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Synthesis and characterization of NiCoMnCuFe1.96O4 for circulator application
Journal of Magnetism and Magnetic Materials 323 (2011) 1593–1598
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis and characterization of NiCoMnCuFe1.96O4 for circulator application T. Ramesh a, R.S. Shinde b, S.R. Murthy a,n a b
Department of Physics, Osmania University, Hyderabad, India Ferrite Laboratory, RRCAT, Indore, India
a r t i c l e in f o
abstract
Article history: Received 7 October 2010 Received in revised form 3 December 2010 Available online 23 December 2010
Modern accelerator design practice includes the use of high-quality ferrites for circulator applications with ever-increasing requirements on power handling ability. Modeling studies of new designs are of increasing economic importance, but are frequently hindered by lack of measured values of the ceramic loss factors. We have developed a nanocrystalline ferrite material with composition Ni0.94Co0.03Mn0.04Cu0.03Fe1.96O4. Nanocrystalline NiCoMnCu ferrite powders were synthesized using a microwavehydrothermal method at 160 1C for 40 min. The ferrite formation conditions, such as pH, temperature and time, were optimized. The phase of the samples was identified by X-ray diffraction and was characterized by Fourier transformation infrared spectroscopy. The size of the nanocrystalline ferrite of as-synthesized powders was 10 nm. The powder was densified at different temperatures using a microwave sintering method. The complex permittivity and permeability of the sintered samples were measured over a frequency range from 10 kHz to 1.8 GHz at room temperature. The applicability of the samples for circulators was tested via the measurement of the ferromagnetic resonance linewidth and the results are presented. & 2010 Elsevier B.V. All rights reserved.
Keywords: Microwave hydrothermal Nanocrystalline ferrite Microwave sintering Complex permittivity Complex permeability FMR linewidth
1. Introduction Nickel cobalt ferrites have attracted attention of researchers for a long time and have been developed as a replacement for yttrium iron garnet (YIG) due to their low cost. They have proved to be very useful for many microwave applications because of their relatively low magnetic and dielectric losses, high-Curie temperature, high saturation magnetization, hysteresis loop, good temperature stability of saturation magnetization and high power handling capability [1] properties offer a performance advantage over other spinel structures. However most ferrites have been produced via the conventional process that is known to have some inherent drawbacks such as poor compositional control, chemical inhomogenity, coarser particle size and introduction of various impurities during ball milling [2]. Furthermore, treatment at high-temperatures induced lower magnetization due to the precipitation of aFe2O3 or the formation of Fe3O4 [3]. In view of this, large scale application of ferrites have prompted the development of various chemical methods, which include hydrothermal, co-precipitation, alkoxide hydrolysis, freeze drying, precursor and sol–gel for the preparations of the desirable stoichiometric and chemically pure spinel ferrites. Several papers have thus far been reported with regard to the structural, electrical and magnetic properties of nickel cobalt ferrite synthesized by different methods [4–7]. The addition of stoichiometrically a small
n
Corresponding author. Tel.: + 914024040217; fax: + 914027090020. E-mail address: [email protected] (S.R. Murthy).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.12.010
amount of Co (0.03) to Ni ferrite controls superparamagnetic behavior and increases resonance frequency significantly [8,9]. Similarly, small amount of Mn has been added to Ni ferrite to enhance saturation magnetization and to control magnetostriction coefficient [10]. In order to improve the density of Ni ferrite, small amount of Cu was added to it. The introduction of Cu improves not only the grain size but also the real permittivity and permeability, which are desirable characteristics for circulator applications. In this study, we attempted a novel method of synthesis called the microwave hydrothermal to produce Co, Cu and Mn-substituted nickel ferrite powders with a composition of Ni0.94Co0.03Mn0.04Cu0.03Fe1.96O4. Synthesized powders were characterized using XRD, TEM and FTIR. Then, the nano-ferrite powders were sintered at three different temperatures using a microwave sintering method [11]. The characterization of sintered samples is done by studying their complex permittivity and complex permeability over a wide range of frequencies from 10 kHz to 1.8 GHz and the results obtained are discussed in this paper.
2. Experimental method Nickel nitrate [Ni(NO3)3 6H2O], cobalt nitrate [Co(NO3)3 6H2O], manganese nitrate [Mn(NO3)3 9H2O], copper nitrate [Cu(NO3)3 6H2O] and ferric nitrate [Fe(NO3)3 9H2O] were measured to their stoichiometric proportions and dissolved in 50 ml of deionized water to obtain a composition: Ni0.94Co0.03Mn0.04Cu0.03Fe1.96O4. An aqueous NaOH solution was added to this mixture until the desired pH ( 9.5) value was obtained. Then the mixture was transferred into a
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recorded using a Brucker tensor 27 DTGS TEC detector spectrophotometer from 1200 to 375 cm 1 by the KBr pellet method. The obtained ferrite powder was mixed with 2 wt% polyvinyl alcohol as a binder. Then the powder was uniaxially pressed at a pressure of 1.5 MPa to form green pellet and toroidal specimens. After the binder was burnt out at 300 1C the compacts were microwave sintered (MS) at three different temperatures, i.e., 750 1C/30 min (MS1), 850 1C/30 min (MS2) and 950 1C/30 min (MS3), in air. The microwave sintering process was carried out using a specially designed applicator that consists of a domestic microwave oven having an output power level tunable up to a maximum of 800 W and operating frequency of 2.45 GHz. The sintering temperatures of the samples were measured using a platinum-13% pt-Rh thermocouple with an accuracy of 71 1C. The rate of heating and cooling during the microwave sintering was 20 1C/min. Complex permittivity and permeability of the sintered samples were measured in the frequency range of 10 kHz–1.8 GHz using an LCR meter and Agilent 4291B impedance analyzer. Magnetic properties such as saturation magnetization (Ms) and coercivity (Hc) are obtained from recorded hysteresis loops using a vibration sample magnetometer (VSM, Lakeshore, Model 7300). Microwave properties were characterized by the out-of-plane and in-plane ferromagnetic resonance (FMR) measurements using a shorted waveguide. The external magnetic field was swept at a fixed frequency using a TE10 mode propagation in a Ka-band (26–40 GHz) waveguide. The microwave magnetic field hrf was applied in the plane of the ferrite disk, which is always normal to the external field.
Teflon lined vessel and treated using a microwave digestion system (Model MDS-2000, CEM Corp., Mathews, NC) at 160 1C/40 min. Our microwave system uses 2.45 GHz microwaves and can operate at 0–100% of its full power (630750 W). The products obtained were filtered and washed repeatedly with deionized water followed by drying overnight. Then the synthesized powder was weighted; the percentage yield was calculated and found to be 96%. The synthesized powder was characterized through powder X-ray diffraction (XRD) and particle size and morphology were determined using transmission electron microscopy (TEM; Model JEM-2010, JEOL, Tokyo, Japan). The spectra of the present samples obtained from Fourier transformation infrared spectroscopy (FTIR) have been
AS
300 250 200 150 100 50 30
40
50
70
60
80
2θ Fig. 1. XRD pattern of M-H as-synthesized powder.
3. Results and discussion
MS3 (6 2 0)
(4 4 0)
(3 3 3)
(4 0 0)
(2 2 2)
(3 1 1)
1200 1000 800 600 400 200 0 600 500 400 300 200 100 0 600 500 400 300 200 100 0
(4 2 2)
Fig. 1 shows powder XRD result of microwave-hydrothermally as-synthesized (AS) powder. It is clear from the broadening of the peak that the sample is nanocrystalline in nature. The crystallite size of as-synthesized ferrite powder has been estimated with the help of XRD pattern using Scherer’s equation Dm ¼Kl/b cos y, where K is a constant, b is the full width half maxima (FWHM) and l is the wavelength of X-rays used. The average value of crystallite size obtained from XRD is 15 nm. Fig. 2 shows the TEM picture of the assynthesized powder. It can be seen from the figure that the powder particles were uniformly and homogeneously distributed. The powder obtained was free from agglomeration and exhibited high reactivity. The particle size using TEM was found to be 10 nm. Figs. 3 and 4 show the XRD patterns and SEM pictures, respectively, for the three microwave sintered samples, i.e. MS1, MS2 and
Fig. 2. TEM picture of as-synthesized powder.
(2 2 0)
20
Intensity (Counts)
Intensity (counts)
350
MS2
MS1
20
25
30
35
40
45 50 Angle (2θ)
55
Fig. 3. XRD patterns of microwave sintered samples.
60
65
70
75
T. Ramesh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1593–1598
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Fig. 4. SEM micrographs of (a) MS1, (b) MS2 and (c) MS3.
Table 1 Preparation data for microwave sintered samples. Serial no.
Sintering temperature (1C/min)
Bulk density (g/cm3)
TD (%)
Grain size (nm)
Saturation magnetization Ms (emu/g)
Coercivity Hc (Oe)
MS1 MS2 MS3
750 850 950
5.07 5.15 5.27
94 97 98
75 82 95
65 68 71
60 52 40
Fig. 6. EDAX pattern of MS3 sample. Fig. 5. FTIR spectra on microwave sintered samples.
MS3. It can be seen from figures that all samples contain only one single phase. The average grain size was estimated from XRD and SEM photographs and results are presented in Table 1. Grain sizes for microwave sintered samples increase with an increase of the sintering temperature. The bulk densities of the samples were accurately measured using the Archimedes principle and presented in the table. It can be seen from the table that microwave sintering process needs only 30 min to reach 750 1C and to obtain density as high as 94% of theoretical density (TD). The density of microwave sintered samples increases to 98% of TD with an increase of sintering temperature from 750 to 950 1C. Thus, higher densification can be achieved in shorter period using the microwave sintering process. In the present experiment, ferrites have densities in the range of 5.07–5.27 g/cm3. The variation of density with temperature is small compared to the variation of grain size (75–95 nm) of a grain consisting of approximately fifty particles on average. Therefore, grain size, not density, has the dominant effect on various properties. The average value of lattice constant for the presently investigated samples is 8.6423 A˚ and the average value of porosity is 7%. Fig. 5 shows FTIR spectra for all sintered samples under investigation. It can be observed from the figure that two bands are in the range of 400–1000 cm 1; the high frequency band (n1) is in the range 550–610 cm 1 and the low frequency band (n2) is in the range of 410–450 cm 1. These bands are common features for all the ferrites. The vibrations in the tetrahedral (A) sites and octahedral (B)
sites of the unit cell of the cubic spinel cause absorption bands. The absorption band at n1 is due to the stretching vibrations of the tetrahedral metal–oxygen bonds and the absorption band at n2 is due to the stretching vibrations in octahedral metal–oxygen bonds. The shifting of bands towards high frequency is attributed to the decrease in the unit cell dimensions. The increase in the frequency of the absorption bands is attributed to the creation of lattice vacancies. These vacancies retard the vibration of octahedral and tetrahedral groups [12,13]. EDAX analysis has been carried out on all microwave sintered samples to confirm the final composition and it was found that the final composition is same as the starting composition. Thus, high purity samples can be obtained by M-H method. Fig. 6 shows the EDAX pattern of the MS3 sample. Fig. 7 gives the frequency dependence of the real (e0 ) and imaginary (e00 ) parts of permittivity on frequency at room temperature. It can be seen from the figure that the values of e0 and e00 for present ferrites are low and remain constant as the frequency increases from 10 kHz to 500 MHz. With further increase of frequency, the values of e0 and e00 increase with an increase of frequency and show resonance at 1 GHz. This behavior of frequency dependence of e0 can be understood with the help of hopping mechanism. Several investigators observed similar behavior in many ferrite systems [14–16]. In general the performance of the ferrites can be estimated from the studies of dependence of permeability on the frequency. Hence,
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ε
11
ε
1
we have measured the complex permeability spectra with frequency for all the ferrites under investigation and the results obtained are plotted in Fig. 8. It can be seen from the figure that the value of real part of permeability (m0 ) is found to increase from 475 to 655 with an increase of sintering temperature from 750 to 950 1C. From the figure, it can also be observed that the value of real part of permeability (m0 ) for all the samples under investigation remains constant up to 20 MHz. With further increase of frequency the value of m0 is found to increase slightly and shows domain wall dispersion at about 110 MHz. The imaginary part of permeability (m00 ) remained constant up to 20 MHz and then onwards gradually increased with an increase of frequency and took a broad maximum at a frequency of 110 MHz, where the real permeability shows domain wall dispersion. Table 2 gives the values of e0 and e00 for present ferrites at 1 and 100 MHz and the values of m0 and m00 at 1 and 10 MHz at room temperature. The
160 140 120 100 80 60 40 20 200 180 160 140 120 100 80 60 40 20 0 -20 10k
MS1 MS2 MS3
100k
1M
100M
10M
1G
Frequency (Hz)
value of real part of permeability increases with increase in sintering temperature. The complex permeability of the sintered ferrite is related to two different magnetizing mechanisms: the spin rotational magnetization and domain wall motion [17–19]. When these theories are applied to our present ferrites, it can be concluded that the frequency dependence of complex permeability can be understood with the help of domain wall motion contribution. The magnetization measurements for the presently investigated specimens were carried out using a vibrating sample magnetometer (VSM) at room temperature with an applied magnetic field of 12 kOe to reach saturation values. It indicates that ferrites synthesized by microwave hydrothermal method and followed by microwave sintering at 950 1C/30 min exhibit optimum magnetic properties like saturation magnetization of 75 emu/g, remanent magnetization of 18 emu/g and intrinsic coercive force of 40 Oe. Such a high saturation magnetization at 950 1C may be attributed to the high phase purity and well-defined crystallinity in the samples. Thermal variation of initial permeability (mi) for all the microwave sintered samples under investigation has been measured and obtained results are presented in Fig. 9. The initial permeability is found to increase with an increase of sintering temperature for all the samples. As expected at a temperature of 770 K called the Curie temperature (Tc) the value of mi attains a maximum value after which it drops to a small value at nearly 2 K. The Curie temperature is the temperature above which the thermal agitation overcomes the alignment of magnetic moments and causes the material to become paramagnetic. The sharp drop in the initial permeability near the Curie point [20] indicates that our samples possess a good homogeneity. The variation of mi with temperature observed in the presently investigated samples can be understood using the Globus model [21]. The average value of Curie point for the present ferrites is 77071 K.
Fig. 7. Plot of real (e0 ) and imaginary (e00 ) parts of permittivity versus frequency.
3500 1
MS1(μ ) 1 MS2(μ ) 1 MS3(μ ) 11 MS1(μ ) 11 MS2(μ ) 11 MS3(μ )
700 600 500 400 300 200 100
MS3 MS2 MS1
3000 Initial permeability (μi)
Real & Imaginary part of permeability
800
2500 2000 1500 1000
0
500
-100
0
-200 100k
1M
10M
100M
300
1G
400
500
600
700
800
Temperature (K)
Frequency (Hz)
Fig. 9. Temperature variation of initial permeability (mi).
Fig. 8. Plot of frequency variation of complex permeability.
Table 2 Dielectric and permeability data for microwave sintered ferrites. Serial no.
Sample
e0 , 1 MHz
e0 , 100 MHz
ee00 , 1 MHz
ee00 , 100 MHz
m0 , 1 MHz
m0 , 10 MHz
m00 , 1 MHz
m00 , 10 MHz
1 2 3
MS1 MS2 MS3
17 18 19
17 18 19
0.3 0.3 0.04
0.3 0.04 0.04
466 491 647
466 491 647
2.30 2.35 2.26
2.30 2.35 2.26
T. Ramesh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1593–1598
The samples sintered at different temperatures show different behaviors depending on micro-structural differences. Out of all the samples of present investigation, sample MS3 possesses good homogeneity, lowest porosity, high value of saturation magnetization, low dielectric loss and high permeability. Thus, the sample MS3 is selected for further studies. Fig. 10 presents the results of microwave measurements on MS3 sample. The FMR measurements were carried out at Ka band (27–40 GHz) with an external static field H0 applied perpendicular to the plane of sample. FMR frequency f versus external field (H0) is plotted in Fig. 10(a). Here, the resonance frequency decreases with increasing external field. This is related to an unsaturated magnetizing process in which the external field is not beyond 15.5 kOe because the anisotropy [22] field (Ha) of the ferrite [23] is 16.1 kOe. It is observed that a broad distribution of FMR linewidth exists where DH ranges from 2.5 to 3.6 kOe (see Fig. 10(b)). The linewidths broaden with increasing resonance frequency. A similar trend in linewidth with frequency has previously been described in Refs. [24,25]. A detailed FMR discussion is now focused on sample MS3. In this case, we conducted FMR measurements in two different configurations of field, i.e., when the external field was aligned perpendicular and then parallel to the sample plane. The microwave field is always normal to the static field (H0). It is noticed that either
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perpendicular or parallel FMR measurements cause the resonance frequency to decrease with increasing external field. The dependence of resonance frequency on the external field is similar to that of sample MS2. For sample MS3, however, perpendicular FMR measurement exhibits a small shift (500 Oe) in resonance fields forward to a higher field in contrast to the parallel situation. This shift is due to a shape-related demagnetizing field. In the present work, all of the samples are randomly oriented polycrystalline ferrites. The measurements describe FMR behavior for unsaturated magnetization of the nanocrystalline samples. We are unable to observe a linear correlation of resonance and external field over the frequency range 27–40 GHz, which can be described by Kittel’s formulae [22]. The linear part probably appears at higher frequencies or fields that are beyond the present measurement. Since ferrite consists of a number of misaligned grains, the FMR measurement should contain many modes representing different angles between H0 and the c-axes. It is evident from Fig. 10 that the experimental points are distributed in the range of angles, yD ¼801–901. In brief, only the grains having the angle close to 901 enable ferromagnetic resonances observed at Ka band. Fig. 10(b) shows the variation of FMR linewidth with resonance frequency for sample MS3. It is seen that the linewidths vary almost linearly with frequency. The lowest linewidth DH¼ 1.2 kOe appears at 28 GHz and reaches 3.5 kOe when the frequency is 40 GHz. Earlier studies have indicated that for polycrystalline materials the linewidth predominantly stems out from the extrinsic contributions such as porosity, misalignment of anisotropy fields, etc. [26–28], whereas the intrinsic contribution to linewidth is essentially too small to be considered, especially for the randomly oriented polycrystalline materials.
4. Conclusions NiCoMnCu ferrite nanopowders were synthesized using the microwave hydrothermal (M-H) method at 160 1C/40 min. These samples were sintered at low temperatures (750 1C/30 min, 850 1C/30 min and 950 1C/30 min) using microwave sintering method. Real and imaginary parts of permittivity almost remain constant with increase of frequency from 10 kHz to 500 MHz. The value of real part of permeability is found to increase from 475 to 655 with increase of sintering temperature from 750 to 950 1C. Microwave properties of the samples with out-of-plane and inplane ferromagnetic resonance measurements using a shorted waveguide were studied. The resonance frequency decreases with increasing external field and linewidth varies almost linearly with frequency.
Acknowledgment We greatly appreciate the financial support from DAE-BRNS, Mumbai, granting us to perform this work. References
Fig. 10. (a) Resonance frequency (f) versus applied field (H0) for MS3 and MS2 samples and (b) FMR linewidth versus resonance frequency.
[1] Nidhi Saxena Bhattacharyya, G.P. Srivastava, J. Magn. Magn. Mater. 262 (2003) 212. [2] P.S. Anil Kumar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Mater. Lett. 27 (1996) 293. [3] P.D. Baba, G.M. Argentina, IEEE Trans. Microwave Theory Technol. 22 (1974) 652. [4] V.L. Mathe, R.B. Kamble, Mater. Res. Bull. 43 (8–9) (2008) 2160. [5] M.K. Shobana, V. Rajendra, K. Jeyasubramanian, N. Suresh Kumar, Mater. Lett.. 61 (13) (2007) 2616. [6] J.J. Lee, S. Bae, Y.K. Hong, J. Jalli, G.S. Abo, W.M. Seong, S.H. Park, C.J. Choi, J.G. Lee, J. Appl. Phys. 105 (2009) 07A514. [7] R.S. Devan, et al., J. Phys.: Condens. Matter 18 (2006) 9809.
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[8] Nobuhiro Matsushita, Tatsuro Nakamura, Masanori Abe, J. Appl. Phys. 93 (2003) 7133. [9] M. Nagalakshmi, Ph.D. Thesis, Osmani University, Hyderabad, 1999. [10] A. Bhaskar, B. Rajini Kanth, S.R. Murthy, J. Magn. Magn. Mater. 283 (2004) 109. [11] S.R. Murthy, J. Mater. Sci. Lett. 21 (2002) 657. [12] A.K. Ghatage, S.C. Choudhari, S.A. Patil, J. Mater. Sci. Lett. 15 (1996) 1548. [13] A. Adam, Z. Ali, E. Abdeltwab, Y. Abbas, J. Ovonic Res. 5 (2009) 157. [14] A.M. Shaikh, S.S. Bellad, B.K. Chougule, J. Magn. Magn. Mater. 195 (1999) 384. [15] A. Verma, T.C. Goel, R.G. Mendiratta, M.I. Alam, Mater. Sci. Eng. B 60 (1999) 156. [16] A. Verma, T.C. Goel, R.G. Mendiratta, J. Magn. Magn. Mater. 210 (2000) 274. [17] Y. Naito, in: Proceedings of the First International Conference on Ferrites, Tokyo, Japan, 1970, p. 558. [18] T. Nakamura, J. Magn. Magn. Mater. 168 (1996) 285. [19] O.F. Caltun, L. Spinu, A. Stancu, IEEE Trans. Magn. 37 (4) (2001) 2353.
[20] R.F. Soohoo, Theory and Application of Ferrites, Prentice-Hall, USA, 1960. [21] J.L. Doorman, D. Florian, Advances in Chemical Physics, vol. XCVII, John Wiley, New York, 1997. [22] Y. Chen, A.L. Geiler, T. Chen, T. Sakai, C. Vittoria, V.G. Harris, J. Appl. Phys. 101 (2007) 09M501. [23] N. Mo, J.J. Green, P. Krivosik, C.E. Patton, J. Appl. Phys. 101 (2007) 023914. [24] S.V. Lebedev, C.E. Patton, M.A. Wittenauer, L.V. Saraf, R. Ramesh, J. Appl. Phys. 91 (2002) 4426. [25] C. Vittoria, Microwave Properties of Magnetic Films, World Scientific, Singapore, 1993. [26] Y. Chen, T. Sakai, T. Chen, S.D. Yoon, A.L. Geiler, C. Vittoria, V.G. Harris, Appl. Phys. Lett. 88 (2006) 062516-3. [27] U. Hoeppe, H. Benner, Phys. Rev. B. 71 (2005) 144403. [28] B. Parvatheeswara Rao, Ovidiu Caltun, Ioan Dumitru, Leonard Spinu, J. Magn. Magn. Mater. 304 (2) (2006) 752.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis and characterization of NiCoMnCuFe1.96O4 for circulator application T. Ramesh a, R.S. Shinde b, S.R. Murthy a,n a b
Department of Physics, Osmania University, Hyderabad, India Ferrite Laboratory, RRCAT, Indore, India
a r t i c l e in f o
abstract
Article history: Received 7 October 2010 Received in revised form 3 December 2010 Available online 23 December 2010
Modern accelerator design practice includes the use of high-quality ferrites for circulator applications with ever-increasing requirements on power handling ability. Modeling studies of new designs are of increasing economic importance, but are frequently hindered by lack of measured values of the ceramic loss factors. We have developed a nanocrystalline ferrite material with composition Ni0.94Co0.03Mn0.04Cu0.03Fe1.96O4. Nanocrystalline NiCoMnCu ferrite powders were synthesized using a microwavehydrothermal method at 160 1C for 40 min. The ferrite formation conditions, such as pH, temperature and time, were optimized. The phase of the samples was identified by X-ray diffraction and was characterized by Fourier transformation infrared spectroscopy. The size of the nanocrystalline ferrite of as-synthesized powders was 10 nm. The powder was densified at different temperatures using a microwave sintering method. The complex permittivity and permeability of the sintered samples were measured over a frequency range from 10 kHz to 1.8 GHz at room temperature. The applicability of the samples for circulators was tested via the measurement of the ferromagnetic resonance linewidth and the results are presented. & 2010 Elsevier B.V. All rights reserved.
Keywords: Microwave hydrothermal Nanocrystalline ferrite Microwave sintering Complex permittivity Complex permeability FMR linewidth
1. Introduction Nickel cobalt ferrites have attracted attention of researchers for a long time and have been developed as a replacement for yttrium iron garnet (YIG) due to their low cost. They have proved to be very useful for many microwave applications because of their relatively low magnetic and dielectric losses, high-Curie temperature, high saturation magnetization, hysteresis loop, good temperature stability of saturation magnetization and high power handling capability [1] properties offer a performance advantage over other spinel structures. However most ferrites have been produced via the conventional process that is known to have some inherent drawbacks such as poor compositional control, chemical inhomogenity, coarser particle size and introduction of various impurities during ball milling [2]. Furthermore, treatment at high-temperatures induced lower magnetization due to the precipitation of aFe2O3 or the formation of Fe3O4 [3]. In view of this, large scale application of ferrites have prompted the development of various chemical methods, which include hydrothermal, co-precipitation, alkoxide hydrolysis, freeze drying, precursor and sol–gel for the preparations of the desirable stoichiometric and chemically pure spinel ferrites. Several papers have thus far been reported with regard to the structural, electrical and magnetic properties of nickel cobalt ferrite synthesized by different methods [4–7]. The addition of stoichiometrically a small
n
Corresponding author. Tel.: + 914024040217; fax: + 914027090020. E-mail address: [email protected] (S.R. Murthy).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.12.010
amount of Co (0.03) to Ni ferrite controls superparamagnetic behavior and increases resonance frequency significantly [8,9]. Similarly, small amount of Mn has been added to Ni ferrite to enhance saturation magnetization and to control magnetostriction coefficient [10]. In order to improve the density of Ni ferrite, small amount of Cu was added to it. The introduction of Cu improves not only the grain size but also the real permittivity and permeability, which are desirable characteristics for circulator applications. In this study, we attempted a novel method of synthesis called the microwave hydrothermal to produce Co, Cu and Mn-substituted nickel ferrite powders with a composition of Ni0.94Co0.03Mn0.04Cu0.03Fe1.96O4. Synthesized powders were characterized using XRD, TEM and FTIR. Then, the nano-ferrite powders were sintered at three different temperatures using a microwave sintering method [11]. The characterization of sintered samples is done by studying their complex permittivity and complex permeability over a wide range of frequencies from 10 kHz to 1.8 GHz and the results obtained are discussed in this paper.
2. Experimental method Nickel nitrate [Ni(NO3)3 6H2O], cobalt nitrate [Co(NO3)3 6H2O], manganese nitrate [Mn(NO3)3 9H2O], copper nitrate [Cu(NO3)3 6H2O] and ferric nitrate [Fe(NO3)3 9H2O] were measured to their stoichiometric proportions and dissolved in 50 ml of deionized water to obtain a composition: Ni0.94Co0.03Mn0.04Cu0.03Fe1.96O4. An aqueous NaOH solution was added to this mixture until the desired pH ( 9.5) value was obtained. Then the mixture was transferred into a
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T. Ramesh et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1593–1598
recorded using a Brucker tensor 27 DTGS TEC detector spectrophotometer from 1200 to 375 cm 1 by the KBr pellet method. The obtained ferrite powder was mixed with 2 wt% polyvinyl alcohol as a binder. Then the powder was uniaxially pressed at a pressure of 1.5 MPa to form green pellet and toroidal specimens. After the binder was burnt out at 300 1C the compacts were microwave sintered (MS) at three different temperatures, i.e., 750 1C/30 min (MS1), 850 1C/30 min (MS2) and 950 1C/30 min (MS3), in air. The microwave sintering process was carried out using a specially designed applicator that consists of a domestic microwave oven having an output power level tunable up to a maximum of 800 W and operating frequency of 2.45 GHz. The sintering temperatures of the samples were measured using a platinum-13% pt-Rh thermocouple with an accuracy of 71 1C. The rate of heating and cooling during the microwave sintering was 20 1C/min. Complex permittivity and permeability of the sintered samples were measured in the frequency range of 10 kHz–1.8 GHz using an LCR meter and Agilent 4291B impedance analyzer. Magnetic properties such as saturation magnetization (Ms) and coercivity (Hc) are obtained from recorded hysteresis loops using a vibration sample magnetometer (VSM, Lakeshore, Model 7300). Microwave properties were characterized by the out-of-plane and in-plane ferromagnetic resonance (FMR) measurements using a shorted waveguide. The external magnetic field was swept at a fixed frequency using a TE10 mode propagation in a Ka-band (26–40 GHz) waveguide. The microwave magnetic field hrf was applied in the plane of the ferrite disk, which is always normal to the external field.
Teflon lined vessel and treated using a microwave digestion system (Model MDS-2000, CEM Corp., Mathews, NC) at 160 1C/40 min. Our microwave system uses 2.45 GHz microwaves and can operate at 0–100% of its full power (630750 W). The products obtained were filtered and washed repeatedly with deionized water followed by drying overnight. Then the synthesized powder was weighted; the percentage yield was calculated and found to be 96%. The synthesized powder was characterized through powder X-ray diffraction (XRD) and particle size and morphology were determined using transmission electron microscopy (TEM; Model JEM-2010, JEOL, Tokyo, Japan). The spectra of the present samples obtained from Fourier transformation infrared spectroscopy (FTIR) have been
AS
300 250 200 150 100 50 30
40
50
70
60
80
2θ Fig. 1. XRD pattern of M-H as-synthesized powder.
3. Results and discussion
MS3 (6 2 0)
(4 4 0)
(3 3 3)
(4 0 0)
(2 2 2)
(3 1 1)
1200 1000 800 600 400 200 0 600 500 400 300 200 100 0 600 500 400 300 200 100 0
(4 2 2)
Fig. 1 shows powder XRD result of microwave-hydrothermally as-synthesized (AS) powder. It is clear from the broadening of the peak that the sample is nanocrystalline in nature. The crystallite size of as-synthesized ferrite powder has been estimated with the help of XRD pattern using Scherer’s equation Dm ¼Kl/b cos y, where K is a constant, b is the full width half maxima (FWHM) and l is the wavelength of X-rays used. The average value of crystallite size obtained from XRD is 15 nm. Fig. 2 shows the TEM picture of the assynthesized powder. It can be seen from the figure that the powder particles were uniformly and homogeneously distributed. The powder obtained was free from agglomeration and exhibited high reactivity. The particle size using TEM was found to be 10 nm. Figs. 3 and 4 show the XRD patterns and SEM pictures, respectively, for the three microwave sintered samples, i.e. MS1, MS2 and
Fig. 2. TEM picture of as-synthesized powder.
(2 2 0)
20
Intensity (Counts)
Intensity (counts)
350
MS2
MS1
20
25
30
35
40
45 50 Angle (2θ)
55
Fig. 3. XRD patterns of microwave sintered samples.
60
65
70
75
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Fig. 4. SEM micrographs of (a) MS1, (b) MS2 and (c) MS3.
Table 1 Preparation data for microwave sintered samples. Serial no.
Sintering temperature (1C/min)
Bulk density (g/cm3)
TD (%)
Grain size (nm)
Saturation magnetization Ms (emu/g)
Coercivity Hc (Oe)
MS1 MS2 MS3
750 850 950
5.07 5.15 5.27
94 97 98
75 82 95
65 68 71
60 52 40
Fig. 6. EDAX pattern of MS3 sample. Fig. 5. FTIR spectra on microwave sintered samples.
MS3. It can be seen from figures that all samples contain only one single phase. The average grain size was estimated from XRD and SEM photographs and results are presented in Table 1. Grain sizes for microwave sintered samples increase with an increase of the sintering temperature. The bulk densities of the samples were accurately measured using the Archimedes principle and presented in the table. It can be seen from the table that microwave sintering process needs only 30 min to reach 750 1C and to obtain density as high as 94% of theoretical density (TD). The density of microwave sintered samples increases to 98% of TD with an increase of sintering temperature from 750 to 950 1C. Thus, higher densification can be achieved in shorter period using the microwave sintering process. In the present experiment, ferrites have densities in the range of 5.07–5.27 g/cm3. The variation of density with temperature is small compared to the variation of grain size (75–95 nm) of a grain consisting of approximately fifty particles on average. Therefore, grain size, not density, has the dominant effect on various properties. The average value of lattice constant for the presently investigated samples is 8.6423 A˚ and the average value of porosity is 7%. Fig. 5 shows FTIR spectra for all sintered samples under investigation. It can be observed from the figure that two bands are in the range of 400–1000 cm 1; the high frequency band (n1) is in the range 550–610 cm 1 and the low frequency band (n2) is in the range of 410–450 cm 1. These bands are common features for all the ferrites. The vibrations in the tetrahedral (A) sites and octahedral (B)
sites of the unit cell of the cubic spinel cause absorption bands. The absorption band at n1 is due to the stretching vibrations of the tetrahedral metal–oxygen bonds and the absorption band at n2 is due to the stretching vibrations in octahedral metal–oxygen bonds. The shifting of bands towards high frequency is attributed to the decrease in the unit cell dimensions. The increase in the frequency of the absorption bands is attributed to the creation of lattice vacancies. These vacancies retard the vibration of octahedral and tetrahedral groups [12,13]. EDAX analysis has been carried out on all microwave sintered samples to confirm the final composition and it was found that the final composition is same as the starting composition. Thus, high purity samples can be obtained by M-H method. Fig. 6 shows the EDAX pattern of the MS3 sample. Fig. 7 gives the frequency dependence of the real (e0 ) and imaginary (e00 ) parts of permittivity on frequency at room temperature. It can be seen from the figure that the values of e0 and e00 for present ferrites are low and remain constant as the frequency increases from 10 kHz to 500 MHz. With further increase of frequency, the values of e0 and e00 increase with an increase of frequency and show resonance at 1 GHz. This behavior of frequency dependence of e0 can be understood with the help of hopping mechanism. Several investigators observed similar behavior in many ferrite systems [14–16]. In general the performance of the ferrites can be estimated from the studies of dependence of permeability on the frequency. Hence,
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ε
11
ε
1
we have measured the complex permeability spectra with frequency for all the ferrites under investigation and the results obtained are plotted in Fig. 8. It can be seen from the figure that the value of real part of permeability (m0 ) is found to increase from 475 to 655 with an increase of sintering temperature from 750 to 950 1C. From the figure, it can also be observed that the value of real part of permeability (m0 ) for all the samples under investigation remains constant up to 20 MHz. With further increase of frequency the value of m0 is found to increase slightly and shows domain wall dispersion at about 110 MHz. The imaginary part of permeability (m00 ) remained constant up to 20 MHz and then onwards gradually increased with an increase of frequency and took a broad maximum at a frequency of 110 MHz, where the real permeability shows domain wall dispersion. Table 2 gives the values of e0 and e00 for present ferrites at 1 and 100 MHz and the values of m0 and m00 at 1 and 10 MHz at room temperature. The
160 140 120 100 80 60 40 20 200 180 160 140 120 100 80 60 40 20 0 -20 10k
MS1 MS2 MS3
100k
1M
100M
10M
1G
Frequency (Hz)
value of real part of permeability increases with increase in sintering temperature. The complex permeability of the sintered ferrite is related to two different magnetizing mechanisms: the spin rotational magnetization and domain wall motion [17–19]. When these theories are applied to our present ferrites, it can be concluded that the frequency dependence of complex permeability can be understood with the help of domain wall motion contribution. The magnetization measurements for the presently investigated specimens were carried out using a vibrating sample magnetometer (VSM) at room temperature with an applied magnetic field of 12 kOe to reach saturation values. It indicates that ferrites synthesized by microwave hydrothermal method and followed by microwave sintering at 950 1C/30 min exhibit optimum magnetic properties like saturation magnetization of 75 emu/g, remanent magnetization of 18 emu/g and intrinsic coercive force of 40 Oe. Such a high saturation magnetization at 950 1C may be attributed to the high phase purity and well-defined crystallinity in the samples. Thermal variation of initial permeability (mi) for all the microwave sintered samples under investigation has been measured and obtained results are presented in Fig. 9. The initial permeability is found to increase with an increase of sintering temperature for all the samples. As expected at a temperature of 770 K called the Curie temperature (Tc) the value of mi attains a maximum value after which it drops to a small value at nearly 2 K. The Curie temperature is the temperature above which the thermal agitation overcomes the alignment of magnetic moments and causes the material to become paramagnetic. The sharp drop in the initial permeability near the Curie point [20] indicates that our samples possess a good homogeneity. The variation of mi with temperature observed in the presently investigated samples can be understood using the Globus model [21]. The average value of Curie point for the present ferrites is 77071 K.
Fig. 7. Plot of real (e0 ) and imaginary (e00 ) parts of permittivity versus frequency.
3500 1
MS1(μ ) 1 MS2(μ ) 1 MS3(μ ) 11 MS1(μ ) 11 MS2(μ ) 11 MS3(μ )
700 600 500 400 300 200 100
MS3 MS2 MS1
3000 Initial permeability (μi)
Real & Imaginary part of permeability
800
2500 2000 1500 1000
0
500
-100
0
-200 100k
1M
10M
100M
300
1G
400
500
600
700
800
Temperature (K)
Frequency (Hz)
Fig. 9. Temperature variation of initial permeability (mi).
Fig. 8. Plot of frequency variation of complex permeability.
Table 2 Dielectric and permeability data for microwave sintered ferrites. Serial no.
Sample
e0 , 1 MHz
e0 , 100 MHz
ee00 , 1 MHz
ee00 , 100 MHz
m0 , 1 MHz
m0 , 10 MHz
m00 , 1 MHz
m00 , 10 MHz
1 2 3
MS1 MS2 MS3
17 18 19
17 18 19
0.3 0.3 0.04
0.3 0.04 0.04
466 491 647
466 491 647
2.30 2.35 2.26
2.30 2.35 2.26
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The samples sintered at different temperatures show different behaviors depending on micro-structural differences. Out of all the samples of present investigation, sample MS3 possesses good homogeneity, lowest porosity, high value of saturation magnetization, low dielectric loss and high permeability. Thus, the sample MS3 is selected for further studies. Fig. 10 presents the results of microwave measurements on MS3 sample. The FMR measurements were carried out at Ka band (27–40 GHz) with an external static field H0 applied perpendicular to the plane of sample. FMR frequency f versus external field (H0) is plotted in Fig. 10(a). Here, the resonance frequency decreases with increasing external field. This is related to an unsaturated magnetizing process in which the external field is not beyond 15.5 kOe because the anisotropy [22] field (Ha) of the ferrite [23] is 16.1 kOe. It is observed that a broad distribution of FMR linewidth exists where DH ranges from 2.5 to 3.6 kOe (see Fig. 10(b)). The linewidths broaden with increasing resonance frequency. A similar trend in linewidth with frequency has previously been described in Refs. [24,25]. A detailed FMR discussion is now focused on sample MS3. In this case, we conducted FMR measurements in two different configurations of field, i.e., when the external field was aligned perpendicular and then parallel to the sample plane. The microwave field is always normal to the static field (H0). It is noticed that either
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perpendicular or parallel FMR measurements cause the resonance frequency to decrease with increasing external field. The dependence of resonance frequency on the external field is similar to that of sample MS2. For sample MS3, however, perpendicular FMR measurement exhibits a small shift (500 Oe) in resonance fields forward to a higher field in contrast to the parallel situation. This shift is due to a shape-related demagnetizing field. In the present work, all of the samples are randomly oriented polycrystalline ferrites. The measurements describe FMR behavior for unsaturated magnetization of the nanocrystalline samples. We are unable to observe a linear correlation of resonance and external field over the frequency range 27–40 GHz, which can be described by Kittel’s formulae [22]. The linear part probably appears at higher frequencies or fields that are beyond the present measurement. Since ferrite consists of a number of misaligned grains, the FMR measurement should contain many modes representing different angles between H0 and the c-axes. It is evident from Fig. 10 that the experimental points are distributed in the range of angles, yD ¼801–901. In brief, only the grains having the angle close to 901 enable ferromagnetic resonances observed at Ka band. Fig. 10(b) shows the variation of FMR linewidth with resonance frequency for sample MS3. It is seen that the linewidths vary almost linearly with frequency. The lowest linewidth DH¼ 1.2 kOe appears at 28 GHz and reaches 3.5 kOe when the frequency is 40 GHz. Earlier studies have indicated that for polycrystalline materials the linewidth predominantly stems out from the extrinsic contributions such as porosity, misalignment of anisotropy fields, etc. [26–28], whereas the intrinsic contribution to linewidth is essentially too small to be considered, especially for the randomly oriented polycrystalline materials.
4. Conclusions NiCoMnCu ferrite nanopowders were synthesized using the microwave hydrothermal (M-H) method at 160 1C/40 min. These samples were sintered at low temperatures (750 1C/30 min, 850 1C/30 min and 950 1C/30 min) using microwave sintering method. Real and imaginary parts of permittivity almost remain constant with increase of frequency from 10 kHz to 500 MHz. The value of real part of permeability is found to increase from 475 to 655 with increase of sintering temperature from 750 to 950 1C. Microwave properties of the samples with out-of-plane and inplane ferromagnetic resonance measurements using a shorted waveguide were studied. The resonance frequency decreases with increasing external field and linewidth varies almost linearly with frequency.
Acknowledgment We greatly appreciate the financial support from DAE-BRNS, Mumbai, granting us to perform this work. References
Fig. 10. (a) Resonance frequency (f) versus applied field (H0) for MS3 and MS2 samples and (b) FMR linewidth versus resonance frequency.
[1] Nidhi Saxena Bhattacharyya, G.P. Srivastava, J. Magn. Magn. Mater. 262 (2003) 212. [2] P.S. Anil Kumar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Mater. Lett. 27 (1996) 293. [3] P.D. Baba, G.M. Argentina, IEEE Trans. Microwave Theory Technol. 22 (1974) 652. [4] V.L. Mathe, R.B. Kamble, Mater. Res. Bull. 43 (8–9) (2008) 2160. [5] M.K. Shobana, V. Rajendra, K. Jeyasubramanian, N. Suresh Kumar, Mater. Lett.. 61 (13) (2007) 2616. [6] J.J. Lee, S. Bae, Y.K. Hong, J. Jalli, G.S. Abo, W.M. Seong, S.H. Park, C.J. Choi, J.G. Lee, J. Appl. Phys. 105 (2009) 07A514. [7] R.S. Devan, et al., J. Phys.: Condens. Matter 18 (2006) 9809.
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[8] Nobuhiro Matsushita, Tatsuro Nakamura, Masanori Abe, J. Appl. Phys. 93 (2003) 7133. [9] M. Nagalakshmi, Ph.D. Thesis, Osmani University, Hyderabad, 1999. [10] A. Bhaskar, B. Rajini Kanth, S.R. Murthy, J. Magn. Magn. Mater. 283 (2004) 109. [11] S.R. Murthy, J. Mater. Sci. Lett. 21 (2002) 657. [12] A.K. Ghatage, S.C. Choudhari, S.A. Patil, J. Mater. Sci. Lett. 15 (1996) 1548. [13] A. Adam, Z. Ali, E. Abdeltwab, Y. Abbas, J. Ovonic Res. 5 (2009) 157. [14] A.M. Shaikh, S.S. Bellad, B.K. Chougule, J. Magn. Magn. Mater. 195 (1999) 384. [15] A. Verma, T.C. Goel, R.G. Mendiratta, M.I. Alam, Mater. Sci. Eng. B 60 (1999) 156. [16] A. Verma, T.C. Goel, R.G. Mendiratta, J. Magn. Magn. Mater. 210 (2000) 274. [17] Y. Naito, in: Proceedings of the First International Conference on Ferrites, Tokyo, Japan, 1970, p. 558. [18] T. Nakamura, J. Magn. Magn. Mater. 168 (1996) 285. [19] O.F. Caltun, L. Spinu, A. Stancu, IEEE Trans. Magn. 37 (4) (2001) 2353.
[20] R.F. Soohoo, Theory and Application of Ferrites, Prentice-Hall, USA, 1960. [21] J.L. Doorman, D. Florian, Advances in Chemical Physics, vol. XCVII, John Wiley, New York, 1997. [22] Y. Chen, A.L. Geiler, T. Chen, T. Sakai, C. Vittoria, V.G. Harris, J. Appl. Phys. 101 (2007) 09M501. [23] N. Mo, J.J. Green, P. Krivosik, C.E. Patton, J. Appl. Phys. 101 (2007) 023914. [24] S.V. Lebedev, C.E. Patton, M.A. Wittenauer, L.V. Saraf, R. Ramesh, J. Appl. Phys. 91 (2002) 4426. [25] C. Vittoria, Microwave Properties of Magnetic Films, World Scientific, Singapore, 1993. [26] Y. Chen, T. Sakai, T. Chen, S.D. Yoon, A.L. Geiler, C. Vittoria, V.G. Harris, Appl. Phys. Lett. 88 (2006) 062516-3. [27] U. Hoeppe, H. Benner, Phys. Rev. B. 71 (2005) 144403. [28] B. Parvatheeswara Rao, Ovidiu Caltun, Ioan Dumitru, Leonard Spinu, J. Magn. Magn. Mater. 304 (2) (2006) 752.