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Effect of 200 MeV Ag15+ ion irradiation on structural and magnetic properties of Mg0.95Mn0.05Fe2O4 ferrite thin film
Surface & Coatings Technology 203 (2009) 2707–2711
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of 200 MeV Ag15+ ion irradiation on structural and magnetic properties of Mg0.95Mn0.05Fe2O4 ferrite thin film S.K. Sharma a, Shalendra Kumar b,d,⁎, Alimuddin b, M. Knobel a, R.J. Choudhary c, D.M. Phase c, C.G. Lee d, Ravi Kumar e a
Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, (UNICAMP) Campinas, 13.083-970, SP, Brazil Department of Applied Physics, Aligarh Muslim University, Aligarh 202 002, India UGC-DAE Consortium for Scientific Research, Indore 452 017, India d School of Nano & Advanced Materials Engineering, Changwon National University, 9 Sarim dong, Changwon-641-773, South Korea e Materials Science Division, Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India b c
a r t i c l e
i n f o
Available online 9 March 2009 PACS: 75.50.Gg 61.80.-x 75.60.Ox 68.37.Ps Keywords: Ferrite thin films Irradiation effects Superparamagnetism Magnetization Magnetic force microscopy
a b s t r a c t Nanocrystalline Mg0.95Mn0.05Fe2O4 ferrite thin films, prepared by pulsed laser deposition technique on a glass substrate coated with indium tin oxide, are irradiated with 200 MeV Ag15+ ions at different fluence values in the range from 1 × 1011 to 1 × 1012 ions/cm2. The as-deposited and irradiated thin films are investigated using X-ray diffraction, dc magnetization and atomic force microscopy techniques. X-ray diffraction analysis of the as-deposited as well as irradiated thin film indicates the single phase cubic structure as the main composition. The crystallite size evaluated from Scherrer's equation is found to be decreased from 26 nm for as-deposited thin films to 17 nm for irradiated at a fluence of 1 × 1012 ions/cm2. The decrease in crystallite size in all the thin film samples after irradiation indicates a distortion in the lattice structure caused by stress-induced defects. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations have been recorded in a low field of 100 Oe and they show a typical behavior of superparamagnetic particles. This is further supported by the magnetization hysteresis (M–H) curve taken at 300 K, for the as-deposited thin film, which shows zero coercivity and remanence. The blocking temperatures calculated from the maxima of ZFC are found to decrease with the increase in irradiation fluence, which is consistent with XRD results. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the recent past, there has been a lot of interest in the study of nanocrystalline ferrite thin films due to their wide range of technological applications in various fields such as audio and video recording heads, memory devices, electronic devices, digital systems etc. This interest stems partly from the fact that the magnetic properties in the case of ferrite thin films are quite different from the bulk counterparts of the same nominal compositions [1–5]. Several deposition techniques such as an RF magnetron sputtering method, sol–gel pyrolysis method, and molecular beam epitaxial are used to synthesize ferrite thin films [6–8]. The pulsed laser deposition (PLD) technique is one of the widely used techniques to grow thin films of such materials. In PLD, the main advantage is the preservation of the stoichiometry of the bulk target, if the conditions during
⁎ Corresponding author. Department of Applied Physics, Aligarh Muslim University, Aligarh 202 002, UP, India. E-mail address: [email protected] (S. Kumar). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.130
deposition are judiciously picked. The growth of thin films using PLD [9] is now an extensively used and most popular technique for the processing of complex multicomponent thin films ranging from high temperature superconductors [10] to biomaterials [11]. Ferrite thin films have been grown using PLD [12,13] and these films have been found to have magnetic properties in good agreement with bulk systems [13]. It has been well established that the structural and magnetic properties of ferrite materials are extremely sensitive to external pressure or stress/strain induced in the system, originated in various ways such as changing the ionic radii of the dopant atoms, creating oxygen vacancies or more recently by swift heavy ion (SHI) irradiation [14–16]. The effect of SHI irradiation indicates significant modifications in their structural and magnetic properties. SHI irradiation provides several interesting and unique aspects in understanding the damage structure and material modification. The effect of energetic ions on the materials depends on the ion energy, fluence and ion species. In fact, it is well established and reported that swift heavy ions lose their energy via two nearly independent processes. As swift heavy ions pass through the material, the ions either excite or ionize the atoms by inelastic collisions or displace atoms of the target by
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elastic collisions. Elastic collisions are dominant in the low energy regime, whereas inelastic collision processes dominate at high-energy regimes where elastic collisions are insignificant. From the literature, it is evident that electronic energy loss Se due to inelastic collision is able to generate point/cluster defects if Se is less than the threshold value of electronic energy loss Seth. If Se is greater than Seth, then the energetic ions can create a columnar amorphization. The stress/strain developed due to the created defects and amorphization is responsible for the modification in the different properties of the materials [17–20]. In the present paper, we are presenting the results of the influence of 200 MeV Ag15+ ions on the structural and magnetic properties of Mg0.95Mn0.05Fe2O4 nanocrystalline ferrite thin film deposited by PLD technique. 2. Experimental details The polycrystalline bulk target of Mg0.95Mn0.05Fe2O4 was synthesized by conventional solid-state reaction technique (for details see Ref. [21]). The KrF excimer laser (Lambda Physik model COMPEX-201) of wavelength 248 nm and pulse duration of 20 ns was employed for the deposition on glass substrates coated with indium tin oxide (ITO). During the deposition, the laser energy density at the target surface was kept at 2 J/cm2 and the repetition rate at 10 Hz. The substrate temperature was maintained at 300 °C and the background pressure was 1 mTorr of oxygen during the deposition. The focused laser beam was incident on the target surface at an angle of 45°. The target was rotated at about 10 rpm and the substrate was mounted opposite to the target at a distance of 5.5 cm. The substrate was mounted on a heater plate using a silver paint. After deposition, the thin films were cooled slowly to room temperature at the rate of 5 °C/min, maintaining the oxygen pressure in the vacuum chamber to 1 mTorr. The film thickness was about 170 nm as measured by Tallystep profilometer (Ambios Inc USA) with 0.5 nm resolution. For irradiation, the deposited thin film was cut into four pieces each 5 mm × 5 mm in size. This set of four pieces was used for irradiation and further study in order to keep the growth conditions uniform for all the studied samples. One piece of the film was kept asdeposited, while the other pieces of the film were irradiated at room temperature with a 200 MeV Ag15+ ion beam using the 15 UD tandem accelerator at the Inter-University Accelerator Centre, New Delhi, with different fluence values of 1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2. The irradiation was performed under high vacuum condition (base pressure, 2 × 10− 6 Torr). The incident angle of the ion beam was kept slightly away from the surface normal of the sample to avoid channeling effects and also the beam current was kept at 0.1 pnA (particle nano ampere, ~ 6.25 × 108 ions/cm2/s) to avoid heating. The ion beam was focused to a spot of 1 mm diameter and scanned over the entire area of the film using a magnetic scanner. The fluence values were determined by measuring the charge arriving on the sample surface, using secondary electron suppression geometry. The sample current was measured with a digital current integrator and a scaler counter. The structure of the thin films was examined by standard θ/2θ X-ray diffraction (XRD) geometry using Bruker AXE D8 X-ray diffractometer with Cu Kα radiation at room temperature. Magnetization measurements were performed using a Quantum Design SQUID magnetometer. The magnetizations versus field loops were measured at room temperature by applying the maximum field of 2 kOe. The diamagnetic contribution from ITO was subtracted from the measured data by measuring the M–H loop of the same substrate having similar dimensions to that of the thin film samples in the same field range. The uncertainty in measuring the absolute value of magnetization for the film was about 1%. The magnetic domain structure was investigated using magnetic force microscopy (MFM) (Digital Nanoscope-III).
3. Results and discussion Fig. 1 shows the XRD patterns of Mg0.95Mn0.05Fe2O4 as-deposited and thin films irradiated at different fluences of 1 × 1011, 5 × 1011, and 1 × 1012 ions/cm2 respectively. The XRD pattern for the as-deposited and irradiated thin films shows the single-phase cubic spinel structure. The average grain size was estimated from the broadening of X-ray diffraction peak using Scherrer's equation, d=
0:89λ β cos θB
where β = (β2M − β2i )1/2. Here λ is the X-ray wavelength (1.54 Å for Cu Kα), βM and βi are the measured and instrumental broadening in radians respectively and θB is the Bragg's angle in degrees. The calculated average grain size for the as-deposited thin film sample is about ~ 26 ± 3 nm. It is observed that the structure remains practically intact after irradiation by 200 MeV Ag15+ ions. However, we observed that after irradiation, the peak intensities decreased slightly, while the broadening of peaks increased. From these observations, it is clear that the SHI irradiation has generated some defect states in the system. Also, the peak positions are shifted to somewhat higher 2θ values, indicating a compression in lattice structure. From the XRD results of irradiated thin films, the calculated average grain size decreased systematically from 26 nm to 15 nm for the fluence of 1 × 1011 ions/ cm2 and then to 13 nm and 11 nm for 5 × 1011 and 1 × 1012 ions/cm2 respectively with an uncertainty of ±3 nm. The decrease in grain size in all the thin film samples after irradiation indicates a distortion in the lattice structure caused by stress-induced defects.
Fig. 1. X-ray diffraction pattern of as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films.
S.K. Sharma et al. / Surface & Coatings Technology 203 (2009) 2707–2711
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Fig. 2. M–T curves in zero-field-cooled (ZFC) and field-cooled (FC) modes taken in an applied magnetic field of 100 Oe for as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films with fluences (1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2).
To further study the effect of 200 MeV Ag15+ ion irradiation on the magnetic behavior of these thin film samples, we obtained the magnetization (M) versus temperature (T) curves in the range from 50 to 300 K under an external magnetic field of 100 Oe recorded in the zero-field-cooled (ZFC) and field-cooled (FC) modes for the asdeposited film as well as for the thin films irradiated at different fluences (see Fig. 2(a–d)). In the ZFC mode the sample was cooled in zero field from 300 K to 50 K and after stabilization of the temperature, a measuring field of 100 Oe was applied. The data were then recorded while heating the sample. In the FC mode the sample was cooled from 300 K to 50 K in the presence of a magnetic field of 100 Oe and then the measurements were carried out while heating the sample in the same field. From Fig. 2, we clearly see the bifurcation of ZFC and FC curves at a certain temperature (see Fig. 2(a–d)), which is one of the characteristic features of a superparamagnetic (SPM) system. However, a coinciding broad maximum was observed on the ZFC curves at a slightly lower temperature (denoted as TMEAN) than TSEP. Such behavior usually signifies a certain particle size distribution in the as-deposited thin film, wherein a fraction of the largest particles already freeze at TSEP, the majority fraction of the nanocrystallites is being blocked at TMEAN, resulting in a distribution of the blocking temperatures TB in these thin film samples. The maxima of ZFC curve are located at TMEAN = 240 K with an uncertainty of ±5 K for the asdeposited thin film sample. After irradiation by 200 MeV Ag15+ ions, the blocking temperature decreases systematically and reaches a
value of 210 K at the highest fluence (1 × 1012 ions/cm2). This may be attributed to the reduction in grain size after irradiation, as observed in the X-ray studies.
Fig. 3. M–H hysteresis curve for as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films with fluences (1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2) at 300 K.
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To further see the superparamagnetic behavior, we have taken the M–H curve at room temperature for the as-deposited samples, as well as those irradiated at 1 × 1011 and 1 × 1012 ions/cm2 at room temperature (see Fig. 3). Clearly, there is no magnetic hysteresis observed at room temperature and the curve passes through the origin (H = 0). Also, the curve gets saturated in a field of 2 KOe. However, it is clearly seen that after irradiation by SHI, the magnetization increases systematically with irradiation fluence and the curve does not saturate even in the same applied magnetic field. In addition to this, the curves get saturated in the same applied field in contrast with that of the as-deposited thin film. This may be due to the redistribution of cations after irradiation by SHI, as has been earlier reported in the literature [21]. In order to further authenticate our results, we have also performed the magnetic force microscopy (MFM) measurements at various lift heights to study the effects of 200 MeV Ag15+ ions on magnetic domains. Fig. 4 shows the results of magnetic force microscopy (MFM) measured at a lift at 20 nm for the as-deposited and irradiated Mg0.95Mn0.05Fe2O4 thin films. From the analysis of MFM data, we have calculated the corresponding root mean square (RMS) phase shifts after irradiation. The value of phase shift is found to be increased from 0.348° for the asdeposited thin film to 0.693° for films irradiated with the fluence of 1 × 1011°ions/cm2, which undoubtedly indicates that the magnetic signal increases with irradiation. Further increase in the irradiation
fluence to 1 × 1011°ions/cm2, causes the phase shift to increase systematically and have a value of 1.26° at the highest fluence. The observed results are in good agreement with the M–H curves. This is further clearly seen using magnetic contrast in Fig. 4, where the films are irradiated with 200 MeV Ag15+ and the magnetic contrast seems to increase with the increase in the fluence. 4. Conclusions To summarize, we have shown that 200 MeV Ag15+ irradiation can modify the structural and magnetic properties of Mg0.95Mn0.05Fe2O4 thin films. XRD studies indicate that irradiation causes a reduction in grain size, presumably due to distortion in lattice structure caused by stress-induced defects. This is further supported by our magnetization results where the blocking temperature decreases with irradiation fluence. The increase in magnetization after irradiation is explained on the basis of redistribution of cations after irradiations. The observed results are consistent with the MFM data. Acknowledgement The authors are thankful to the Director and Pelletron Group of Inter-University Accelerator Centre, New Delhi for providing the
Fig. 4. Magnetic force microscopy (MFM) images taken at a lift height of 20 nm for (a) as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films with fluences of (b) 1 × 1011 ions/cm2, (c) 5 × 1011 ions/cm2 and (d) 1 × 1012 ions/cm2.
S.K. Sharma et al. / Surface & Coatings Technology 203 (2009) 2707–2711
necessary characterization and irradiation facilities. This work was supported by the Korean Research Foundation Grant funded by the Korea Government (MOEHRD) (KRF – 2008 – 005 – J02703). Authors (SKS & MK)) are very grateful to FAPESP and CNPq (Brazil) for providing financial support (Grant No.06/06792-2). References [1] J. Dash, S. Prasad, N. Venkataramani, P. Kishan, N. Kumar, S.D. Kulkarani, S.K. Date, J. Appl. Phys. 86 (1999) 3303. [2] B.R. Acarya, R. Krishan, S. Prasad, N. Venkataramani, A. Ajan, N. Shringi, Appl. Phys. Lett. 64 (1994) 1579. [3] A. Morisako, M. Matsumoto, M. Naoe, IEEE Trans. Magn. 24 (1998) 3024. [4] T.L. Hylton, M.A. Parker, J.K. Howard, Appl. Phys. Lett. 61 (1992) 867. [5] X. Sui, M.H. Kryder, Appl. Phys. Lett. 63 (1993) 1582. [6] H.S. Cho, M.H. Kim, H.J. Kim, J. Mater. Res. 9 (9) (1994) 2425. [7] S. Venzke, R.B. van Dover, J.M. Philips, E.M. Gyorgy, T. Siegrist, C.H. Chen, D. Werder, R.M. Fleming, R.J. Felder, E. Coleman, R. Opila, J. Mater. Res. 11 (5) (1996) 1187. [8] Ravi Kumar, M. Wasi Khan, J.P. Srivastava, S.K. Arora, R.G.S. Sofin, R.J. Choudhary, I.V. Shevets, J. Appl. Phys. 100 (2006) 100703. [9] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [10] H.S. Newman, D.B. Chrisey, J.S. Horwitz, B.D. Weaver, M.E. Reeves, IEEE Trans. Magn. 27 (1991) 2540.
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[11] C.M. Cotell, D.B. Chrisey, K.S. Grabowski, J.S. Sprague, C.R. Gossett, J. Biomaterials 3 (1992) 87. [12] C.A. Carosella, D.B. Chrisey, P. Lubitz, J.S. Horwitz, P. Dorsey, R. Seed, C. Vitoria, J. Appl. Phys. 71 (1992) 5107. [13] C.M. Williams, D.B. Chrisey, P. Lubitz, K.S. Grabowski, C.M. Cotell, J. Appl. Phys. 75 (1994) 1676. [14] J.J. Neumeier, M.F. Hundley, J.D. Thompson, R.H. Heffner, Phys. Rev. B 52 (1995) R7006. [15] AnjaliS Ogale, S.R. Shinde, V.N. Kulkarani, J. Higgins, R.J. Choudhary, Drashan C. Kundaliya, T. Pelleto, S.B. Ogale, R.L. Greene, T. Venkatesan, Phys. Rev. B 69 (2004) 235101. [16] S.B. Ogale, K. Ghosh, J.Y. Gu, R. Shreekala, S.R. Shinde, M. Downes, M. Rajeswari, R.P. Sharma, R.L. Green, T. Venkatesan, R. Ramesh, Ravi Bathe, S.I. Patil, Ravi Kumar, S.K. Arora, G.K. Mehta, J. Appl. Phys. 84 (1988) 6255. [17] F. Studer, M. Toulmonde, Nucl. Instrum. Methods B 65 (1992) 560. [18] C. Houpert, F. Studer, D. Groult, M. Toulmonde, Nucl. Instrum. Methods B 39 (1989) 720Y723. [19] R. Kumar, S.B. Samantra, S.K. Arora, A. Gupta, D. Kanjilal, R. Pinto, A.V. Narlikar, Solid State Commun. 106 (12) (1998) 805Y810. [20] R. Kumar, S.K. Arora, D. Kanjilal, G.K. Mehta, R. Bache, S.K. Date, S.R. Shinde, L.V. Saraf, S.B. Ogale, S.I. Patil, Radiat. Eff. Defects Solids 147 (1999) 187. [21] F. Studer, C.h. Houpert, D. Groult, J.Y. Fan, A. Meftah, M. Toulemonde, Nucl. Instrum. Methods B 82 (1993) 91.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of 200 MeV Ag15+ ion irradiation on structural and magnetic properties of Mg0.95Mn0.05Fe2O4 ferrite thin film S.K. Sharma a, Shalendra Kumar b,d,⁎, Alimuddin b, M. Knobel a, R.J. Choudhary c, D.M. Phase c, C.G. Lee d, Ravi Kumar e a
Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, (UNICAMP) Campinas, 13.083-970, SP, Brazil Department of Applied Physics, Aligarh Muslim University, Aligarh 202 002, India UGC-DAE Consortium for Scientific Research, Indore 452 017, India d School of Nano & Advanced Materials Engineering, Changwon National University, 9 Sarim dong, Changwon-641-773, South Korea e Materials Science Division, Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India b c
a r t i c l e
i n f o
Available online 9 March 2009 PACS: 75.50.Gg 61.80.-x 75.60.Ox 68.37.Ps Keywords: Ferrite thin films Irradiation effects Superparamagnetism Magnetization Magnetic force microscopy
a b s t r a c t Nanocrystalline Mg0.95Mn0.05Fe2O4 ferrite thin films, prepared by pulsed laser deposition technique on a glass substrate coated with indium tin oxide, are irradiated with 200 MeV Ag15+ ions at different fluence values in the range from 1 × 1011 to 1 × 1012 ions/cm2. The as-deposited and irradiated thin films are investigated using X-ray diffraction, dc magnetization and atomic force microscopy techniques. X-ray diffraction analysis of the as-deposited as well as irradiated thin film indicates the single phase cubic structure as the main composition. The crystallite size evaluated from Scherrer's equation is found to be decreased from 26 nm for as-deposited thin films to 17 nm for irradiated at a fluence of 1 × 1012 ions/cm2. The decrease in crystallite size in all the thin film samples after irradiation indicates a distortion in the lattice structure caused by stress-induced defects. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations have been recorded in a low field of 100 Oe and they show a typical behavior of superparamagnetic particles. This is further supported by the magnetization hysteresis (M–H) curve taken at 300 K, for the as-deposited thin film, which shows zero coercivity and remanence. The blocking temperatures calculated from the maxima of ZFC are found to decrease with the increase in irradiation fluence, which is consistent with XRD results. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the recent past, there has been a lot of interest in the study of nanocrystalline ferrite thin films due to their wide range of technological applications in various fields such as audio and video recording heads, memory devices, electronic devices, digital systems etc. This interest stems partly from the fact that the magnetic properties in the case of ferrite thin films are quite different from the bulk counterparts of the same nominal compositions [1–5]. Several deposition techniques such as an RF magnetron sputtering method, sol–gel pyrolysis method, and molecular beam epitaxial are used to synthesize ferrite thin films [6–8]. The pulsed laser deposition (PLD) technique is one of the widely used techniques to grow thin films of such materials. In PLD, the main advantage is the preservation of the stoichiometry of the bulk target, if the conditions during
⁎ Corresponding author. Department of Applied Physics, Aligarh Muslim University, Aligarh 202 002, UP, India. E-mail address: [email protected] (S. Kumar). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.130
deposition are judiciously picked. The growth of thin films using PLD [9] is now an extensively used and most popular technique for the processing of complex multicomponent thin films ranging from high temperature superconductors [10] to biomaterials [11]. Ferrite thin films have been grown using PLD [12,13] and these films have been found to have magnetic properties in good agreement with bulk systems [13]. It has been well established that the structural and magnetic properties of ferrite materials are extremely sensitive to external pressure or stress/strain induced in the system, originated in various ways such as changing the ionic radii of the dopant atoms, creating oxygen vacancies or more recently by swift heavy ion (SHI) irradiation [14–16]. The effect of SHI irradiation indicates significant modifications in their structural and magnetic properties. SHI irradiation provides several interesting and unique aspects in understanding the damage structure and material modification. The effect of energetic ions on the materials depends on the ion energy, fluence and ion species. In fact, it is well established and reported that swift heavy ions lose their energy via two nearly independent processes. As swift heavy ions pass through the material, the ions either excite or ionize the atoms by inelastic collisions or displace atoms of the target by
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S.K. Sharma et al. / Surface & Coatings Technology 203 (2009) 2707–2711
elastic collisions. Elastic collisions are dominant in the low energy regime, whereas inelastic collision processes dominate at high-energy regimes where elastic collisions are insignificant. From the literature, it is evident that electronic energy loss Se due to inelastic collision is able to generate point/cluster defects if Se is less than the threshold value of electronic energy loss Seth. If Se is greater than Seth, then the energetic ions can create a columnar amorphization. The stress/strain developed due to the created defects and amorphization is responsible for the modification in the different properties of the materials [17–20]. In the present paper, we are presenting the results of the influence of 200 MeV Ag15+ ions on the structural and magnetic properties of Mg0.95Mn0.05Fe2O4 nanocrystalline ferrite thin film deposited by PLD technique. 2. Experimental details The polycrystalline bulk target of Mg0.95Mn0.05Fe2O4 was synthesized by conventional solid-state reaction technique (for details see Ref. [21]). The KrF excimer laser (Lambda Physik model COMPEX-201) of wavelength 248 nm and pulse duration of 20 ns was employed for the deposition on glass substrates coated with indium tin oxide (ITO). During the deposition, the laser energy density at the target surface was kept at 2 J/cm2 and the repetition rate at 10 Hz. The substrate temperature was maintained at 300 °C and the background pressure was 1 mTorr of oxygen during the deposition. The focused laser beam was incident on the target surface at an angle of 45°. The target was rotated at about 10 rpm and the substrate was mounted opposite to the target at a distance of 5.5 cm. The substrate was mounted on a heater plate using a silver paint. After deposition, the thin films were cooled slowly to room temperature at the rate of 5 °C/min, maintaining the oxygen pressure in the vacuum chamber to 1 mTorr. The film thickness was about 170 nm as measured by Tallystep profilometer (Ambios Inc USA) with 0.5 nm resolution. For irradiation, the deposited thin film was cut into four pieces each 5 mm × 5 mm in size. This set of four pieces was used for irradiation and further study in order to keep the growth conditions uniform for all the studied samples. One piece of the film was kept asdeposited, while the other pieces of the film were irradiated at room temperature with a 200 MeV Ag15+ ion beam using the 15 UD tandem accelerator at the Inter-University Accelerator Centre, New Delhi, with different fluence values of 1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2. The irradiation was performed under high vacuum condition (base pressure, 2 × 10− 6 Torr). The incident angle of the ion beam was kept slightly away from the surface normal of the sample to avoid channeling effects and also the beam current was kept at 0.1 pnA (particle nano ampere, ~ 6.25 × 108 ions/cm2/s) to avoid heating. The ion beam was focused to a spot of 1 mm diameter and scanned over the entire area of the film using a magnetic scanner. The fluence values were determined by measuring the charge arriving on the sample surface, using secondary electron suppression geometry. The sample current was measured with a digital current integrator and a scaler counter. The structure of the thin films was examined by standard θ/2θ X-ray diffraction (XRD) geometry using Bruker AXE D8 X-ray diffractometer with Cu Kα radiation at room temperature. Magnetization measurements were performed using a Quantum Design SQUID magnetometer. The magnetizations versus field loops were measured at room temperature by applying the maximum field of 2 kOe. The diamagnetic contribution from ITO was subtracted from the measured data by measuring the M–H loop of the same substrate having similar dimensions to that of the thin film samples in the same field range. The uncertainty in measuring the absolute value of magnetization for the film was about 1%. The magnetic domain structure was investigated using magnetic force microscopy (MFM) (Digital Nanoscope-III).
3. Results and discussion Fig. 1 shows the XRD patterns of Mg0.95Mn0.05Fe2O4 as-deposited and thin films irradiated at different fluences of 1 × 1011, 5 × 1011, and 1 × 1012 ions/cm2 respectively. The XRD pattern for the as-deposited and irradiated thin films shows the single-phase cubic spinel structure. The average grain size was estimated from the broadening of X-ray diffraction peak using Scherrer's equation, d=
0:89λ β cos θB
where β = (β2M − β2i )1/2. Here λ is the X-ray wavelength (1.54 Å for Cu Kα), βM and βi are the measured and instrumental broadening in radians respectively and θB is the Bragg's angle in degrees. The calculated average grain size for the as-deposited thin film sample is about ~ 26 ± 3 nm. It is observed that the structure remains practically intact after irradiation by 200 MeV Ag15+ ions. However, we observed that after irradiation, the peak intensities decreased slightly, while the broadening of peaks increased. From these observations, it is clear that the SHI irradiation has generated some defect states in the system. Also, the peak positions are shifted to somewhat higher 2θ values, indicating a compression in lattice structure. From the XRD results of irradiated thin films, the calculated average grain size decreased systematically from 26 nm to 15 nm for the fluence of 1 × 1011 ions/ cm2 and then to 13 nm and 11 nm for 5 × 1011 and 1 × 1012 ions/cm2 respectively with an uncertainty of ±3 nm. The decrease in grain size in all the thin film samples after irradiation indicates a distortion in the lattice structure caused by stress-induced defects.
Fig. 1. X-ray diffraction pattern of as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films.
S.K. Sharma et al. / Surface & Coatings Technology 203 (2009) 2707–2711
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Fig. 2. M–T curves in zero-field-cooled (ZFC) and field-cooled (FC) modes taken in an applied magnetic field of 100 Oe for as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films with fluences (1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2).
To further study the effect of 200 MeV Ag15+ ion irradiation on the magnetic behavior of these thin film samples, we obtained the magnetization (M) versus temperature (T) curves in the range from 50 to 300 K under an external magnetic field of 100 Oe recorded in the zero-field-cooled (ZFC) and field-cooled (FC) modes for the asdeposited film as well as for the thin films irradiated at different fluences (see Fig. 2(a–d)). In the ZFC mode the sample was cooled in zero field from 300 K to 50 K and after stabilization of the temperature, a measuring field of 100 Oe was applied. The data were then recorded while heating the sample. In the FC mode the sample was cooled from 300 K to 50 K in the presence of a magnetic field of 100 Oe and then the measurements were carried out while heating the sample in the same field. From Fig. 2, we clearly see the bifurcation of ZFC and FC curves at a certain temperature (see Fig. 2(a–d)), which is one of the characteristic features of a superparamagnetic (SPM) system. However, a coinciding broad maximum was observed on the ZFC curves at a slightly lower temperature (denoted as TMEAN) than TSEP. Such behavior usually signifies a certain particle size distribution in the as-deposited thin film, wherein a fraction of the largest particles already freeze at TSEP, the majority fraction of the nanocrystallites is being blocked at TMEAN, resulting in a distribution of the blocking temperatures TB in these thin film samples. The maxima of ZFC curve are located at TMEAN = 240 K with an uncertainty of ±5 K for the asdeposited thin film sample. After irradiation by 200 MeV Ag15+ ions, the blocking temperature decreases systematically and reaches a
value of 210 K at the highest fluence (1 × 1012 ions/cm2). This may be attributed to the reduction in grain size after irradiation, as observed in the X-ray studies.
Fig. 3. M–H hysteresis curve for as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films with fluences (1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2) at 300 K.
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To further see the superparamagnetic behavior, we have taken the M–H curve at room temperature for the as-deposited samples, as well as those irradiated at 1 × 1011 and 1 × 1012 ions/cm2 at room temperature (see Fig. 3). Clearly, there is no magnetic hysteresis observed at room temperature and the curve passes through the origin (H = 0). Also, the curve gets saturated in a field of 2 KOe. However, it is clearly seen that after irradiation by SHI, the magnetization increases systematically with irradiation fluence and the curve does not saturate even in the same applied magnetic field. In addition to this, the curves get saturated in the same applied field in contrast with that of the as-deposited thin film. This may be due to the redistribution of cations after irradiation by SHI, as has been earlier reported in the literature [21]. In order to further authenticate our results, we have also performed the magnetic force microscopy (MFM) measurements at various lift heights to study the effects of 200 MeV Ag15+ ions on magnetic domains. Fig. 4 shows the results of magnetic force microscopy (MFM) measured at a lift at 20 nm for the as-deposited and irradiated Mg0.95Mn0.05Fe2O4 thin films. From the analysis of MFM data, we have calculated the corresponding root mean square (RMS) phase shifts after irradiation. The value of phase shift is found to be increased from 0.348° for the asdeposited thin film to 0.693° for films irradiated with the fluence of 1 × 1011°ions/cm2, which undoubtedly indicates that the magnetic signal increases with irradiation. Further increase in the irradiation
fluence to 1 × 1011°ions/cm2, causes the phase shift to increase systematically and have a value of 1.26° at the highest fluence. The observed results are in good agreement with the M–H curves. This is further clearly seen using magnetic contrast in Fig. 4, where the films are irradiated with 200 MeV Ag15+ and the magnetic contrast seems to increase with the increase in the fluence. 4. Conclusions To summarize, we have shown that 200 MeV Ag15+ irradiation can modify the structural and magnetic properties of Mg0.95Mn0.05Fe2O4 thin films. XRD studies indicate that irradiation causes a reduction in grain size, presumably due to distortion in lattice structure caused by stress-induced defects. This is further supported by our magnetization results where the blocking temperature decreases with irradiation fluence. The increase in magnetization after irradiation is explained on the basis of redistribution of cations after irradiations. The observed results are consistent with the MFM data. Acknowledgement The authors are thankful to the Director and Pelletron Group of Inter-University Accelerator Centre, New Delhi for providing the
Fig. 4. Magnetic force microscopy (MFM) images taken at a lift height of 20 nm for (a) as-deposited and 200 MeV Ag15+ ion irradiated Mg0.95Mn0.05Fe2O4 thin films with fluences of (b) 1 × 1011 ions/cm2, (c) 5 × 1011 ions/cm2 and (d) 1 × 1012 ions/cm2.
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necessary characterization and irradiation facilities. This work was supported by the Korean Research Foundation Grant funded by the Korea Government (MOEHRD) (KRF – 2008 – 005 – J02703). Authors (SKS & MK)) are very grateful to FAPESP and CNPq (Brazil) for providing financial support (Grant No.06/06792-2). References [1] J. Dash, S. Prasad, N. Venkataramani, P. Kishan, N. Kumar, S.D. Kulkarani, S.K. Date, J. Appl. Phys. 86 (1999) 3303. [2] B.R. Acarya, R. Krishan, S. Prasad, N. Venkataramani, A. Ajan, N. Shringi, Appl. Phys. Lett. 64 (1994) 1579. [3] A. Morisako, M. Matsumoto, M. Naoe, IEEE Trans. Magn. 24 (1998) 3024. [4] T.L. Hylton, M.A. Parker, J.K. Howard, Appl. Phys. Lett. 61 (1992) 867. [5] X. Sui, M.H. Kryder, Appl. Phys. Lett. 63 (1993) 1582. [6] H.S. Cho, M.H. Kim, H.J. Kim, J. Mater. Res. 9 (9) (1994) 2425. [7] S. Venzke, R.B. van Dover, J.M. Philips, E.M. Gyorgy, T. Siegrist, C.H. Chen, D. Werder, R.M. Fleming, R.J. Felder, E. Coleman, R. Opila, J. Mater. Res. 11 (5) (1996) 1187. [8] Ravi Kumar, M. Wasi Khan, J.P. Srivastava, S.K. Arora, R.G.S. Sofin, R.J. Choudhary, I.V. Shevets, J. Appl. Phys. 100 (2006) 100703. [9] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [10] H.S. Newman, D.B. Chrisey, J.S. Horwitz, B.D. Weaver, M.E. Reeves, IEEE Trans. Magn. 27 (1991) 2540.
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