- Email: [email protected]
Effect of grain size on the Néel temperature of nanocrystalline nickel ferrite
Materials Letters 64 (2010) 1144–1146
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Effect of grain size on the Néel temperature of nanocrystalline nickel ferrite A.T. Raghavender a,⁎, Kreso Zadro a, Damir Pajic a, Zeljko Skoko a, Nikola Biliškov b a b
Department of Physics, Faculty of Science, University of Zagreb, Bijenička c. 32, HR-10000, Zagreb, Croatia Laboratory of Molecular Spectroscopy, Ruđer Bošković Institute, Bijenička c. 54, HR-10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 29 January 2010 Accepted 11 February 2010 Available online 20 February 2010 Keywords: Nanomaterials Structural properties Magnetic properties Néel temperature
a b s t r a c t Thermally induced changes of nanocrystalline NiFe2O4 spinel synthesized by sol–gel auto-ignition method are investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and magnetization measurements (VSM). The average grain size of NiFe2O4 increases from about 29 to 50 nm as the calcination temperature increases from 500 to 1000 °C. The IR spectra show the absorption bands corresponding to stretching vibrations of tetrahedral and octahedral bonds. The Néel temperature of NiFe2O4 for various grain sizes were determined by the direct measurement of magnetization. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The synthesis of nanosized magnetic oxide particles, such as spinel nanoferrites of the type MFe2O4 (M is a divalent metal cation), is intensively investigated in terms of their applications in high-density magnetic recording media and magnetic fluids. These materials are also largely used in electric and electronic devices and in catalysis. In order to improve sinterability and magnetic properties, the investigation of alternative, nonconventional synthesis methods to obtain ferrites in the form of nanostructured powders is the current subject [1]. It is well known that the method of preparation plays a very vital role in determining the chemical, structural and magnetic properties of spinel ferrites [2–5]. Nickel ferrite (NiFe2O4) is an important member of the spinel family and it is found to be the most versatile technological materials suited for high-frequency applications due to its high resistivity [6]. In the bulk state, this material possesses an inverse spinel structure, in which tetrahedral (A) sites are occupied by Fe3+ ions and octahedral [B] sites by Fe3+ and Ni2+ ions. It exhibits ferrimagnetism that originates from the antiparallel orientation of spins on (A) and [B] sites. In our previous work [7], nanocrystalline NiFe2O4 was synthesized by sol–gel auto-ignition method. In the present study, we are reporting on its structural and magnetic behavior with the change in calcination temperature. The nanocrystalline NiFe2O4 was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The Néel temperatures of NiFe2O4 samples were
⁎ Corresponding author. Department of Physics, Faculty of Science, University of Zagreb, Bijenicka C.32, HR-10000, Zagreb, Croatia. Tel.: + 385 1 460 5555; fax: + 385 1 4680 336. E-mail address: [email protected] (A.T. Raghavender). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.031
determined using vibrating sample magnetometer (VSM) with a small applied field of 10 Oe. 2. Experimental Nanocrystalline powders of NiFe2O4 were prepared by sol–gel auto-ignition method [7]. The a.r. grade citric acid (C6H8O7 H2O), nickel nitrate (Ni(NO3)2 6H2O), ferric nitrate (Fe(NO3)3 9H2O) from Sigma Aldrich were used as starting materials. The synthesis technique is described in detail elsewhere [7]. To study the response of NiFe2O4 nanoparticles to changes in temperature, the as-prepared powders were heat treated separately at different temperatures ranging from 500 to 1000 °C for 4 h. The spinel ferrite phase was confirmed by X-ray powder diffraction (XRD) measurements using Cu-Kα radiation. The average particle size (d) was calculated using the most intense XRD peak (311) corresponding to the ferrite phase, employing the Scherer formula. IR spectra was recorded in the range 800–200 cm− 1 using ABB Bomem MB 102 infrared spectrometer equipped with CsI optics and DTGS detector. The Néel temperatures were determined from magnetization measured using a PARC EG&G vibrating sample magnetometer VSM 4500. 3. Results and discussions 3.1. Grain size and lattice parameters Fig.1 shows the XRD pattern of NiFe2O4 samples taken after their heat treatment at various temperatures ranging from 500 to 1000 °C. As seen, NiFe2O4 with the spinel structure has been formed already at 500 °C. It was noted that a small amount of α–Fe2O3 phase had also been formed, which disappeared at calcination temperature above 700 °C (Fig. 1). Several authors [2–5] have observed the hematite
A.T. Raghavender et al. / Materials Letters 64 (2010) 1144–1146
1145
3.2. Néel temperature versus grain size Néel temperatures estimated for NiFe2O4 samples with various grain sizes found by the most direct unambiguous manner possible, observation of the spontaneous change in the magnetization as the temperature is varied through TN with a small applied field of 10 Oe are as shown in Fig.2 and Table 1. As seen from the figure, the values of Néel temperatures of the samples are found to be less than that of the bulk sample [11]. It is decreased with decreasing particle grain. The decreases in the value of Néel temperature with grain size reduction in the case of NiFe2O4 and MnFe2O4 has been explained on the basis of finite size effects [12,13]. Similarly, in our case the decrease in TN with grain size could be explained on the basis of finite size scaling theory as seen from Fig.3. According to this theory, the shift in the Néel temperature from that of the bulk is given by
Fig. 1. X-ray diffractogram of nanocrystalline NiFe 2O 4 at various calcination temperatures.
phase even up to the calcination temperature of 900 °C. It is clearly seen in Fig. 1 that the XRD peaks corresponding to the spinel phase became sharper and narrower with increasing calcination temperature, indicating the enhancement of crystallinity of the ferrite. It was found that the crystallite size (d) of NiFe2O4 increases from about 29 to 50 nm with the corresponding increase in calcination temperature from 500 °C to 1000 °C. The lattice parameter (a) was calculated from the XRD data for the NiFe2O4 samples calcined at several temperatures ranging from 500 to 1000 °C. It was found that there is no significant change in the lattice parameter values of the samples thermally treated at various temperatures (see Table 1). Thus, independent of the calcination temperature, the a values of NiFe2O4 samples with various crystallite sizes are close to the lattice parameter of bulk NiFe2O4 (8.337 Å) reported in the (JCPDS database). To confirm the formation of NiFe2O4 phases, the IR spectra were recorded in the range 800–200 cm− 1. The observed absorption band positions of the samples are listed in Table 1. NiFe2O4 gives rise to two main absorption envelopes, consisting of metal-oxygen stretching bands v1 and v2, in the range 600–560 cm− 1 and 400–386 cm− 1, respectively. The v1 band corresponds to intrinsic stretching vibrations of tetrahedral Fe3+–O, while v2 is assigned to Fe3+–O and Ni2+–O bond stretching vibrations of octahedral sites [8]. These bands linearly change their positions towards the higher wavenumbers with the increase in annealing temperature, this may be as a result of particle aggregation due to the magnetostatic interaction of the magnetic moments of the particles [9]. This suggests that the increase of the calcination temperature could cause an increase of the magnetic moments of particles. All of these changes are correlated with particle size, so it can be concluded that the change of the size of nanoparticles causes the variation of positions of v1 and v2 infrared bands, which is frequently reported in the literature [10].
TN ðdÞ−TN ðbulkÞ = TN ðbulkÞ
d d0
−1 v
ð1Þ
where TN (d) is Néel temperature for given particle size d, TN (bulk) is the Néel temperature of the bulk samples, d0 is a constant an v is the critical exponent of the correlation length. The straight line in Fig.3 is obtained as a result of fitting the experimental data using Eq. (1). The parameters obtained from the best fit were TN ðbulkÞ = 863 4K; v = 0:75 0:1; d0 = 2:9 0:1 nm: The value of TN (bulk) of 863 K obtained through the fitting is nearer when compared to the value of 858 K [11]. The fit values of v = 0.75 ± 0.1 and d0 = 2.9 ± 0.1 nm are consistent with the finite size scaling reported earlier [12,13]. One important observation is that the obtained TN values for all the grain sizes are slightly smaller than the standard bulk value, revealing that with the calcination temperature the NiFe2O4 samples has returned to the structural state similar to the bulk one in conformity with the obtained lattice values. Therefore, we conclude that our results are consistent with finite size scaling as compared to earlier works.
Table 1 Grain size (d), lattice constant (a), IR frequency (v1 and v2) and Néel temperature (TN). Temperature
d (nm)
a (Å)
v1 (cm− 1)
v2 (cm− 1)
TN (K)
500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C
29 33 37 43 47 50
8.298 8.301 8.309 8.309 8.307 8.299
579 587 589 593 595 601
388 392 394 394 400 403
818 830 833 840 841 842
Fig. 2. Néel temperature versus average grain size for nanocrystalline NiFe2O4. The continuous line is guide to the eye.
1146
A.T. Raghavender et al. / Materials Letters 64 (2010) 1144–1146
decrease with decreasing grain size and are in accordance with finite size scaling. Acknowledgements One of the authors (A.T.R) thanks “The National Foundation for Science, Higher Education and Technological Development of the Republic of Croatia” for awarding Postdoctoral Fellowship under Brain Gain for foreign researchers. Special thanks are due to Prof. M. Cindric and Mr. M. Mustapic (University of Zagreb) for providing the instrument for the synthesis. References
Fig. 3. Finite scale analysis of Néel temperature versus grain size. Solid line is obtained by fitting the data for all the grain sizes.
[1] [2] [3] [4] [5] [6] [7]
4. Conclusions The grain size decreases with increasing calcination temperature. The IR analysis supported the formation of spinel ferrite phase with the presence of high-frequency metal-oxygen bands corresponding to intrinsic stretching vibrations of tetrahedral and octahedral sites. From the size dependent studies of the calcined samples we conclude that the Néel temperatures of nanocrystalline NiFe2O4 are found to
[8] [9] [10] [11] [12] [13]
Da Silva JB, Mohallem ND. S. J Magn Magn Mater 2001;226–230:1393. Azadmanjiri J. Seyyed Ebrahimi S A, Salehani H K. Ceram Int 2007;33:1623. Cheng Y, Zheng Y, Wang Y, Bao F, Qin Y. J Solid State Chem 2005;178:2394. Yang JM, Tsuo WJ, Yen FS. J Solid State Chem 1999;145:50. Aphesteguy JC, Bercoff PG, Jacobo SE. Physica B 2007;398:200. Albuquerque AS, Ardisson JD, Macedo WAA, Lopez JL, Paniago R, Persiano AIC. J Magn Magn Mater 2001;226:1379. Raghavender AT, Pajic D, Zadro K, Milekovic T. Venkateshwar Rao P, Jadhav KM. Ravinder D J Magn Magn Mater 2007;316:1. Waldron RD. Phys. Rev 1955;99:1727. Andrés - Vergés M, De Julián C, González JM, Serna CJ. J Mater Sci 1993;28:2962. Lo´peza FA, Lo´pez-Delgado A, Mart´ın de Vidales JL, Vila E. J Alloys Comp 1998;265:291. Smith J. Wijn H P J. Philips Technical Library, Eindhoven, The Netherlands: Ferrites; 1959. p. 157. Chinnasamy CN, Narayanasamy A, Ponpandian N, Justin Joseyphus R, Jeyadevan B, Tohji K, Chattopadhyay K. J Magn Magn Mate 2003;238:281. Chen JP, Sorenson CM, Klabunde KJ, Hadjipanayis GC, Devilin E, Kostikas A. Phys Rev B 1996;54:9288.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Effect of grain size on the Néel temperature of nanocrystalline nickel ferrite A.T. Raghavender a,⁎, Kreso Zadro a, Damir Pajic a, Zeljko Skoko a, Nikola Biliškov b a b
Department of Physics, Faculty of Science, University of Zagreb, Bijenička c. 32, HR-10000, Zagreb, Croatia Laboratory of Molecular Spectroscopy, Ruđer Bošković Institute, Bijenička c. 54, HR-10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 29 January 2010 Accepted 11 February 2010 Available online 20 February 2010 Keywords: Nanomaterials Structural properties Magnetic properties Néel temperature
a b s t r a c t Thermally induced changes of nanocrystalline NiFe2O4 spinel synthesized by sol–gel auto-ignition method are investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and magnetization measurements (VSM). The average grain size of NiFe2O4 increases from about 29 to 50 nm as the calcination temperature increases from 500 to 1000 °C. The IR spectra show the absorption bands corresponding to stretching vibrations of tetrahedral and octahedral bonds. The Néel temperature of NiFe2O4 for various grain sizes were determined by the direct measurement of magnetization. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The synthesis of nanosized magnetic oxide particles, such as spinel nanoferrites of the type MFe2O4 (M is a divalent metal cation), is intensively investigated in terms of their applications in high-density magnetic recording media and magnetic fluids. These materials are also largely used in electric and electronic devices and in catalysis. In order to improve sinterability and magnetic properties, the investigation of alternative, nonconventional synthesis methods to obtain ferrites in the form of nanostructured powders is the current subject [1]. It is well known that the method of preparation plays a very vital role in determining the chemical, structural and magnetic properties of spinel ferrites [2–5]. Nickel ferrite (NiFe2O4) is an important member of the spinel family and it is found to be the most versatile technological materials suited for high-frequency applications due to its high resistivity [6]. In the bulk state, this material possesses an inverse spinel structure, in which tetrahedral (A) sites are occupied by Fe3+ ions and octahedral [B] sites by Fe3+ and Ni2+ ions. It exhibits ferrimagnetism that originates from the antiparallel orientation of spins on (A) and [B] sites. In our previous work [7], nanocrystalline NiFe2O4 was synthesized by sol–gel auto-ignition method. In the present study, we are reporting on its structural and magnetic behavior with the change in calcination temperature. The nanocrystalline NiFe2O4 was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The Néel temperatures of NiFe2O4 samples were
⁎ Corresponding author. Department of Physics, Faculty of Science, University of Zagreb, Bijenicka C.32, HR-10000, Zagreb, Croatia. Tel.: + 385 1 460 5555; fax: + 385 1 4680 336. E-mail address: [email protected] (A.T. Raghavender). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.031
determined using vibrating sample magnetometer (VSM) with a small applied field of 10 Oe. 2. Experimental Nanocrystalline powders of NiFe2O4 were prepared by sol–gel auto-ignition method [7]. The a.r. grade citric acid (C6H8O7 H2O), nickel nitrate (Ni(NO3)2 6H2O), ferric nitrate (Fe(NO3)3 9H2O) from Sigma Aldrich were used as starting materials. The synthesis technique is described in detail elsewhere [7]. To study the response of NiFe2O4 nanoparticles to changes in temperature, the as-prepared powders were heat treated separately at different temperatures ranging from 500 to 1000 °C for 4 h. The spinel ferrite phase was confirmed by X-ray powder diffraction (XRD) measurements using Cu-Kα radiation. The average particle size (d) was calculated using the most intense XRD peak (311) corresponding to the ferrite phase, employing the Scherer formula. IR spectra was recorded in the range 800–200 cm− 1 using ABB Bomem MB 102 infrared spectrometer equipped with CsI optics and DTGS detector. The Néel temperatures were determined from magnetization measured using a PARC EG&G vibrating sample magnetometer VSM 4500. 3. Results and discussions 3.1. Grain size and lattice parameters Fig.1 shows the XRD pattern of NiFe2O4 samples taken after their heat treatment at various temperatures ranging from 500 to 1000 °C. As seen, NiFe2O4 with the spinel structure has been formed already at 500 °C. It was noted that a small amount of α–Fe2O3 phase had also been formed, which disappeared at calcination temperature above 700 °C (Fig. 1). Several authors [2–5] have observed the hematite
A.T. Raghavender et al. / Materials Letters 64 (2010) 1144–1146
1145
3.2. Néel temperature versus grain size Néel temperatures estimated for NiFe2O4 samples with various grain sizes found by the most direct unambiguous manner possible, observation of the spontaneous change in the magnetization as the temperature is varied through TN with a small applied field of 10 Oe are as shown in Fig.2 and Table 1. As seen from the figure, the values of Néel temperatures of the samples are found to be less than that of the bulk sample [11]. It is decreased with decreasing particle grain. The decreases in the value of Néel temperature with grain size reduction in the case of NiFe2O4 and MnFe2O4 has been explained on the basis of finite size effects [12,13]. Similarly, in our case the decrease in TN with grain size could be explained on the basis of finite size scaling theory as seen from Fig.3. According to this theory, the shift in the Néel temperature from that of the bulk is given by
Fig. 1. X-ray diffractogram of nanocrystalline NiFe 2O 4 at various calcination temperatures.
phase even up to the calcination temperature of 900 °C. It is clearly seen in Fig. 1 that the XRD peaks corresponding to the spinel phase became sharper and narrower with increasing calcination temperature, indicating the enhancement of crystallinity of the ferrite. It was found that the crystallite size (d) of NiFe2O4 increases from about 29 to 50 nm with the corresponding increase in calcination temperature from 500 °C to 1000 °C. The lattice parameter (a) was calculated from the XRD data for the NiFe2O4 samples calcined at several temperatures ranging from 500 to 1000 °C. It was found that there is no significant change in the lattice parameter values of the samples thermally treated at various temperatures (see Table 1). Thus, independent of the calcination temperature, the a values of NiFe2O4 samples with various crystallite sizes are close to the lattice parameter of bulk NiFe2O4 (8.337 Å) reported in the (JCPDS database). To confirm the formation of NiFe2O4 phases, the IR spectra were recorded in the range 800–200 cm− 1. The observed absorption band positions of the samples are listed in Table 1. NiFe2O4 gives rise to two main absorption envelopes, consisting of metal-oxygen stretching bands v1 and v2, in the range 600–560 cm− 1 and 400–386 cm− 1, respectively. The v1 band corresponds to intrinsic stretching vibrations of tetrahedral Fe3+–O, while v2 is assigned to Fe3+–O and Ni2+–O bond stretching vibrations of octahedral sites [8]. These bands linearly change their positions towards the higher wavenumbers with the increase in annealing temperature, this may be as a result of particle aggregation due to the magnetostatic interaction of the magnetic moments of the particles [9]. This suggests that the increase of the calcination temperature could cause an increase of the magnetic moments of particles. All of these changes are correlated with particle size, so it can be concluded that the change of the size of nanoparticles causes the variation of positions of v1 and v2 infrared bands, which is frequently reported in the literature [10].
TN ðdÞ−TN ðbulkÞ = TN ðbulkÞ
d d0
−1 v
ð1Þ
where TN (d) is Néel temperature for given particle size d, TN (bulk) is the Néel temperature of the bulk samples, d0 is a constant an v is the critical exponent of the correlation length. The straight line in Fig.3 is obtained as a result of fitting the experimental data using Eq. (1). The parameters obtained from the best fit were TN ðbulkÞ = 863 4K; v = 0:75 0:1; d0 = 2:9 0:1 nm: The value of TN (bulk) of 863 K obtained through the fitting is nearer when compared to the value of 858 K [11]. The fit values of v = 0.75 ± 0.1 and d0 = 2.9 ± 0.1 nm are consistent with the finite size scaling reported earlier [12,13]. One important observation is that the obtained TN values for all the grain sizes are slightly smaller than the standard bulk value, revealing that with the calcination temperature the NiFe2O4 samples has returned to the structural state similar to the bulk one in conformity with the obtained lattice values. Therefore, we conclude that our results are consistent with finite size scaling as compared to earlier works.
Table 1 Grain size (d), lattice constant (a), IR frequency (v1 and v2) and Néel temperature (TN). Temperature
d (nm)
a (Å)
v1 (cm− 1)
v2 (cm− 1)
TN (K)
500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C
29 33 37 43 47 50
8.298 8.301 8.309 8.309 8.307 8.299
579 587 589 593 595 601
388 392 394 394 400 403
818 830 833 840 841 842
Fig. 2. Néel temperature versus average grain size for nanocrystalline NiFe2O4. The continuous line is guide to the eye.
1146
A.T. Raghavender et al. / Materials Letters 64 (2010) 1144–1146
decrease with decreasing grain size and are in accordance with finite size scaling. Acknowledgements One of the authors (A.T.R) thanks “The National Foundation for Science, Higher Education and Technological Development of the Republic of Croatia” for awarding Postdoctoral Fellowship under Brain Gain for foreign researchers. Special thanks are due to Prof. M. Cindric and Mr. M. Mustapic (University of Zagreb) for providing the instrument for the synthesis. References
Fig. 3. Finite scale analysis of Néel temperature versus grain size. Solid line is obtained by fitting the data for all the grain sizes.
[1] [2] [3] [4] [5] [6] [7]
4. Conclusions The grain size decreases with increasing calcination temperature. The IR analysis supported the formation of spinel ferrite phase with the presence of high-frequency metal-oxygen bands corresponding to intrinsic stretching vibrations of tetrahedral and octahedral sites. From the size dependent studies of the calcined samples we conclude that the Néel temperatures of nanocrystalline NiFe2O4 are found to
[8] [9] [10] [11] [12] [13]
Da Silva JB, Mohallem ND. S. J Magn Magn Mater 2001;226–230:1393. Azadmanjiri J. Seyyed Ebrahimi S A, Salehani H K. Ceram Int 2007;33:1623. Cheng Y, Zheng Y, Wang Y, Bao F, Qin Y. J Solid State Chem 2005;178:2394. Yang JM, Tsuo WJ, Yen FS. J Solid State Chem 1999;145:50. Aphesteguy JC, Bercoff PG, Jacobo SE. Physica B 2007;398:200. Albuquerque AS, Ardisson JD, Macedo WAA, Lopez JL, Paniago R, Persiano AIC. J Magn Magn Mater 2001;226:1379. Raghavender AT, Pajic D, Zadro K, Milekovic T. Venkateshwar Rao P, Jadhav KM. Ravinder D J Magn Magn Mater 2007;316:1. Waldron RD. Phys. Rev 1955;99:1727. Andrés - Vergés M, De Julián C, González JM, Serna CJ. J Mater Sci 1993;28:2962. Lo´peza FA, Lo´pez-Delgado A, Mart´ın de Vidales JL, Vila E. J Alloys Comp 1998;265:291. Smith J. Wijn H P J. Philips Technical Library, Eindhoven, The Netherlands: Ferrites; 1959. p. 157. Chinnasamy CN, Narayanasamy A, Ponpandian N, Justin Joseyphus R, Jeyadevan B, Tohji K, Chattopadhyay K. J Magn Magn Mate 2003;238:281. Chen JP, Sorenson CM, Klabunde KJ, Hadjipanayis GC, Devilin E, Kostikas A. Phys Rev B 1996;54:9288.