Eur. Phys. J. Appl. Phys. (2013) 61: 10302 DOI: 10.1051/epjap/2012120329
THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS
Regular Article
Optical and electrical characterization of Ni-doped orthoferrites thin films prepared by sol-gel process Feroz Ahmad Mir1,a , Javid A. Banday2 , Christian Chong3 , Pierre Dahoo3 , and Fayaz A. Najar4 1 2 3 4
Department of Physics, National Institute of Technology, Srinagar 190006, India Department of Chemistry, National Institute of Technology, Srinagar 190006, India ´ Faculty of Science, Department of Physics, University of Versailles (UVSQ), 45 av. des Etats-Unis, 78035 Versailles Cedex, France Department of Physics, University of Kashmir, Srinagar-190006, India Received: 18 August 2012 / Received in final form: 24 November 2012 / Accepted: 6 December 2012 c EDP Sciences 2013 Published online: 25 January 2013 – Abstract. This paper presents a low-temperature route for producing RFe0.6 Ni0.4 O3 (where R = Pr, Nd and Sm) thin films by an aqueous inorganic sol-gel process. The films produced were characterized by X-ray diffraction (XRD) for structural, four probes for electrical and UV-vis spectroscopy for optical properties. As-deposited films were amorphous and after annealing them at 650 ◦ C, crystallinity appears and shows an orthorhombic structure. From UV-vis spectroscopy, variation in optical band gap and transmission is seen with change of rare-earth ions. From electrical resistivity measurement, semiconducting behavior is observed. The difference in activation energy is observed. This variation could be due to the orthorhombic distortion caused by size of rare-earth ion and which may impact the Fe-O-Fe or Fe-O-Ni or Ni-O-Ni bond angle, and hence affects the single particle band width in the present system.
1 Introduction Rare-earth orthoferrites RFeO3 (R = lanthanide) constitute a family of Dzyaloshinsky interaction antiferromagnets which exhibit an unusual variety of magnetic properties and structural changes [1–3]. From a technological viewpoint, orthoferrites show a variety of physical properties, for instance good transparency, i.e., high magneto-optic figure of merit, high mobility of domain walls with the highest limit of the domain wall velocity, high Neel temperatures; which provide the opportunity to create a large family of various sensors as well as devices for optical communications and data processing [4]. Also, these compounds have been proposed for use in resonance-based devices such as logic and memory elements, and as lasers and light modulators in optical applications [5,6]. These materials are also known to be catalytically active for the complex oxidation of hydrocarbons and as combustion catalysts [7–9]. These compounds contain only trivalent metals in the structure, making them attractive systems for investigations of homovalent substitutions. In particular, the replacement of Fe3+ by other trivalent transition metal ion and its effect on structural, magnetic and other properties have been reported by several groups [10–14]. In several orthoferrites, the partial replacement of Fe by Ni or Cr leads to a reduction in a
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Neel temperature (TN ) from 750 K in LaFeO3 to 280 K in LaCrO3 . A reduction in cell parameters is also observed on successive replacement of iron by chromium/nickel as indicated by Vegard’s law [12,13]. On searching for the variation of the physical properties of orthoferrites (especially in thin film), it was found that, there is still need of study especially in the field of transport and spectroscopy. This encourages us to build up a skeleton for the present work like firstly to develop a simple route for thin preparation and then to study the effect of rare-earth ion on the structural, electrical and spectroscopic parameters of RFe0.6 Ni0.4 O3 using XRD, four-probe technique and UV-vis spectroscopy. This study could help in opening a new window (or existing) for thin film growth of orthoferrites and related compounds. Also this process can be easily scaled up to produce large area coatings of uniform and stable thin films for use in devices mentioned above.
2 Experimental 2.1 Preparation of RFe0.6 Ni0.4 O3 (where R = Pr, Nd and Sm) thin films All reagents were of analytical grade and used without further purification. The fabrication procedure of RFe0.6 Ni0.4 O3 (where R = Pr, Nd and Sm) is illustrated
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The European Physical Journal Applied Physics 0.006 mol of iron nitrate and 0.004 mol of nickel nitrate
0.01 mol of Rare-earth nitrate (R)
PrFe Ni O
0.02 mol of citric acid + 0.04 mol of glycine
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0.4
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210 002
Adding distilled water
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100 ml mixing Solution 0
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Drying at 60 -70 C
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2θ/Degree
sol
Coated on Glass substrate using Spin coater Annealing at different temperatures RFe0.6Ni0.4O3 thin films
20
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35
2θ/Degree
Fig. 1. Shows flow chart involved for thin-film preparation.
in the flowchart in Figure 1. Firstly, appropriate amounts of Pr(NO3 )3 · 6H2 O, Nd(NO3 )3 · 6H2 O, Sm(NO3 )3 · 6H2 O (purity 98.5%), Fe(NO3 )3 · 9H2 O, Ni(NO3 )2 · 6H2 O (purity 98.5%), citric acid (99%) and glycine (purity 99.5%) were added into distilled water. The mixtures were thoroughly stirred by magnetic mixer to eliminate the water at 60–70 ◦ C until the homogeneous sol-like solution was formed. For the solution preparation, a more detailed prescription is given in reference [15]. For RFe0.6 Ni0.4 O3 films, the substrates (pyrex, 1 × 1 cm2 ) were etched by the 10% HF solution for 3 min and dried. The coating solution as prepared above was coated with spin coater as follows: 1 step − 500 rpm, 10 s → 2 step − 1500 rpm, 20 s. The coated films were dried at 150 ◦ C for 1 h in a drying oven. Finally, the films were treated at 650 ◦ C for 3 h in an electrical furnace. X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advance X-ray diffraction using Cu Kα radiation. Atomic force microscopy (AFM) was done by using Nanoscope-E in contact mode. The electrical resistivity of films was measured using a standard four-probe method from 150 to 300 K. A Hitachi U3300 spectrophotometer (dual beam) was used to perform optical measurements in the present work.
3 Results and discussion 3.1 Structural study The powder X-ray diffraction patterns of as-deposited orthoferrite thin films show amorphous nature (not shown here). The films were then heated to higher temperature and then again XRD was performed. The onset of crystallization takes place at approximately 650 ◦ C. Figure 2 shows typical XRD patterns of representative Ni-doped
Fig. 2. XRD pattern of film annealed at 650 RFe0.6 Ni0.4 O3 (where R = Pr and Nd) in air.
◦
C of
orthoferrite [RFe0.6 Ni0.4 O3 , where R = Pr and Nd] films heated to 650 ◦ C for 3 h. It is seen that heating to 650 ◦ C and above resulted in complete crystallization of the orthoferrite films. The characteristic XRD reflections, (1 2 1) and (0 0 2), appeared in the region (2θ = 30◦ –50◦ ), in addition to other reflections corresponding to the perovskite structure. The films were randomly oriented (polycrystalline) and did not show any preferred crystallographic orientation. It is clear from these figures that there is a complete absence of reflections from phases other than the orthoferrites indicating compositionally pure samples. The crystalline orthoferrite phase forms directly without any phase segregation from the amorphous phase. In the present sol-gel process, the molecular level mixing of metal ions is retained at various stages of processing which enable the formation of desired compounds at relatively low temperatures. Further to note here that the broad halo seen in the 2Θ ≤ 25◦ is characteristic of the glass plate used to deposit these thin films (see Fig. 2). From the XRD patterns the unit cell parameters were calculated, which are listed in Table 1. As seen from Table 1, the “a” and “c” parameters decrease for rareearth ions (R) of smaller size. However, the “b” parameter shows only minor variations and it increases slightly. These structural variations are the result of the size mismatch between the trivalent rare-earth and Ni3+ /Fe3+ ions distorting the perovskite lattice to give optimum coordination to R ions [16,17]. Therefore, the decrease of the unit cell volume (V = a × b × c) agrees with the decrease of size volume of the involved rare-earth ions (r3+ Pr > r3+ Nd > r3+ Sm). The R-O bond length is enlarged by a cooperative buckling of corner shared Fe/NiO6 octahedra to match Fe/Ni-O bond length. The buckling of Fe/NiO6 results in a decreased metal-oxygenmetal bond angle from 180◦ and the orthoferrites having smaller rare-earth ions adopt the space group Pbnm. Figure 3 shows the AFM of (a) PrFe0.6 Ni0.4 O3 , (b) NdFe0.6 Ni0.4 O3 and (c) SmFe0.6 Ni0.4 O3 annealed thin
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F.A. Mir et al.: Optical and electrical characterization of Ni-doped orthoferrites thin films by sol-gel process Table 1. Shows different important parameters of RFe0.6 Ni0.4 O3 (where R = Pr, Nd and Sm) film annealed at 650 ◦ C in air. Compound PrFe0.6 Ni0.4 O3 NdFe0.6 Ni0.4 O3 SmFe0.6 Ni0.4 O3
Lattice (a) ˚ A 5.552 5.392 5.345
Lattice (b) ˚ A 5.563 5.5717 5.604
Lattice (c) ˚ A 7.862 7.713 7.669
Volume ˚ A3 242.82 230.43 229.71
Optical band gap Eg (eV) 3.65 3.59 3.47
Acivation energy Ea (meV) 121 148 134
Fig. 3. (Color online) Shows the AFM (scan area 2 × 2 μm) of (a) PrFe0.6 Ni0.4 O3 , (b) NdFe0.6 Ni0.4 O3 and (c) SmFe0.6 Ni0.4 O3 annealed at 650 ◦ C thin films respectively.
80
Sm
70
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60
Transmission %
10.0
50
(α. hv)1/2(eV-cm)1/2
films respectively. Image of PrFe0.6 Ni0.4 O3 shows (Fig. 3a) a smoother surface with smaller grains with root mean square (RMS) of around 5 nm. However, some nanopits are seen in the scanned area. Image of NdFe0.6 Ni0.4 O3 shows (Fig. 3b) a rough surface with larger grains with root mean square (RMS) of around 12 nm. Also image of SmFe0.6 Ni0.4 O3 shows (Fig. 3c) a smoother surface with smaller grains with root mean square (RMS) of around 7 nm. In most of the cases, it has been seen that the grain size and surface roughness can influence the different physical properties of the films [18].
7.5
Pr
40 30 20
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3.2 Optical study
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E(eV)
Figure 4 (inset) shows the optical transmission spectrum for the annealed (at 650 ◦ C) sample in the 200–800 nm wavelength range at room temperature. The SmFe0.6 Ni0.4 O3 film is found to have high levels of transparency (>70%) in the 450–750 nm range. However, there is decrease in transparency for NdFe0.6 Ni0.4 O3 (55% in 400–700 nm) and PrFe0.6 Ni0.4 O3 (45% in 550–750 nm) thin films. The 315–346 nm optical absorption edge (wavelength at which the percentage transmission is zero; λcut−off ) encountered in the present studies is in agreement with the values reported for the other transparent orthoferrites [19]. The absorption coefficient α(υ), near the edge of each spectrum, was calculated using the relation [20]; α(υ) = (1/d) ln(I0 /It ), (1) where d is the thickness of the film, and I0 and It are the intensities of incident and transmitted radiation, respectively. Mott and Davis proposed the following relation for
Fig. 4. (Color online) Shows UV-vis spectra for RFe0.6 Ni0.4 O3 (where R = Pr, Nd and Sm) thin films. Inset shows transmittance for the same films.
amorphous materials [21]; αhυ = B 2 (hυ − Eg )β ,
(2)
where Eg is the optical band gap, β is the index, which can have different values (2, 3, 1/2 and 1/3) corresponding to indirect allowed, in direct forbidden, direct allowed and direct forbidden transitions, respectively. B is a constant called the band tailing parameter and hυ is the incident photon energy. In glass samples, equation (2) depicts a straight line where β = 2. The variation of (αhυ)1/2 with hυ (Tauc plot) is shown in Figure 4 for these films. The values of Eg for the sample have been calculated by extrapolating the linear region of the curves to meet the
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√ t = (RA + RO )/ 2(RB + RO ),
7 6
ln(Reistivity)(Ohm-cm)
hυ axis at (αhυ)1/2 = 0 and are presented in Table 1. Since the band gap of pure rare-earth orthoferrites prepared by a similar method is 3.1 eV. However, the observed band gap in our case is 3.47 eV (for SmFe0.6 Ni0.4 O3 , 3.59 eV (for NdFe0.6 Ni0.4 O3 ) and 3.65 eV (for PrFe0.6 Ni0.4 O3 ), and seems greater than this value and shows red shift with decrease in size of rare-earth ion, which could be attributed to the nature of the excitation. The orthorhombic distortion could also be one of the reasons for this observation [22]. Also the Goldsmith factor or tolerance factor (t) for ABO3 -type perovskite is expressed as [23],
5 4 3
NdFe0.6Ni0.4O3
2 SmFe0.6Ni0.4O3
1 PrFe0.6Ni0.4O3
0
(3)
140
160
180
200
220
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T(K)
Fig. 5. Variation of resistivity vs temperature for RFe0.6 Ni0.4 O3 (where R = Pr, Nd and Sm) thin films in measured temperature range.
6
PrFe0.6Ni0.4O3 5 4
SmFe0.6Ni0.4O3 3 2 1 0 3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
-1
1000/K (K)
Fig. 6. Shows variation of ln(ρ) vs. 1000/T for these thin films.
Arrhenius relation. The activation energy can be calculated by the Arrhenius plot of the conductance as follows:
3.3 Electrical study A plot of resistance versus temperature for the RFe0.6 Ni0.4 O3 is shown in Figure 5. The resistances decrease with increasing temperature and show semiconducting behavior. The highest resistance was found for NdFe0.6 Ni0.4 O3 -based thin film, while the smallest for PrFe0.6 Ni0.4 O3 . Differences in resistance originate not only from the size variations of the R-cation but could also originate from the grain boundary resistance which is a main factor, influencing the conduction of a semiconductor [25,26]. Other factors like effect of substrate, thin-film processing, etc., may also be responsible for this behavior. Figure 6 depicts the relationship between ln(ρ) versus 1000/T for the present thin films. It indicates that the conductivity is thermally activated according to the
NdFe0.6Ni0.4O3
7
ln(Resistivity)(Ohm-cm)
where RA , RB and RO are the ionic radii of the A, B and oxygen ions respectively in this ABO3 perovskite (where A = rare earth, B = transition metal). This factor “t” characterizes the size mismatch that occurs when the A-site cations are too small to fill the space in the threedimensional network of BO6 octahedra. The bond-length mismatch created by a t < 1 is accommodated by cooperative rotations of the BO6 octahedra; these rotations are accompanied by a corresponding shift of the A cations. Since “t” lies between 1 < t < 1 (t = 1 = cubic and t < 1 orthorhombic phases), the BO6 tilting angle (ω) changes accordingly. Further, it is believed that this BO6 octahedra is not rigid, so any strain or pressure at B site may lead to in-phase or out-of-phase tilting of BO6 . In addition, Ni occupies the iron site of FeO6 octahedra, thus leading to the change of bond angle and bond length (as ionic radii of Fe3+ and Ni3+ are 0.056 and 0.132 nm respectively). On the other hand, Fe-O binding energy will decrease. It is favorable for the formation of more oxygen vacancy traps on the surface. The band-gap values fall in the range of a number of absorption features observed in the magneto-optic spectra of single crystals of single crystal orthoferrites. These can be attributed to the charge transfer or charge transfer enhanced crystal field transitions associated with the octahedrally coordinated Fe3+ ions [24].
or, ρ = ρ0 exp(ΔE/kB T ) σ = σ0 exp(−ΔE/kB T ),
(4) (5)
where σ is the conductance, kB is the Boltzmann’s constant, T is the absolute temperature and ΔE is the activation energy [20]. The activation energy obtained from Figure 6 is 148 meV, 134 meV and 121 meV for NdFe0.6 Ni0.4 O3 , SmFe0.6 Ni0.4 O3 and PrFe0.6 Ni0.4 O3 respectively. It showed that NdFe0.6 Ni0.4 O3 had the highest activation energy, while PrFe0.6 Ni0.4 O3 had the smallest activation energy. When Ni3+ partly substitutes for Fe3+ in RFeO3 , there will be some defects of acceptor in the crystal lattice and some acceptor levels will appear on the top of valence band. The difference in activation energy may be due to the difference in band gap between the valence band and the defect energy levels. Also the
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observed activated energy PrFe0.6 Ni0.4 O3 thin deposited on LaAlO3 by pulse later ablation was 111 meV, a little bit smaller than that calculated here [27]. Another possible reason for this observation may be the orthorhombic distortion which may be influenced by size of rare-earth ions [16,17]. Effect of distortion on single particle band width may also be one reason for this behavior. However, comprehensive detailed theoretical studies should be carried out to elucidate observations noticed in the present system.
4 Conclusion This paper reports a low-temperature and low economical route for producing Ni-doped orthoferrite thin films by an aqueous inorganic sol-gel process. Almost successful attempt has been made to produce good quality thin films of the above under study materials. The films produced were characterized by XRD for structural, four probes for electrical measurement and UV-vis for optical measurement. The following conclusion was drawn. 1. As a deposited film was amorphous, after heat treatment at 650 ◦ C crystallinity becomes visible, and orthorhombic structure is formed, which is confirmed by XRD data. Also the “a” and “c” parameters decrease for rare-earth ions of smaller size. However, the “b” parameter shows only minor variations and it increases slightly in the region Pr3+ -Sm3+ . 2. From UV-vis spectroscopy, SmFe0.6 Ni0.4 O3 shows the highest transmittance of around 75% from 450 to 750 nm. However, there is decrease in transparency for NdFe0.6 Ni0.4 O3 (55% in 400–700 nm) and PrFe0.6 Ni0.4 O3 (45% in 550–750 nm) thin films. In addition, a slight variation (red shift) in the optical band gap is observed with increase in size of rare-earth ions. 3. From dc conductivity measurement, a semiconducting behavior is observed. Further, the resistivity and activation energy are less for PrFe0.6 Ni0.4 O3 and greater for NdFe0.6 Ni0.4 O3 . The difference in activation energy may be due to the difference in band gap between the valence band and the defect energy levels. Size of rareearth ions and orthorhombic distortion may also influence the Fe-O-Fe or Fe-O-Ni or Ni-O-Ni angle, which consequently affects the single particle band width of the present system.
Authors would like to thank IUAC-New Delhi for providing experimental facilities. Authors also would like to thank Dr M. Ikram (NIT Srinagar, India) for his encouragement during this work.
References 1. M. Eibschutz, S. Shtrikman, D. Treves, Phys. Rev. 156, 562 (1967) 2. R.L. White, J. Appl. Phys. 40, 1061 (1968) 3. N.S. Ovanesyan, A.T. Trukhtanov, JETP Lett. Engl. Transl. 17, 67 (1973) 4. P. Ripka, Magnetic Sensors and Magnetometers (Artech House, Boston, Mass, 2001), p. 425 5. C. Vasques, P. Kogerler, A. Lopez-Quintela, J. Mater. Res. 13, 451 (1998) 6. A.S. Moskvin, N.S. Ovanesyan, V.A. Trukhtanov, Hyperfine Interact. 1, 265 (1975) 7. G. Karlsson, Electrochim. Acta 30, 1555 (1985) 8. T. Nekamura, G. Petzow, L.J. Gaucker, Mater. Res. Bull. 14, 649 (1979) 9. J. Mizusaki, T. Sisamoto, W.K. Cannon, H. Kent Bowen, J. Am. Ceram. Soc. 65, 363 (1982) 10. N.S. Ovanesyan, V.A. Trukhtanov, Proceedings of the International Conference on Mossbauer Spectroscopy, Fifth, 1975, p. 157 11. H. Taguchi, J. Solid State Chem. 131, 108 (1997) 12. A. Belayachi, E. Loudghiri, M. El Yamani, M. Nouges, J.L. Dormann, M. Taibi, Ann. Chim. Sci. Mater. 23, 297 (1998) 13. A. Belayachi, M. Nogues, J.L. Dormann, M. Taibi, Eur. J. Solid State Inorg. Chem. 33, 1039 (1996) 14. Y. Nishihara, J. Phys. Soc. Jpn. 38, 710 (1975) 15. Y. Wang, J. Zhu, L. Zhang, X. Yang, L. Lu, X. Wang, Y. Wang, J. Zhu, L. Zhang, X. Yang, L. Lu, X. Wang, Mater. Lett. 60, 1767 (2006) 16. J.B. Goodenough, J.M. Longo, Magnetic Oxides and Related Compounds (Springer-verlag, New York, 1970) 17. D.J. Craik, Structure and Properties of Magnetic Materials (Pion Ltd, London, UK, 1971) 18. F. Ahmad Mir, M. Ikram, R. Kumar, Appl. Radiat. Isotopes 70, 2409 (2012) 19. A. Hartridge, M. Ghanashyam Krishna, A.K. Bhattacharya, J. Phys. Chem. Solids 59, 859 (1998) 20. E.A. Davis, N.F. Mott, Philos. Mag. 22, 903 (1970) 21. N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, 2nd edn. (Clarendon Press, Oxford, 1979) 22. J.-S. Zhou, J.B. Goodenough, Phys. Rev. Lett. 96, 247202 (2006) 23. J.L.G. Fierro, L.G. Tejuca (eds.), Properties and Applications of Perovskite-type Oxides (Marcel Dekker, New York, 1993) 24. F.J. Kahn, P.S. Pershan, J.P. Remeika, Phys. Rev. Lett. 21, 804 (1968) 25. J.C.C. Abrantes, J.A. Labrincha, J.R. Frade, J. Eur. Ceram. Soc. 20, 603 (2000) 26. H. Suo, J. Wang, E. Wu, G. Liu, B. Xu, M. Zhao, J. Solid State Chem. 130, 52 (1997) 27. F. Ahmad Mir, M. Ikram, R. Kumar, Philosophical Magazine 92, 1058 (2012)
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