Indian Journal of Pure & Applied Physics Vol. 47, April 2009, pp. 277-281 Effect of dopants on barium hexaferrite S V Salvi* & V H Joshi + *Material Physics Lab, Department of Physics, Birla College, Kalyan, Mumbai, 421 304 + Department of Physics, Bhavans H S College, Chowpatty, Mumbai 400 007 E- mail: [email protected]Received 11 August 2008; revised 8 December 2008; accepted 16 February 2009 Samples of Ba 1.266 Ti 2.956 Fe 7.6 Li 0.844 O 19 hexaferrite have been prepared using anatase and rutile phases of TiO 2 by classical ceramic route. A pure sample of Ba Fe 12 O 19 has also been prepared for comparison. The structural, electrical and magnetic characteristics have been studied by XRD, SEM, FTIR, two probe method, hysteresis loop and susceptibility curves. The samples exhibit hexagonal magnetoplumbic structure with spacegroup P6 3 /mmc and p type ferrimagnetic behaviour. BaFe 12 O 19 is a hard ferrite, however, the substituted hexaferrites are soft ferrites. The substitution causes increase in grain size and particle size. The Mc value, coercive force Hc, squareness M R /Ms and Curie temperature T c are large for rutile TiO 2 based samples as compared to anatase TiO 2 based samples. However, the anatase allotrope based samples enhance the grain growth. These materials can be used in the applications related to magnetic recording, microwave absorption and EM absorption. Keywords: Polycrystalline ferrites, Hexaferrites, Curie temperature, Magnetoplumbic 1 Introduction A study of phase equilibria in the BaO:Fe 2 O 3 :TiO 2 system reveals the existence of a number of ternary compounds that form between the high dielectric- constant polytitanates and magnetic barium hexaferrite having properties suitable for a wide variety of electronic applications, including wireless system 1 . Barium haxaferrite, a well-known permanent magnet with great technical importance, has attracted extensive attention for the last few decades. Significant variations in magnetic properties have been reported in Bi, Zn doped BaFe 12 O 19 2,3 . Co-Ti substituted BaFe 12 O 19 particles lead to archivally stable media for magnetic recording 4 . Lithium ferrite has attractive electric and magnetic properties for microwave and memory core applications 5 . The bulk properties of anatase and rutile allotropes of TiO 2 are quite different due to vertex and edge sharing in the respective phase. The variation in the structural and electrical properties has been reported in mixed oxide systems using anatase and rutile 6 TiO 2 . In this paper, the influence of titanium and lithium substitution on the properties of barium ferrite has been studied. 2 Experimental Details The hexaferrites were prepared by usual high- temperature solid-state technique by homogeneously mixing AR grade powders of BaCO 3, Fe 2 O 3, Li 2 CO 3 and anatase/ rutile TiO 2 and using step sintering for 6 hr each at 600°, 800° and 1000° C and final sintering at 1150° C for 24 hr to ensure decomposition of carbonates and a homogeneous reaction. BaFe 12 O 19 (BFO) was also synthesized in the same manner to facilitate comparison. The hexaferrites Ba 1.266 Ti 2.955 Fe 7.6 Li 0.844 O 19 (anatase) [BTF7.6LO(A)] and Ba 1.266 Ti 2.955 Fe 7.6 Li 0.844 O 19 (rutile) [BTF7.6LO(R)] maintain the charge balance and therefore, are cation deficient. The XRD patterns for the samples were obtained using a highly sophisticated microprocessor based JEOL-JDX 8030 diffractometer using a copper target. The IR spectra of the compounds were recorded on the Nicolet Instrument Corporation, USA MAGNA 550 Spectro Photometer at room temperature, in the range of 50 to 4000 cm -1 . Fine coat ion sputter JFC-1100 and JSM-840 Jeol Scanning Microscope were used to obtain the SEMs. For dielectric measurements, the samples were polished to obtain flat, smooth and parallel surfaces. The two surfaces were electroded with high purity ultrafine silver paste by firing at 500°C for 5 min. The dielectric properties were studied in the temperature range 300-650 K at 100Hz and 1 kHz using Aplab 4910 autocompute LCR-Q meter. The variation in capacitance(C) and loss factor (tanδ) with the frequency in the range 10 kHz to 1000 kHz was also recorded at room temperature using a HP 4277 ALCZ
Transcript
Indian Journal of Pure & Applied Physics Vol. 47, April 2009, pp. 277-281
Effect of dopants on barium hexaferrite
S V Salvi* & V H Joshi+ *Material Physics Lab, Department of Physics, Birla College, Kalyan, Mumbai, 421 304
+Department of Physics, Bhavans H S College, Chowpatty, Mumbai 400 007 E- mail: [email protected]
Received 11 August 2008; revised 8 December 2008; accepted 16 February 2009
Samples of Ba1.266Ti2.956 Fe7.6Li0.844O19 hexaferrite have been prepared using anatase and rutile phases of TiO2 by classical ceramic route. A pure sample of Ba Fe12O19 has also been prepared for comparison. The structural, electrical and magnetic characteristics have been studied by XRD, SEM, FTIR, two probe method, hysteresis loop and susceptibility curves. The samples exhibit hexagonal magnetoplumbic structure with spacegroup P63/mmc and p type ferrimagnetic behaviour. BaFe12O19 is a hard ferrite, however, the substituted hexaferrites are soft ferrites. The substitution causes increase in grain size and particle size. The Mc value, coercive force Hc, squareness MR/Ms and Curie temperature Tc are large for rutile TiO2 based samples as compared to anatase TiO2 based samples. However, the anatase allotrope based samples enhance the grain growth. These materials can be used in the applications related to magnetic recording, microwave absorption and EM absorption.
A study of phase equilibria in the BaO:Fe2O3:TiO2 system reveals the existence of a number of ternary compounds that form between the high dielectric-constant polytitanates and magnetic barium hexaferrite having properties suitable for a wide variety of electronic applications, including wireless system1. Barium haxaferrite, a well-known permanent magnet with great technical importance, has attracted extensive attention for the last few decades. Significant variations in magnetic properties have been reported in Bi, Zn doped BaFe12O19
2,3. Co-Ti substituted BaFe12O19 particles lead to archivally stable media for magnetic recording4. Lithium ferrite has attractive electric and magnetic properties for microwave and memory core applications5. The bulk properties of anatase and rutile allotropes of TiO2 are quite different due to vertex and edge sharing in the respective phase. The variation in the structural and electrical properties has been reported in mixed oxide systems using anatase and rutile6 TiO2. In this paper, the influence of titanium and lithium substitution on the properties of barium ferrite has been studied. 2 Experimental Details
The hexaferrites were prepared by usual high-temperature solid-state technique by homogeneously mixing AR grade powders of BaCO3, Fe2O3, Li2CO3
and anatase/ rutile TiO2 and using step sintering for 6 hr each at 600°, 800° and 1000° C and final sintering at 1150° C for 24 hr to ensure decomposition of carbonates and a homogeneous reaction. BaFe12O19 (BFO) was also synthesized in the same manner to facilitate comparison. The hexaferrites Ba1.266Ti2.955Fe7.6Li0.844O19 (anatase) [BTF7.6LO(A)] and Ba1.266Ti2.955Fe7.6Li0.844O19 (rutile) [BTF7.6LO(R)] maintain the charge balance and therefore, are cation deficient. The XRD patterns for the samples were obtained using a highly sophisticated microprocessor based JEOL-JDX 8030 diffractometer using a copper target. The IR spectra of the compounds were recorded on the Nicolet Instrument Corporation, USA MAGNA 550 Spectro Photometer at room temperature, in the range of 50 to 4000 cm-1. Fine coat ion sputter JFC-1100 and JSM-840 Jeol Scanning Microscope were used to obtain the SEMs. For dielectric measurements, the samples were polished to obtain flat, smooth and parallel surfaces. The two surfaces were electroded with high purity ultrafine silver paste by firing at 500°C for 5 min. The dielectric properties were studied in the temperature range 300-650 K at 100Hz and 1 kHz using Aplab 4910 autocompute LCR-Q meter. The variation in capacitance(C) and loss factor (tanδ) with the frequency in the range 10 kHz to 1000 kHz was also recorded at room temperature using a HP 4277 ALCZ
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LCR meter. The d c resistivity at room temperature and the a c resistivity(1kHz) in the temperature range 300 – 838 K were determined by two - probe method using the same electrode system that was used in dielectric studies. Magnetic properties were obtained using Hysteresis loop tracer and susceptibility apparatus.
3 Results and Discussion All the X-ray patterns indicate a single-phase
hexagonal magnetoplumbite structure having the space group P63/mmc7,8 as per the JCPDS data. Figure 1 shows one such pattern corresponding to BTF7.6LO(R). The structural properties of all the samples are presented in Table 1. The a and c values calculated for all the samples are pertaining to the P63/mmc hexagonal unit cell structure9. When compared with the most intense (114) plane, intensities corresponding to the other planes in the substituted samples increase. This may be attributed to the presence of lithium having low atomic number or the absence of a cation in the (114) plane.
The SEMs (Fig. 2) are dense and homogeneous. The substitution results in almost doubling of the grain size (Table 1). The grains are in the single domain limit10 (<1µm) suitable for electronic applications.
FTIR spectra show two prominent bands between 590 and 435 cm-1 in these hexaferrites, agreeing well with the reports11. The bands in the substituted samples are broadened because of random distribution of (Ba2+, Li1+, Ti4+) cations. The substitution brings the two bands closer implying a stronger interaction between the two sites, which may be responsible for the significant growth in the grain/particle size (Table 1).
The dielectric properties of polycrystalline ferrite composites arise mainly due to interfacial polarization (space charge) and intrinsic electric dipole polarization12,13. The relaxation spectra and the dielectric loss of these hexaferrites at room temperature (100 Hz to 1000 kHz) show a decrease in dielectric constant as well as dielectric loss with the
Table 1 — Structural properties of the hexaferrites
Sample Properties
BTF7.6LO(A) BTF7.6LO(R) BFO
Lat. Const. a= b Å 5.882 5.882 5.8904 Lat. Const. c Å 23.240 23.23 23.176 Avg. Debye part. size Å 256 225 203 Avg. grain size µm 0.65 0.627 0.38 Density Experimentalg/cc
Fig. 1 — XRD of Ba1.266Ti2.955Fe7.6Li0.844O19 (rutile)
Fig 2 — SEMs of BFO and BTF7.6LO(R)
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increase in frequency, agreeing well with the Maxwell’s Wagner Model for interfacial polarization. In general, on substitution, the space charge, tan δ and K” increase (Table 2) due to the random distribution of cations having different valencies and larger grain size. However, the use of anatase in place of rutile allotrope of TiO2 results in further increase in the dielectric constant and decrease of tan δ and K”. Interestingly, the relative intensities of majority of planes are more enhanced in anatase as compared to the rutile counterpart and (1 1 4) plane.
The plots of dielectric constant at 1kHz. versus temperature of all the hexaferrites are shown in Figs 3 (a-c). In both the substituted samples, there is a decrease in the dielectric constant initially from 300 to about 350 K which becomes more significant in BTF7.6LO(A) sample. The sharpness of the fall is
characterized by the temperature coefficient of the dielectric constant (K’T) (Table 2), which is considerably larger for both the substituted samples suggesting that the substituted samples may be used as transducers near room temperatures. At higher temperatures, the dielectric constant exhibits a hysteresis associated with a transition temperature (Tt = 554K – 629K (Fig. 3)), after which it increases in all the hexaferrites. This transition is likely to be due to the magnetic Curie temperature14, which is around 750 K for BFO.
Both the dc and ac resistivities for the BTF7.6LO(R) samples are larger than those for BTF7.6LO(A) samples (Table 3). The corresponding dielectric constant (Table 2) is smaller for BTF7.6LO(R) sample implying that the space charge plays significant role in the conduction at low frequencies. The dc resistivity is observed to increase for decreasing grain size (Tables 1 and 3) of the samples. Since the d c resistivity is correlated with the grain boundary, it appears that as the grain boundary increases, the thickness of the boundary also increases.
Table 3Electrical parameters of hexaferrites
a.c. (ρa.c.) resistivity at room
d.c. (ρd.c.) resistivity at room
Activation Energy Parameters
temp temp
PTCR Coefficient =dρ/ (ρ dT)
Region II Region III Region IV Sample (KΩ.cm) (KΩ.cm) Region I (380 K -460K) (495k-550k) (550-700k) × 106 × 106 (K-1) (eV) (eV) (eV) BTF7.6LO(A) 2.82 3.55 0.5103 0.17 1.20 1.20 BTF7.6LO(R) 8.25 4.49 0.2634 0.14 0.72 0.24 BFO 6.40 5.20 0.0805 0.24 0.54 0.54
Table 2 — Dielectric parameters of all the samples
Sample Parameters BTF7.6LO(A) BTF7.6LO(R) BFO
Dielectric constant (K’) at room temp. and at 100 Hz.
540 300 300
Tan δ at room temp. and at 100 Hz.
1.2 3.25 0.84
Dielectric loss (K’’) at room temp. and at 100Hz.
648 975 252
Dielectric constant (K’) at room temp. and at 1 KHz.
175 120 150
Tan δ at room temp. and at 1KHz.
0.98 2.10 0.64
Dielectric loss (K’’) at room temp. and at 1KHz.
172 252 98
Dielectric constant (K’) at room temp. and at 1MHz.
Fig. 3 — (a) Dielectric constant for BTF7.6LO(A) (b) — ielectric constant for BTF7.6LO(R), (c ) Dielectric constant for BFO
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The conduction in ferrite is attributed to the simultaneous presence of Fe2+ and Fe3+ ions on equivalent sites or Fe3+ and an electron on vacant site, which remain fv1 and 2b sites present in the R-block of M-type barium ferrite12. The inverse correlation between the resistivity and the dielectric constant both measured at 1 kHz. (Tables 2 and 3) suggests the similar presence of space charge.
The results of resistivity versus temperature measurements of all the hexaferrite samples are shown in Figs 4. In these figures log of resistivity in kΩ.m at 1kHz is plotted against inverse absolute temperature. Just above room temperature, all the hexaferrites show PTCR effect (Table 3). It is observed that the PTCR effect is the least in BFO. PTCR effect may be due to oxygen vacancies. It is significantly larger on substitution. Interestingly, it is maximum for the BTF7.6LO(R) sample. It is noted that the semiconducting behaviour (region II) of the BTF7.6LO(R) sample commences at temperatures lower than that of the BTF7.6LO(A) sample. The regions III and IV appear to correspond to ferrimagnetic and paramagnetic states, respectively. However, a significant decrease in the activation energy corresponding to post Curie temperature of BTF7.6LO(R) sample means considerable increase in mobility. The activation energies (region III) appear
to depend on the site preference by the dopants viz., lithium and titanium and their distributions, as is indicated by the fact that the intensities of reflections corresponding to BTF7.6LO(A) is more than that of BTF7.6LO(R) sample.
The normalized susceptibility (X/XT) vs temperature curves for all the samples and one pattern for the magnetization vs field curve at room temperature for BTF7.6LO(R) sample are shown in Figs 5 and 6, respectively. The results of room temperature saturation magnetization, remenance, coercivity, remenance ratio and Curie temperature for all the samples are given in Table 4. The Curie point for BFO (Fig. 5) is close to the reported value14 of 750 K. It is lowered on substitution. The decrease in the Curie point for the substituted samples (Fig. 5, Table 4) is attributed to the fact that with addition of Ti and Li and decrease of Fe content in the M-type phase, the magnetic coupling becomes weak15. BFO shows hard16 p type ferrimagnetic features. On the
Fig. 4— (a) Log(ρ) versus 1/T for BTF7.6LO(A), (b) Log(ρ) versus 1/ T for BTF7.6LO(R), (c) Log(ρ) versus 1/ T for BFO
Table 4 — Summary of the magnetic properties of hexaferrites at room temperature
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other hand, both the substituted samples show soft17 p type ferrimagnetic features8 (Table 4). Oxygen vacancies may be responsible for the soft features17. The value of saturation magnetization for the polycrystalline BFO sample is 34.95 emu/g at room temperature, which is expectedly lower than the value of the saturation magnetization18,19 (Ms) of 55 emu/g and 72 emu/g19 reported for single crystals of BaFe12O19 since the orientation, surface and inhomogeneity of the particles18 and preparation method17,20-22 affects Ms, Mr/Ms and Hc. A decrease in the values of MR, Ms, MR /Ms and Tc with substitution may be because the Li1+ ions replace Fe3+ ions of the octahedral 12k sites and hexahedral 2b sites, while Ti4+ ions replace the spin up Fe3+ ions of the octahedral 4fv1 and 2a sites as the intensities of certain planes indicate.
All the curves (Fig.5) show a peak before Curie temperature, indicating single domain states23 and even the grain size considerations support the existence of single domains (Table 1). The magnetic
transition for BTF7.6LO(R) sample at Curie temperature is the sharpest. The drastic change in activation energies (Table 3) at the Curie temperature corresponding to BTF7.6LO(R) sample may be correlated with the sharp transition. The cation distribution and the site preference of the cations seems to play an important role in the magnetic properties of the rutile based hexaferrites.
4 Conclusions All the hexaferrites exhibit hexagonal
magnetoplumbic structure with spacegroup P63/mmc. The structural and electrical parameters indicate large grain size and PTCR coefficient whereas magnetic parameters indicate soft ferrite behaviour for the substituted samples. It can be concluded that allotropes of TiO2, i.e. vertices sharing in anatase and edge sharing in the rutile, site preferences and cation distribution of different cations result in marked variation in the properties of the titanium and lithium substituted hexaferrite samples. References
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