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Observation of enhanced dielectric permittivity in Bi3+ doped BaFe12O19 ferrite
Journal of Magnetism and Magnetic Materials 80 (1989) 241-245 North-Holland, Amsterdam
241
OBSERVATION OF ENHANCED DIELECTRIC PERMITTIVITY I N Bi 3+ D O P E D BaFe12019 F E R R I T E Shanker R A M 1 Department of Materials Science and Engineering, McMaster Universtty, Hamilton, Ontario LSS 4L7, Canada
Received 20 May 1988; in revised form 22 March 1989
The dielectric properties of a 2.5 mol% Bi203 doped Ba-ferrite (BaFex2019) have been investigated. The specimen shows a drastically enhanced ( - 105 times) permittivity (c) through local polarization of Fe 3+ electronic charges activated with nearby Bi3+ sites. The c value also depends reasonably on thermal annealing (causes grain growth) of the samples. The specimens comprising large particle sizes (1-20 ~tm) usually exhibit smaller ¢ values and low dielectric losses.
1. Introduction The oxide additives of A g 2 0 , Bi203, P205 or As203 added by suitable amounts (~< 2.5 mol%) to the crystallization reaction of BaFe12Oa9 ferrite in a B a O - F e 2 0 3 - B 2 0 3 glass o r / a n d to the stoichiometric composition, BaO + 6Fe203 ~ BaFea2019 , in processing the ferrite by high temperature solid state reaction process, have been useful in crystallizing out platelet-like BaFea2019 ferrite particles [1-4]. These particles exhibit superior magnetic and other physical properties to most applications of BaFea2019 ferrite in devices. Bi203 additives appeared to be particularly interesting in the in-
vestigation series of BaFe12019 ferrite processed b y solid state reaction process. Besides activating the crystallization of ferrite particles, the Bi203 incorporates in the ferrite lattice following BaFeaE_xBixO19 type substitution reaction, with x 0.35. The structural and magnetic properties of this composition series has been reported in an earlier publication [4]. In the present article we report on the dielectric permittivity (c) because results reveal interestingly enhanced (105 times) c values b y B i 2 0 3 incorporation.
2. Experimental BaFe12_xBixO19 (x ~ 0.35) is prepared b y solid state reaction (performed at 1 1 0 0 - 1 3 0 0 ° C ) of
1 Visiting Scientist. Table 1 Summary of experimental results Sample no.
Sintering
Ms (#B)
Density (g/cm3)
Grain size (~m)
¢'a) ( × 103)
tan 8 a)
A1 B1 B2 B3 B4
1300 ° C/12 h 1100 o C/4 h 1100 o C/8 h 1100 ° C/12 h 1300 ° C/12 h
14.50 12.67 12.72 13.25 12.72
4.97 4.89 4.92 4.95 4.98
2.0 1.2 1.4 1.5 20
0.005 111.7 264.8 6.1 1.2
0.8 3.45 2.1 2.0 0'.9
A refers to pure BaFe12019 ferrite composition. The sample series B contains 2.5 mol% Bi203 added (for Fe203) in BaFe12019 ferrite composition (A). a) Measured at f = 100 Hz. 0 3 0 4 - 8 8 5 3 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
242
S. Ram / Dielectric permittivity in Bi J + doped ferrite
BaCO3, Fe203 and Bi203 mixed in stoichiometric ratio. The formation of the Ba-ferrite phase has been verified by X-ray powder diffraction patterns recorded on a Rich-Seifert Isodebyeflex 2002 diffractometer with filtered CrKa radiation. Dielectric permittivity (c) measurements of the various samples (table 1) are carried out in the range of 100 Hz to 13 MHz using an impedance bridge (model 1608A) of General Radio Company, USA. Other experimental details appeared elsewhere [4].
3. Results and discussion
X-ray diffraction analysis of the solid state reaction of BaCO3 and 6Fe203, performed at temperatures between 1100 and 1300 °C demonstrates crystallization of Ba-hexaferrite (BaFe12019) through -/-Fe203 and BaFe204 as the intermediate reaction products [4-6]. We continue sintering of the samples (based on our previous experience of synthesising similar ferrites [2,3]) for 4, 8 and 12 h, to ensure about 100% utilization of the reactants/ intermediate phases in the reaction. The optimum period of sintering thus required to accomplish the ferritization reaction was found to be --12 h. When 2.5 mol% Bi203 was incorporated in the composition, it acted as an internal additive, i.e. the additive increases nucleation and growth of ferrite grains (of sizes up to 20 ~m shown by the micrographs in fig. 1) as well as it participates in the formation of the ferrite. The samples sintered with Bi203 for small periods of 4 and 8 h also comprise single phase Ba-ferrite. In table 1 we have summarized all the samples comprising single phase Ba-ferrite, as verified by X-ray diffractometry, selected for our dielectric permittivity measurements. The dissolution of Bi 3÷ (Bi203) in the ferrite interstitial sites has been demonstrated by the variation of sintered density of the bulk containing Bi203 additives [4]. Optimum densities of 4.967 and 4.981 g / c m 3, respectively, were found for the pure Ba-ferrite and the sample containing 2.5 mol% Bi203, by sintering at = 1300°C for 12 h. X-ray diffraction patterns of both these samples exhibit the same diffraction lines (fig. 2) but they reasonably differ in the peak intensities. For ex-
Fig. 1. Micrographs of BaFe12019 ferrite containing 2.5 mol% Bi203 in samples (a) B3 and (b) B4 sintered at 1100 and 1300 o C, respectively, for 12 h. The sample of pure BaFe12019 ferrite shows the structure similar to (a) with average grain size 0.5 and 2.0 ~tm for sintering at two respective temperatures. The bar refers to 150 ixm.
ample, the intensity for (129), (1011) and (107) lines is considerably enhanced and that for a (213) line is considerably diminished after the incorporation of Bi 3+ in the ferrite core. The Bi203 is not completely incorporated into the ferrite core by sintering at l l 0 0 ° C , in the case of sample B3 (comprises bulk density=4.95 g/cm3), and the diffraction patterns shown in fig. 2 do not differ much from those for pure Ba-ferrite. The features also suggest that the crystal structure of the system is essentially hexagonal (space group P63/mmc [7]) and is not changed by the incorporation of Bi203 impurities. A maximum amount of Bi203 (2.5 mol%) incorporates in Ba-ferrite lattice corresponding to a sintering at 1300 ° C. As a consequence, the samples show considerable decrease in the saturation
S. Ram / Dielectric permittioity in Bi ~ + doped ferrite
243
(I(
(304) (12~ (114)
(129)
(127)
"E
I
t
(lml)
~.
I
I
I-
I~
~(126) (1110) 300) --
A
d~ 0
(220)
; 1'v'1-°'/I •
ov
.r
._.
(205) (118) (206)
1
(OO8) 110)
~
(200) (100)1
II ~fl010)l ~ (116(203)~]
A.JLJL
JLJ
**
(0061
L_
U j
Sample B4
(114)1I (220) I I
I_L
105
(304) I (127)
90
.,to, 1.88 l
(205)
,o0,
(203) I . . . . II
(107)
B3 (006)
I
I
_^ 2.~9 dhk I (~)----e.2.99 l
75 60 Diffraction Angle, 20 (degree)
45
AI 4.4;
30
Fig. 2. X-ray powder diffraction patterns showing the effect of Bi203 additives on the characteristic lines of Ba,Fe12019 hexaferrite. The lines are marked by the (hkl) values. The crystal structure is hexagonal with unit cell parameter a = 5.9 A and c = 23.2 ,~ [8]. * lines are not assigned firmly.
magnetization M s (cf. the data in table 1). For example, sample B4 (sintered at 1300°C/12 h) exhibits a lowest M s (12.72/~s/f.u.) and the highest bulk density of 4.98 g / c m 3 (consistent with X-ray density = 5 g/cm3). All the Bi203 (2.5 mol%) added in the starting composition is believed to be utilized in this substitution reaction. The expected composition of the sample in this model appears to be BaFe12_xBixO19, with x ~<0.35 (equivalent to 2.5 mol% Bi203 addition). The substitution accounts well for the decrease of M s, compared to that for pure Ba-ferrite, if Bi 3+ is supposed to substitute on for the octahedral Fe 3+ sites (also see the magnetic distribution for twelve Fe 3+ ions in BaFe12019, proposed by Collomb et al. [7]). The ionic size of Bi 3+ (=0.96 ,~) is reasonably large and it expectedly can only go in the octahedral Fe 3+ sites. In BaFe12019 , the Fe 3+ ions bear different octahedral and tetrahedral sites of sizes 0.65 and 0.49 .~, respectively [8]. The results of dielectric measurements obtained on the various samples are reported in table 1. The values for the parameters ~' and c" (real and imaginary parts of the complex permittivity, respectively) for Ba-hexaferrite are drastically mad-
ified (exhibit magnitudes enhanced by a factor of ---105) when Bi203 is incorporated in the composition system (sample series B). This variation could not be accounted for by effects of impurities. The expected impurities due to BiFeO 3 [9], BaFe204 [5,6], etc. (though they were not indicated by the X-ray diffraction measurements) are not expected to exhibit much larger c values than for Ba-hexaferrites [10]. Moreover, the BiFeO 3 which is the most probable impurity for the sample series B and is known to be a ferroelectric phase [11], comprises --- 103 times smaller values [12] than had been observed for the Bi 3+ substituted Ba-hexa-ferrite samples (table 1). The anomalously large ~ values observed with the sample series B (containing Bi203) may be understood by invoking Bi3+--, Fe 3+ incorporation in BaFe12_xBixO19 ferrite and the distortion of Bi 3+ substituted Fe 3+ sites [forming (FeO6) polyhedron]. The distortion of Bi 3+ substituted Fe 3+ sites is very phenomenological because of the characteristically larger ionic size (0.96 ~,) of Bi 3+ than Fe 3+ (0.65 /k). Samples A1 and B4 had the same heat-treatment (1300°C/12 h) and c increased by a factor of 200. The locally distorted
244
S. Ram / Dielectric permittivity in Bi 3 + doped ferrite
B i 3 + ~ F e 3÷ sites would, in general, possess spontaneous dipole moments of varying magnitudes and orientations frozen in microcrystals of the polycrystalline Ba-ferrite material. As a matter of fact, the c values should also depend on grain size (cf. table 1). The samples B1, B2 and B3, which had been given heat treatment at the same temperature of 1100 ° C for 4, 8 and 12 h, respectively, exhibit gradually increasing grain size of 1.2 to 1.5 ~m. They reveal c values of 112 × 103, 265 × 103 and 6 × 103, respectively, following a trend of decreasing with an increase in grain size. Sample B4 that contains Ba-ferrite crystallites of a maximum size of = 20 ~m (also contains the maximum Bi 3+ incorporation) presents ~ = 1.2 × 103. The apparently smaller c value was, however, shown in the case of sample B1. It is likely due to reasonably small Bi 3÷ incorporation in the Ba-ferrite interstitial sites, as evident by a smaller bulk density (table 1) shown for this sample. It is interesting that,the concentration of Bi 3÷ distorted (FeO 6) polyhedra in these samples is too small (~< 3%) and the increase of c (owing to Bi 3÷ incorporation) appears by as much as 105 . It ultimately suggests their mixing with the nearby Fe 3+ sites, possible via spin-orbit coupling [13]. The change in c could occur from wave function mixing of the Fe 3÷ (3d 5) wave function with that of the ligand fields. This is characteristically weak for d 5 orbitals and it is because the pure Ba-hexaferrite exhibits smaller c values (cf. table 1). The Bi3+(6s 2) having very extended wave functions offers an intimate mixing with the nearby Fe 3÷ ions and then in turns to the ligand fields forming "Bi3+-Fe 3÷ " coupled pairs [14]. A large permittivity change associated with the Ba-hexaferrite particles with BizO 3 addition is thus expected in this model. Fig. 3 represents the variation of c' as a function of frequency in the 100-107 Hz region. ~' usually decreases with increasing frequency and follows different decay paths for the different samples. The samples B4, which is believed to contain maximum (2.5 tool%) Bi203 incorporated into the BaFea20~9 ferrite (and does not indicate presence of unreacted BizO3), shows an almost unchanged variation of c' with frequency between 100-103.5 Hz and then a steady fall of e' to 107
_
B2
1
O~
A1
2
L
3
4
5
6
Log (Hz) Fig, 3. The complex permittivity (~') of annealed Ba-hexaferrite samples.
Hz. The other samples comprise much faster decay of c' with frequency, indicating large spinorbit a n d / o r spin-lattice relaxation losses on higher frequencies. These samples expectedly (present relatively small bulk densities given in table 1) contain traces of unreacted Fe203 and Bi203. The Ba-ferrite in these cases could have impurities of Fe3+/Fe 2+ and Bi 3÷ cations trapped interstitially (not on Fe 3+ ferrite sites) in the system. These impurity cations being comparatively loosely bound in the lattice could impart a larger relaxation loss with increase of frequency. The effects appear to be saturated after f>~ 10 4 n z and as a consequence e' shows less dependence onward frequencies. An expectedly small frequency dependence (and small values) of c' observed with sample A1 containing no additive of Bi203 is consistent with the earlier reported (BaFe12019) resuits [10]. The Bi 3+ substituted Ba-hexaferrite (except sample B1) shows very little dielectric loss (defined by tan 8 = c " / c ' ) particularly in the high frequency region (cf. fig. 4). They also reveal a low eddy current loss and a high enough resistivity [4]. A peculiar feature is observed for sample B1. It exhibits a peak (inferring increase of permittivity as well the loss factor tan 8) at f - 105 Hz in the plots for log c' and tan 8 as a function of frequency ( f ) . With the data available here we have not been able to ascertain the origin and the mechanism responsible for this abnormal behaviour. It is
S. Ram / Dielectric permittivity in Bi 3 + doped ferrite
245
0.35) type substitution. The results are consistent with the X - r a y diffractometry and variation of the bulk densities of the samples.
3-
2 . . . .
...93
Acknowledgement
The author is thankful to Dr. K. Shahi (I.I.T. K a n p u r , India) for providing the laboratory facilities for the dielectric measurements. 0
L
2
I
3
I
/4
I
5
I
6
I
7
Log f (Hz)
Fig. 4. Dielectric loss (tan 8) behaviour for the samples in fig. 3. sure that this sample comprises comparatively small grains of BaFea2019 with less s t r a i n / distortion defects because the reaction of ferrite formation was processed at a lower temperature ( ~ 1 1 0 0 ° C ) of sintering and for shorter (4 h) period.
4. Conclusions
The dielectric permittivity (c) of BaFe12019 ferrite samples obtained b y solid state reaction of the stoichiometric composition (BaCO 3 and Fe203) containing = 2.5 mol% B i 2 0 3 is measured. The c value has been found to be enhanced, as m u c h as 10 5 times, after the use of B i 2 0 3. The samples exhibit considerably small values of saturation magnetization (Ms) as c o m p a r e d to pure BaFea2O19 ferrite. The decrease of M s suggests incorporation of Bi 3+ (Bi203) cations in the ferrite core (substituting for Fe 3 + on the octahedral interstitial sites), according to BaFe12_xBixO19 (with x---
References
[1] H. Kojima and M. Sugimoto, Ferromagnetic Materials, vol. 3, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1982). [2] S. Ram, D. Chakravorty and D. Bahadur, J. Magn. Magn. Mat.~62 (1986) 221. [3] S. Ram, D. Bahadur and D. Chakravorty, J. Magn. Magn. Mat. 67 (1987) 378. [4] S. Ram, H. Krishnan, K.N. Rai and K.A. Narayan, Japan. J. Appl. Phys. 28 (1989) 604. [5] J. Beretka and M.J. Ridge, J. Chem. Soc. 3A (1968) 2463. [6] C.G. Bye and C.R. Howard, J. Appl. Chem. Biotechnol. 21 (1971) 319. [7] A. Collomb, P. Wolfers and X. Obradors, J. Magn. Magn. Mat. 62 (1986) 57. [8] A. Collomb, O. Abdelkader, P. Wolfers, J.C. Guitel and D. Samaras, J. Magn. Magn. Mat. 58 (1986) 247. [9] J.L. Mukherjee and F.F.Y. Wang, J. Am. Ceram. Soc. 54 (1971) 31. [10] H. Haberey and H.P.J. Wijn, Phys. Stat. Sol. 26 (1968) 231. [11] J.M. Moreau, C. Michel, R. Gerson and W.J. James, J. Phys. Chem. Solids 32 (1971) 1315. [12] M. Polomska, W. Kaczmarek and Z. Pajak, Phys. Stat. Sol. (a)23' (1974) 567. [13] D.K. Sardar, M.D. Shinn and W.A. Sibley, Phys. Rev. B26 (1982) 2382. [14] D.E. Lacklison, G.B. Scott and J.L. Page, Solid State Com~tm. 14 (1974) 861.
241
OBSERVATION OF ENHANCED DIELECTRIC PERMITTIVITY I N Bi 3+ D O P E D BaFe12019 F E R R I T E Shanker R A M 1 Department of Materials Science and Engineering, McMaster Universtty, Hamilton, Ontario LSS 4L7, Canada
Received 20 May 1988; in revised form 22 March 1989
The dielectric properties of a 2.5 mol% Bi203 doped Ba-ferrite (BaFex2019) have been investigated. The specimen shows a drastically enhanced ( - 105 times) permittivity (c) through local polarization of Fe 3+ electronic charges activated with nearby Bi3+ sites. The c value also depends reasonably on thermal annealing (causes grain growth) of the samples. The specimens comprising large particle sizes (1-20 ~tm) usually exhibit smaller ¢ values and low dielectric losses.
1. Introduction The oxide additives of A g 2 0 , Bi203, P205 or As203 added by suitable amounts (~< 2.5 mol%) to the crystallization reaction of BaFe12Oa9 ferrite in a B a O - F e 2 0 3 - B 2 0 3 glass o r / a n d to the stoichiometric composition, BaO + 6Fe203 ~ BaFea2019 , in processing the ferrite by high temperature solid state reaction process, have been useful in crystallizing out platelet-like BaFea2019 ferrite particles [1-4]. These particles exhibit superior magnetic and other physical properties to most applications of BaFea2019 ferrite in devices. Bi203 additives appeared to be particularly interesting in the in-
vestigation series of BaFe12019 ferrite processed b y solid state reaction process. Besides activating the crystallization of ferrite particles, the Bi203 incorporates in the ferrite lattice following BaFeaE_xBixO19 type substitution reaction, with x 0.35. The structural and magnetic properties of this composition series has been reported in an earlier publication [4]. In the present article we report on the dielectric permittivity (c) because results reveal interestingly enhanced (105 times) c values b y B i 2 0 3 incorporation.
2. Experimental BaFe12_xBixO19 (x ~ 0.35) is prepared b y solid state reaction (performed at 1 1 0 0 - 1 3 0 0 ° C ) of
1 Visiting Scientist. Table 1 Summary of experimental results Sample no.
Sintering
Ms (#B)
Density (g/cm3)
Grain size (~m)
¢'a) ( × 103)
tan 8 a)
A1 B1 B2 B3 B4
1300 ° C/12 h 1100 o C/4 h 1100 o C/8 h 1100 ° C/12 h 1300 ° C/12 h
14.50 12.67 12.72 13.25 12.72
4.97 4.89 4.92 4.95 4.98
2.0 1.2 1.4 1.5 20
0.005 111.7 264.8 6.1 1.2
0.8 3.45 2.1 2.0 0'.9
A refers to pure BaFe12019 ferrite composition. The sample series B contains 2.5 mol% Bi203 added (for Fe203) in BaFe12019 ferrite composition (A). a) Measured at f = 100 Hz. 0 3 0 4 - 8 8 5 3 / 8 9 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
242
S. Ram / Dielectric permittivity in Bi J + doped ferrite
BaCO3, Fe203 and Bi203 mixed in stoichiometric ratio. The formation of the Ba-ferrite phase has been verified by X-ray powder diffraction patterns recorded on a Rich-Seifert Isodebyeflex 2002 diffractometer with filtered CrKa radiation. Dielectric permittivity (c) measurements of the various samples (table 1) are carried out in the range of 100 Hz to 13 MHz using an impedance bridge (model 1608A) of General Radio Company, USA. Other experimental details appeared elsewhere [4].
3. Results and discussion
X-ray diffraction analysis of the solid state reaction of BaCO3 and 6Fe203, performed at temperatures between 1100 and 1300 °C demonstrates crystallization of Ba-hexaferrite (BaFe12019) through -/-Fe203 and BaFe204 as the intermediate reaction products [4-6]. We continue sintering of the samples (based on our previous experience of synthesising similar ferrites [2,3]) for 4, 8 and 12 h, to ensure about 100% utilization of the reactants/ intermediate phases in the reaction. The optimum period of sintering thus required to accomplish the ferritization reaction was found to be --12 h. When 2.5 mol% Bi203 was incorporated in the composition, it acted as an internal additive, i.e. the additive increases nucleation and growth of ferrite grains (of sizes up to 20 ~m shown by the micrographs in fig. 1) as well as it participates in the formation of the ferrite. The samples sintered with Bi203 for small periods of 4 and 8 h also comprise single phase Ba-ferrite. In table 1 we have summarized all the samples comprising single phase Ba-ferrite, as verified by X-ray diffractometry, selected for our dielectric permittivity measurements. The dissolution of Bi 3÷ (Bi203) in the ferrite interstitial sites has been demonstrated by the variation of sintered density of the bulk containing Bi203 additives [4]. Optimum densities of 4.967 and 4.981 g / c m 3, respectively, were found for the pure Ba-ferrite and the sample containing 2.5 mol% Bi203, by sintering at = 1300°C for 12 h. X-ray diffraction patterns of both these samples exhibit the same diffraction lines (fig. 2) but they reasonably differ in the peak intensities. For ex-
Fig. 1. Micrographs of BaFe12019 ferrite containing 2.5 mol% Bi203 in samples (a) B3 and (b) B4 sintered at 1100 and 1300 o C, respectively, for 12 h. The sample of pure BaFe12019 ferrite shows the structure similar to (a) with average grain size 0.5 and 2.0 ~tm for sintering at two respective temperatures. The bar refers to 150 ixm.
ample, the intensity for (129), (1011) and (107) lines is considerably enhanced and that for a (213) line is considerably diminished after the incorporation of Bi 3+ in the ferrite core. The Bi203 is not completely incorporated into the ferrite core by sintering at l l 0 0 ° C , in the case of sample B3 (comprises bulk density=4.95 g/cm3), and the diffraction patterns shown in fig. 2 do not differ much from those for pure Ba-ferrite. The features also suggest that the crystal structure of the system is essentially hexagonal (space group P63/mmc [7]) and is not changed by the incorporation of Bi203 impurities. A maximum amount of Bi203 (2.5 mol%) incorporates in Ba-ferrite lattice corresponding to a sintering at 1300 ° C. As a consequence, the samples show considerable decrease in the saturation
S. Ram / Dielectric permittioity in Bi ~ + doped ferrite
243
(I(
(304) (12~ (114)
(129)
(127)
"E
I
t
(lml)
~.
I
I
I-
I~
~(126) (1110) 300) --
A
d~ 0
(220)
; 1'v'1-°'/I •
ov
.r
._.
(205) (118) (206)
1
(OO8) 110)
~
(200) (100)1
II ~fl010)l ~ (116(203)~]
A.JLJL
JLJ
**
(0061
L_
U j
Sample B4
(114)1I (220) I I
I_L
105
(304) I (127)
90
.,to, 1.88 l
(205)
,o0,
(203) I . . . . II
(107)
B3 (006)
I
I
_^ 2.~9 dhk I (~)----e.2.99 l
75 60 Diffraction Angle, 20 (degree)
45
AI 4.4;
30
Fig. 2. X-ray powder diffraction patterns showing the effect of Bi203 additives on the characteristic lines of Ba,Fe12019 hexaferrite. The lines are marked by the (hkl) values. The crystal structure is hexagonal with unit cell parameter a = 5.9 A and c = 23.2 ,~ [8]. * lines are not assigned firmly.
magnetization M s (cf. the data in table 1). For example, sample B4 (sintered at 1300°C/12 h) exhibits a lowest M s (12.72/~s/f.u.) and the highest bulk density of 4.98 g / c m 3 (consistent with X-ray density = 5 g/cm3). All the Bi203 (2.5 mol%) added in the starting composition is believed to be utilized in this substitution reaction. The expected composition of the sample in this model appears to be BaFe12_xBixO19, with x ~<0.35 (equivalent to 2.5 mol% Bi203 addition). The substitution accounts well for the decrease of M s, compared to that for pure Ba-ferrite, if Bi 3+ is supposed to substitute on for the octahedral Fe 3+ sites (also see the magnetic distribution for twelve Fe 3+ ions in BaFe12019, proposed by Collomb et al. [7]). The ionic size of Bi 3+ (=0.96 ,~) is reasonably large and it expectedly can only go in the octahedral Fe 3+ sites. In BaFe12019 , the Fe 3+ ions bear different octahedral and tetrahedral sites of sizes 0.65 and 0.49 .~, respectively [8]. The results of dielectric measurements obtained on the various samples are reported in table 1. The values for the parameters ~' and c" (real and imaginary parts of the complex permittivity, respectively) for Ba-hexaferrite are drastically mad-
ified (exhibit magnitudes enhanced by a factor of ---105) when Bi203 is incorporated in the composition system (sample series B). This variation could not be accounted for by effects of impurities. The expected impurities due to BiFeO 3 [9], BaFe204 [5,6], etc. (though they were not indicated by the X-ray diffraction measurements) are not expected to exhibit much larger c values than for Ba-hexaferrites [10]. Moreover, the BiFeO 3 which is the most probable impurity for the sample series B and is known to be a ferroelectric phase [11], comprises --- 103 times smaller values [12] than had been observed for the Bi 3+ substituted Ba-hexa-ferrite samples (table 1). The anomalously large ~ values observed with the sample series B (containing Bi203) may be understood by invoking Bi3+--, Fe 3+ incorporation in BaFe12_xBixO19 ferrite and the distortion of Bi 3+ substituted Fe 3+ sites [forming (FeO6) polyhedron]. The distortion of Bi 3+ substituted Fe 3+ sites is very phenomenological because of the characteristically larger ionic size (0.96 ~,) of Bi 3+ than Fe 3+ (0.65 /k). Samples A1 and B4 had the same heat-treatment (1300°C/12 h) and c increased by a factor of 200. The locally distorted
244
S. Ram / Dielectric permittivity in Bi 3 + doped ferrite
B i 3 + ~ F e 3÷ sites would, in general, possess spontaneous dipole moments of varying magnitudes and orientations frozen in microcrystals of the polycrystalline Ba-ferrite material. As a matter of fact, the c values should also depend on grain size (cf. table 1). The samples B1, B2 and B3, which had been given heat treatment at the same temperature of 1100 ° C for 4, 8 and 12 h, respectively, exhibit gradually increasing grain size of 1.2 to 1.5 ~m. They reveal c values of 112 × 103, 265 × 103 and 6 × 103, respectively, following a trend of decreasing with an increase in grain size. Sample B4 that contains Ba-ferrite crystallites of a maximum size of = 20 ~m (also contains the maximum Bi 3+ incorporation) presents ~ = 1.2 × 103. The apparently smaller c value was, however, shown in the case of sample B1. It is likely due to reasonably small Bi 3÷ incorporation in the Ba-ferrite interstitial sites, as evident by a smaller bulk density (table 1) shown for this sample. It is interesting that,the concentration of Bi 3÷ distorted (FeO 6) polyhedra in these samples is too small (~< 3%) and the increase of c (owing to Bi 3÷ incorporation) appears by as much as 105 . It ultimately suggests their mixing with the nearby Fe 3+ sites, possible via spin-orbit coupling [13]. The change in c could occur from wave function mixing of the Fe 3÷ (3d 5) wave function with that of the ligand fields. This is characteristically weak for d 5 orbitals and it is because the pure Ba-hexaferrite exhibits smaller c values (cf. table 1). The Bi3+(6s 2) having very extended wave functions offers an intimate mixing with the nearby Fe 3÷ ions and then in turns to the ligand fields forming "Bi3+-Fe 3÷ " coupled pairs [14]. A large permittivity change associated with the Ba-hexaferrite particles with BizO 3 addition is thus expected in this model. Fig. 3 represents the variation of c' as a function of frequency in the 100-107 Hz region. ~' usually decreases with increasing frequency and follows different decay paths for the different samples. The samples B4, which is believed to contain maximum (2.5 tool%) Bi203 incorporated into the BaFea20~9 ferrite (and does not indicate presence of unreacted BizO3), shows an almost unchanged variation of c' with frequency between 100-103.5 Hz and then a steady fall of e' to 107
_
B2
1
O~
A1
2
L
3
4
5
6
Log (Hz) Fig, 3. The complex permittivity (~') of annealed Ba-hexaferrite samples.
Hz. The other samples comprise much faster decay of c' with frequency, indicating large spinorbit a n d / o r spin-lattice relaxation losses on higher frequencies. These samples expectedly (present relatively small bulk densities given in table 1) contain traces of unreacted Fe203 and Bi203. The Ba-ferrite in these cases could have impurities of Fe3+/Fe 2+ and Bi 3÷ cations trapped interstitially (not on Fe 3+ ferrite sites) in the system. These impurity cations being comparatively loosely bound in the lattice could impart a larger relaxation loss with increase of frequency. The effects appear to be saturated after f>~ 10 4 n z and as a consequence e' shows less dependence onward frequencies. An expectedly small frequency dependence (and small values) of c' observed with sample A1 containing no additive of Bi203 is consistent with the earlier reported (BaFe12019) resuits [10]. The Bi 3+ substituted Ba-hexaferrite (except sample B1) shows very little dielectric loss (defined by tan 8 = c " / c ' ) particularly in the high frequency region (cf. fig. 4). They also reveal a low eddy current loss and a high enough resistivity [4]. A peculiar feature is observed for sample B1. It exhibits a peak (inferring increase of permittivity as well the loss factor tan 8) at f - 105 Hz in the plots for log c' and tan 8 as a function of frequency ( f ) . With the data available here we have not been able to ascertain the origin and the mechanism responsible for this abnormal behaviour. It is
S. Ram / Dielectric permittivity in Bi 3 + doped ferrite
245
0.35) type substitution. The results are consistent with the X - r a y diffractometry and variation of the bulk densities of the samples.
3-
2 . . . .
...93
Acknowledgement
The author is thankful to Dr. K. Shahi (I.I.T. K a n p u r , India) for providing the laboratory facilities for the dielectric measurements. 0
L
2
I
3
I
/4
I
5
I
6
I
7
Log f (Hz)
Fig. 4. Dielectric loss (tan 8) behaviour for the samples in fig. 3. sure that this sample comprises comparatively small grains of BaFea2019 with less s t r a i n / distortion defects because the reaction of ferrite formation was processed at a lower temperature ( ~ 1 1 0 0 ° C ) of sintering and for shorter (4 h) period.
4. Conclusions
The dielectric permittivity (c) of BaFe12019 ferrite samples obtained b y solid state reaction of the stoichiometric composition (BaCO 3 and Fe203) containing = 2.5 mol% B i 2 0 3 is measured. The c value has been found to be enhanced, as m u c h as 10 5 times, after the use of B i 2 0 3. The samples exhibit considerably small values of saturation magnetization (Ms) as c o m p a r e d to pure BaFea2O19 ferrite. The decrease of M s suggests incorporation of Bi 3+ (Bi203) cations in the ferrite core (substituting for Fe 3 + on the octahedral interstitial sites), according to BaFe12_xBixO19 (with x---
References
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