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Structural and magnetic study of a dilute magnetic semiconductor: Fe doped CeO2 nanoparticles

This paper reports the effect of Fe doping on the structure and room temperature ferromagnetism of CeO2 nanoparticles. X-ray diffraction and selective area electron diffraction measurement reflects that Ce1-xFexO2 (x = 0.0 - 0.07) nanoparticles exhibit single phase nature with cubic structure and none of the sample showed the presence of any secondary phase. The mean particle size calculated by using a transmission electron microscopy measurement was found to increase with increase in Fe content. DC magnetization measurements performed at room temperature indicates that all the samples exhibit ferromagnetism. The saturation magnetic moment has been found to increase with an increase in the Fe content.


INTRODUCTION
Dilute magnetic semiconductors (DMSs) have drawn considerable attention in the past few years in view of their projected potential for the development of the magneto-optoelectronics devices. [1][2][3] It is generally accepted that ferromagnetism in semiconductors requires a few percent of the transition metal (TM) ions dopants that have partially filled shell of d or f electrons to mediate ferromagnetism. 4 However, certain DMS systems based on III-V and II-VI semiconductors appear to have been recognized as to have intrinsic (carrier-induced) ferromagnets 5 , but the situation with the oxide based system is still controversial, many promising compounds such as: transition metal (TM) doped ZnO, TiO 2 , CeO 2 and HfO 2 etc, have been observed to show room temperature ferromagnetism (RT-FM). However, for most of the experimental results, doubt arose about the real origination of the FM. [6][7][8][9][10][11] Some of the report claims that the observed FM is induced by the segregation of metallic clusters or due to the secondary phase, while in some systems it is due to the different valence state of TM ions. Although, the high T C FM observed in TM doped insulating oxides has been explained using different models even in the same system by the different groups. In the early stage, the origin of FM in oxide based DMSs were explained by the carrier mediated exchange. 5 Recently, Croy et al 10 have been proposed donor impurity band exchange in DMSs. According to this model ferromagnetic exchange in DMSs are mediated by the shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. More recently, the origin of high temperature FM in insulating oxides has been explained by the oxygen vacancy mediated FM or so-called F-centre exchange coupling (FCE) mechanism. 12 In FCE mechanism an electron is trapped in oxygen vacancy which acts as a coupling centre, via which doped magnetic ions align in ferromagnetic order.
In the last few years, CeO 2 containing materials have received a lot of attention due to its diversity of applications such as: catalysis 13 , in solid oxide fuel cell 14 , in ceramic materials 15 and in spintronics. [16][17][18][19][20] CeO 2 nanoparticles have been prepared using several methods such as hydrothermal synthesis 21 , hexamethylenetetramine based homogeneous precipitation 22 , coprecipitation 23 and force hydrolysis 24 etc. However, there are copious methods to synthesize CeO 2 nanoparticles, but a very small work has been carried out to investigate the effect of the TM doping on the magnetic properties of CeO 2 . The discovery of FM in CeO 2 tiles the way for the realization spintronics devices. CeO 2 is an insulating material with high dielectric constant (~26). Since CeO 2 has FCC structure like fluoride crystal structure and closely lattice matches with silicon, therefore, the RT-FM in CeO 2 will make possible the integration spintronics devices with the advanced silicon based microelectronics. In the present work, we have prepared Fe doped CeO 2 nanoparticles using co-precipitation technique. The effect of Fe doping has been studied using x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and DC magnetization measurements. The results of XRD, FTIR and TEM measurements clearly indicate that Fe ions are substituting Ce ions in the CeO 2 matrix. The DC magnetization measurement also confirms that all the samples display RT-FM.

EXPERIMENTAL DETAILS
Nanoparticles of Fe doped CeO 2 have been synthesized using the co-precipitation method.   TEM micrographs indicate that the nanoparticles have spherical shape. The particle size distribution obtained from TEM micrographs using Image J 1.3.2 J software reflects that particle size increases with Fe doping. It is observed that 1 and 7% Fe doped CeO 2 nanoparticles have particle size distribution from 3.5 -8 and 4 -12 nm respectively. The mean particle size calculated for 1 and 7% Fe doped CeO 2 nanoparticles from the fitting of the particle distribution histogram using Lorentzian function are found 5.7 and 7.5 nm respectively. SAED measurement were performed to further study the single phase nature of Fe doped CeO 2 nanoparticles, since, it is difficult to detect any impurity phase below 5% using XRD measurements. Inset in Fig. 3(a) and (b) shows the SAED pattern of Fe doped CeO 2 nanoparticles. Rings in SAED patterns indicate clearly the randomly oriented single crystals, hence ruled out the presence of any impurity phase and indicate that each nanoparticle is indeed in single phase.
FTIR measurement of Fe doped CeO 2 nanoparticles carried out in the wave number range 2000 to 400 cm -1 is shown in Fig. 4. The FTIR measurements been performed to investigate the effect of Fe doping on the Ce-O-Ce bonding. FTIR measurements were done using KBr method at RT.
The absorption band observed around 1625 cm -1 is attributed to the stretching mode hydroxyl. 27 The peak at 846 cm -1 is due to the vibrations associated with the incordination of the absorbed  (see table). It can be clearly seen from the inset in Thurber el al 26 also reported that both the saturation magnetization and remanent magnetization increases with Ni doped CeO 2 nanoparticles up to 4% and then decreases with further increase in the Ni doping. The magnetic ordering in the insulating materials is generally originated by the super-exchange interaction, which results antiferromagnetic ordering. However, recently another conceivable mechanism has been proposed to explain the FM in transparent oxide is FCE mechanism. 12 According to the FCE mechanism, the magnetic ion and oxygen vacancy are two necessary requirements which results the FM ordering. The FM observed in the present system can be explained in the light of the FCE mechanism. In FCE mechanism an F centre consists of an electron trapped in the oxygen vacancy.