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Defect induced modification of structural, topographical and magnetic properties of zinc ferrite thin films by swift heavy ion irradiation
Nuclear Instruments and Methods in Physics Research B 396 (2017) 68–74
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Defect induced modification of structural, topographical and magnetic properties of zinc ferrite thin films by swift heavy ion irradiation Lisha Raghavan a,d, P.A. Joy b, B. Varma Vijaykumar c, R.V. Ramanujan c, M.R. Anantharaman a,⇑ a
Department of Physics, Cochin University of Science and Technology, Cochin 682022, India National Chemical Laboratory, Pune, India c School of Materials Science and Engineering, Nanyang Technological University, Singapore d Inter University Accelerator Center, New Delhi 110067, India b
a r t i c l e
i n f o
Article history: Received 10 June 2016 Received in revised form 16 December 2016 Accepted 23 January 2017 Available online 1 February 2017 Keywords: Zinc ferrite Swift heavy ion Spin glass
a b s t r a c t Swift heavy ion irradiation provides unique ways to modify physical and chemical properties of materials. In ferrites, the magnetic properties can change significantly as a result of swift heavy ion irradiation. Zinc ferrite is an antiferromagnet with a Neel temperature of 10 K and exhibits anomalous magnetic properties in the nano regime. Ion irradiation can cause amorphisation of zinc ferrite thin films; thus the role of crystallinity on magnetic properties can be examined. The influence of surface topography in these thin films can also be studied. Zinc ferrite thin films, of thickness 320 nm, prepared by RF sputtering were irradiated with 100 MeV Ag ions. Structural characterization showed amorphisation and subsequent reduction in particle size. The change in magnetic properties due to irradiation was correlated with structural and topographical effects of ion irradiation. A rough estimation of ion track radius is done from the magnetic studies. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Design and fabrication of new materials with novel properties is one of the main objectives of material scientists. Material scientists adopt a wide variety of processing techniques to produce such materials. One such technique is to modify the material properties by subjecting the material to ion irradiation. Ion beams can be employed for synthesis, modifications and characterization of materials. When ions traverse through a material, ions can impart their energy and momentum to the material. Depending on the energy of ions, two regimes can be defined viz, nuclear energy regime (below 10 keV/amu) and electronic energy regime (greater than 100 keV/amu). Slow energetic ions lose energy mainly via nuclear energy loss while high energetic ions lose energy via electronic energy loss. Ions can pass through the material when the range of ions is greater than the thickness of the material. Swift heavy ions can cause amorphisation, recrystallisation and nanostructuring in materials [1–2]. The formation of ion tracks by swift heavy ions depends on both the material properties and the energy of incoming ion [3]. Track formation occurs beyond a particular threshold of electronic energy loss, track size can be controlled by ion energy and fluence ⇑ Corresponding author. E-mail address: [email protected] (M.R. Anantharaman). http://dx.doi.org/10.1016/j.nimb.2017.01.046 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
[3,4]. The formation of ion tracks can greatly influence magnetic properties as the stress generated by the tracks can affect anisotropy and permeability [5–7]. Ion induced modifications of ferrites has been studied extensively by researchers [3,6,8–11]. Ferrites are an important class of material having spinel, inverse spinel or mixed spinel structure. The magnetic properties are structure sensitive and the degree of structural disorder determines the magnetic order. In a normal spinel structure represented by 3+ 2M2+ A Fe2BO4 , the divalent cation M occupies the tetrahedral vacancy (A site) and the trivalent cation occupies the octahedral vacancy (B site). The interaction of ions in A and B site of the spinel structure leads to JAB interaction giving rise to ferrimagnetic ordering while JAA and JBB lead to antiferromagnetic ordering. Small structural deviation can induce glassy behaviour. The threshold electronic energy loss for track formation in ferrites is 20 keV/nm [12]. Many studies have been carried out on ion induced modification in inverse and substituted ferrites which are already ferrimagnetic [5,8,11]. In nickel zinc ferrite (Ni0.65Zn0.375In0.25Ti0.025Fe1.70O4) and magnesium zinc ferrites (Mn0.75Zn0.18Fe2.07O4), ions are reported to cause structural and magnetic deviation [8,11]. Zinc ferrite (ZnFe2O4) is a normal spinel and exhibits antiferromagnetic properties in the bulk [13]. However, thin film and nano forms of zinc ferrite exhibit altogether different properties and this has been attributed to cation redistribution [14–19]. Shenoy et al. have observed room temperature superparamagnetic behaviour in
L. Raghavan et al. / Nuclear Instruments and Methods in Physics Research B 396 (2017) 68–74
ball milled zinc ferrite [14]. Hoffmann et al. observed glassy behaviour in nano zinc ferrite, the blocking temperature was found to be influenced by particle size [20]. Jeong et al. reported that nanocrystalline zinc ferrite exhibited ferrimagnetism up to 460 K and co-existent ferrimagnetic and antiferromagnetic ordering at 10 K. The competition between Fe ions among A and B sites to interact via JAB and JBB results in canted spin structure of Fe ions in B site [21]. Nakashima et al. have reported high magnetisation in zinc ferrite thin films prepared by RF sputtering [15]. Bohra et al. have studied the properties of zinc ferrite thin films prepared by RF sputtering and Pulsed Laser Deposition (PLD) [19]. Recently, Liang et al. have observed a magnetisation of 1 memu/cc in zinc ferrite thin films [22]. Authors of this paper have reported a magnetisation of 18 emu/cc at room temperature in zinc ferrite thin films of thickness 120 nm prepared by RF sputtering [23]. The altogether different properties of zinc ferrite, which depends mainly on cation redistribution, make zinc ferrite an interesting candidate for research. The effect of amorphisation induced by SHI in zinc ferrite thin films has not yet been studied and is the objective of this investigation. The defects and changes in surface morphology induced as a result of irradiation, which can alter the magnetic properties, is also interrogated in this study. 2. Experimental Zinc ferrite thin films were prepared by RF sputtering from a phase pure target synthesized by sol gel auto combustion method. The films were deposited on naturally oxidized Si substrates at an RF power of 150 W for 90 min. Cross sectional TEM operated at 200 kV in the imaging mode is used to determine the film thickness. The films were annealed at 600 °C and then irradiated using 100 MeV Ag ions at fluences of 1 1012, 1 1013 and 3 1013 ions/cm2. The irradiation was performed using 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi. The range and energy loss of 100 MeV Ag ions in zinc ferrite films were simulated using the SRIM code [24]. The electronic energy loss of 100 MeV Ag ions in zinc ferrite, as calculated, was 16 keV/nm (Fig. 1). The range of ions was 11 lm. Structural characterization was done using a Glancing Angle X Ray Diffractometer (GXRD) Bruker Discover D-8 with Cu Ka (k = 1.5406 Å) at a glancing angle of 10. The crystallite size D is calculated using the Scherrer formula , where k is the wavelength of X rays used, b full width D ¼ b0:9k cos h at half maximum and h is the diffraction angle. Atomic Force
Fig. 1. SRIM simulation of electronic and nuclear energy loss of Ag ions in zinc ferrite.
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Microscope (AFM) Nanoscope IIIa Digital Instruments, Veeco was employed to study the morphological evolution of sample with ion irradiation. Roughness and Power Spectral Density (PSD) were deduced from AFM images. Magnetic studies were carried out with a 7T SQUID VSM. M-H curves at room temperature and at 5 K were recorded. Field Cooled-Zero Field Cooled (FC-ZFC) measurements were performed from 300 K to 5 K at a cooling field of 50 Oe and 100 Oe.
3. Results and discussion The film thickness calculated from cross sectional TEM (Fig. 2) was found to be 320 nm. The range of ions is greater than film thickness, so the ions pass through the film and get terminated in the substrate. The GXRD images of pristine and irradiated films are shown in Fig. 3. Each sample was subjected to GXRD before and after irradiation to avoid sample to sample variation. The crystallite size calculated using the Scherrer formula, was found to be 21 nm for pristine film. The crystallite size varied with ion fluence. At a fluence of 1 1012 ions/cm2, crystallite size reduced to 15 nm and on irradiation at highest fluence of 3 1013 ions/cm2 crystallite size increased to 17 nm. The decrease in crystallite size with ion fluence has been observed in many systems and is explained on the basis of amorphisation of the material on irradiation [8,25]. The amorphisation of the crystalline material can be explained by the thermal spike model; the high energy imparted by ions creates localized high temperature zones (105 K) along the ion path, which can melt the material, this is followed by sudden cooling (1013 ps) resulting in amorphisation along the ion path [26,27]. However, in our study complete amorphisation have not taken place. The high energy imparted can also cause break up of crystallites. These crystallites can flow along the melted track and rejoin to form larger crystallite. The observed increase in crystallite size at higher fluence is attributed to aggregation of these broken crystallites [28]. This is further evident from the AFM analysis discussed in the following section. The 2D and 3D AFM images are shown in Figs. 4 and 5. The grain size was observed to decrease at lower fluence and increase at highest fluence, consistent with the GXRD studies. For the 3 1013 ions/cm2 irradiated samples one can observe agglomeration of grains, resulting in increase in grain size.
Fig. 2. Cross sectional TEM image of pristine zinc ferrite film.
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Fig. 3. GXRD images of pristine and irradiated films.
From AFM analysis, we can obtain information of rms roughness Rq, maximum roughness Rmax, average roughness Ra, skewness and kurtosis. Ra is the arithmetic average of the absolute values of the surface height deviations measured from the mean
plane. Rmax is the maximum vertical distance between the highest and lowest data points in the image following the plane fit. Rq is the root mean square average of height deviations taken from the mean image data plane. Skewness is indicative of symmetry in distribution of surface features. For a symmetric distribution of surface features having a Gaussian distribution, the skewness value is 0. A surface with flat peaks and deep valleys has a negative skewness while a surface with wide distribution of peaks has a positive skewness. The sharpness of surface features is given by the kurtosis value. The kurtosis value tends to 3 for a Gaussian surface; whereas a higher value of kurtosis indicates the presence of sharper surface features in the films. When the surface features are more or less flatly distributed, the kurtosis value will be less than 3 [29]. The rms roughness Rq, maximum roughness Rmax, average roughness Ra, skewness and kurtosis are tabulated in table 1. Rq and Rmax decreases at lower fluence and thereafter increases for the highest fluence. At the highest fluence the grains are agglomerated and cause an increase in surface roughness. The high energy imparted to the material initially breaks the grain and at higher fluence they diffuse through the surface and the grains are agglomerated. The formation of hills and valleys are also predominant at a fluence of 3 1013 ions/cm2. This may be because of surface diffusion due to irradiation. The skewness value increases at lower fluence and decreases for higher fluence. The film irradiated at 3 1013 ions/cm2 shows a skewness value slightly less than 0 and kurtosis value less than 3. The negative skewness indicates presence of hills and valleys in the film surface. A kurtosis value
Fig. 4. AFM images of a) pristine and film irradiated at fluences b) 1 1012 c) 1 1013 and d) 3 1013 ions/cm2.
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Fig. 5. 3D AFM images of a) pristine and film irradiated at fluences b) 1 1012 c) 1 1013 and d) 3 1013 ions/cm2.
Table 1 Roughness values of pristine and irradiated films. Sample
Rq
Ra
Rmax
Skewness
Kurtosis
Pristine 1 1012 1 1013 3 1013
3.61 ± 0.02 3.37 ± 0.01 3.31 ± 0.01 3.46 ± 0.04
2.88 ± 0.01 2.58 ± 0.01 2.64 ± 0.01 2.76 ± 0.03
27.6 ± 2.7 28.5 ± 2.6 25.9 ± 2.7 27.4 ± 3.7
0.220 ± 0.01 0.834 ± 0.01 0.286 ± 0.01 -0.0197 ± 0.01
3.01 ± 0.06 4.77 ± 0.02 3.11 ± 0.05 2.97 ± 0.01
of 3 is obtained for the pristine and 1 1013 ions/cm2 irradiated films and the skewness value is 0.2 which is indicative of symmetry in surface distribution. 1 1012 ions/cm2 irradiated sample has a skewness of 0.8 and kurtosis of 4. This denotes sharpness in the distribution of surface features. To understand the mechanism of surface evolution due to ion irradiation, the AFM images were subjected to power spectral density analysis. Power spectral density function provides a representation of the amplitude of surface roughness as a function of spatial frequency of the roughness. Power spectral density analysis decomposes the surface profile into spatial spectral frequencies and is quantitatively obtained by the Fourier transformation of the surface [30].
PSDðf Þ ¼
2 Z Z 2 1 d r if :r e hðrÞ h i 2 2p L
ð1Þ
where, L is the scan length, h(r) is the line profile, f is the spatial frequency. 2D isotropic PSD data is extracted from the AFM analysis software. The log-log plot of PSD usually shows a low frequency region representing noise and the high frequency region corresponds to surface evolution. This region obeys a power law given n
by PSDðf Þ ¼ Af , where n is the slope of the PSD spectra in the high frequency region [31]. The PSD spectra of pristine and irradiated films are shown in Fig. 6. The slope n value of the high frequency region indicates various surface evolution mechanisms. n values of
Fig. 6. PSD spectra of a) pristine and film irradiated at fluences b) 1 1012 c) 1 1013 and d) 3 1013 ions/cm2.
1, 2, 3 and 4 represents surface evolution due to plastic flow driven by surface tension, evaporation condensation, volume diffusion and surface diffusion [32,33]. For the pristine film the slope value is 2.4 and on increasing the fluence the slope decreases. The possible surface mechanism is evaporation condensation and the decrease in slope value with fluence indicates change of surface mechanism to plastic flow as a result of irradiation.
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M-H loops were measured at room temperature and at 5 K of pristine and irradiated films (Fig. 7). The pristine films show near saturation with a magnetisation of 45 emu/cc indicating ferrimagnetic behaviour of films. The 1 1012 ions/cm2 irradiated samples show almost similar behaviour as that of pristine sample. On irradiating the sample at a fluence of 1 1013 ions/cm2 magnetisation was lowered and at fluence of 3 1013 ions/cm2 magnetisation increased to 22 emu/cc. MH loops at 5 K is shown in Fig. 7b. Contrary to room temperature ferrimagnetic behaviour, the loops of pristine and film irradiated at 1 1012 ions/cm2 were not saturating even at the maximum applied field of 60,000 Oe. However films irradiated at 1 1013 ions/cm2 and 3 1013 ions/cm2 saturated at a field of 15,000 Oe. The variation of magnetisation with fluence at 5 K was similar to that of room temperature magnetisation with the pristine sample exhibiting higher value of saturation magnetisation. As mentioned earlier, Nakshima et al. have observed room temperature ferrimagnetism in zinc ferrite thin films [15] while Bohra et al. observed that the films were paramagnetic at room temperature [19]. Our earlier studies on zinc ferrite thin films prepared by RF sputtering exhibited room temperature ferrimagnetic behaviour [23]. The magnetism of zinc ferrite thin films is greatly influenced by the cation distribution in the octahedral and tetrahedral sites. The ferrimagnetic behaviour of zinc ferrite thin films is due to redistribution of Fe3+ ions into A site which leads to strong AOB interaction. The decrease in magnetisation with ion fluence can be attributed to ion induced defects which alters cation redistribution. GXRD and AFM results indicate that crystallite size decreases at initial fluence and increases at highest fluence. The decrease in crystallite size leads to surface state pinning of domains, which results in decrease in magnetisation. At a fluence of 3 1013 ions/cm2 crystallite size increases and surface state pinning of domains is reduced which can enhance magnetisation [6,34]. The lack of saturation at 5 K can be attributed to antiferromagnetic ordering of the films below the Neel temperature. On ion irradiation, saturation magnetisation decreases, and at higher fluences of 1 1013 ions/cm2 and 3 1013 ions/cm2 the films saturate at lower applied fields. The irradiation has decreased the antiferromagnetic component; however, the decrease in saturation magnetisation is due to decrease in grain size. The ion induced modification has been explained using the thermal spike model. Jitendra et al. have calculated the track radius in zinc ferrite using the relation [28,35]
t ¼ 0:94
ðse Þ0:578 E0:156
ð2Þ
where E is the energy in MeV and Se is the electronic energy loss in keV/nm and t the track radius in nm. Using this model, we obtain a track radius of 2.3 nm for 100 MeV Ag ions in zinc ferrite. The damage cross section arising due to ion irradiation can be found from magnetic measurements [36]. The variation of saturation magnetisation with fluence is related by the expression
Ms ¼ 1 expðAutÞ Ms;0
ð3Þ
where, Ms is the saturation magnetisation after irradiation, Mso saturation magnetisation of non irradiated sample, A is the damage cross section and ut is the ion fluence. The variation of magnetisation with fluence at room temperature is shown in Fig. 8. The damage cross section was obtained as (2.46 ± 0.13) 1013 cm2. The damage cross section is related to track radius by the expression [36]
Re ¼
rffiffiffiffi A
p
:
ð4Þ
The estimated track radius from magnetic measurements was obtained as 2.7 ± 0.07 nm. This value is in close agreement with that obtained from thermal spike calculation. The magnetic measurements at 5 K also shows similar trend and the track radius obtained was 2.8 ± 0.05 nm.
Fig. 8. Variation of magnetisation with fluence.
Fig. 7. MH loops of pristine and irradiated films a) at room temperature and b) at 5 K.
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Field Cooled-Zero Field Cooled (FC-ZFC) curves of pristine and irradiated samples for cooling fields of 50Oe and 100Oe are shown in Fig. 9. Tirr, the bifurcation temperature of FC and ZFC, shifts to lower temperatures on increasing cooling field. This is indicative of glassy ordering. The wide discrepancy between FC and ZFC curves is also indicative of glassy behaviour. Nakashima et al. have reported spin glass behaviour in zinc ferrite films [15]. The studies indicate that the cation redistribution is responsible for glassy behaviour. Our earlier report on zinc ferrite thin films suggest the existence of glassy behaviour [23]. The existence of superparamagnetic behaviour can be ruled out as the films exhibit a defined value of coercivity and also the films saturates as seen from the MH results. The interaction of Fe2+ ions in the A site leads to antiferromagnetic ordering and the interaction of Fe2+ ions in A and B sites leads to ferrimagnetic ordering. As a result of cation redistribution, the competing interaction of Fe ions in both A and B sites leads to the observed glassy behaviour in the system. The irradiated samples also shows glassy behaviour. For pristine and 3 1013 ions/ cm2 irradiated samples, the ZFC maxima exhibits a very broad peak. Tung et al. have reported ZFC peaks becomes broader with increasing particle size in zinc ferrite nanoparticles due to the presence of wide distribution of relaxation times for metastable magnetic state [37]. In the present case, the pristine sample has largest grain size; on irradiation at a fluence of 1 1012 ions/
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cm2, the grain size reduces and at higher fluence of 3 1013 ions/cm2, the grain size increases. The broadness of ZFC curve can be attributed to variation in grain size with irradiation.
4. Conclusions Zinc ferrite films of thickness 320 nm were prepared by RF sputtering; the films were annealed at 600 °C and then irradiated with 100 MeV Ag ions at fluences of 1 1012, 1 1013 and 3 1013 ions/cm2. Amorphisation of films were observed with irradiation, and for the highest fluence of 3 1013 ions/cm2, there was an increase in crystallite size. AFM images also indicate the reduction and then the increase of grain size. Power spectral density analysis indicates the possible mechanism of surface evolution as evaporation condensation for the pristine film and on irradiation the mechanism changes to plastic flow. The saturation magnetisation was observed to decrease with ion fluence which is due to surface state pinning of domains as a result of reduced particle size. The damage cross section and track radius was calculated from magnetic measurements and was found to be in agreement with that obtained from thermal spike calculations. Thus the structural, topographical and magnetic properties of zinc ferrite thin films were tailored by swift heavy ions.
Fig. 9. Field Cooled-Zero Field Cooled (FC-ZFC) of a) pristine and film irradiated at fluences of b) 1 1012 and c) 3 1013 ions/cm2 at different cooling fields.
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Acknowledgement LR and MRA are thankful to IUAC, New Delhi for providing financial assistance in the form of UFUP project (IUAC/XIII.7 / UFR-48306). LR and MRA is thankful to Dr. D K Avasthi, Dr. Indra Sulania, Dr. G B V S Lakshmi, V V Sivakumar, Scientist, Abhilash, Engineer, IUAC New Delhi and Dr. V Ganesan, Centre Director, Dr. Ram Janay Choudhary, Scientist E, Mr Pankaj Pandey, Research Scholar, UGC DAE CSR, Indore for help in carrying out characterizations and measurements. LR acknowledges Shareef Muhammad and Shamlath Alavi, Research Scholars, Central University of Kerala for their help during irradiation experiments. MRA also thank DST Nanomission for financial assistance. References [1] I.P. Jain, G. Agarwal, Ion beam induced surface and interface engineering, Surf. Sci. Rep. 66 (2011) 77. [2] D.K. Avasthi, G.K. Mehta, Swift Heavy Ions for Materials Engineering and Nanostructuring, Springer series, 2011, G K Mehta. [3] S.J. Zinkle, Track formation and dislocation loop interaction in spinel irradiated with swift heavy ions, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 141 (2008) 737. [4] A. Colder, O. Marty, B. Canut, M. Levalois, P. Marie, X. Portier, S.M.M. Ramos, M. Toulemonde, Latent track formation in germanium irradiated with 20 30 and 40 MeV fullerenes in the electronic regime, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 174 (2001) 491. [5] J.-M. Costantini, F. Studer, J.-C. Peuzin, Modifications of the magnetic properties of ferrites by swift heavy ion irradiations, J. Appl. Phys. 90 (2001) 126. [6] S.K. Sharma, R. Kumar, V.V. Siva Kumar, M. Knobel, V.R. Reddy, A. Gupta, M. Singh, Role of electronic energy loss on the magnetic properties of Mg0.95Mn0.05Fe2O4 nanoparticles, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 248 (2006) 37. [7] J.P. Nozieres, M. Ghidini, N.M. Dempsey, B. Gervais, D. Givord, G. Suran, J.M.D. Coey, Swift heavy ions for magnetic nanostructures, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 146 (1998) 250. [8] G. Dixit, J.P. Singh, R.C. Srivastava, H.M. Agrawal, Study of 200 MeV Ag15+ ion induced amorphisation in nickel ferrite thin films, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 269 (2011) 133. [9] R.C. Srivastava, J.P. Singh, H.M. Agrawal, R. Kumar, Tripathi, Tripathi R P, Reddy V R and Gupta A, 57 Fe Mössbauer investigation of nanostructured zinc ferrite irradiated by 100 MeV oxygen beam, J. Phys: Conf. Ser. 217 (2010), 01210912109. [10] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G. Marest, Influence of 85 MeV oxygen ion irradiation on magnetization behavior of micron-sized and nano-sized powders of strontium ferrite (SrFe2O3), J. Magn. Magn. Mater. 186 (1998) 342. [11] B.P. Rao, K.H. Rao, P.S.V. Subba Rao, A. Mahesh Kumar, Y.L.N. Murthy, K. Asokan, V.V. Siva Kumar, R. Kumar, N.S. Gajbhiye, O.F. Caltun, Swift heavy ions irradiation studies on some ferrite nanoparticles, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 244 (2006) 27. [12] F. Studer, C. Houpert, D. Groult, A. Meftah, M. Toulemonde, C. Cedex, Spontaneous magnetization induced in the spinel ZnFe204 by heavy ion irradiation in the electronic stopping power regime, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 82 (1993) 91. [13] F.G. Brockman, The cation distribution in ferrites with spinel structure, Phys. Rev. 77 (1950) 841. [14] S D Shenoy, P A Joy, M R Anantharaman, Effect of mechanical milling on the structural, magnetic and dielectric properties of coprecipitated ultrafine zinc ferrite, J. Magn. Magn. Mater. 269 (2004) 217.
[15] S. Nakashima, K. Fujita, K. Tanaka, K. Hirao, High magnetization and the hightemperature superparamagnetic transition with intercluster interaction in disordered zinc ferrite thin film, J. Phys.: Condens. Matter 17 (2005) 137. [16] K. Tanaka, S. Nakashima, K. Fujita, K. Hirao, Large Faraday effect in a short wavelength range for disordered zinc ferrite thin films, J. Appl. Phys. 99 (2006) 106103–106111. [17] S. Nakashima, K. Fujita, K. Tanaka, K. Hirao, T. Yamamoto, I. Tanaka, Thermal annealing effect on magnetism and cation distribution in disordered ZnFe2O4 thin films deposited on glass substrates, J. Magn. Magn. Mater. 310 (2007) 2543. [18] D. Guo, J. Zhu, Y. Yang, X. Fan, G. Chai, W. Sui, Z. Zhang, D. Xue, High-frequency magnetic properties of Zn ferrite films deposited by magnetron sputtering, J. Appl. Phys. 107 (2010), 043903-1. [19] M. Bohra, S. Prasad, N. Kumar, D.S. Misra, S.C. Sahoo, N. Venkataramani, R. Krishnan, Large room temperature magnetization in nanocrystalline zinc ferrite thin films, Appl. Phys. Lett. 88 (2006), 262506-1. [20] M. Hofmann, S.J. Campbell, H. Ehrhardt, R. Feyerherm, The magnetic behaviour of nanostructured zinc ferrite, J. Mater. Sci. 39 (2004) 5057. [21] J. Shim, S. Lee, J. Park, S.-J. Han, Y. Jeong, Y. Cho, Coexistence of ferrimagnetic and antiferromagnetic ordering in Fe-inverted zinc ferrite investigated by NMR, Phys. Rev. B 73 (2006), 064404-1. [22] Y. Liang, H. Hsia, Growth and crystallographic feature-dependent characterization of spinel zinc ferrite thin films by RF sputtering, Nanoscale Res. Lett. 1 (2013) 1. [23] L. Raghavan, G. Pookat, H. Thomas, S. Ojha, D.K. Avasthi, M.R. Anantharaman, Room temperature ferrimagnetism and low temperature disorder effects in zinc ferrite thin films, J. Magn. Magn. Mater. 385 (2015) 265. [24] J F Ziegler, J P Biersack, U Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, SRIM code: http://www.srim.org. [25] D.K. Shukla, R. Kumar, S. Mollah, R.J. Choudhary, P. Thakur, S.K. Sharma, N.B. Brookes, M. Knobel, Swift heavy ion irradiation induced magnetism in magnetically frustrated BiMn2O5 thin films, Phys. Rev. B 82 (2010) 174432– 174441. [26] G. Szenes, Z. Horváth, B. Pécz, F. Pászti, L. Tóth, Tracks induced by swift heavy ions in semiconductors, Phys. Rev. B 65 (2002), 045206-1. [27] C. Dufour, E. Paumier, M. Toulemonde, Electron-phonon coupling and the sensitivity of metals to irradiation with swift heavy ions, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 122 (1997) 68. [28] J. Pal, G. Dixit, H. Kumar, R.C. Srivastava, H.M. Agrawal, R. Kumar, Formation of latent tracks and their effects on the magnetic properties of nanosized zinc ferrite, J. Magn. Magn. Mater. 352 (2014) 36. [29] G. Yildirim, S. Bal, M. Gulen, Varilci, E. Budak, M. Akdogan, Substrate effect on microstructure and optical performance of sputter-deposited TiO2 thin films, Cryst. Res. Technol. 47 (2012) 195. [30] R. Gavrila, A. Dinescu, D. Mardare, A power spectral density study of thin films morphology based on AFM profiling, Rom. J. Inf. Sci. Technol. 10 (2007) 291. [31] P. Dash, P. Mallick, H. Rath, A. Tripathi, J. Prakash, D.K. Avasthi, S. Mazumder, S. Varma, P.V. Satyam, N.C. Mishra, Roughness and power spectral density study of SHI irradiated ultra-thin gold films, Appl. Surf. Sci. 256 (2009) 558. [32] C. Herring, Effect of change of scale on sintering phenomena, J. Appl. Phys. 21 (1950) 301. [33] W.W. Mullins, Flattening of a nearly plane solid surface due to capillarity, J. Appl. Phys. 30 (1959) 77. [34] R. Kumar, S.K. Sharma, A. Dogra, V.V. Siva Kumar, S.N. Dolia, A. Gupta, M. Knobel, M. Singh, Magnetic study of nanocrystalline ferrites and the effect of swift heavy ion irradiation, Hyperfine Interact. 160 (2005) 143. [35] T.A. Tombrello, Distribution of damage along an MeV ion track, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 83 (1993) 508. [36] J.M. Costantini, F. Rrisard, J.L. Flament, A. Meftah, M. Toulemonde, M. Hage-Ali, High energy heavy ion irradiation damage in yttrium iron garnet, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 65 (1992) 568. [37] L.D. Tung, V. Kolesnichenko, G. Caruntu, D. Caruntu, Y. Remond, V.O. Golub, C.J. O’Connor, L. Spinu, Annealing effects on the magnetic properties of nanocrystalline zinc ferrite, Phys. B 319 (2002) 116.
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Defect induced modification of structural, topographical and magnetic properties of zinc ferrite thin films by swift heavy ion irradiation Lisha Raghavan a,d, P.A. Joy b, B. Varma Vijaykumar c, R.V. Ramanujan c, M.R. Anantharaman a,⇑ a
Department of Physics, Cochin University of Science and Technology, Cochin 682022, India National Chemical Laboratory, Pune, India c School of Materials Science and Engineering, Nanyang Technological University, Singapore d Inter University Accelerator Center, New Delhi 110067, India b
a r t i c l e
i n f o
Article history: Received 10 June 2016 Received in revised form 16 December 2016 Accepted 23 January 2017 Available online 1 February 2017 Keywords: Zinc ferrite Swift heavy ion Spin glass
a b s t r a c t Swift heavy ion irradiation provides unique ways to modify physical and chemical properties of materials. In ferrites, the magnetic properties can change significantly as a result of swift heavy ion irradiation. Zinc ferrite is an antiferromagnet with a Neel temperature of 10 K and exhibits anomalous magnetic properties in the nano regime. Ion irradiation can cause amorphisation of zinc ferrite thin films; thus the role of crystallinity on magnetic properties can be examined. The influence of surface topography in these thin films can also be studied. Zinc ferrite thin films, of thickness 320 nm, prepared by RF sputtering were irradiated with 100 MeV Ag ions. Structural characterization showed amorphisation and subsequent reduction in particle size. The change in magnetic properties due to irradiation was correlated with structural and topographical effects of ion irradiation. A rough estimation of ion track radius is done from the magnetic studies. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Design and fabrication of new materials with novel properties is one of the main objectives of material scientists. Material scientists adopt a wide variety of processing techniques to produce such materials. One such technique is to modify the material properties by subjecting the material to ion irradiation. Ion beams can be employed for synthesis, modifications and characterization of materials. When ions traverse through a material, ions can impart their energy and momentum to the material. Depending on the energy of ions, two regimes can be defined viz, nuclear energy regime (below 10 keV/amu) and electronic energy regime (greater than 100 keV/amu). Slow energetic ions lose energy mainly via nuclear energy loss while high energetic ions lose energy via electronic energy loss. Ions can pass through the material when the range of ions is greater than the thickness of the material. Swift heavy ions can cause amorphisation, recrystallisation and nanostructuring in materials [1–2]. The formation of ion tracks by swift heavy ions depends on both the material properties and the energy of incoming ion [3]. Track formation occurs beyond a particular threshold of electronic energy loss, track size can be controlled by ion energy and fluence ⇑ Corresponding author. E-mail address: [email protected] (M.R. Anantharaman). http://dx.doi.org/10.1016/j.nimb.2017.01.046 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
[3,4]. The formation of ion tracks can greatly influence magnetic properties as the stress generated by the tracks can affect anisotropy and permeability [5–7]. Ion induced modifications of ferrites has been studied extensively by researchers [3,6,8–11]. Ferrites are an important class of material having spinel, inverse spinel or mixed spinel structure. The magnetic properties are structure sensitive and the degree of structural disorder determines the magnetic order. In a normal spinel structure represented by 3+ 2M2+ A Fe2BO4 , the divalent cation M occupies the tetrahedral vacancy (A site) and the trivalent cation occupies the octahedral vacancy (B site). The interaction of ions in A and B site of the spinel structure leads to JAB interaction giving rise to ferrimagnetic ordering while JAA and JBB lead to antiferromagnetic ordering. Small structural deviation can induce glassy behaviour. The threshold electronic energy loss for track formation in ferrites is 20 keV/nm [12]. Many studies have been carried out on ion induced modification in inverse and substituted ferrites which are already ferrimagnetic [5,8,11]. In nickel zinc ferrite (Ni0.65Zn0.375In0.25Ti0.025Fe1.70O4) and magnesium zinc ferrites (Mn0.75Zn0.18Fe2.07O4), ions are reported to cause structural and magnetic deviation [8,11]. Zinc ferrite (ZnFe2O4) is a normal spinel and exhibits antiferromagnetic properties in the bulk [13]. However, thin film and nano forms of zinc ferrite exhibit altogether different properties and this has been attributed to cation redistribution [14–19]. Shenoy et al. have observed room temperature superparamagnetic behaviour in
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ball milled zinc ferrite [14]. Hoffmann et al. observed glassy behaviour in nano zinc ferrite, the blocking temperature was found to be influenced by particle size [20]. Jeong et al. reported that nanocrystalline zinc ferrite exhibited ferrimagnetism up to 460 K and co-existent ferrimagnetic and antiferromagnetic ordering at 10 K. The competition between Fe ions among A and B sites to interact via JAB and JBB results in canted spin structure of Fe ions in B site [21]. Nakashima et al. have reported high magnetisation in zinc ferrite thin films prepared by RF sputtering [15]. Bohra et al. have studied the properties of zinc ferrite thin films prepared by RF sputtering and Pulsed Laser Deposition (PLD) [19]. Recently, Liang et al. have observed a magnetisation of 1 memu/cc in zinc ferrite thin films [22]. Authors of this paper have reported a magnetisation of 18 emu/cc at room temperature in zinc ferrite thin films of thickness 120 nm prepared by RF sputtering [23]. The altogether different properties of zinc ferrite, which depends mainly on cation redistribution, make zinc ferrite an interesting candidate for research. The effect of amorphisation induced by SHI in zinc ferrite thin films has not yet been studied and is the objective of this investigation. The defects and changes in surface morphology induced as a result of irradiation, which can alter the magnetic properties, is also interrogated in this study. 2. Experimental Zinc ferrite thin films were prepared by RF sputtering from a phase pure target synthesized by sol gel auto combustion method. The films were deposited on naturally oxidized Si substrates at an RF power of 150 W for 90 min. Cross sectional TEM operated at 200 kV in the imaging mode is used to determine the film thickness. The films were annealed at 600 °C and then irradiated using 100 MeV Ag ions at fluences of 1 1012, 1 1013 and 3 1013 ions/cm2. The irradiation was performed using 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi. The range and energy loss of 100 MeV Ag ions in zinc ferrite films were simulated using the SRIM code [24]. The electronic energy loss of 100 MeV Ag ions in zinc ferrite, as calculated, was 16 keV/nm (Fig. 1). The range of ions was 11 lm. Structural characterization was done using a Glancing Angle X Ray Diffractometer (GXRD) Bruker Discover D-8 with Cu Ka (k = 1.5406 Å) at a glancing angle of 10. The crystallite size D is calculated using the Scherrer formula , where k is the wavelength of X rays used, b full width D ¼ b0:9k cos h at half maximum and h is the diffraction angle. Atomic Force
Fig. 1. SRIM simulation of electronic and nuclear energy loss of Ag ions in zinc ferrite.
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Microscope (AFM) Nanoscope IIIa Digital Instruments, Veeco was employed to study the morphological evolution of sample with ion irradiation. Roughness and Power Spectral Density (PSD) were deduced from AFM images. Magnetic studies were carried out with a 7T SQUID VSM. M-H curves at room temperature and at 5 K were recorded. Field Cooled-Zero Field Cooled (FC-ZFC) measurements were performed from 300 K to 5 K at a cooling field of 50 Oe and 100 Oe.
3. Results and discussion The film thickness calculated from cross sectional TEM (Fig. 2) was found to be 320 nm. The range of ions is greater than film thickness, so the ions pass through the film and get terminated in the substrate. The GXRD images of pristine and irradiated films are shown in Fig. 3. Each sample was subjected to GXRD before and after irradiation to avoid sample to sample variation. The crystallite size calculated using the Scherrer formula, was found to be 21 nm for pristine film. The crystallite size varied with ion fluence. At a fluence of 1 1012 ions/cm2, crystallite size reduced to 15 nm and on irradiation at highest fluence of 3 1013 ions/cm2 crystallite size increased to 17 nm. The decrease in crystallite size with ion fluence has been observed in many systems and is explained on the basis of amorphisation of the material on irradiation [8,25]. The amorphisation of the crystalline material can be explained by the thermal spike model; the high energy imparted by ions creates localized high temperature zones (105 K) along the ion path, which can melt the material, this is followed by sudden cooling (1013 ps) resulting in amorphisation along the ion path [26,27]. However, in our study complete amorphisation have not taken place. The high energy imparted can also cause break up of crystallites. These crystallites can flow along the melted track and rejoin to form larger crystallite. The observed increase in crystallite size at higher fluence is attributed to aggregation of these broken crystallites [28]. This is further evident from the AFM analysis discussed in the following section. The 2D and 3D AFM images are shown in Figs. 4 and 5. The grain size was observed to decrease at lower fluence and increase at highest fluence, consistent with the GXRD studies. For the 3 1013 ions/cm2 irradiated samples one can observe agglomeration of grains, resulting in increase in grain size.
Fig. 2. Cross sectional TEM image of pristine zinc ferrite film.
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Fig. 3. GXRD images of pristine and irradiated films.
From AFM analysis, we can obtain information of rms roughness Rq, maximum roughness Rmax, average roughness Ra, skewness and kurtosis. Ra is the arithmetic average of the absolute values of the surface height deviations measured from the mean
plane. Rmax is the maximum vertical distance between the highest and lowest data points in the image following the plane fit. Rq is the root mean square average of height deviations taken from the mean image data plane. Skewness is indicative of symmetry in distribution of surface features. For a symmetric distribution of surface features having a Gaussian distribution, the skewness value is 0. A surface with flat peaks and deep valleys has a negative skewness while a surface with wide distribution of peaks has a positive skewness. The sharpness of surface features is given by the kurtosis value. The kurtosis value tends to 3 for a Gaussian surface; whereas a higher value of kurtosis indicates the presence of sharper surface features in the films. When the surface features are more or less flatly distributed, the kurtosis value will be less than 3 [29]. The rms roughness Rq, maximum roughness Rmax, average roughness Ra, skewness and kurtosis are tabulated in table 1. Rq and Rmax decreases at lower fluence and thereafter increases for the highest fluence. At the highest fluence the grains are agglomerated and cause an increase in surface roughness. The high energy imparted to the material initially breaks the grain and at higher fluence they diffuse through the surface and the grains are agglomerated. The formation of hills and valleys are also predominant at a fluence of 3 1013 ions/cm2. This may be because of surface diffusion due to irradiation. The skewness value increases at lower fluence and decreases for higher fluence. The film irradiated at 3 1013 ions/cm2 shows a skewness value slightly less than 0 and kurtosis value less than 3. The negative skewness indicates presence of hills and valleys in the film surface. A kurtosis value
Fig. 4. AFM images of a) pristine and film irradiated at fluences b) 1 1012 c) 1 1013 and d) 3 1013 ions/cm2.
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Fig. 5. 3D AFM images of a) pristine and film irradiated at fluences b) 1 1012 c) 1 1013 and d) 3 1013 ions/cm2.
Table 1 Roughness values of pristine and irradiated films. Sample
Rq
Ra
Rmax
Skewness
Kurtosis
Pristine 1 1012 1 1013 3 1013
3.61 ± 0.02 3.37 ± 0.01 3.31 ± 0.01 3.46 ± 0.04
2.88 ± 0.01 2.58 ± 0.01 2.64 ± 0.01 2.76 ± 0.03
27.6 ± 2.7 28.5 ± 2.6 25.9 ± 2.7 27.4 ± 3.7
0.220 ± 0.01 0.834 ± 0.01 0.286 ± 0.01 -0.0197 ± 0.01
3.01 ± 0.06 4.77 ± 0.02 3.11 ± 0.05 2.97 ± 0.01
of 3 is obtained for the pristine and 1 1013 ions/cm2 irradiated films and the skewness value is 0.2 which is indicative of symmetry in surface distribution. 1 1012 ions/cm2 irradiated sample has a skewness of 0.8 and kurtosis of 4. This denotes sharpness in the distribution of surface features. To understand the mechanism of surface evolution due to ion irradiation, the AFM images were subjected to power spectral density analysis. Power spectral density function provides a representation of the amplitude of surface roughness as a function of spatial frequency of the roughness. Power spectral density analysis decomposes the surface profile into spatial spectral frequencies and is quantitatively obtained by the Fourier transformation of the surface [30].
PSDðf Þ ¼
2 Z Z 2 1 d r if :r e hðrÞ h i 2 2p L
ð1Þ
where, L is the scan length, h(r) is the line profile, f is the spatial frequency. 2D isotropic PSD data is extracted from the AFM analysis software. The log-log plot of PSD usually shows a low frequency region representing noise and the high frequency region corresponds to surface evolution. This region obeys a power law given n
by PSDðf Þ ¼ Af , where n is the slope of the PSD spectra in the high frequency region [31]. The PSD spectra of pristine and irradiated films are shown in Fig. 6. The slope n value of the high frequency region indicates various surface evolution mechanisms. n values of
Fig. 6. PSD spectra of a) pristine and film irradiated at fluences b) 1 1012 c) 1 1013 and d) 3 1013 ions/cm2.
1, 2, 3 and 4 represents surface evolution due to plastic flow driven by surface tension, evaporation condensation, volume diffusion and surface diffusion [32,33]. For the pristine film the slope value is 2.4 and on increasing the fluence the slope decreases. The possible surface mechanism is evaporation condensation and the decrease in slope value with fluence indicates change of surface mechanism to plastic flow as a result of irradiation.
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M-H loops were measured at room temperature and at 5 K of pristine and irradiated films (Fig. 7). The pristine films show near saturation with a magnetisation of 45 emu/cc indicating ferrimagnetic behaviour of films. The 1 1012 ions/cm2 irradiated samples show almost similar behaviour as that of pristine sample. On irradiating the sample at a fluence of 1 1013 ions/cm2 magnetisation was lowered and at fluence of 3 1013 ions/cm2 magnetisation increased to 22 emu/cc. MH loops at 5 K is shown in Fig. 7b. Contrary to room temperature ferrimagnetic behaviour, the loops of pristine and film irradiated at 1 1012 ions/cm2 were not saturating even at the maximum applied field of 60,000 Oe. However films irradiated at 1 1013 ions/cm2 and 3 1013 ions/cm2 saturated at a field of 15,000 Oe. The variation of magnetisation with fluence at 5 K was similar to that of room temperature magnetisation with the pristine sample exhibiting higher value of saturation magnetisation. As mentioned earlier, Nakshima et al. have observed room temperature ferrimagnetism in zinc ferrite thin films [15] while Bohra et al. observed that the films were paramagnetic at room temperature [19]. Our earlier studies on zinc ferrite thin films prepared by RF sputtering exhibited room temperature ferrimagnetic behaviour [23]. The magnetism of zinc ferrite thin films is greatly influenced by the cation distribution in the octahedral and tetrahedral sites. The ferrimagnetic behaviour of zinc ferrite thin films is due to redistribution of Fe3+ ions into A site which leads to strong AOB interaction. The decrease in magnetisation with ion fluence can be attributed to ion induced defects which alters cation redistribution. GXRD and AFM results indicate that crystallite size decreases at initial fluence and increases at highest fluence. The decrease in crystallite size leads to surface state pinning of domains, which results in decrease in magnetisation. At a fluence of 3 1013 ions/cm2 crystallite size increases and surface state pinning of domains is reduced which can enhance magnetisation [6,34]. The lack of saturation at 5 K can be attributed to antiferromagnetic ordering of the films below the Neel temperature. On ion irradiation, saturation magnetisation decreases, and at higher fluences of 1 1013 ions/cm2 and 3 1013 ions/cm2 the films saturate at lower applied fields. The irradiation has decreased the antiferromagnetic component; however, the decrease in saturation magnetisation is due to decrease in grain size. The ion induced modification has been explained using the thermal spike model. Jitendra et al. have calculated the track radius in zinc ferrite using the relation [28,35]
t ¼ 0:94
ðse Þ0:578 E0:156
ð2Þ
where E is the energy in MeV and Se is the electronic energy loss in keV/nm and t the track radius in nm. Using this model, we obtain a track radius of 2.3 nm for 100 MeV Ag ions in zinc ferrite. The damage cross section arising due to ion irradiation can be found from magnetic measurements [36]. The variation of saturation magnetisation with fluence is related by the expression
Ms ¼ 1 expðAutÞ Ms;0
ð3Þ
where, Ms is the saturation magnetisation after irradiation, Mso saturation magnetisation of non irradiated sample, A is the damage cross section and ut is the ion fluence. The variation of magnetisation with fluence at room temperature is shown in Fig. 8. The damage cross section was obtained as (2.46 ± 0.13) 1013 cm2. The damage cross section is related to track radius by the expression [36]
Re ¼
rffiffiffiffi A
p
:
ð4Þ
The estimated track radius from magnetic measurements was obtained as 2.7 ± 0.07 nm. This value is in close agreement with that obtained from thermal spike calculation. The magnetic measurements at 5 K also shows similar trend and the track radius obtained was 2.8 ± 0.05 nm.
Fig. 8. Variation of magnetisation with fluence.
Fig. 7. MH loops of pristine and irradiated films a) at room temperature and b) at 5 K.
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Field Cooled-Zero Field Cooled (FC-ZFC) curves of pristine and irradiated samples for cooling fields of 50Oe and 100Oe are shown in Fig. 9. Tirr, the bifurcation temperature of FC and ZFC, shifts to lower temperatures on increasing cooling field. This is indicative of glassy ordering. The wide discrepancy between FC and ZFC curves is also indicative of glassy behaviour. Nakashima et al. have reported spin glass behaviour in zinc ferrite films [15]. The studies indicate that the cation redistribution is responsible for glassy behaviour. Our earlier report on zinc ferrite thin films suggest the existence of glassy behaviour [23]. The existence of superparamagnetic behaviour can be ruled out as the films exhibit a defined value of coercivity and also the films saturates as seen from the MH results. The interaction of Fe2+ ions in the A site leads to antiferromagnetic ordering and the interaction of Fe2+ ions in A and B sites leads to ferrimagnetic ordering. As a result of cation redistribution, the competing interaction of Fe ions in both A and B sites leads to the observed glassy behaviour in the system. The irradiated samples also shows glassy behaviour. For pristine and 3 1013 ions/ cm2 irradiated samples, the ZFC maxima exhibits a very broad peak. Tung et al. have reported ZFC peaks becomes broader with increasing particle size in zinc ferrite nanoparticles due to the presence of wide distribution of relaxation times for metastable magnetic state [37]. In the present case, the pristine sample has largest grain size; on irradiation at a fluence of 1 1012 ions/
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cm2, the grain size reduces and at higher fluence of 3 1013 ions/cm2, the grain size increases. The broadness of ZFC curve can be attributed to variation in grain size with irradiation.
4. Conclusions Zinc ferrite films of thickness 320 nm were prepared by RF sputtering; the films were annealed at 600 °C and then irradiated with 100 MeV Ag ions at fluences of 1 1012, 1 1013 and 3 1013 ions/cm2. Amorphisation of films were observed with irradiation, and for the highest fluence of 3 1013 ions/cm2, there was an increase in crystallite size. AFM images also indicate the reduction and then the increase of grain size. Power spectral density analysis indicates the possible mechanism of surface evolution as evaporation condensation for the pristine film and on irradiation the mechanism changes to plastic flow. The saturation magnetisation was observed to decrease with ion fluence which is due to surface state pinning of domains as a result of reduced particle size. The damage cross section and track radius was calculated from magnetic measurements and was found to be in agreement with that obtained from thermal spike calculations. Thus the structural, topographical and magnetic properties of zinc ferrite thin films were tailored by swift heavy ions.
Fig. 9. Field Cooled-Zero Field Cooled (FC-ZFC) of a) pristine and film irradiated at fluences of b) 1 1012 and c) 3 1013 ions/cm2 at different cooling fields.
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Acknowledgement LR and MRA are thankful to IUAC, New Delhi for providing financial assistance in the form of UFUP project (IUAC/XIII.7 / UFR-48306). LR and MRA is thankful to Dr. D K Avasthi, Dr. Indra Sulania, Dr. G B V S Lakshmi, V V Sivakumar, Scientist, Abhilash, Engineer, IUAC New Delhi and Dr. V Ganesan, Centre Director, Dr. Ram Janay Choudhary, Scientist E, Mr Pankaj Pandey, Research Scholar, UGC DAE CSR, Indore for help in carrying out characterizations and measurements. LR acknowledges Shareef Muhammad and Shamlath Alavi, Research Scholars, Central University of Kerala for their help during irradiation experiments. MRA also thank DST Nanomission for financial assistance. References [1] I.P. Jain, G. Agarwal, Ion beam induced surface and interface engineering, Surf. Sci. Rep. 66 (2011) 77. [2] D.K. Avasthi, G.K. Mehta, Swift Heavy Ions for Materials Engineering and Nanostructuring, Springer series, 2011, G K Mehta. [3] S.J. Zinkle, Track formation and dislocation loop interaction in spinel irradiated with swift heavy ions, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 141 (2008) 737. [4] A. Colder, O. Marty, B. Canut, M. Levalois, P. Marie, X. Portier, S.M.M. Ramos, M. Toulemonde, Latent track formation in germanium irradiated with 20 30 and 40 MeV fullerenes in the electronic regime, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 174 (2001) 491. [5] J.-M. Costantini, F. Studer, J.-C. Peuzin, Modifications of the magnetic properties of ferrites by swift heavy ion irradiations, J. Appl. Phys. 90 (2001) 126. [6] S.K. Sharma, R. Kumar, V.V. Siva Kumar, M. Knobel, V.R. Reddy, A. Gupta, M. Singh, Role of electronic energy loss on the magnetic properties of Mg0.95Mn0.05Fe2O4 nanoparticles, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 248 (2006) 37. [7] J.P. Nozieres, M. Ghidini, N.M. Dempsey, B. Gervais, D. Givord, G. Suran, J.M.D. Coey, Swift heavy ions for magnetic nanostructures, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 146 (1998) 250. [8] G. Dixit, J.P. Singh, R.C. Srivastava, H.M. Agrawal, Study of 200 MeV Ag15+ ion induced amorphisation in nickel ferrite thin films, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 269 (2011) 133. [9] R.C. Srivastava, J.P. Singh, H.M. Agrawal, R. Kumar, Tripathi, Tripathi R P, Reddy V R and Gupta A, 57 Fe Mössbauer investigation of nanostructured zinc ferrite irradiated by 100 MeV oxygen beam, J. Phys: Conf. Ser. 217 (2010), 01210912109. [10] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G. Marest, Influence of 85 MeV oxygen ion irradiation on magnetization behavior of micron-sized and nano-sized powders of strontium ferrite (SrFe2O3), J. Magn. Magn. Mater. 186 (1998) 342. [11] B.P. Rao, K.H. Rao, P.S.V. Subba Rao, A. Mahesh Kumar, Y.L.N. Murthy, K. Asokan, V.V. Siva Kumar, R. Kumar, N.S. Gajbhiye, O.F. Caltun, Swift heavy ions irradiation studies on some ferrite nanoparticles, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 244 (2006) 27. [12] F. Studer, C. Houpert, D. Groult, A. Meftah, M. Toulemonde, C. Cedex, Spontaneous magnetization induced in the spinel ZnFe204 by heavy ion irradiation in the electronic stopping power regime, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 82 (1993) 91. [13] F.G. Brockman, The cation distribution in ferrites with spinel structure, Phys. Rev. 77 (1950) 841. [14] S D Shenoy, P A Joy, M R Anantharaman, Effect of mechanical milling on the structural, magnetic and dielectric properties of coprecipitated ultrafine zinc ferrite, J. Magn. Magn. Mater. 269 (2004) 217.
[15] S. Nakashima, K. Fujita, K. Tanaka, K. Hirao, High magnetization and the hightemperature superparamagnetic transition with intercluster interaction in disordered zinc ferrite thin film, J. Phys.: Condens. Matter 17 (2005) 137. [16] K. Tanaka, S. Nakashima, K. Fujita, K. Hirao, Large Faraday effect in a short wavelength range for disordered zinc ferrite thin films, J. Appl. Phys. 99 (2006) 106103–106111. [17] S. Nakashima, K. Fujita, K. Tanaka, K. Hirao, T. Yamamoto, I. Tanaka, Thermal annealing effect on magnetism and cation distribution in disordered ZnFe2O4 thin films deposited on glass substrates, J. Magn. Magn. Mater. 310 (2007) 2543. [18] D. Guo, J. Zhu, Y. Yang, X. Fan, G. Chai, W. Sui, Z. Zhang, D. Xue, High-frequency magnetic properties of Zn ferrite films deposited by magnetron sputtering, J. Appl. Phys. 107 (2010), 043903-1. [19] M. Bohra, S. Prasad, N. Kumar, D.S. Misra, S.C. Sahoo, N. Venkataramani, R. Krishnan, Large room temperature magnetization in nanocrystalline zinc ferrite thin films, Appl. Phys. Lett. 88 (2006), 262506-1. [20] M. Hofmann, S.J. Campbell, H. Ehrhardt, R. Feyerherm, The magnetic behaviour of nanostructured zinc ferrite, J. Mater. Sci. 39 (2004) 5057. [21] J. Shim, S. Lee, J. Park, S.-J. Han, Y. Jeong, Y. Cho, Coexistence of ferrimagnetic and antiferromagnetic ordering in Fe-inverted zinc ferrite investigated by NMR, Phys. Rev. B 73 (2006), 064404-1. [22] Y. Liang, H. Hsia, Growth and crystallographic feature-dependent characterization of spinel zinc ferrite thin films by RF sputtering, Nanoscale Res. Lett. 1 (2013) 1. [23] L. Raghavan, G. Pookat, H. Thomas, S. Ojha, D.K. Avasthi, M.R. Anantharaman, Room temperature ferrimagnetism and low temperature disorder effects in zinc ferrite thin films, J. Magn. Magn. Mater. 385 (2015) 265. [24] J F Ziegler, J P Biersack, U Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, SRIM code: http://www.srim.org. [25] D.K. Shukla, R. Kumar, S. Mollah, R.J. Choudhary, P. Thakur, S.K. Sharma, N.B. Brookes, M. Knobel, Swift heavy ion irradiation induced magnetism in magnetically frustrated BiMn2O5 thin films, Phys. Rev. B 82 (2010) 174432– 174441. [26] G. Szenes, Z. Horváth, B. Pécz, F. Pászti, L. Tóth, Tracks induced by swift heavy ions in semiconductors, Phys. Rev. B 65 (2002), 045206-1. [27] C. Dufour, E. Paumier, M. Toulemonde, Electron-phonon coupling and the sensitivity of metals to irradiation with swift heavy ions, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 122 (1997) 68. [28] J. Pal, G. Dixit, H. Kumar, R.C. Srivastava, H.M. Agrawal, R. Kumar, Formation of latent tracks and their effects on the magnetic properties of nanosized zinc ferrite, J. Magn. Magn. Mater. 352 (2014) 36. [29] G. Yildirim, S. Bal, M. Gulen, Varilci, E. Budak, M. Akdogan, Substrate effect on microstructure and optical performance of sputter-deposited TiO2 thin films, Cryst. Res. Technol. 47 (2012) 195. [30] R. Gavrila, A. Dinescu, D. Mardare, A power spectral density study of thin films morphology based on AFM profiling, Rom. J. Inf. Sci. Technol. 10 (2007) 291. [31] P. Dash, P. Mallick, H. Rath, A. Tripathi, J. Prakash, D.K. Avasthi, S. Mazumder, S. Varma, P.V. Satyam, N.C. Mishra, Roughness and power spectral density study of SHI irradiated ultra-thin gold films, Appl. Surf. Sci. 256 (2009) 558. [32] C. Herring, Effect of change of scale on sintering phenomena, J. Appl. Phys. 21 (1950) 301. [33] W.W. Mullins, Flattening of a nearly plane solid surface due to capillarity, J. Appl. Phys. 30 (1959) 77. [34] R. Kumar, S.K. Sharma, A. Dogra, V.V. Siva Kumar, S.N. Dolia, A. Gupta, M. Knobel, M. Singh, Magnetic study of nanocrystalline ferrites and the effect of swift heavy ion irradiation, Hyperfine Interact. 160 (2005) 143. [35] T.A. Tombrello, Distribution of damage along an MeV ion track, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 83 (1993) 508. [36] J.M. Costantini, F. Rrisard, J.L. Flament, A. Meftah, M. Toulemonde, M. Hage-Ali, High energy heavy ion irradiation damage in yttrium iron garnet, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 65 (1992) 568. [37] L.D. Tung, V. Kolesnichenko, G. Caruntu, D. Caruntu, Y. Remond, V.O. Golub, C.J. O’Connor, L. Spinu, Annealing effects on the magnetic properties of nanocrystalline zinc ferrite, Phys. B 319 (2002) 116.