XAS and XMCD investigation of zinc ferrite nanoparticles irradiated with 100 MeV O beam

Abstract

Synchrotron radiation based techniques have been considered superior to characterize the materials at atomic scale and to solve puzzles of physics, chemistry and biology in present decades [1, 2]. Techniques based on this radiation not only provide a description of atomic empty states but also give information about their bulk behavior. Thus simultaneous information of material characteristics at both scales by these techniques open a pathway to develop a theory regarding the behavior of materials. For more than two decades, swift heavy ion interaction with matter has been a most interesting field for researcher to understand the mechanism of the interaction which is still in debate [3, 4]. Thus investigation of this interaction with suitable techniques is demand for present era. With this motivation, synchrotron based techniques X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) have been carried out to investigate the interaction of 100 MeV O7+ ions with zinc ferrite nanoparticles of size ∼ 12 nm. The nanoparticles were synthesized using nitrates of zinc and iron in presence of citric acid followed by moderate thermal treatment of 450°C, and subsequently irradiated by 100 MeV oxygen beam in vacuum (∼ 10−6 Torr) with Pelletron accelerator installed at Inter University Accelerator Centre, New Delhi, India. Fluence of irradiation was kept at 5×1013 ions/cm2. Although as-deposited sample exhibits well-ordered cubic spinel phase, a slight degradation in crystalline phase is observed after irradiation. This behavior is analogues to that are reported for zinc ferrite with similar size and can be explained on the basis of distortion in the atomic position in effect of deposited energy inside the material [5]. To get glimpse of irradiation induced effects on electronic structure, XAS at O K-edge, Fe L-edge, and Zn L-edge of these samples have been measured. An O K-edge spectrum reflects approximately unoccupied oxygen p projected density of states in materials. Spectral features appear at 531, 532, 538, 541, 548, and 561 eV in O K-edge spectra for pristine and irradiated samples. Major changes occur in the pre-peak region for irradiated sample. Zn L-edge spectra shows spectral feature at 1026, 1030, 1035, 1041, and 1053 eV for pristine and irradiated samples [6]. Fe L-edge XAS spectra exhibit spectral features positioned around 707, 709, 720, and 722 eV. The spectral features at Fe L-edge occur due to Fe 2p core-level which gives rise to degenerate state of 2p 3/2 and 2p 1/2 showing multi-plets centered on ∼708 and 721 eV in effect of spin-orbit coupling. The octahedral crystal field lifts the degeneracy of 2p 3/2 and 2p 1/2 levels so that two levels with t 2 and e symmetry are created as indicated by two structures at about 707, 709 eV (L 3 ) and 720, 722 eV (L g Presence of these spectral features indicates that Fe remains in the ferromagnetic high spin 3d configuration. Intensities of various spectral features for XAS spectra at different edges were reduced after irradiation. This has been associated with the increase of broken bonds in the irradiated samples. Bond breaking in irradiated materials is consequences of energy deposition in the form of thermal spikes [7]. Further, to investigate magnetic properties, we have recorded the Fe L-edge transitions under magnetic field induced by right and left circularly polarized light. Recorded Fe L-edge XAS spectra (denoted by μ and μ− for right and left circurly polarized light in Figure 1) at 300 K for pristine and irradiated samples are very much similar to that are reported for zinc ferrite thin films and nanoparticles [8]. XMCD to XAS signal intensity ratio is estimated as 1.96 to 2.98 employing increase of 3d magnetic moment after irradiation. To explore the nature of magnetic interaction, XMCD spectral features A, B1 and B 2 of L 3 -edge were utilized. XMCD signals for pristine sample exhibit positive peak corresponding to feature A and negative peaks corresponding to B 1 , B 2 features (Figure 1). These features are associated with presence of Fe3+ ions at tetrahedral site and octahedral sites, respectively. Combined area of B1 and B 2 peaks represent total concentration of Fe3+ ions at octahedral site. From the area ratio of A and B 1 , B 2 features, Fe3+ ratio for tetrahedral to octahedral site is estimated as ∼0.20. This ratio is similar to that are reported for zinc ferrite samples with similar thermal treatment [9]. Apart from this, irradiated sample exhibits negative peak for feature A indicating change in magnetic interaction with irradiation. The observed results have been discussed on the basis of existing theories.