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Magnetic behaviour of nanocrystalline Ni–Cu ferrite and the effect of irradiation by 100 MeV Ni ions
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Current Applied Physics 8 (2008) 620–625 www.elsevier.com/locate/cap www.kps.or.kr
Magnetic behaviour of nanocrystalline Ni–Cu ferrite and the effect of irradiation by 100 MeV Ni ions S.N. Dolia a,*, Ravi Kumar b, S.K. Sharma c, M.P. Sharma a, Subhash Chander d, M. Singh c b
a Department of Physics, University of Rajasthan, Jaipur 302 004, India Material Science Division, Inter-University Accelerator Centre, New Delhi 110 067, India c Department of Physics, H.P. University, Shimla 171 005, India d Sobhasaria Engineering College, Sikar 332 021, India
Received 14 April 2007; received in revised form 2 October 2007; accepted 6 November 2007 Available online 17 November 2007
Abstract The effects of 100 MeV Ni ion irradiation on magnetic properties of nanoparticles of Ni0.8Cu0.2Fe2O4 with average particle sizes of ˚ and 60 A ˚ , synthesized by chemical co-precipitation method have been studied. The spinel cubic structures were confirmed by XRD. 40 A The average particle size estimated by XRD and by Langevin function fitting are in good agreement for both the pristine and irradiated samples. The blocking temperature increases with particle size and does not change after irradiation. On irradiation by 100 MeV Ni ions, significant changes in the hysteresis loop features are observed, which may be attributed to formation of cluster of defects in the nanocrystalline samples due to swift heavy ion (SHI) irradiation. It is also found that SHI irradiation produces more dominant changes in the ˚ as compared to that of 60 A ˚. hysteresis loop of smaller particle size of 40 A Ó 2007 Elsevier B.V. All rights reserved. PACS: 75.75.+a; 75.50.Tt; 75.50.G; 61.80.Jh Keywords: Magnetic properties; Nanoparticles; Spinel ferrites; SHI irradiations
1. Introduction SHI irradiation is known to generate controlled defects of various types such as point, cluster and columnar defects in materials [1] and for the past two decades, the magnetic oxides and ferrites have been extensively studied [2–4]. The nature of defects is highly dependent upon the electronic energy loss (Se) in the materials. If the Se is less than the threshold value, Seth, then only the point/cluster of defects will be generated in the materials [2] which alter the cation distribution, produce strain/stress in the lattice structure and amorphize materials. The possible modifications are, in general, responsible to modify the magnetic properties in magnetic oxide materials. In recent years, *
Corresponding author. Tel.: +91 1412706378; fax: +91 1412711049. E-mail address: [email protected] (S.N. Dolia).
1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.11.004
nanocrystalline magnetic materials have been investigated extensively due to their unique properties such as superparamagnetism and spin glass behaviour which are generally attributed to surface (rather than the volume) disorder [5]. There have been reports on increase/decrease of magnetization in bulk spinel ferrites after irradiation by high energy heavy ions attributed to cation displacements induced by amorphization–recrystalline processes [4]. It is also reported that increase in coercive force [6], magnetization enhancement [7] and changes in hysteresis loop features [8] etc. in small particles of ferrites on SHI irradiation. The present work has been taken up to study the effect of 100 Mev Ni ions irradiation on the size dependent magnetic properties of nanocrystalline Ni0.8Cu0.2Fe2O4 ferrites synthesized by chemical co-precipitation method.
S.N. Dolia et al. / Current Applied Physics 8 (2008) 620–625
A ferrofluid comprised of Ni0.8Cu0.2Fe2O4 in the nanosize range has been prepared using wet chemical process using oleic acid as surfactant and kerosene has been used as the dispersing medium. In order to obtain narrow distribution of particle sizes, the resulting fluid was centrifuged at 12,000 rpm. For obtaining dried particles of this ferrite, carrier liquid from the ferrofluid has been removed by repetitive washing with acetone [9]. The powder so obtained was then heated at two temperatures of 425 °C and 450 °C. The two samples are hereafter referred as S40 and S60, respectively. X-Ray diffraction pattern has been recorded at 300 K using Fe Ka radiation, on a Philips make powder diffractometer PW-1840. The dc magnetization measurements of nanoparticles were performed using PARC make vibrating sample magnetometer (VSM) model 155. The samples were irradiated with 100 MeV Ni ions with fluence values of 5 1012 ions/cm2 and 1 1013 ions/cm2 using 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi. In order to prepare sample holder for the irradiation work, the sample powder was spreaded in a hole of 1 cm2 area on aluminum sheet where aluminum foil is put at the bottom of the hole on the sheet and then powder is pressed softly to form a layer of few lm. From the SRIM calculation, the thickness of the sample is less than the range of ions so that the ion passage through the entire material is dominated by electronic energy loss. 3. Results Fig. 1a and b show the X-ray diffraction (XRD) patterns of nanocrystalline pristine and irradiated samples S40 and
S60 with fluence value of 1 1013 ions/cm2, respectively. All the Bragg reflections have been indexed, which confirms the formation of cubic spinel structure in single phase ˚ and 8.32 A ˚ [10]. The values of lattice parameters are 8.34 A for the two pristine samples, respectively. Considerably broadened lines in the XRD patterns are indicative of the presence of nano-sized particles. Average particle sizes estimated with the help of Scherer equation [10] using the width of 311 reflections are shown in Table 1. It is observed that the basic spinel structure remains practically the same after irradiation and the average particle size does not change after irradiation. Fig. 2a and b show Langevin function fitting on the M–H data at 300 K (at 300 K samples are in superparamagnetic state, see the insets of Fig. 3) for pristine and irradiated samples of S40 and S60, respectively. The Langevin function fitting has been obtained by assuming a log-normal distribution for the particle sizes [9]. The values of median diameters and standard deviations for pristine and irradiated samples provided by the fitting are shown in the Table 1. In lognormal distribution, the distribution function is given by [9] " # 1 ðln D ln Dm Þ2 f ðDÞdD ¼ pffiffiffiffiffiffi exp dD; 2r2 2prD where pffiffiffiffiffiffi r is the standard deviation of ln D, the factor 1= 2prD is a normalization factor, D is particle diameter and Dm is median diameter. It is evident from the Table 1 that mean diameter of the nanoparticles obtained by the Langevin function fitted data is in good agreement with that obtained by XRD patterns. Several studies have shown that the annealing temperatures tend to increase the particle size and also the probability of larger particle size distribution
Intensity (arb. units)
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(400)
(311)
(220)
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(440)
2. Experimental details
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13
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b
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13
1x10
20
30
40
50
60
70
ions/cm2
80
2
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2θ Fig. 1. The X-ray diffraction patterns for (a) pristine and irradiated samples of S40 and (b) pristine and irradiated samples of S60 recorded with Fe Ka radiation.
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Table 1 Magnetization M, coercivity Hc, obtained from hysteresis curves recorded at 20 K and median diameter Dm and sigma, obtained from Langevin function fitted curves at 300 K for pristine and irradiated samples of nanocrystalline Ni0.8Cu0.2Fe2O4 ˚ ) by XRD ˚ ) by Langevin fit Sample Fluence (ions/cm2) Average size (A Size (Dm) (A Sigma (r) Mat 8.5 kOe (emu/g) Hc (Oe) S40
Pristine 5 1012 1 1013
40 40 40
41 40 39
0.35 0.33 0.32
34 26 23
300 340 454
S60
Pristine 5 1012 1 1013
60 60 60
61 60 59
0.44 0.42 0.41
39 34 31
660 699 708
35 pristine
30 25 13
2
1x10 ions/cm
20 15
Moment (emu/g)
10 5
a
0 pristine
20 16 13
2
1x10 ions/cm
12 8 4
b
0 0
2
4
6
8
Field (k Oe) Fig. 2. Magnetization-field curves recorded at 300 K for (a) pristine and irradiated samples of S40 and (b) pristine and irradiated samples of S60; along with Langevin function fitting shown by open circles and open triangles.
[11] due to coalescence that increases with increasing temperature of anneal [12]. From the Table 1, the value of r is more for the sample S60 as compared to S40 also suggests that the particle size distribution is broader in the sample S60 as compared to S40 as annealing temperature is more in S60. Fig. 3 shows the hysteresis loops recorded at 20 K for the pristine and irradiated samples (a) for S40 and (b) for S60. The 300 K curves shown in insets of Fig. 3 show zero coercivity and zero remenance, which are suggestive of superparamagnetic behaviour at 300 K. The magnetization at 8.5 kOe at 20 K are shown in Table 1. It is clearly evi-
dent from the Table 1 that the obtained values of magnetization at 8.5 kOe for the pristine and irradiated samples are much less than 58 emu/g, the value for bulk sample of Ni0.8Cu0.2Fe2O4 [13]. Taking the analogy observed in NiFe2O4 [14] and in our earlier work on nanoparticles of ˚ size [9] by the magnetization meaNi0.8Cu0.2Fe2O4 of 30 A surements, we may attribute this to the much reduced magnetization at 8.5 kOe in the nanoparticle-pristine sample and irradiated samples to the frozen disordered spins at the surface. Fig. 4 shows a schematic representation of arrangements of spins in a magnetic ferrite nanoparticle. It is also observed that the magnetization at 8.5 kOe decreases and the coercivity increases with the fluence values after irradiation by 100 Mev Ni ions for both the samples S40 and S60. It could be explained on the basis of formation of cluster of defects by SHI irradiation as 8.5 kOe field is insufficient to overcome the clusters. Such created defects might be responsible for modifications of magnetic properties of materials [8,15–20]. From the SRIM based calculation, the electronic energy loss; nuclear energy loss and projectile range for the 100 MeV Ni ions in Ni0.8Cu0.2Fe2O4 system are 10.8 keV/nm, 22.7 eV/nm, 13.2 lm. From the calculated parameters, it is clear that the dominant process is the electronic energy loss in the present system. However, the threshold to create columnar defect Seth 13 keV/nm, which indicate that there is no possibility to create columnar defects, and only point/clusters of defects will be formed. However, these calculations are for bulk materials and thin films. Further, it is also found that SHI irradiation has more dominant effect on ˚ compared to that of 60 A ˚ . This smaller particle size of 40 A is attributed to the fact that in the nanoparticles the surface to volume energy ratio is dominating and hence plays an important role in their overall physical properties [21]. In the sample S40, the surface to volume ratio is more than the S60 sample and hence SHI irradiation has more dominant role. Fig. 5 shows variation of magnetization M as function of temperature T in the range 20 K–300 K in an external field of 45 Oe recorded in zero filed cooling (ZFC) and field cooling (FC) modes for all the pristine samples. In the ZFC mode the sample was cooled in the zero field from 300 K to 20 K and after stabilization of the temperature, a measuring field of 45 Oe was applied. The data were then recorded while heating the sample. In the FC mode the sample was
S.N. Dolia et al. / Current Applied Physics 8 (2008) 620–625
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-8
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-6
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0
2
4
6
8
Field (k Oe)
-45 -8
-6
-4
-2
0
2
4
6
8
Field (k Oe) Fig. 3. Hysteresis loops recorded at constant temperatures of 20 K for (a) pristine and irradiated samples of S40 (b) pristine and irradiated samples of S60. The insets show M–H curves for pristine samples at room temperature.
cooled from 300 K to 20 K in the presence of a magnetic field of 45 Oe and then measurements were carried out while heating in the same field. Appearance of peaks in the ZFC curves due to blocking mechanism owing to the competition between the thermal energy and the magnetic anisotropy energy of nanoparticles. The broader peaks in the ZFC curves are due to large distribution of magnetic anisotropy constant (K) and distribution of particle sizes.
Thus broader peak in the ZFC curve of S60 sample as compared to the S40 sample suggesting larger distribution of particle size in S60 sample and also corroborated by the Langevin fitting. The blocking temperatures (TB) obtained are 130 K and 225 K for the samples S40 and S60, respectively, implying that the TB increases with the increase in particle size [11,22]. Fig. 5 also shows a departure of FC curves from the ZFC curves in all the samples,
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which is a characteristic feature of superparamagnetic behaviour and such a departure is suggestive of temporal relaxation. The ZFC curves for irradiated samples with fluence value of 1 1013 ions/cm2 indicates that the blocking temperatures nearly same for both the samples S40 and S60 after irradiation. 4. Conclusions
Fig. 4. Schematic representation (not up to scale) of arrangement of spins in a magnetic ferrite nanoparticle. Each unit cell of a ferrite molecule gives ˚ size contains 200 ferrite one spin moment and one nanoparticle of 60 A molecules.
0.30
FC
0.25
0.20
Acknowledgements
0.15
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0.10
Moment (emu/g)
The dc magnetization measurements have been made on pristine and irradiated nanoparticle samples of Ni0.8Cu0.2Fe2O4 with two different average particle sizes. XRD study and Langevin function analysis show good agreement in particle size for both the samples and also after irradiation. It has been shown that the irradiation of the nanoparticle samples S40 and S60 of Ni0.8Cu0.2Fe2O4 by 100 MeV Ni ions modifies the magnetic properties. The observed changes in the hysteresis loops are ascribed to the formation of cluster of defects in the nanocrystalline samples. However, irradiation by Ni ions causes no discernible changes regarding the particle size and blocking temperature. Further, it is also found that SHI irradiation has more dominant effect on smaller particle ˚ compared to that of 60 A ˚. size of 40 A
0.05
a
0.00
Authors are grateful to Prof. A. Krishnamurthy and Prof. B. K. Srivastava for the helpful discussions. Authors are thankful to Accelerator group of IUAC, New Delhi for providing the irradiation facilities. Authors are also thankful to referees for constructive comments resulted in thorough revision of the paper. References
2.0
FC
1.5
1.0
ZFC
0.5
b 0.0 0
50
100
150
200
250
300
Field (k Oe) Fig. 5. Magnetization-temperature curves recorded in zfc and fc modes in an external magnetic field of 45 Oe for (a) pristine (solid circles) and irradiated samples of S40 with 1 1013 ions/cm2 (open circles) and (b) pristine (solid circles) and irradiated samples of S60 with 1 1013 ions/ cm2 (open circles).
[1] M. Toulemonde, Nucl. Instrum. Meth. B 156 (1999) 1. [2] F. Studer, M. Toulemonde, Nucl. Instrum. Meth. B 65 (1992) 560. [3] F. Studer, H. Pascard, D. Groult, C. Houpert, N. Nguyen, M. Toulemonde, Nucl. Instrum. Meth. B 32 (1989) 389. [4] H. Pascard, F. Studer, J. De Phys. IV France 7 (1997) C1–211. [5] B. Martinez, X. Obradors, L. Balcells, A. Rouanet, C. Monty, Phys. Rev. Lett. 80 (1998) 181. [6] R. Nawathey-Dikshit, S.R. Shinde, S.B. Ogale, S.D. Kulkarni, S.R. Sainkar, S.K. Date, Appl. Phys. Lett. 68 (24) (1996) 3491. [7] J.P. Chen, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Phys. Rev. B 51 (17) (1995) 11527. [8] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G. Marest, J. Magn. Magn. Mater. 186 (1998) 342. [9] S.N. Dolia, S.K. Jain, Hyperfine Inter. 160 (2005) 219–225. [10] B.D. Culity, Elements of X-ray Diffraction, Addison Wesley, Reading, Mass, 1959, p. 132. [11] S. Dey, A. Roy, J. Ghose, R.N. Bhowmik, R. Rangnathan, J. Appl. Phys. 90 (8) (2001) 4139. [12] K. Maaz, Arif Mutaz, S.K. Hasanain, Abdullah Ceylan, J. Magn. Magn. Mater. 308 (2007) 289. [13] S. Manjura Hoque, Md. Amanullah Choudhury, Fakhrul Islam, J. Magn. Mater. 251 (2002) 292. [14] R.H. Kodama, A.E. Berkowitz, Phys. Rev. B 59 (1999) 6321. [15] M. Singh, A. Dogra, R. Kumar, Nucl. Instrum. Meth. B 191 (2002) 315.
S.N. Dolia et al. / Current Applied Physics 8 (2008) 620–625 [16] R. Kumar, S.K. Arora, D. Kanjilal, G.K. Mehta, R. Bathe, S.K. Bathe, S.K. Date, S.R. Shinde, L.V. Saraf, S.B. Ogale, S.I. Patil, Radiat. Eff. Defect Solids 147 (1998) 187. [17] F. Gao, W.J. Weber, R. Devanathan, Nucl. Instrum. Meth. B 191 (2002) 487. [18] C.G. Lee, Y. Takeda, N. Kishimoto, Nucl. Instrum. Meth. B 191 (2002) 591.
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[19] M. Singh, A. Dogra, R. Kumar, Nucl. Instrum. Meth. B 196 (2002) 315. [20] A. Dogra, M. Singh, R. Kumar, Nucl. Instrum. Meth. B 207 (2003) 296. [21] S.K. Sharma, Ravi Kumar, V.V. Siva Kumar, M. Knobel, V.R. Reddy, A. Gupta, M. Singh, Nucl. Instrum. Meth. B 248 (2006) 37. [22] Chao Liu, Adam J. Rondinone, Z. John Zhang, Pure Appl. Chem. 72 (1–2) (2000) 37.
Current Applied Physics 8 (2008) 620–625 www.elsevier.com/locate/cap www.kps.or.kr
Magnetic behaviour of nanocrystalline Ni–Cu ferrite and the effect of irradiation by 100 MeV Ni ions S.N. Dolia a,*, Ravi Kumar b, S.K. Sharma c, M.P. Sharma a, Subhash Chander d, M. Singh c b
a Department of Physics, University of Rajasthan, Jaipur 302 004, India Material Science Division, Inter-University Accelerator Centre, New Delhi 110 067, India c Department of Physics, H.P. University, Shimla 171 005, India d Sobhasaria Engineering College, Sikar 332 021, India
Received 14 April 2007; received in revised form 2 October 2007; accepted 6 November 2007 Available online 17 November 2007
Abstract The effects of 100 MeV Ni ion irradiation on magnetic properties of nanoparticles of Ni0.8Cu0.2Fe2O4 with average particle sizes of ˚ and 60 A ˚ , synthesized by chemical co-precipitation method have been studied. The spinel cubic structures were confirmed by XRD. 40 A The average particle size estimated by XRD and by Langevin function fitting are in good agreement for both the pristine and irradiated samples. The blocking temperature increases with particle size and does not change after irradiation. On irradiation by 100 MeV Ni ions, significant changes in the hysteresis loop features are observed, which may be attributed to formation of cluster of defects in the nanocrystalline samples due to swift heavy ion (SHI) irradiation. It is also found that SHI irradiation produces more dominant changes in the ˚ as compared to that of 60 A ˚. hysteresis loop of smaller particle size of 40 A Ó 2007 Elsevier B.V. All rights reserved. PACS: 75.75.+a; 75.50.Tt; 75.50.G; 61.80.Jh Keywords: Magnetic properties; Nanoparticles; Spinel ferrites; SHI irradiations
1. Introduction SHI irradiation is known to generate controlled defects of various types such as point, cluster and columnar defects in materials [1] and for the past two decades, the magnetic oxides and ferrites have been extensively studied [2–4]. The nature of defects is highly dependent upon the electronic energy loss (Se) in the materials. If the Se is less than the threshold value, Seth, then only the point/cluster of defects will be generated in the materials [2] which alter the cation distribution, produce strain/stress in the lattice structure and amorphize materials. The possible modifications are, in general, responsible to modify the magnetic properties in magnetic oxide materials. In recent years, *
Corresponding author. Tel.: +91 1412706378; fax: +91 1412711049. E-mail address: [email protected] (S.N. Dolia).
1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.11.004
nanocrystalline magnetic materials have been investigated extensively due to their unique properties such as superparamagnetism and spin glass behaviour which are generally attributed to surface (rather than the volume) disorder [5]. There have been reports on increase/decrease of magnetization in bulk spinel ferrites after irradiation by high energy heavy ions attributed to cation displacements induced by amorphization–recrystalline processes [4]. It is also reported that increase in coercive force [6], magnetization enhancement [7] and changes in hysteresis loop features [8] etc. in small particles of ferrites on SHI irradiation. The present work has been taken up to study the effect of 100 Mev Ni ions irradiation on the size dependent magnetic properties of nanocrystalline Ni0.8Cu0.2Fe2O4 ferrites synthesized by chemical co-precipitation method.
S.N. Dolia et al. / Current Applied Physics 8 (2008) 620–625
A ferrofluid comprised of Ni0.8Cu0.2Fe2O4 in the nanosize range has been prepared using wet chemical process using oleic acid as surfactant and kerosene has been used as the dispersing medium. In order to obtain narrow distribution of particle sizes, the resulting fluid was centrifuged at 12,000 rpm. For obtaining dried particles of this ferrite, carrier liquid from the ferrofluid has been removed by repetitive washing with acetone [9]. The powder so obtained was then heated at two temperatures of 425 °C and 450 °C. The two samples are hereafter referred as S40 and S60, respectively. X-Ray diffraction pattern has been recorded at 300 K using Fe Ka radiation, on a Philips make powder diffractometer PW-1840. The dc magnetization measurements of nanoparticles were performed using PARC make vibrating sample magnetometer (VSM) model 155. The samples were irradiated with 100 MeV Ni ions with fluence values of 5 1012 ions/cm2 and 1 1013 ions/cm2 using 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi. In order to prepare sample holder for the irradiation work, the sample powder was spreaded in a hole of 1 cm2 area on aluminum sheet where aluminum foil is put at the bottom of the hole on the sheet and then powder is pressed softly to form a layer of few lm. From the SRIM calculation, the thickness of the sample is less than the range of ions so that the ion passage through the entire material is dominated by electronic energy loss. 3. Results Fig. 1a and b show the X-ray diffraction (XRD) patterns of nanocrystalline pristine and irradiated samples S40 and
S60 with fluence value of 1 1013 ions/cm2, respectively. All the Bragg reflections have been indexed, which confirms the formation of cubic spinel structure in single phase ˚ and 8.32 A ˚ [10]. The values of lattice parameters are 8.34 A for the two pristine samples, respectively. Considerably broadened lines in the XRD patterns are indicative of the presence of nano-sized particles. Average particle sizes estimated with the help of Scherer equation [10] using the width of 311 reflections are shown in Table 1. It is observed that the basic spinel structure remains practically the same after irradiation and the average particle size does not change after irradiation. Fig. 2a and b show Langevin function fitting on the M–H data at 300 K (at 300 K samples are in superparamagnetic state, see the insets of Fig. 3) for pristine and irradiated samples of S40 and S60, respectively. The Langevin function fitting has been obtained by assuming a log-normal distribution for the particle sizes [9]. The values of median diameters and standard deviations for pristine and irradiated samples provided by the fitting are shown in the Table 1. In lognormal distribution, the distribution function is given by [9] " # 1 ðln D ln Dm Þ2 f ðDÞdD ¼ pffiffiffiffiffiffi exp dD; 2r2 2prD where pffiffiffiffiffiffi r is the standard deviation of ln D, the factor 1= 2prD is a normalization factor, D is particle diameter and Dm is median diameter. It is evident from the Table 1 that mean diameter of the nanoparticles obtained by the Langevin function fitted data is in good agreement with that obtained by XRD patterns. Several studies have shown that the annealing temperatures tend to increase the particle size and also the probability of larger particle size distribution
Intensity (arb. units)
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(400)
(311)
(220)
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40
50
60
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2θ Fig. 1. The X-ray diffraction patterns for (a) pristine and irradiated samples of S40 and (b) pristine and irradiated samples of S60 recorded with Fe Ka radiation.
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Table 1 Magnetization M, coercivity Hc, obtained from hysteresis curves recorded at 20 K and median diameter Dm and sigma, obtained from Langevin function fitted curves at 300 K for pristine and irradiated samples of nanocrystalline Ni0.8Cu0.2Fe2O4 ˚ ) by XRD ˚ ) by Langevin fit Sample Fluence (ions/cm2) Average size (A Size (Dm) (A Sigma (r) Mat 8.5 kOe (emu/g) Hc (Oe) S40
Pristine 5 1012 1 1013
40 40 40
41 40 39
0.35 0.33 0.32
34 26 23
300 340 454
S60
Pristine 5 1012 1 1013
60 60 60
61 60 59
0.44 0.42 0.41
39 34 31
660 699 708
35 pristine
30 25 13
2
1x10 ions/cm
20 15
Moment (emu/g)
10 5
a
0 pristine
20 16 13
2
1x10 ions/cm
12 8 4
b
0 0
2
4
6
8
Field (k Oe) Fig. 2. Magnetization-field curves recorded at 300 K for (a) pristine and irradiated samples of S40 and (b) pristine and irradiated samples of S60; along with Langevin function fitting shown by open circles and open triangles.
[11] due to coalescence that increases with increasing temperature of anneal [12]. From the Table 1, the value of r is more for the sample S60 as compared to S40 also suggests that the particle size distribution is broader in the sample S60 as compared to S40 as annealing temperature is more in S60. Fig. 3 shows the hysteresis loops recorded at 20 K for the pristine and irradiated samples (a) for S40 and (b) for S60. The 300 K curves shown in insets of Fig. 3 show zero coercivity and zero remenance, which are suggestive of superparamagnetic behaviour at 300 K. The magnetization at 8.5 kOe at 20 K are shown in Table 1. It is clearly evi-
dent from the Table 1 that the obtained values of magnetization at 8.5 kOe for the pristine and irradiated samples are much less than 58 emu/g, the value for bulk sample of Ni0.8Cu0.2Fe2O4 [13]. Taking the analogy observed in NiFe2O4 [14] and in our earlier work on nanoparticles of ˚ size [9] by the magnetization meaNi0.8Cu0.2Fe2O4 of 30 A surements, we may attribute this to the much reduced magnetization at 8.5 kOe in the nanoparticle-pristine sample and irradiated samples to the frozen disordered spins at the surface. Fig. 4 shows a schematic representation of arrangements of spins in a magnetic ferrite nanoparticle. It is also observed that the magnetization at 8.5 kOe decreases and the coercivity increases with the fluence values after irradiation by 100 Mev Ni ions for both the samples S40 and S60. It could be explained on the basis of formation of cluster of defects by SHI irradiation as 8.5 kOe field is insufficient to overcome the clusters. Such created defects might be responsible for modifications of magnetic properties of materials [8,15–20]. From the SRIM based calculation, the electronic energy loss; nuclear energy loss and projectile range for the 100 MeV Ni ions in Ni0.8Cu0.2Fe2O4 system are 10.8 keV/nm, 22.7 eV/nm, 13.2 lm. From the calculated parameters, it is clear that the dominant process is the electronic energy loss in the present system. However, the threshold to create columnar defect Seth 13 keV/nm, which indicate that there is no possibility to create columnar defects, and only point/clusters of defects will be formed. However, these calculations are for bulk materials and thin films. Further, it is also found that SHI irradiation has more dominant effect on ˚ compared to that of 60 A ˚ . This smaller particle size of 40 A is attributed to the fact that in the nanoparticles the surface to volume energy ratio is dominating and hence plays an important role in their overall physical properties [21]. In the sample S40, the surface to volume ratio is more than the S60 sample and hence SHI irradiation has more dominant role. Fig. 5 shows variation of magnetization M as function of temperature T in the range 20 K–300 K in an external field of 45 Oe recorded in zero filed cooling (ZFC) and field cooling (FC) modes for all the pristine samples. In the ZFC mode the sample was cooled in the zero field from 300 K to 20 K and after stabilization of the temperature, a measuring field of 45 Oe was applied. The data were then recorded while heating the sample. In the FC mode the sample was
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-6
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Field (k Oe) Fig. 3. Hysteresis loops recorded at constant temperatures of 20 K for (a) pristine and irradiated samples of S40 (b) pristine and irradiated samples of S60. The insets show M–H curves for pristine samples at room temperature.
cooled from 300 K to 20 K in the presence of a magnetic field of 45 Oe and then measurements were carried out while heating in the same field. Appearance of peaks in the ZFC curves due to blocking mechanism owing to the competition between the thermal energy and the magnetic anisotropy energy of nanoparticles. The broader peaks in the ZFC curves are due to large distribution of magnetic anisotropy constant (K) and distribution of particle sizes.
Thus broader peak in the ZFC curve of S60 sample as compared to the S40 sample suggesting larger distribution of particle size in S60 sample and also corroborated by the Langevin fitting. The blocking temperatures (TB) obtained are 130 K and 225 K for the samples S40 and S60, respectively, implying that the TB increases with the increase in particle size [11,22]. Fig. 5 also shows a departure of FC curves from the ZFC curves in all the samples,
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which is a characteristic feature of superparamagnetic behaviour and such a departure is suggestive of temporal relaxation. The ZFC curves for irradiated samples with fluence value of 1 1013 ions/cm2 indicates that the blocking temperatures nearly same for both the samples S40 and S60 after irradiation. 4. Conclusions
Fig. 4. Schematic representation (not up to scale) of arrangement of spins in a magnetic ferrite nanoparticle. Each unit cell of a ferrite molecule gives ˚ size contains 200 ferrite one spin moment and one nanoparticle of 60 A molecules.
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FC
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Acknowledgements
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Moment (emu/g)
The dc magnetization measurements have been made on pristine and irradiated nanoparticle samples of Ni0.8Cu0.2Fe2O4 with two different average particle sizes. XRD study and Langevin function analysis show good agreement in particle size for both the samples and also after irradiation. It has been shown that the irradiation of the nanoparticle samples S40 and S60 of Ni0.8Cu0.2Fe2O4 by 100 MeV Ni ions modifies the magnetic properties. The observed changes in the hysteresis loops are ascribed to the formation of cluster of defects in the nanocrystalline samples. However, irradiation by Ni ions causes no discernible changes regarding the particle size and blocking temperature. Further, it is also found that SHI irradiation has more dominant effect on smaller particle ˚ compared to that of 60 A ˚. size of 40 A
0.05
a
0.00
Authors are grateful to Prof. A. Krishnamurthy and Prof. B. K. Srivastava for the helpful discussions. Authors are thankful to Accelerator group of IUAC, New Delhi for providing the irradiation facilities. Authors are also thankful to referees for constructive comments resulted in thorough revision of the paper. References
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FC
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Field (k Oe) Fig. 5. Magnetization-temperature curves recorded in zfc and fc modes in an external magnetic field of 45 Oe for (a) pristine (solid circles) and irradiated samples of S40 with 1 1013 ions/cm2 (open circles) and (b) pristine (solid circles) and irradiated samples of S60 with 1 1013 ions/ cm2 (open circles).
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