the host TiO2 still play a major role in deciding the DMS properties. The primary aim of this work is to figure out the precise role and impact of defects and Fe and Cr dopants on the observed RTFM in our Fe:TiO2 and Cr:TiO2 nanostructures, which is supported by structural and optical measurements.