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Structural, magnetic and microstructural study of Sr2Ni2Fe12O22
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
Structural, magnetic and microstructural study of Sr2Ni2Fe12O22 M.Y. Salunkhe*, D.K. Kulkarni Department of Physics, Institute of Science, R.T. Road, Nagpur 440010, Maharashtra, India
Abstract The substitution of divalent nickel ions in Sr–Y type hexaferrite is considered to be important for the enhancement of its magnetic properties. The nickel substituted Sr–Y hexaferrite with chemical formula Sr2 Ni2 Fe12 O22 is prepared using analar grade reactants in the proper molar ratio by well-known solid-state reaction method. Structural characterization of the compound has been carried out from X-ray powder pattern. The compound is in single hexagonal phase without ( and c ¼ 43:52 A. ( The real the traces of the unreacting ambiguous reflections. The lattice parameters are a ¼ 5:881 A density of the compound is calculated from helium picnometry, which agrees with the theoretically calculated value of X-ray density. The surface morphology was studied by scanning electron microscopy. Clearly hexagonal particles are observed with faster crystallization rate. The average particle size is also estimated. The compound is studied magnetically using Gouy’s method. The results show the compound is ferrimagnetic at room temperature with transition temperature TC ¼ 698 K: The high value of TC is as expected due to the presence of Ni2þ and Fe3þ ions in the compound. From the paramagnetic behaviour, the Curie molar constant is worked out. Electrical conductivity measurement approves the dual conduction mechanism in the compound. The compound shows semiconducting behaviour with room temperature resistivity 8:935 109 O cm: r 2004 Elsevier B.V. All rights reserved. Keywords: Hexaferrite; X-ray diffraction; SEM; Gouy’s balance; Electrical conductivity
1. Introduction Hexagonal ferrites have large utility in the field of materials science. The family of hexagonal ferrites can be classified on the basis of chemical composition and crystal structure. These are subdivided into five fundamental, simplest structural types: M, W, Y, X and Z [1]. Gorter [2] made the first attempt to determine the position of the magnetic ions and orientation of the spins in the *Corresponding author. Tel.: +7122542845. E-mail address: [email protected] (M.Y. Salunkhe).
crystal lattice by considering exchange interactions. He observed that the spins are collinear in the basel plane particularly in Y structure. The structure of Y type hexaferrite has space group (R3% m) and is often designated as— (TS)’’|(TS)(TS)’(TS)’’|(TS)—, where the prime means the block is rotated 120 around the c-axis [3]. The structure consists of three Y blocks (formula units). Each Y block consists of twolayered spinel S block and four-layered antiferromagnetic T block. The metallic cations in the blocks occupy either tetrahedral or octahedral sites between the oxygen polyhedra [4]. The knowledge
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.01.046
ARTICLE IN PRESS M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
of the distribution of cations is utmost importance in understanding their physical and chemical properties. Among the Y type hexagonal ferrites, our attention has been focused on Strontium Y type hexaferrite with nickel as a divalent substitution with the aim of achieving better understanding of this substitution on structural, microstructural magnetic and electrical properties.
2. Experimental Single-phase polycrystalline sample with the composition corresponding to the formula Sr2 Ni2 Fe12 O22 was prepared. The well-known solid-state reaction method [5] was adopted for the preparation of this Y type hexagonal ferrite. The analar grade reactants SrCO3 ; NiO and Fe2 O3 were mixed in the proper molar ratio using A.R. grade acetone in agate mortar for about 627 h to achieve uniform grain size and homogenization. The pellets of thoroughly grounded mixture are prepared by adding 5% Polyvinylacetate in analar acetone as a binder, applying 10 ton of pressure per square inch. The pellets thus obtained are slowly heated in an electric furnace at 600 C for 6 h to remove the binder [6]. The temperature of the furnace is then slowly raised to 1200 C for sintering and kept constant for 120 h: The cooling rate of the furnace is kept 20 C=h upto 1000 C and 60 C=h upto 500 C and then by natural way to room temperature. These processed ferrite pellets are finely powdered, sieved through fine sieve (200 mesh) and used for further characterization. The completion of the solid-state reaction and formation of the compound was confirmed by Xray diffraction carried out on the powdered sample on Philips PW-1710 X-ray diffractometer using ( The X-ray patCu2Ka radiation ðl ¼ 1:542 A). tern was indexed and lattice parameters were obtained by usual procedure given by Henri and Lipson [7]. The structural data is included in Table 1. The true density of the compound was performed on micrometrics multi-volume helium Picnometer 1305. The scanning electron microscopy was carried out on a Jeol instrument JXA,
65
Table 1 X-ray diffraction results of Sr2 Ni2 Fe12 O22 dobs
dcal
Iobs
h
k
l
4.821 3.754 3.668 3.284 3.168 2.963 2.759 2.694 2.513 2.490 2.413 2.202 2.166 2.041 1.995 1.837 1.693 1.688 1.596 1.484 1.452
4.836 3.718 3.627 3.309 3.125 2.941 2.798 2.664 2.513 2.480 2.418 2.198 2.184 2.027 2.001 1.838 1.694 1.686 1.590 1.470 1.440
19.5 15.2 31.2 43.0 27.7 69.1 26.3 86.2 100.0 43.0 22.5 21.3 74.6 28.4 23.1 29.0 45.6 23.7 16.2 33.4 47.4
0 0 0 1 0 1 1 0 1 0 0 0 1 0 0 2 1 3 0 2 2
0 1 0 0 1 1 0 1 1 2 0 2 0 2 1 1 1 0 1 2 1
9 8 12 10 11 0 13 14 9 4 18 10 18 13 20 7 21 3 26 0 19
( c ¼ 43:521 A. ( Lattice parameter: a ¼ 5:881 A,
840-A operating at 5 KeV: The magnetic susceptibility measurements were performed on Gouy’s balance [8] in the temperature range 3002900 K: The electrical conductivity s is measured by determining the resistance of the compound using bridge type circuit developed in the laboratory using two probes up to 900 K [9]. The voltage drop across a standard resistance was measured by digital multimeter (input impedance 109 O).
3. Results and discussion The X-ray diffractograms (Fig. 1) of the polycrystalline sample Sr2 Ni2 Fe12 O22 hexaferrite reveal the formation of the single hexagonal phase without the traces of unreacting ambiguous reflections. The structural data shows the compound has Y type hexagonal structure with lattice ( and c ¼ 43:52 A, ( which parameters a ¼ 5:881 A ( and c ¼ 43:6 A ( [10]. The agrees with a ¼ 5:9 A slight variation may be due to the replacement of ( by strontium cation barium (ionic radius 1:34 A)
ARTICLE IN PRESS 66
M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
Fig. 1. X-ray diffraction pattern for Sr2 Ni2 Fe12 O22 :
( Also out of six different (ionic radius 1:12 A). cationic sites nickel ion likely to occupy the 6c ( positions at the boundaries of the T (z ¼ 0:152 A) blocks. The presence of nickel at these sites alters the length and angles of the exchange bonds between the ions in the neighbouring blocks [11]. ( 3 : The true The unit-cell volume is 1303:53 ðAÞ density of the compound was calculated by helium Picnometer method and was found to be 5:008 gm=cc; which is 99.68% of the theoretically calculated value of X-ray density ð5:024 gm=ccÞ: But is much less than reported value 5:40 gm=cc in case of Ba2 Ni2 Fe12 O22 [1]. This may be due to the effect of the non-diffusing gas atmosphere, which may trapped within the pores [12]. The sintering process rearranges the fine ferrite particles in the presence of the liquid phase. Surface tension forces rotate the hexagonal particles. Due to their anisotropy these particles arrange themselves in a direction, which increases the basic anisotropy of the matrix. If wetting of the grains is effective, a dense, homogeneous, welloriented fine-grained microstructure can be obtained. The surface morphology of the compound was studied by scanning electron microscopy. As generally known the strontium hexaferrites are prone to anomalous grain growth [13,14] but if the
cooling rate is kept sufficiently slow, some grains increase their size becomes larger, after achieving critical magnitude the growth seems to be accelerated. Fig. 2 shows a typical scanning electron microscopic photographs of Sr2 Ni2 Fe12 O22 ferrite on finely polished pellet surface. The grain growth seems to be normal with the growth of the seed crystal. The crystallization rate seems to be faster. Clearly hexagonal particles with the start of secondary growth on the top of some particles are observed with the existence of porosity. The particle size distribution of the particles of several samples was determined by evaluation of SEM images (magnification from 1000 to 10000). The average particle size measured on micrographs is 4:746 mm: The maximum and minimum particle size is 31.536 and 2:596 mm; respectively. These particles may contain number of crystallites calculated from X-ray diffractometry (average crystallite size was 0:209 mm on 100% peak at 2y ¼ 35:705 ) [15]. The plot of inverse molar susceptibility versus temperature (Fig. 3) above the Curie temperature is linear shows the paramagnetic behaviour of the compound. From the linearity region the Curie molar constant ðCM Þ is calculated (54.41) which is in good agreement with the theoretically calculated
ARTICLE IN PRESS M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
67
15 14
1/χm
13 12 11 10 9 8 600
650
700
750
800
850
900
950
T˚K
Fig. 3. Variation of inverse molar susceptibility versus T K for Sr2 Ni2 Fe12 O22 :
103/T 1.35 -5
1.4
1.45
1.5
1.55
1.6
-5.2
log σ
-5.4 -5.6 -5.8 -6 -6.2
Fig. 4. Variation of log s versus 103 =T for Sr2 Ni2 Fe12 O22 :
Fig. 2. SEM of Sr2 Ni2 Fe12 O22 :
spin only values of CM (54.56). The compound is ferrimagnetic at room temperature with Curie temperature TC ¼ 698 K; such a high value of
TC is as expected due to substitution of magnetic nickel and iron cations in the structure. But it is comparatively higher than Ba2 Ni2 Fe12 O22 ðTC Þ ¼ 663 KÞ [1,16] may be due to substitution of strontium cations. The DC electrical conductivity measurement verifies the dual conductivity mechanisms in the compound as observed in NiFe2 O4 spinel ferrite 3þ [17] and in Ba2 Fe2þ 2 Fe12 O22 hexagonal ferrite [18]. 3 The log s versus 10 =T graph (Fig. 4) shows change in slope at about Curie temperature of the compound ð698 KÞ: The activation energies were found to be 0.839 and 0:881 eV in ferrimagnetic and paramagnetic regions, respectively. The high activation energy approves the high resistivity of the compound at room temperature. The room temperature resistivity of the compound was
ARTICLE IN PRESS 68
M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
calculated and was found to be 8:935 109 O cm: The increase in the activation energy in the paramagnetic region suggests the electrical conduction takes place due to hopping of charge carriers between occupied and unoccupied sites or between ions of different valences at equivalent crystallographic positions [19] according to the equation Ni2þ þ Fe3þ #Ni3þ þ Fe2þ : Here with rise in temperature Ni2þ turns into Ni3þ while Fe3þ into Fe2þ : As such a transition in the 3d shell takes place for a very short interval of time, is not detectable by any ordinary method [20].
References [1] J. Smit, H.P.J Wijn, in: Ferrites, Philip’s Tech. Library, New York, 1959. [2] E.W. Gorter, Proc. IEE (London) 104B 5 (Suppl.) (1957) 255. [3] G. Albanese, M. Carbucicchio, A. Deriu, G. Asti, S. Rinaldi, Appl. Phys. 7 (1975) 227.
[4] L.R. Bickford, Phys. Rev. 119 (1960) 1000. [5] J. Economous, Am. Ceram. Soc. 38 (1955) 241. [6] D.B. Ghare, A.P.B. Sinha, J. Phys. Chem. Solids 29 (1968) 885. [7] N.F.M. Henry, H. Lipson, W.A. Wooster, in: Interpretation of X-ray Diffraction Photographs, Mc Millan and Co., London, 1961. [8] L.F. Bates, in: Modern Magnetism, Cambridge University Press, Cambridge, 1939. [9] S.P. Yawale, S.V. Pakade, J. Mater. Sci. 28 (1993) 5451. [10] P.B. Braun, Philips Res. Rep. 12 (1957) 491. [11] R.A. Sizov, Soviet Phys.—Solid State 23 (1981) 1092. [12] R.L. Coble, J. Appl. Phys. 32 (1964) 787. [13] F. Kools, Adv. Ceramics 15 (1985) 177. [14] C. Lacour, M. Paulus, Phys. Stat. Sol. (A) 27 (1975) 441. [15] R.C. Puller, S.G. Appleton, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) 313. [16] M.A. Vinnik, Sov. Phys.—Crystallogr. 12 (1967) 344. [17] L.G. VanUitert, J. Chem. Phys. 23 (1955) 1883. [18] M.A. Hadj Farhat, J.C. Joubert, J. Magn. Magn. Mater. 62 (1986) 353. [19] M.A. El Hiti, A.M. Abo El Ata, J. Magn. Magn. Mater. 195 (1999) 667. [20] K.G. Revatkar, C.S. Prakash, D.K. Kulkarni, Mater. Lett. 28 (1996) 365.
Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
Structural, magnetic and microstructural study of Sr2Ni2Fe12O22 M.Y. Salunkhe*, D.K. Kulkarni Department of Physics, Institute of Science, R.T. Road, Nagpur 440010, Maharashtra, India
Abstract The substitution of divalent nickel ions in Sr–Y type hexaferrite is considered to be important for the enhancement of its magnetic properties. The nickel substituted Sr–Y hexaferrite with chemical formula Sr2 Ni2 Fe12 O22 is prepared using analar grade reactants in the proper molar ratio by well-known solid-state reaction method. Structural characterization of the compound has been carried out from X-ray powder pattern. The compound is in single hexagonal phase without ( and c ¼ 43:52 A. ( The real the traces of the unreacting ambiguous reflections. The lattice parameters are a ¼ 5:881 A density of the compound is calculated from helium picnometry, which agrees with the theoretically calculated value of X-ray density. The surface morphology was studied by scanning electron microscopy. Clearly hexagonal particles are observed with faster crystallization rate. The average particle size is also estimated. The compound is studied magnetically using Gouy’s method. The results show the compound is ferrimagnetic at room temperature with transition temperature TC ¼ 698 K: The high value of TC is as expected due to the presence of Ni2þ and Fe3þ ions in the compound. From the paramagnetic behaviour, the Curie molar constant is worked out. Electrical conductivity measurement approves the dual conduction mechanism in the compound. The compound shows semiconducting behaviour with room temperature resistivity 8:935 109 O cm: r 2004 Elsevier B.V. All rights reserved. Keywords: Hexaferrite; X-ray diffraction; SEM; Gouy’s balance; Electrical conductivity
1. Introduction Hexagonal ferrites have large utility in the field of materials science. The family of hexagonal ferrites can be classified on the basis of chemical composition and crystal structure. These are subdivided into five fundamental, simplest structural types: M, W, Y, X and Z [1]. Gorter [2] made the first attempt to determine the position of the magnetic ions and orientation of the spins in the *Corresponding author. Tel.: +7122542845. E-mail address: [email protected] (M.Y. Salunkhe).
crystal lattice by considering exchange interactions. He observed that the spins are collinear in the basel plane particularly in Y structure. The structure of Y type hexaferrite has space group (R3% m) and is often designated as— (TS)’’|(TS)(TS)’(TS)’’|(TS)—, where the prime means the block is rotated 120 around the c-axis [3]. The structure consists of three Y blocks (formula units). Each Y block consists of twolayered spinel S block and four-layered antiferromagnetic T block. The metallic cations in the blocks occupy either tetrahedral or octahedral sites between the oxygen polyhedra [4]. The knowledge
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.01.046
ARTICLE IN PRESS M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
of the distribution of cations is utmost importance in understanding their physical and chemical properties. Among the Y type hexagonal ferrites, our attention has been focused on Strontium Y type hexaferrite with nickel as a divalent substitution with the aim of achieving better understanding of this substitution on structural, microstructural magnetic and electrical properties.
2. Experimental Single-phase polycrystalline sample with the composition corresponding to the formula Sr2 Ni2 Fe12 O22 was prepared. The well-known solid-state reaction method [5] was adopted for the preparation of this Y type hexagonal ferrite. The analar grade reactants SrCO3 ; NiO and Fe2 O3 were mixed in the proper molar ratio using A.R. grade acetone in agate mortar for about 627 h to achieve uniform grain size and homogenization. The pellets of thoroughly grounded mixture are prepared by adding 5% Polyvinylacetate in analar acetone as a binder, applying 10 ton of pressure per square inch. The pellets thus obtained are slowly heated in an electric furnace at 600 C for 6 h to remove the binder [6]. The temperature of the furnace is then slowly raised to 1200 C for sintering and kept constant for 120 h: The cooling rate of the furnace is kept 20 C=h upto 1000 C and 60 C=h upto 500 C and then by natural way to room temperature. These processed ferrite pellets are finely powdered, sieved through fine sieve (200 mesh) and used for further characterization. The completion of the solid-state reaction and formation of the compound was confirmed by Xray diffraction carried out on the powdered sample on Philips PW-1710 X-ray diffractometer using ( The X-ray patCu2Ka radiation ðl ¼ 1:542 A). tern was indexed and lattice parameters were obtained by usual procedure given by Henri and Lipson [7]. The structural data is included in Table 1. The true density of the compound was performed on micrometrics multi-volume helium Picnometer 1305. The scanning electron microscopy was carried out on a Jeol instrument JXA,
65
Table 1 X-ray diffraction results of Sr2 Ni2 Fe12 O22 dobs
dcal
Iobs
h
k
l
4.821 3.754 3.668 3.284 3.168 2.963 2.759 2.694 2.513 2.490 2.413 2.202 2.166 2.041 1.995 1.837 1.693 1.688 1.596 1.484 1.452
4.836 3.718 3.627 3.309 3.125 2.941 2.798 2.664 2.513 2.480 2.418 2.198 2.184 2.027 2.001 1.838 1.694 1.686 1.590 1.470 1.440
19.5 15.2 31.2 43.0 27.7 69.1 26.3 86.2 100.0 43.0 22.5 21.3 74.6 28.4 23.1 29.0 45.6 23.7 16.2 33.4 47.4
0 0 0 1 0 1 1 0 1 0 0 0 1 0 0 2 1 3 0 2 2
0 1 0 0 1 1 0 1 1 2 0 2 0 2 1 1 1 0 1 2 1
9 8 12 10 11 0 13 14 9 4 18 10 18 13 20 7 21 3 26 0 19
( c ¼ 43:521 A. ( Lattice parameter: a ¼ 5:881 A,
840-A operating at 5 KeV: The magnetic susceptibility measurements were performed on Gouy’s balance [8] in the temperature range 3002900 K: The electrical conductivity s is measured by determining the resistance of the compound using bridge type circuit developed in the laboratory using two probes up to 900 K [9]. The voltage drop across a standard resistance was measured by digital multimeter (input impedance 109 O).
3. Results and discussion The X-ray diffractograms (Fig. 1) of the polycrystalline sample Sr2 Ni2 Fe12 O22 hexaferrite reveal the formation of the single hexagonal phase without the traces of unreacting ambiguous reflections. The structural data shows the compound has Y type hexagonal structure with lattice ( and c ¼ 43:52 A, ( which parameters a ¼ 5:881 A ( and c ¼ 43:6 A ( [10]. The agrees with a ¼ 5:9 A slight variation may be due to the replacement of ( by strontium cation barium (ionic radius 1:34 A)
ARTICLE IN PRESS 66
M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
Fig. 1. X-ray diffraction pattern for Sr2 Ni2 Fe12 O22 :
( Also out of six different (ionic radius 1:12 A). cationic sites nickel ion likely to occupy the 6c ( positions at the boundaries of the T (z ¼ 0:152 A) blocks. The presence of nickel at these sites alters the length and angles of the exchange bonds between the ions in the neighbouring blocks [11]. ( 3 : The true The unit-cell volume is 1303:53 ðAÞ density of the compound was calculated by helium Picnometer method and was found to be 5:008 gm=cc; which is 99.68% of the theoretically calculated value of X-ray density ð5:024 gm=ccÞ: But is much less than reported value 5:40 gm=cc in case of Ba2 Ni2 Fe12 O22 [1]. This may be due to the effect of the non-diffusing gas atmosphere, which may trapped within the pores [12]. The sintering process rearranges the fine ferrite particles in the presence of the liquid phase. Surface tension forces rotate the hexagonal particles. Due to their anisotropy these particles arrange themselves in a direction, which increases the basic anisotropy of the matrix. If wetting of the grains is effective, a dense, homogeneous, welloriented fine-grained microstructure can be obtained. The surface morphology of the compound was studied by scanning electron microscopy. As generally known the strontium hexaferrites are prone to anomalous grain growth [13,14] but if the
cooling rate is kept sufficiently slow, some grains increase their size becomes larger, after achieving critical magnitude the growth seems to be accelerated. Fig. 2 shows a typical scanning electron microscopic photographs of Sr2 Ni2 Fe12 O22 ferrite on finely polished pellet surface. The grain growth seems to be normal with the growth of the seed crystal. The crystallization rate seems to be faster. Clearly hexagonal particles with the start of secondary growth on the top of some particles are observed with the existence of porosity. The particle size distribution of the particles of several samples was determined by evaluation of SEM images (magnification from 1000 to 10000). The average particle size measured on micrographs is 4:746 mm: The maximum and minimum particle size is 31.536 and 2:596 mm; respectively. These particles may contain number of crystallites calculated from X-ray diffractometry (average crystallite size was 0:209 mm on 100% peak at 2y ¼ 35:705 ) [15]. The plot of inverse molar susceptibility versus temperature (Fig. 3) above the Curie temperature is linear shows the paramagnetic behaviour of the compound. From the linearity region the Curie molar constant ðCM Þ is calculated (54.41) which is in good agreement with the theoretically calculated
ARTICLE IN PRESS M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
67
15 14
1/χm
13 12 11 10 9 8 600
650
700
750
800
850
900
950
T˚K
Fig. 3. Variation of inverse molar susceptibility versus T K for Sr2 Ni2 Fe12 O22 :
103/T 1.35 -5
1.4
1.45
1.5
1.55
1.6
-5.2
log σ
-5.4 -5.6 -5.8 -6 -6.2
Fig. 4. Variation of log s versus 103 =T for Sr2 Ni2 Fe12 O22 :
Fig. 2. SEM of Sr2 Ni2 Fe12 O22 :
spin only values of CM (54.56). The compound is ferrimagnetic at room temperature with Curie temperature TC ¼ 698 K; such a high value of
TC is as expected due to substitution of magnetic nickel and iron cations in the structure. But it is comparatively higher than Ba2 Ni2 Fe12 O22 ðTC Þ ¼ 663 KÞ [1,16] may be due to substitution of strontium cations. The DC electrical conductivity measurement verifies the dual conductivity mechanisms in the compound as observed in NiFe2 O4 spinel ferrite 3þ [17] and in Ba2 Fe2þ 2 Fe12 O22 hexagonal ferrite [18]. 3 The log s versus 10 =T graph (Fig. 4) shows change in slope at about Curie temperature of the compound ð698 KÞ: The activation energies were found to be 0.839 and 0:881 eV in ferrimagnetic and paramagnetic regions, respectively. The high activation energy approves the high resistivity of the compound at room temperature. The room temperature resistivity of the compound was
ARTICLE IN PRESS 68
M.Y. Salunkhe, D.K. Kulkarni / Journal of Magnetism and Magnetic Materials 279 (2004) 64–68
calculated and was found to be 8:935 109 O cm: The increase in the activation energy in the paramagnetic region suggests the electrical conduction takes place due to hopping of charge carriers between occupied and unoccupied sites or between ions of different valences at equivalent crystallographic positions [19] according to the equation Ni2þ þ Fe3þ #Ni3þ þ Fe2þ : Here with rise in temperature Ni2þ turns into Ni3þ while Fe3þ into Fe2þ : As such a transition in the 3d shell takes place for a very short interval of time, is not detectable by any ordinary method [20].
References [1] J. Smit, H.P.J Wijn, in: Ferrites, Philip’s Tech. Library, New York, 1959. [2] E.W. Gorter, Proc. IEE (London) 104B 5 (Suppl.) (1957) 255. [3] G. Albanese, M. Carbucicchio, A. Deriu, G. Asti, S. Rinaldi, Appl. Phys. 7 (1975) 227.
[4] L.R. Bickford, Phys. Rev. 119 (1960) 1000. [5] J. Economous, Am. Ceram. Soc. 38 (1955) 241. [6] D.B. Ghare, A.P.B. Sinha, J. Phys. Chem. Solids 29 (1968) 885. [7] N.F.M. Henry, H. Lipson, W.A. Wooster, in: Interpretation of X-ray Diffraction Photographs, Mc Millan and Co., London, 1961. [8] L.F. Bates, in: Modern Magnetism, Cambridge University Press, Cambridge, 1939. [9] S.P. Yawale, S.V. Pakade, J. Mater. Sci. 28 (1993) 5451. [10] P.B. Braun, Philips Res. Rep. 12 (1957) 491. [11] R.A. Sizov, Soviet Phys.—Solid State 23 (1981) 1092. [12] R.L. Coble, J. Appl. Phys. 32 (1964) 787. [13] F. Kools, Adv. Ceramics 15 (1985) 177. [14] C. Lacour, M. Paulus, Phys. Stat. Sol. (A) 27 (1975) 441. [15] R.C. Puller, S.G. Appleton, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) 313. [16] M.A. Vinnik, Sov. Phys.—Crystallogr. 12 (1967) 344. [17] L.G. VanUitert, J. Chem. Phys. 23 (1955) 1883. [18] M.A. Hadj Farhat, J.C. Joubert, J. Magn. Magn. Mater. 62 (1986) 353. [19] M.A. El Hiti, A.M. Abo El Ata, J. Magn. Magn. Mater. 195 (1999) 667. [20] K.G. Revatkar, C.S. Prakash, D.K. Kulkarni, Mater. Lett. 28 (1996) 365.