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Magnetic and magnetostrictive properties of Cu substituted Co-ferrites
Author’s Accepted Manuscript Magnetic and magnetostrictive properties of CU substituted CO-ferrites B. Chandra Sekhar, G.S.N. Rao, O.F. Caltun, B. Dhana Lakshmi, B. Parvatheeswara Rao, P.S.V. Subba Rao www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(15)30572-2 http://dx.doi.org/10.1016/j.jmmm.2015.09.028 MAGMA60636
To appear in: Journal of Magnetism and Magnetic Materials Received date: 23 July 2015 Revised date: 24 August 2015 Accepted date: 7 September 2015 Cite this article as: B. Chandra Sekhar, G.S.N. Rao, O.F. Caltun, B. Dhana Lakshmi, B. Parvatheeswara Rao and P.S.V. Subba Rao, Magnetic and magnetostrictive properties of CU substituted CO-ferrites, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.09.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic and magnetostrictive properties of Cu substituted Co-ferrites B. Chandra Sekhar1, G.S.N. Rao2, O.F. Caltun3, B. Dhana Lakshmi4, B. Parvatheeswara Rao4* and P.S.V. Subba Rao4 1
Vignan’s Institute of Engineering for Women, Visakhapatnam 530046, India 2 Dr. V. S. Krishna Govt. Degree College, Visakhapatnam, India 3 Department of Physics, A.I. Cuza University, Iasi, Romania 4 Department of Physics, Andhra University, Visakhapatnam 530003, India
Abstract Copper substituted cobalt ferrite, Co1-xCuxFe2O4 (x=0.00-0.25), nanoparticles were synthesized by sol-gel autocombustion method. X-ray diffraction analysis on the samples was done to confirm the cubic spinel structures and Scherrer equation was used to estimate the mean crystallite size as 40 nm. Using the obtained nanoparticles, fabrication of the sintered pellets was done by standard ceramic technique. Magnetic and magnetostrictive measurements on the samples were made by strain gauge and vibrating sample magnetometer techniques, respectively. Maximum magnetostriction and strain derivative values were deduced from the field dependent magnetostriction curves while the magnetic parameters such as saturation magnetization (51.7-61.9 emu/g) and coercivity (1045-1629 Oe) on the samples were estimated from the obtained magnetic hysteresis loops. Curie temperature values (457-315 oC) were measured by a built in laboratory set-up. Copper substituted cobalt ferrites have shown improved strain derivative values as compared to the pure cobalt ferrite and thus making them suitable for stress sensing applications. The results have been explained on the basis of cationic distributions, strength of exchange interactions and net decreased anisotropic contributions due to the increased presence of Co2+ ions in B-sites as a result of Cu substitutions. Keywords: Co-Cu ferrite nanoparticles, Sol-gel autocombustion, Maximum magnetostriction, Strain derivative, Curie temperature, Torque sensor applications *Corresponding author e-mail: [email protected]
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1.
Introduction Ferrites exhibit a wide range of electromagnetic properties useful for a variety of
technological applications. Nickel-zinc and manganese-zinc ferrites are the most explored over a wide frequency range for electronic device applications as they exhibit high permeability, high magnetization, high resistivity and low core losses [1]. Metal bonded cobalt ferrites, on the other hand, with high magnetizations and high values of negative magnetostriction are found promising for magnetomechanical stress/strain sensing and actuating applications [2]. Till recently, high magnetostrictive materials, such as Terfenol, SmFe2 and other rare earth iron compounds are widely used as sensing materials [3,4]. However, the rare earth based composites exhibit magnetomechanical hysteresis apart from low sensitivity to stress and thus provide only limited scope for sensing due to their high values of anisotropy besides other disadvantages such as poor mechanical stability and high costs [5]. In order to enhance the magnetostrictive performance, current research is focused on obtaining a substituted cobalt ferrite material, which exhibits higher magnetostrictive strains at lower magnetic field strengths (larger strain derivative). Several efforts have been made by exploring different processing and compositional modifications to the cobalt ferrite to obtain better magnetic and magnetostrictive properties. In this connection, it was reported that the annealing of cobalt ferrite can lead to alteration of the cation distribution among the octahedral and tetrahedral sites and there by leads to resulting in useful magnetic properties [6]. Also from the view point of chemical compositional modifications, it was observed that the substitution of Mn for either Fe or Co was seen as a promising measure to bring in structural changes as well as improvements in strain derivative due to corresponding variations in cation distributions [7]. Moreover, it was also shown that the saturation magnetization, coercivity, Curie temperature and magnetostriction of the cobalt ferrite can be desirably altered by adjusting the Mn content [8]. In order to obtain further improvements in strain derivative, it is now aimed at investigating the effects of copper ions when they replace cobalt ions in Co ferrites. Based on the obtained data and analysis, a suitable composition with the desirable characteristics of high strain derivative, moderate values of maximum magnetostriction and saturation magnetization and low Curie temperature as a good sensing material has been proposed.
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Experimental details Co-Cu ferrite nanoparticles with the chemical formula, Co1-xCuxFe2O4, where x varies
from 0.00 to 0.25 in steps of 0.05, have been prepared by sol-gel autocombustion method. For this purpose, analytical reagent grade cobalt nitrate, copper nitrate, iron nitrate, citric acid and ammonia were used in desired proportions as raw materials. All the chemicals were purchased from Himedia, Mumbai, India and used as-obtained without any further modifications. The solutions of the metal nitrate salts and citric acid in desired proportions were prepared separately by using minimum amounts of deionized water and mixed in 1:1 molar ratio to form an aqueous solution, a method that maximizes molecular mixing of the components [9]. The pH of the mixed solution was adjusted to 7 using ammonia. The solution was then heated up to 80°C while stirring to transform the same into dried gel. At this stage, the temperature of the container was further increased to 110 oC only to be ignited at any point of time. Upon ignition, the dried gel burns in a self propagating combustion manner until all the gels were completely burnt out to form ash like flakes. These flakes were then neatly collected and collapsed by using a glass rod or spatula for ultimately making them to be fluffy loose powders. The X-ray diffraction patterns of the obtained samples confirm single phase cubic spinel structures [10]. Fabrication of ferrite pellets using the obtained nanoparticles (crystallites of about 40 nm in size as estimated from Scherrer equation) was done by standard ceramic procedure [11]. The ferrite nanoparticles of all the samples were lightly ground for a few hours in the presence of methanol, air dried and granulated by using 5% poly vinyl alcohol (PVA) as a binder. The granulated powder was then pressed into pellets of 12 mm diameter and 3mm thickness at a pressure of 150 MPa. The resulting compacts of the ferrite system were then sintered at a temperature of 1050°C for 4 hours in air atmosphere before switching of the furnace and allowing the samples to cool normally. 3
Results and discussion
3.1
Saturation magnetization Magnetic hysteresis loops of the Cu substituted cobalt ferrite samples at room temperature are
shown in Fig. 1. The curves of all the samples exhibit typical soft ferrimagnetic nature with moderate coercivities.
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Fig.1. Room temperature magnetic hysteresis loops of Co1-xCuxFe2O4.
Saturation magnetization and coercivity values for each sample in the ferrite system has been extracted from the corresponding loop and the variation of saturation magnetization as a function of substituent ion concentration is shown in Fig. 2.
Fig. 2.Variation of saturation magnetization with concentration (x) in Co1-xCuxFe2O4.
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It is evident from the figure that the Cu substitution in Co1-xCuxFe2O4 system throughout the range of concentrations investigated has resulted slightly lower values of saturation magnetization compared to the basic cobalt ferrite. Among different Cu concentrations, the samples with x=0.05 and 0.2 are marked by lower values of saturation magnetization. The observed variations in saturation magnetization as a function of substituent concentration can be explained on the basis of super exchange interactions among tetrahedral (A) and octahedral [B] site ions in the spinel lattice. Generally, in ferrites, Neel [12] considered three kinds of exchange interactions between unpaired electrons of the ions lying (i) both at A-sites (A-A interaction), (ii) both at B-sites (B-B interaction) and (iii) one at A-site and another at B-site (A-B interaction). Out of these three interactions, A-B interaction is predominant over B-B and A-A interactions. These interactions tend to align all the magnetic spins at A-site in one direction and those at B-site in the opposite direction. The net magnetic moment of the lattice is therefore the difference between the magnetic moments of B- and A-sublattices, i.e. MB - MA. The exchange interactions in the spinel lattice can greatly be influenced by the nature of the ions present at both A- and B-sublattices. The distribution of cations in tetrahedral and octahedral sites in the spinel structure is necessary to discuss the compositional dependence of the saturation magnetization. It takes a long way to provide the correct distribution of cations from the consideration of our measurements on various properties. Before attaining the accurate cation distribution to the Co1-xCuxFe2O4 system, it is first considered necessary to examine the cation distribution of the CoFe2O4, which can be represented by:
Co
y
Fe1 y Co1 y Fe1 y O4
wherein the value of y is reported to be around 0.19 in many works [13,14]. As per the crystal field stabilization energies for occupation of cations in spinel lattice sites, the Cu2+ ions have specific preferences for B-sites [15].
In the present Co1-xCuxFe2O4 system,
incorporation of Cu2+ ions (1μB) with low magnetic moment per ion in place of Co2+ ions (3μB) with high magnetic moment per ion would naturally be expected to reduce the net magnetic moment of the system in every step of copper substitution. The observed behaviour of saturation magnetization is also marked by lower values for all the concentrations compared to that of the basic cobalt ferrite.
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However, the decrease in magnetization with the Cu substitution is not linear implying that the substituent copper ions are not completely residing in B-sites only. The initial decrease of saturation magnetization however is in support of the argument that Cu2+ ions prefer to occupy the B-sites and thereby decreasing the B-sublattice magnetization as well as net magnetization for this sample. The observed deviation from this trend in resulting relatively higher values of magnetization for the subsequent two concentrations of Cu2+ ions (1μB) up to x=0.15 suggests that the copper substitution might lead to migration of some of the Co2+ ions (3μB) from B-sites to A-sites so as to force an equal amount of Fe3+ ions (5μB) from there moving to the displaced sites in B-sublattice. This kind of distribution helps to retain the B-sublattice magnetization remains similar but decreases the Asublattice magnetization with increased density of Co2+ ions and decreased population of Fe3+ ions in those sites, and thus explains the higher values of magnetization for these concentrations. And, the saturation magnetization behavior at concentrations beyond x=0.15 might be governed by the possibility of occupation of some of the Cu2+ ions in A-sites as well and the consequent changes in chemical environment of the B-sites. The ionic distribution and the corresponding strengths of magnetic moments for this system can be largely expressed with the formulae as under:
Co
2 y
Cuz2 Fe13y z Co12x y Cux2z Fe13y z O4
M ( x) (1 x y ) M Co 1 y z M Fe y M Co (1 y z ) M Fe 3 4 y 10 z 3x Here, the x is the substituted copper content at B-sites, y is the cobalt content at A-sites and z is the copper content at A-sites.
3.2
Maximum magnetostriction Magnetostriction curves of the Co1-xCuxFe2O4 system as a function of applied magnetic field
are shown in Fig. 3. Maximum magnetostriction and strain derivative values have been estimated from these curves. The variation of maximum magnetostriction as a function of substituent concentration is shown in Fig. 4. Contrary to expectation the maximum magnetostriction has been observed to decrease rapidly with increasing copper ion concentration in Co1-xCuxFe2O4 system.
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The observed variation can be discussed on the basis of magnetostrictive contributions of various cations present in the system, their valence states and site occupancy in a given ferrite composition. In cobalt ferrite based systems, the large negative magnetostrictive contributions of Co2+ ions, and large positive and sizeable negative magnetostrictive contribution of Fe2+ and Fe3+ ions are to be considered in explaining the magnetostrictive properties [16].
Fig.3. Magnetostriction curves as a function of magnetic field for Co1-xCuxFe2O4.
Fig. 4. Variation of maximum magnetostriction with concentration (x) in Co1-xCuxFe2O4.
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Besides, the magnetostrictive contributions of the substituted cations, either positive or negative, tend to alter the net magnetostriction of the system because of the corresponding changes in cation distribution arising out of these substitutions. In this context, it may be pointed out that the tetrahedral site magnetostrictive contribution of a cation is known to be lesser and opposite in nature compared to that of the octahedral site contribution for any given ion. As reported in our earlier work of CoMnxFe2-xO4 system [17], wherein also the maximum magnetostriction was observed to decrease with increasing Mn concentration, the manganese exists in Mn3+ state and replaces Fe3+ ions at octahedral sites. However, in the process of Mn ions displacing Fe ions at B-sites, a small amount of Co2+ ions were forced to migrate to A-sites resulting in transfer of an equal amount of Fe ions from there to octahedral sites, and this transfer is assumed to be proportional to Mn concentration. Therefore, the observed decrease in magnetostriction in that system was attributed to the increasing population of Mn3+ ions of positive magnetostrictive contribution and decreasing population of both Co2+ and Fe3+ ions of negative magnetostrictive contributions at B- sites [18]. However, in the present Co1-xCuxFe2O4 system, the decreasing trend of maximum magnetostriction all along the copper substitutions has been rapid and also such incorporation of copper ions in place of cobalt ions does not cause a rapid decrease in the magnetization in this system. Therefore, the observed decrease may be related not only to the occupation of Cu2+ ions in both the lattice sites but also to the John-Teller type of distortion caused by the copper ions in the lattice [19]. There have been several reports indicating the copper ions to cause lattice distortions particularly when they attain in different valence states such as Cu1+ Cu2+ in the same lattice site [20]. In such case, the system should exhibit a gradual decrease in saturation magnetostriction with increasing copper concentration. This argument holds good for the observed decrease of magnetostriction for the entire range of concentrations of copper in the present study. 3.3
Strain Derivative (dλ/dH): Variation of magnetostrictive strain derivative d/dH as a function of substituent
concentration in the Co1-xCuxFe2O4 system is shown in Fig. 5. The strain derivative in Co-Cu ferrite system has been observed to increase initially up to x = 0.15 followed by a decrease for higher concentrations. High value of strain derivative is recorded for copper substitution at x = 0.15. It is
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interesting to note here that the copper containing system exhibited an opposite trend of strain derivative compared to that of the manganese substituted cobalt ferrite system particularly at the initial substituent concentrations [17].
Fig. 5. Variation of strain derivative with concentration (x) in Co1-xCuxFe2O4.
The observed variations in the strain derivative with substituent concentration can be understood on the basis of magnetic anisotropy and magnetostrictive contributions of the cations present in the sample because the strain derivative depends on both the magnetostriction and anisotropy of the material [21]. For small changes of magnetic field (H) and applied stress (σ), a thermodynamic relation [22] exists as
d dB . d H d H Further, it is well-known from the single ion anisotropy study that the strain derivative is proportional to the ratio of saturation magnetostriction λmax to cubic anisotropic constant K [23]. The ratio of magnetostriction to anisotropy is an important factor in determining the sensitivity of the magnetization to stress which can be expressed as
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d B d 2 0 S M d d H NK which implies that the strain derivative is proportional to the ratio of magnetostriction to anisotropy constant, /K. The orbital degeneracy of an ion also plays an important role in determining the magnetic anisotropy. There exists a relation between magnetic anisotropy and orbital degeneracy of the ground state of transition metal ions. Magnetic anisotropy cannot be expected in case the orbital state of an ion is non-degenerate. In cobalt ferrite, the magnetic anisotropy has been found to be proportional to cobalt content. Also, Co2+ ions at octahedral sites are known to create stronger anisotropy through trigonal distortion whereas cobalt ions at tetrahedral sites contribute little anisotropy to the cobalt ferrite compound [24]. In Co1-xCuxFe2O4 system, substitution of Cu2+ ions in place of Co2+ ions in Bsites is not likely to make a straight forward replacement but forces the cobalt ions to migrate into Asites and thereby moving Fe3+ ions from there to B-sites. This process increases the amount of Co2+ ions in the A-sites and the smaller degeneracy associated with Co2+ ions over Fe3+ ions at tetrahedral sites results in a decreased anisotropy and increased λ/K. The observed increase in strain derivative up to x = 0.15 can be explained on the basis of decreased anisotropic contribution of the compound. As discussed earlier in magnetostriction studies, the amount of copper entered into lattice up to x = 0.15 has been considered to be maximum and the same might exist in the subsequent concentrations too in the system. Generally in soft magnetic materials, the presence of a non-magnetic second phase hinders the magnetization processes and decreases the strain derivative in low fields [25]. The pores and segregations can lead to pinning of domain walls thereby affecting the response of the domains to the applied field and reducing the strain derivative at higher concentrations of copper. Moreover, occupation of some of the copper ions in A-sites at higher concentrations of copper also changes the situation described above resulting in modifications to the anisotropy and thereby the reduction of strain derivative for these concentrations as observed in Fig. 5. 3.4
Coercivity The observed variation of coercivity as a function of Cu concentration in Co1-xCuxFe2O4
system is shown in Fig. 6. After an initial increase in coercivity at x = 0.05, a gradual fall for the
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remaining concentrations of copper is observed for the system. And, the magnitude of fall from the basic cobalt ferrite (x=0.0) to the highest copper substituted ferrite (x=0.25) also appears considerable indicating that the anisotropic and the microstructural modifications leading to coercivity variations in the substituted ferrite system play an important role.
Fig. 6. Variation of coercivity with concentration (x) in Co1-xCuxFe2O4.
Besides, coercivity in a ferrite system is known to depend on various parameters like magnetocrystalline anisotropy, lattice imperfections, dislocations, internal strains, particle size and secondary phases [26]. The observed initial increase in coercivity in Co1-xCuxFe2O4 could be due to the segregation of copper at grain boundaries as second phase. The sharp decrease of coercivity and a similar increase in strain derivative, in the range of concentrations from x = 0.05 to 0.15, confirms our argument made earlier in magnetization part on cobalt migration to A-sites. 3.5
Curie temperature The observed variation of Curie temperature for the Co1-xCuxFe2O4 system is shown in Fig. 7.
It has been observed to decrease gradually with increasing Cu concentration up to x=0.2 and then a rapid decrease was observed for x=0.25 sample.
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Fig. 7. Variation of Curie temperature with concentration (x) in Co1-xCuxFe2O4.
It is well known that the Curie temperature depends not only on the occupation of magnetic cations in different sublattices but also on the density of magnetic cations and their corresponding magnetic strengths in the sublattices [27]. In the Co1-xCuxFe2O4 system, since both the substituted (Cu2+) and replaced (Co2+) cations are magnetic only, the density of the magnetic cations remains the same and does not explain the observed Curie temperature variation. On the other hand, the occupation of the cations in different sublattices as well as the exchange interactions between them based on the magnetic strength of the cations involved must be governing the magnitude of the Curie temperature. Among all the exchange interactions, the A-B interaction between (FeA-O-FeB) is the strongest compared to (CoA-O-FeB), (FeA-O-CoB) and (FeA-O-CuB) interactions. Therefore, rapid decrease in Curie temperature can be expected only when there is weakening of (FeA-O-FeB) interaction. Since Cu2+ ions are substituted for Co2+ ions in the present study and the total quantity of Fe3+ ions in the system remains unaltered, the exchange interactions which involving Cu and Co cations are only expected to take place and thus a slow decrease in Curie temperature can be understood due to the difference in the magnetic strength of Co2+ ions (3μB) and Cu2+ ions (1μB). The observed variation of Curie temperature up to x=0.2 is in accordance with this argument. For the sample with x=0.25, migration of a small quantity of Cu or Co ions from B-sites to A-sites is
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expected which in turn forces the A-site Fe ions to move into B-sites resulting in larger reduction in the strength of the (FeA-O-FeB) interaction as well as in Curie temperature. 4
Conclusions Copper substituted cobalt ferrites have been investigated for improved magnetic and
magnetostrictive properties. These materials have shown excellent enhancements in strain derivative up to x=0.15 in Co1-xCuxFe2O4 system. The maximum value of strain derivative obtained in the present study is 4.52 10-9 m/A, which is well suited for stress/torque sensing applications. Copper substitution in cobalt ferrite is quite effective in decreasing the coercivity which is attributed to the decrease in anisotropy. Also, considerable decrease in the Curie temperature was obtained with the partial replacement of Co ions with Cu ions, which is considered desirable as it decreases the temperature dependent magnetomechanical hysteresis. In summary, the Co-Cu ferrite with the properties of moderate values of magnetostriction and magnetization together with the large values of strain derivative and substantial decreases in coercivity and Curie temperature make them very attractive candidates for magnetostrictive sensing applications.
Acknowledgements The authors thank the DST-PURSE and UGC-SAP for the financial support extended during the course of the work.
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References [1] J. Smit, and H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959. [2] R.W. McCallum, K.W. Dennis, D.C. Jiles, J.E. Snyder, and Y.H. Chen, Low Temperature Physics 27 (2001) 266. [3] A. E. Clark, in Ferromagnetic Materials, Vol. 1, edited by E. P. Wohlfarth (North-Holland, Amsterdam,1980), pp. 531. [4] F. E. Pinkerton, T. W. Capehart, J. F. Herbst, E. G. Brewer, and C. B.Murphy, Appl. Phys. Lett. 70 (1997) 2601. [5] W. J. Fleming, Society of Automotive Engineers, 890482 (1989). [6] K.J. Kim, H.K. Kim, and Y.R. Park, J. Korean Phys. Soc. 49 (2006) 1024. [7] S.D. Bhame, and P. A. Joy, J. Appl. Phys. 100 (2006) 2. [8] J.A. Paulsen, A.P. Ring, C.C.H. Lo, J.E. Snyder, and D.C.Jiles, J. Appl. Phys. 97 (2005) 044502. [9] A.C.F.M. Costa, E. Tortellab, M.R. Morelli, R.H.G.A. Kiminami, J. Magn. Magn. Mater. 256 (2003) 174. [10] B. Parvatheeswara Rao, Chong-Oh Kim, CheolGi Kim, I. Dumitru, L. Spinu, and O. F. Caltun, IEEE Trans. Magn. 42 (2006) 2858. [11] B. D. Cullity, Elements of X-ray Diffraction, 2nd ed. Reading, MA: Addison-Wesley, 1978. [12] L. Neel, Ann. Phys. (France) 3(1948) 137. [13] G.A. Petit, and D.W. Forester, Phys. Rev. B. 4 (1971) 3912. [14] G.A. Sawatzky, F. van Der Woude and A.H. Morrish, Phys. Rev. 187 (1969) 747 [15] J.D.Dunitz, and L.E. Orgel, J. Phys. Chem. Solids 3 (1957) 318. [16] G.F.Dionne, J. Appl. Phys. 99 (2006) 08M913 [17] G.S.N. Rao, O.F. Caltun, K.H. Rao, P.S.V.Subba Rao, and B. Parvatheeswara Rao, J. Magn. Magn Mater. 341 (2013) 60. [18] M. Atif, R. Sato Turtelli, R. Grossinger and F. Kubel, J. Appl. Phys. 113 (2013) 153902. [19] K.H. Maria, S. Choudhury, Mohammad Abdul Hakim, Intl. Nano Letters 3 (2013) 42. [20] E. Kester, B. Gillot, P. Perriat, C. Villette, Ph. Taillhades, and A. Rousset, J. Phys. IV France 7, (1997) C1-261.
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[21] Y. Chen, B.K. Kriegermeier-Sutton, J.E. Snyder, K.W. Dennis, R.W. Mccallam, and D.C.Jiles, J. Magn. Magn. Mater. 236 (2001) 131. [22] R.M.Bozorth, “Ferromagnetism”, Van Nostrand, Princeton, NJ, 1951. [23] J.H. van Vleck, and W.G. Penney, Philos. Mag. 17 (1934) 961. [24] S. Chikazumi and S.H. Charap, Physics of magnetism, Krieger Malabar, 1978, p. 248. [25] A. Goldman, Modern Ferrite Technology, van Nostrand Reinhold, New York, 1990. [26] J. Ding, Y.J. Chen, Y. Shi and S. Wang, Appl. Phys. Lett. 77 (2000) 3621. [27] B. Parvatheeswara Rao, P. S.V. Subba Rao and K.H. Rao, IEEE Trans. Magn. 33(1997) 4454. [28] C.H. Lo Chester, IEEE Trans. Magn. 43 (2007) 2367.
Highlights
Sol-gel autocombustion synthesized Co1-xCuxFe2O4 were investigated.
Moderate values of magnetostriction and saturation magnetization were obtained.
Cu substitution in Co ferrite enhances strain derivative and reduces Curie temperature.
Obtained results of Co1-xCuxFe2O4 find themselves suitable for magnetostrictive sensors.
PII: DOI: Reference:
S0304-8853(15)30572-2 http://dx.doi.org/10.1016/j.jmmm.2015.09.028 MAGMA60636
To appear in: Journal of Magnetism and Magnetic Materials Received date: 23 July 2015 Revised date: 24 August 2015 Accepted date: 7 September 2015 Cite this article as: B. Chandra Sekhar, G.S.N. Rao, O.F. Caltun, B. Dhana Lakshmi, B. Parvatheeswara Rao and P.S.V. Subba Rao, Magnetic and magnetostrictive properties of CU substituted CO-ferrites, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.09.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic and magnetostrictive properties of Cu substituted Co-ferrites B. Chandra Sekhar1, G.S.N. Rao2, O.F. Caltun3, B. Dhana Lakshmi4, B. Parvatheeswara Rao4* and P.S.V. Subba Rao4 1
Vignan’s Institute of Engineering for Women, Visakhapatnam 530046, India 2 Dr. V. S. Krishna Govt. Degree College, Visakhapatnam, India 3 Department of Physics, A.I. Cuza University, Iasi, Romania 4 Department of Physics, Andhra University, Visakhapatnam 530003, India
Abstract Copper substituted cobalt ferrite, Co1-xCuxFe2O4 (x=0.00-0.25), nanoparticles were synthesized by sol-gel autocombustion method. X-ray diffraction analysis on the samples was done to confirm the cubic spinel structures and Scherrer equation was used to estimate the mean crystallite size as 40 nm. Using the obtained nanoparticles, fabrication of the sintered pellets was done by standard ceramic technique. Magnetic and magnetostrictive measurements on the samples were made by strain gauge and vibrating sample magnetometer techniques, respectively. Maximum magnetostriction and strain derivative values were deduced from the field dependent magnetostriction curves while the magnetic parameters such as saturation magnetization (51.7-61.9 emu/g) and coercivity (1045-1629 Oe) on the samples were estimated from the obtained magnetic hysteresis loops. Curie temperature values (457-315 oC) were measured by a built in laboratory set-up. Copper substituted cobalt ferrites have shown improved strain derivative values as compared to the pure cobalt ferrite and thus making them suitable for stress sensing applications. The results have been explained on the basis of cationic distributions, strength of exchange interactions and net decreased anisotropic contributions due to the increased presence of Co2+ ions in B-sites as a result of Cu substitutions. Keywords: Co-Cu ferrite nanoparticles, Sol-gel autocombustion, Maximum magnetostriction, Strain derivative, Curie temperature, Torque sensor applications *Corresponding author e-mail: [email protected]
2
1.
Introduction Ferrites exhibit a wide range of electromagnetic properties useful for a variety of
technological applications. Nickel-zinc and manganese-zinc ferrites are the most explored over a wide frequency range for electronic device applications as they exhibit high permeability, high magnetization, high resistivity and low core losses [1]. Metal bonded cobalt ferrites, on the other hand, with high magnetizations and high values of negative magnetostriction are found promising for magnetomechanical stress/strain sensing and actuating applications [2]. Till recently, high magnetostrictive materials, such as Terfenol, SmFe2 and other rare earth iron compounds are widely used as sensing materials [3,4]. However, the rare earth based composites exhibit magnetomechanical hysteresis apart from low sensitivity to stress and thus provide only limited scope for sensing due to their high values of anisotropy besides other disadvantages such as poor mechanical stability and high costs [5]. In order to enhance the magnetostrictive performance, current research is focused on obtaining a substituted cobalt ferrite material, which exhibits higher magnetostrictive strains at lower magnetic field strengths (larger strain derivative). Several efforts have been made by exploring different processing and compositional modifications to the cobalt ferrite to obtain better magnetic and magnetostrictive properties. In this connection, it was reported that the annealing of cobalt ferrite can lead to alteration of the cation distribution among the octahedral and tetrahedral sites and there by leads to resulting in useful magnetic properties [6]. Also from the view point of chemical compositional modifications, it was observed that the substitution of Mn for either Fe or Co was seen as a promising measure to bring in structural changes as well as improvements in strain derivative due to corresponding variations in cation distributions [7]. Moreover, it was also shown that the saturation magnetization, coercivity, Curie temperature and magnetostriction of the cobalt ferrite can be desirably altered by adjusting the Mn content [8]. In order to obtain further improvements in strain derivative, it is now aimed at investigating the effects of copper ions when they replace cobalt ions in Co ferrites. Based on the obtained data and analysis, a suitable composition with the desirable characteristics of high strain derivative, moderate values of maximum magnetostriction and saturation magnetization and low Curie temperature as a good sensing material has been proposed.
3
2
Experimental details Co-Cu ferrite nanoparticles with the chemical formula, Co1-xCuxFe2O4, where x varies
from 0.00 to 0.25 in steps of 0.05, have been prepared by sol-gel autocombustion method. For this purpose, analytical reagent grade cobalt nitrate, copper nitrate, iron nitrate, citric acid and ammonia were used in desired proportions as raw materials. All the chemicals were purchased from Himedia, Mumbai, India and used as-obtained without any further modifications. The solutions of the metal nitrate salts and citric acid in desired proportions were prepared separately by using minimum amounts of deionized water and mixed in 1:1 molar ratio to form an aqueous solution, a method that maximizes molecular mixing of the components [9]. The pH of the mixed solution was adjusted to 7 using ammonia. The solution was then heated up to 80°C while stirring to transform the same into dried gel. At this stage, the temperature of the container was further increased to 110 oC only to be ignited at any point of time. Upon ignition, the dried gel burns in a self propagating combustion manner until all the gels were completely burnt out to form ash like flakes. These flakes were then neatly collected and collapsed by using a glass rod or spatula for ultimately making them to be fluffy loose powders. The X-ray diffraction patterns of the obtained samples confirm single phase cubic spinel structures [10]. Fabrication of ferrite pellets using the obtained nanoparticles (crystallites of about 40 nm in size as estimated from Scherrer equation) was done by standard ceramic procedure [11]. The ferrite nanoparticles of all the samples were lightly ground for a few hours in the presence of methanol, air dried and granulated by using 5% poly vinyl alcohol (PVA) as a binder. The granulated powder was then pressed into pellets of 12 mm diameter and 3mm thickness at a pressure of 150 MPa. The resulting compacts of the ferrite system were then sintered at a temperature of 1050°C for 4 hours in air atmosphere before switching of the furnace and allowing the samples to cool normally. 3
Results and discussion
3.1
Saturation magnetization Magnetic hysteresis loops of the Cu substituted cobalt ferrite samples at room temperature are
shown in Fig. 1. The curves of all the samples exhibit typical soft ferrimagnetic nature with moderate coercivities.
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Fig.1. Room temperature magnetic hysteresis loops of Co1-xCuxFe2O4.
Saturation magnetization and coercivity values for each sample in the ferrite system has been extracted from the corresponding loop and the variation of saturation magnetization as a function of substituent ion concentration is shown in Fig. 2.
Fig. 2.Variation of saturation magnetization with concentration (x) in Co1-xCuxFe2O4.
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It is evident from the figure that the Cu substitution in Co1-xCuxFe2O4 system throughout the range of concentrations investigated has resulted slightly lower values of saturation magnetization compared to the basic cobalt ferrite. Among different Cu concentrations, the samples with x=0.05 and 0.2 are marked by lower values of saturation magnetization. The observed variations in saturation magnetization as a function of substituent concentration can be explained on the basis of super exchange interactions among tetrahedral (A) and octahedral [B] site ions in the spinel lattice. Generally, in ferrites, Neel [12] considered three kinds of exchange interactions between unpaired electrons of the ions lying (i) both at A-sites (A-A interaction), (ii) both at B-sites (B-B interaction) and (iii) one at A-site and another at B-site (A-B interaction). Out of these three interactions, A-B interaction is predominant over B-B and A-A interactions. These interactions tend to align all the magnetic spins at A-site in one direction and those at B-site in the opposite direction. The net magnetic moment of the lattice is therefore the difference between the magnetic moments of B- and A-sublattices, i.e. MB - MA. The exchange interactions in the spinel lattice can greatly be influenced by the nature of the ions present at both A- and B-sublattices. The distribution of cations in tetrahedral and octahedral sites in the spinel structure is necessary to discuss the compositional dependence of the saturation magnetization. It takes a long way to provide the correct distribution of cations from the consideration of our measurements on various properties. Before attaining the accurate cation distribution to the Co1-xCuxFe2O4 system, it is first considered necessary to examine the cation distribution of the CoFe2O4, which can be represented by:
Co
y
Fe1 y Co1 y Fe1 y O4
wherein the value of y is reported to be around 0.19 in many works [13,14]. As per the crystal field stabilization energies for occupation of cations in spinel lattice sites, the Cu2+ ions have specific preferences for B-sites [15].
In the present Co1-xCuxFe2O4 system,
incorporation of Cu2+ ions (1μB) with low magnetic moment per ion in place of Co2+ ions (3μB) with high magnetic moment per ion would naturally be expected to reduce the net magnetic moment of the system in every step of copper substitution. The observed behaviour of saturation magnetization is also marked by lower values for all the concentrations compared to that of the basic cobalt ferrite.
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However, the decrease in magnetization with the Cu substitution is not linear implying that the substituent copper ions are not completely residing in B-sites only. The initial decrease of saturation magnetization however is in support of the argument that Cu2+ ions prefer to occupy the B-sites and thereby decreasing the B-sublattice magnetization as well as net magnetization for this sample. The observed deviation from this trend in resulting relatively higher values of magnetization for the subsequent two concentrations of Cu2+ ions (1μB) up to x=0.15 suggests that the copper substitution might lead to migration of some of the Co2+ ions (3μB) from B-sites to A-sites so as to force an equal amount of Fe3+ ions (5μB) from there moving to the displaced sites in B-sublattice. This kind of distribution helps to retain the B-sublattice magnetization remains similar but decreases the Asublattice magnetization with increased density of Co2+ ions and decreased population of Fe3+ ions in those sites, and thus explains the higher values of magnetization for these concentrations. And, the saturation magnetization behavior at concentrations beyond x=0.15 might be governed by the possibility of occupation of some of the Cu2+ ions in A-sites as well and the consequent changes in chemical environment of the B-sites. The ionic distribution and the corresponding strengths of magnetic moments for this system can be largely expressed with the formulae as under:
Co
2 y
Cuz2 Fe13y z Co12x y Cux2z Fe13y z O4
M ( x) (1 x y ) M Co 1 y z M Fe y M Co (1 y z ) M Fe 3 4 y 10 z 3x Here, the x is the substituted copper content at B-sites, y is the cobalt content at A-sites and z is the copper content at A-sites.
3.2
Maximum magnetostriction Magnetostriction curves of the Co1-xCuxFe2O4 system as a function of applied magnetic field
are shown in Fig. 3. Maximum magnetostriction and strain derivative values have been estimated from these curves. The variation of maximum magnetostriction as a function of substituent concentration is shown in Fig. 4. Contrary to expectation the maximum magnetostriction has been observed to decrease rapidly with increasing copper ion concentration in Co1-xCuxFe2O4 system.
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The observed variation can be discussed on the basis of magnetostrictive contributions of various cations present in the system, their valence states and site occupancy in a given ferrite composition. In cobalt ferrite based systems, the large negative magnetostrictive contributions of Co2+ ions, and large positive and sizeable negative magnetostrictive contribution of Fe2+ and Fe3+ ions are to be considered in explaining the magnetostrictive properties [16].
Fig.3. Magnetostriction curves as a function of magnetic field for Co1-xCuxFe2O4.
Fig. 4. Variation of maximum magnetostriction with concentration (x) in Co1-xCuxFe2O4.
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Besides, the magnetostrictive contributions of the substituted cations, either positive or negative, tend to alter the net magnetostriction of the system because of the corresponding changes in cation distribution arising out of these substitutions. In this context, it may be pointed out that the tetrahedral site magnetostrictive contribution of a cation is known to be lesser and opposite in nature compared to that of the octahedral site contribution for any given ion. As reported in our earlier work of CoMnxFe2-xO4 system [17], wherein also the maximum magnetostriction was observed to decrease with increasing Mn concentration, the manganese exists in Mn3+ state and replaces Fe3+ ions at octahedral sites. However, in the process of Mn ions displacing Fe ions at B-sites, a small amount of Co2+ ions were forced to migrate to A-sites resulting in transfer of an equal amount of Fe ions from there to octahedral sites, and this transfer is assumed to be proportional to Mn concentration. Therefore, the observed decrease in magnetostriction in that system was attributed to the increasing population of Mn3+ ions of positive magnetostrictive contribution and decreasing population of both Co2+ and Fe3+ ions of negative magnetostrictive contributions at B- sites [18]. However, in the present Co1-xCuxFe2O4 system, the decreasing trend of maximum magnetostriction all along the copper substitutions has been rapid and also such incorporation of copper ions in place of cobalt ions does not cause a rapid decrease in the magnetization in this system. Therefore, the observed decrease may be related not only to the occupation of Cu2+ ions in both the lattice sites but also to the John-Teller type of distortion caused by the copper ions in the lattice [19]. There have been several reports indicating the copper ions to cause lattice distortions particularly when they attain in different valence states such as Cu1+ Cu2+ in the same lattice site [20]. In such case, the system should exhibit a gradual decrease in saturation magnetostriction with increasing copper concentration. This argument holds good for the observed decrease of magnetostriction for the entire range of concentrations of copper in the present study. 3.3
Strain Derivative (dλ/dH): Variation of magnetostrictive strain derivative d/dH as a function of substituent
concentration in the Co1-xCuxFe2O4 system is shown in Fig. 5. The strain derivative in Co-Cu ferrite system has been observed to increase initially up to x = 0.15 followed by a decrease for higher concentrations. High value of strain derivative is recorded for copper substitution at x = 0.15. It is
9
interesting to note here that the copper containing system exhibited an opposite trend of strain derivative compared to that of the manganese substituted cobalt ferrite system particularly at the initial substituent concentrations [17].
Fig. 5. Variation of strain derivative with concentration (x) in Co1-xCuxFe2O4.
The observed variations in the strain derivative with substituent concentration can be understood on the basis of magnetic anisotropy and magnetostrictive contributions of the cations present in the sample because the strain derivative depends on both the magnetostriction and anisotropy of the material [21]. For small changes of magnetic field (H) and applied stress (σ), a thermodynamic relation [22] exists as
d dB . d H d H Further, it is well-known from the single ion anisotropy study that the strain derivative is proportional to the ratio of saturation magnetostriction λmax to cubic anisotropic constant K [23]. The ratio of magnetostriction to anisotropy is an important factor in determining the sensitivity of the magnetization to stress which can be expressed as
10
d B d 2 0 S M d d H NK which implies that the strain derivative is proportional to the ratio of magnetostriction to anisotropy constant, /K. The orbital degeneracy of an ion also plays an important role in determining the magnetic anisotropy. There exists a relation between magnetic anisotropy and orbital degeneracy of the ground state of transition metal ions. Magnetic anisotropy cannot be expected in case the orbital state of an ion is non-degenerate. In cobalt ferrite, the magnetic anisotropy has been found to be proportional to cobalt content. Also, Co2+ ions at octahedral sites are known to create stronger anisotropy through trigonal distortion whereas cobalt ions at tetrahedral sites contribute little anisotropy to the cobalt ferrite compound [24]. In Co1-xCuxFe2O4 system, substitution of Cu2+ ions in place of Co2+ ions in Bsites is not likely to make a straight forward replacement but forces the cobalt ions to migrate into Asites and thereby moving Fe3+ ions from there to B-sites. This process increases the amount of Co2+ ions in the A-sites and the smaller degeneracy associated with Co2+ ions over Fe3+ ions at tetrahedral sites results in a decreased anisotropy and increased λ/K. The observed increase in strain derivative up to x = 0.15 can be explained on the basis of decreased anisotropic contribution of the compound. As discussed earlier in magnetostriction studies, the amount of copper entered into lattice up to x = 0.15 has been considered to be maximum and the same might exist in the subsequent concentrations too in the system. Generally in soft magnetic materials, the presence of a non-magnetic second phase hinders the magnetization processes and decreases the strain derivative in low fields [25]. The pores and segregations can lead to pinning of domain walls thereby affecting the response of the domains to the applied field and reducing the strain derivative at higher concentrations of copper. Moreover, occupation of some of the copper ions in A-sites at higher concentrations of copper also changes the situation described above resulting in modifications to the anisotropy and thereby the reduction of strain derivative for these concentrations as observed in Fig. 5. 3.4
Coercivity The observed variation of coercivity as a function of Cu concentration in Co1-xCuxFe2O4
system is shown in Fig. 6. After an initial increase in coercivity at x = 0.05, a gradual fall for the
11
remaining concentrations of copper is observed for the system. And, the magnitude of fall from the basic cobalt ferrite (x=0.0) to the highest copper substituted ferrite (x=0.25) also appears considerable indicating that the anisotropic and the microstructural modifications leading to coercivity variations in the substituted ferrite system play an important role.
Fig. 6. Variation of coercivity with concentration (x) in Co1-xCuxFe2O4.
Besides, coercivity in a ferrite system is known to depend on various parameters like magnetocrystalline anisotropy, lattice imperfections, dislocations, internal strains, particle size and secondary phases [26]. The observed initial increase in coercivity in Co1-xCuxFe2O4 could be due to the segregation of copper at grain boundaries as second phase. The sharp decrease of coercivity and a similar increase in strain derivative, in the range of concentrations from x = 0.05 to 0.15, confirms our argument made earlier in magnetization part on cobalt migration to A-sites. 3.5
Curie temperature The observed variation of Curie temperature for the Co1-xCuxFe2O4 system is shown in Fig. 7.
It has been observed to decrease gradually with increasing Cu concentration up to x=0.2 and then a rapid decrease was observed for x=0.25 sample.
12
Fig. 7. Variation of Curie temperature with concentration (x) in Co1-xCuxFe2O4.
It is well known that the Curie temperature depends not only on the occupation of magnetic cations in different sublattices but also on the density of magnetic cations and their corresponding magnetic strengths in the sublattices [27]. In the Co1-xCuxFe2O4 system, since both the substituted (Cu2+) and replaced (Co2+) cations are magnetic only, the density of the magnetic cations remains the same and does not explain the observed Curie temperature variation. On the other hand, the occupation of the cations in different sublattices as well as the exchange interactions between them based on the magnetic strength of the cations involved must be governing the magnitude of the Curie temperature. Among all the exchange interactions, the A-B interaction between (FeA-O-FeB) is the strongest compared to (CoA-O-FeB), (FeA-O-CoB) and (FeA-O-CuB) interactions. Therefore, rapid decrease in Curie temperature can be expected only when there is weakening of (FeA-O-FeB) interaction. Since Cu2+ ions are substituted for Co2+ ions in the present study and the total quantity of Fe3+ ions in the system remains unaltered, the exchange interactions which involving Cu and Co cations are only expected to take place and thus a slow decrease in Curie temperature can be understood due to the difference in the magnetic strength of Co2+ ions (3μB) and Cu2+ ions (1μB). The observed variation of Curie temperature up to x=0.2 is in accordance with this argument. For the sample with x=0.25, migration of a small quantity of Cu or Co ions from B-sites to A-sites is
13
expected which in turn forces the A-site Fe ions to move into B-sites resulting in larger reduction in the strength of the (FeA-O-FeB) interaction as well as in Curie temperature. 4
Conclusions Copper substituted cobalt ferrites have been investigated for improved magnetic and
magnetostrictive properties. These materials have shown excellent enhancements in strain derivative up to x=0.15 in Co1-xCuxFe2O4 system. The maximum value of strain derivative obtained in the present study is 4.52 10-9 m/A, which is well suited for stress/torque sensing applications. Copper substitution in cobalt ferrite is quite effective in decreasing the coercivity which is attributed to the decrease in anisotropy. Also, considerable decrease in the Curie temperature was obtained with the partial replacement of Co ions with Cu ions, which is considered desirable as it decreases the temperature dependent magnetomechanical hysteresis. In summary, the Co-Cu ferrite with the properties of moderate values of magnetostriction and magnetization together with the large values of strain derivative and substantial decreases in coercivity and Curie temperature make them very attractive candidates for magnetostrictive sensing applications.
Acknowledgements The authors thank the DST-PURSE and UGC-SAP for the financial support extended during the course of the work.
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Highlights
Sol-gel autocombustion synthesized Co1-xCuxFe2O4 were investigated.
Moderate values of magnetostriction and saturation magnetization were obtained.
Cu substitution in Co ferrite enhances strain derivative and reduces Curie temperature.
Obtained results of Co1-xCuxFe2O4 find themselves suitable for magnetostrictive sensors.