Synthesis, structural, Raman scattering and magnetic properties of Fe -doped HfO₂ nanoparticles

The precursor materials, hafnium (IV) chloride (HfCl₄, 99.9%), iron (II) chloride tetrahydrate (FeCl₂.4H₂O, 98%), sodium hydroxide pellets (NaOH, 98%), were procured from Alfa Aesar (USA) and used as received without any further purification. For the synthesis of Hf_1−xFe_xO₂ (0 ≤ x ≤ 0.05) nanoparticles, appropriate amounts of hafnium tetrachloride and iron (II) chloride tetrahydrate were dissolved separately in distilled water in 0.1M and 0.2 M ratio. The two solutions were mixed together, and 0.1 M solution of sodium hydroxide were added slowly to this solution, followed by continuous stirring for 30 min. This resulted in the precipitate of Hf_1−xFe_x(OH)₄, which were recovered using a centrifuge. The resulting precipitate were washed several times with water and dried overnight. The obtained dried powder of Hf_1−xFe_x(OH)₄ were ground and calcined at 600 °C for 3 h. to obtain Hf_1−xFe_xO₂ nanoparticles.

X-ray diffraction studies of the synthesized materials were performed using Rigaku Mini flex-II diffractometer (CuKα radiation with wavelength of 1.5406 angstrom.), at a scan rate of 1°/min. The data was collected at every 0.02°, at room temperature. Raman measurements of the sample were carried out using a Horiba-Yobin T64000 micro-Raman system, and a 514.5 nm line of an argon ion laser was used as excitation source. The morphological characterizations were carried out using a field emission scanning electron microscope (FESEM, JEOL JSM-7500F SEM) at an applied field of 15 KV and elemental analysis were carried out using energy dispersive x-ray spectrophotometer (EDX). The magnetic properties of Hf_1−xFe_xO₂ materials were studied using Quantum Design PPMS DynaCool system.

Figure 1 shows the x-ray diffraction pattern of Hf_1−xFe_xO₂ (x = 0.0, 0.03, 0.05) nanoparticles. The diffraction pattern of pure HfO₂ corresponds to a monoclinic structure. Peak broadening upon increasing concentration of Fe³⁺ indicates that crystallinity of Fe³⁺ doped nanoparticles powders reduces compared to pure HfO₂ nanoparticles. The major characteristic diffraction peaks were observed at 2θ values of 24.5°, 28.4°, 31.76°, 34.68°, 35.61°, 49.68°, 50.48° and 55.96°, which corresponds to (011), (−111), (111), (200), (002), (220), (022), and (310) lattice plane, respectively. These results are in good agreement with earlier literature reports [14, 15]. In addition, the x-ray diffraction pattern of Hf_0.93Fe_0.03O₂ and Hf_0.95Fe_0.05O₂ show an additional peak at 30.48°, which corresponds to (111) plane of tetragonal phase of HfO₂. This is in good agreement with standard PDF card (JCPDF-53-0550) of HfO₂ and reveals the presence of tetragonal and monoclinic phases in Fe doped HfO₂. The coexistence of the monoclinic and tetragonal phase was also reported earlier for Y doped [1], and 20% Fe doped [12] HfO₂. The XRD data of Y doped HfO₂ nanoparticles also show a peak around 30.48^o corresponding to tetragonal phase. These results reveal that the structure of HfO₂ is sensitive with dopant species and their concentrations. More recently, Wan and Zhou [16] obtained the stable tetragonal phase of HfO₂ nanoparticle, synthesized by hydrothermal root. The peak positions from x-ray pattern of Hf_1−xFe_xO₂ were obtained using Fityk 0.9.8 curve fitting program [17]. The average crystallite sizes were found using Scherrer equation

$$\begin{eqnarray*}&&{\rm{D}}={\rm{k}}\lambda /\left(\beta \,\mathrm{Cos}\,\theta \right)\end{eqnarray*}$$

where, k is a constant (0.94), λ is the wavelength of Cu-Kα radiations (1.541 Å), β is the full width at half maximum for diffraction peak and θ is the Bragg's diffraction angle. The average crystallite size for HfO₂, Hf_0.93Fe_0.03O₂, and Hf_0.95Fe_0.05O₂ nanoparticles were found to be 11 nm, 10 nm, and 8 nm, respectively. The crystallite sizes of the synthesized Fe³⁺ doped hafnium oxide materials are comparable to those reported earlier in literature [18]. The various peak positions, crystallite size, and lattice strain are presented in table 1. It can be seen that the crystallite size of Hf_1−xFe_xO₂ decrease upon increasing of Fe³⁺ concentration, and peaks shift towards higher θ values. This may be due to the fact that atomic radius of Fe (156 pm) is much smaller than the atomic radius of Hf (208 pm). Further the lattice strain of Hf_1−xFe_xO₂ nanomaterials were found to increase upon increasing Fe³⁺ concentration. For HfO₂, Hf_0.93Fe_0.03O₂, and Hf_0.95Fe_0.05O₂ the lattice strains were found to be as 0.013, 0.014, and 0.016, respectively. This increment in lattice strain may be due to the reduction of crystallite size [19].

Zoom In Zoom Out Reset image size

The nanoparticles of HfO₂ were characterized by monoclinic symmetry with the point group (m) and space group (P2₁/C) [20]. A group theoretical analysis, considering the site symmetry for the zone center (Γ-point) phonons of monoclinic HfO₂, indicate that the only 18 (9A_g + 9B_1g) modes are Raman active [21]. The room temperature Raman scattering data of Hf_1−xFe_xO₂ (x = 0.0, 0.03, 0.05) nanomaterials were obtained by employing normal backscattering geometries and the results are presented in figures 2(a) and (b). Figure 2(a) represents the Raman spectra of monoclinic HfO₂ nanoparticles at room temperature. We observed a total of 17 Raman modes at 110, 133, 147, 164, 241, 256, 324, 335, 382, 397, 498, 522, 552, 579, 639, 672 and 776 cm⁻¹, which are comparable to earlier reported Raman spectra of HfO₂ with the same monoclinic symmetry [19, 20]. The Raman mode at 110, 133, 147, 256, 335, 382, 498 (with high intensity Raman mode), 579 and 672 were assigned as A_1g Raman modes whereas Raman modes at 133, 164, 241, 324, 397, 522, 552, 639, 776 cm⁻¹ were assigned to B_1g mode. The Fe doped HfO₂ samples (Fe = 0.03 and 0.05) show a Raman mode at 215 cm⁻¹ (figure 2(b)), which may be correspond to tetragonal phase of HfO₂. This clearly indicates the presence of two phases (monoclinic and tetragonal) in Hf_1−xFe_xO₂ (x = 0.03 and 0.05) nanomaterials as were observed in the x ray pattern. A similar Raman mode close to 215 cm⁻¹ in HfO₂ was reported earlier to correspond to tetragonal phase of HfO₂ [1, 20]. We did not observe any evidence of the presence of extra phase of Fe (Fe₂O₃ or Fe₃O₄) in the Raman spectra of Hf_0.93Fe_0.03O₂ and Hf_0.95Fe_0.05O₂.

Zoom In Zoom Out Reset image size

From figure 2(b), we also observed that the FWHM and intensity of all the Raman modes show anomalous changes with increase in Fe³⁺ ions concentration. The phonon frequencies and line widths of high intensity Raman modes of Hf_1−xFe_xO₂ samples were obtained using Peak FIT (version V4) program and are presented in table 2. The steady decrease in Raman peak intensity and an increase in FWHM with increase in Fe ³⁺ concentration indicate the reduction in crystalline size. The broadening of Raman peaks due to the reduction in particle size have been reported earlier for Mn₂O₃, SnO₂ TiO₂ nanoparticles [23–25].

The lower wavenumber Raman modes between 109 to 300 cm⁻¹ are dominated by vibration in Hf–Hf stretching bonds, whereas Raman modes between 300 to 600 cm⁻¹ are dominated by the vibrations in O–Hf–O bending and Hf–O stretching bond [22]. We observed that all the intense Raman modes associated with the monoclinic symmetry of HfO₂ are present in Fe³⁺ ion doped HfO₂ nanoparticles, indicating that all Fe doped samples retained good lattice order. As the concentration of Fe ions increased, the lower Raman modes (Low phonon mode below 300 cm⁻¹) due to stretching vibration in Hf–Hf, shifted towards the lower frequencies (shown in inset of figure 2(b)). This may be due to the large atomic mass difference between Hf and Fe, whereas higher Raman modes due to vibration in O²⁻ ions does not show remarkable Raman shift. These results clearly show that the incorporation of Fe ions occurs at Hf sites of HfO₂ lattice.

X-ray diffraction and micro-Raman studies confirm that there is no other impurity or phase formation in the synthesized Hf_1−xFe_xO₂ (x = 0.0, 0.03, and 0.05) nanoparticles. In order to study the surface morphology of Hf_1−xFe_xO₂ nanoparticles and the existence of the doping element (Fe) in Hf_1−xFe_xO₂, scanning electron microscopic and EDX studies were performed and the results are presented in figure 3. It can be seen from SEM pictures that the materials are in nanosize range. The EDX spectra of pure HfO₂ show the presence of Hf and O atoms, while EDX spectrum of Fe doped samples shows the presence of Hf, Fe and O atoms. Absence of any element other than Hf, Fe, and O confirm the purity of synthesized Hf_1−xFe_xO₂ nanoparticles.

Zoom In Zoom Out Reset image size

Figure 4 shows the room temperature magnetization versus magnetic field (M–H) hysteresis for pure HfO₂ and Fe-doped samples at magnetic field up to ±4000 Oe. All the samples show a superimposed saturating ferromagnetic like behavior and after subtracting the diamagnetic part (the linear component χ_p), the corrected magnetization curves clearly display the ferromagnetic behavior as shown in figure 4. The magnetization curves are almost hysteretic, as is often the case for ferromagnetic oxides. The saturation magnetization value for the pure HfO₂ samples is about 0.1142 emu g⁻¹, which might be due to the presence of defects as also found in the earlier reports [2, 5]. C.D. Pemmaraju and S. Sanvito [26] explained the origin of ferromagnetism in HfO₂ thin film nanostructure using first principle calculation and observed that the isolated cation vacancies site in HfO₂ thin films lead to formation of high spin defect states, resulting in a ferromagnetic state in HfO₂. A Sundram et al [27] suggest that the origin of ferromagnetism in nanoparticles of nonmagnetic oxides such as SnO₂, In₂O₃, CeO₂, Al₂O₃, ZnO may be due to exchange interaction between localized electron spin moments resulting from oxygen vacancies at the surface of nanoparticles.

Zoom In Zoom Out Reset image size

The Fe doped HfO₂ samples showed noticeable increase in ferromagnetic properties as compared to pristine HfO₂ sample. The value of saturation magnetization increased with Fe concentration and reached 0.238 and 1.002 emu g⁻¹ for Hf_1−xFe_xO₂ (x = 0.03) and Hf_1−xFe_xO₂ (x = 0.05), respectively. The inset of figure 4 shows the coercivity of undoped HfO₂ and Fe-doped nanoparticles. The value of coercivity of undoped HfO₂ nanoparticle was observed as 180 Oe at 300 K. The coercivity (Hc) of Hf_1−xFe_xO₂ decreased to 100 Oe and 52 Oe for (x = 0.03) and (x = 0.05), respectively. The decrease in coercivity may be due to the reduction in crystallite size with increase in Fe³⁺ concentration as calculated from the XRD values. A comparative list of the values of saturation magnetization (M_s) and coercive field (H_c) of Hf_1−xFe_xO₂ (x = 0.0, 0.03, 0.05) nanoparticles and similar materials Ti_1−xFe_xO₂ (x = 0.01, 0.02, 0.03) are given in table 3. We found that for both the materials, saturation magnetization (M_s) increases while the coercive field (H_c) decreases upon increasing Fe concentration. These comparative magnetization parameters suggest that room temperature ferromagnetism in Fe doped HfO₂ is driven by the oxygen vacancy, similar to Fe doped TiO₂ and the oxygen vacancies increases upon increasing Fe concentrations [28].

The existence of significant improvement in the saturation magnetization is possibly due to the ferromagnetic nature of Fe nanoparticles and mainly due to the increased magnetic volume fraction in the doped samples. Coey et al [29] proposed a bound magnetic polaron model, in which polarons interact with the magnetic ions present at interstitial sites. The resulting indirect ferromagnetic interaction that arises align the magnetic polarons. Therefore, the ferromagnetism in metal doped nonmagnetic oxides may be due to formation of magnetic polarons. According to this model, low concentrations of metal dopant dominate the local properties of HfO₂ like materials and play an important role in mediating long range magnetic order. We observed anomalous change in frequency (shift up to 10 cm⁻¹) and in FWHM in the lowest phonon mode at 110 cm⁻¹ and in the oxygen ion dominated high frequency phonon modes at 776 cm⁻¹ (marked as * in figure 2(b)) as the Fe concentration in the HfO₂ lattice increased. Gao and coworkers [30] predicted that these two modes are highly localized, and they could be introduced by oxygen vacancies in HfO₂. These observations reveal that oxygen vacancies increase with Fe concentration and this may be a reason for the enhancement of ferromagnetic behavior with increasing Fe concentration in HfO₂.

Figures 5(a) and (b) shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements as a function of temperature obtained at applied magnetic field of 100 Oe for pure HfO₂ and Fe-doped HfO₂ samples, respectively. The Curie temperature (Tc) of pure HfO₂ is estimated to be above 400 K (figure 5(a)), which is consistent with the earlier reports [8, 26].

Zoom In Zoom Out Reset image size

Figure 5(b) shows that the value of magnetization increases with Fe doping concentration as compared to pure HfO₂, which is also consistent with the magnetization versus field analysis. The ZFC and FC magnetization behavior of the Fe-doped HfO₂ samples shows that when temperature increases from about 100 to 400 K, the magnetization decreases very slowly but remains nonzero in the experimental range, which also confirms the ferromagnetic nature of the samples. Further, the ZFC and FC curves of the Fe-doped HfO₂ samples did not show any peak near 45 K and 84 K or abrupt transition near 120 K, which indicates the absence of other phases (such as α- Fe₂O₃ or Fe₃O₄) in Fe doped HfO₂ compounds, as recently reported by Sales and coworkers [12]. This also confirms that the Fe is doped at the Hf sites instead of generating the other phases in the structure, which is supported by the results obtained from Raman spectra (figure 2(b)). Therefore, Fe doping is found to be beneficial for the improvement of magnetization of HfO₂ system.