Solid State Communications 148 (2008) 117–121

Contents lists available at ScienceDirect

Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Permeability of Nb and Ta doped lithium ferrite in high frequency rangeVivek Verma a, S.P. Gairola a, Vibhav Pandey a, R.K. Kotanala a,∗, Hua Su b

a National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, Indiab State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China

a r t i c l e i n f o

Article history:Received 16 June 2008Received in revised form28 July 2008Accepted 28 July 2008by E.V. SampathkumaranAvailable online 3 August 2008

PACS:78.20.Ci81.40.Ef75.20.Hr07.55.–w

Keywords:A. FerritesA. MicrostructureD. Saturation magnetic momentD. Permeability

a b s t r a c t

Nb and Ta doped lithium ferrites with composition of Li0.5Fe2.5O4 + xNb2O5 and Li0.5Fe2.5O5 + xTa2O5(where x = 0, 0.3, 0.6, 0.9, 1.2 and 1.5 wt%) have been prepared by a solid-state reaction method.The effect of Nb2O5 and Ta2O5 on microstructure, magnetic properties like saturation magnetic moment(Ms), coercivity (Hc), retentivity (Mr) and permeability of lithium ferrite have been investigated in thefrequency range 1 MHz to 1 GHz. The permeability of Nb2O5 doped lithium ferrite has been found higher(35) than pure lithium ferrite (25) at 1 MHz.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Lithium ferrite is one of the most versatile magnetic materialfor common use, having many applications in both low and highfrequency devices. It plays a useful role in many technologicalapplications such as microwave devices, power transformers inelectronics, rod antennas, read/write heads for high speed digitaltapes etc. due to high resistivity, mechanical hardness, high Curietemperature and chemical stability [1]. Magnetic and electricalproperties of lithium ferrites are strongly dependent on materialprocessing steps. Small amount of additives in it drasticallyaffects the properties of ferrite [2]. The magnetic propertiesof ferrites in general are determined by chemical composition,porosity and grain size. Some small dopants can affect the grainboundary energy and ultimately acts as the driving force forthe grain growth which improves the microstructure. Among thedifferent measurements of microstructure, grain size is one of themost important parameters affecting the magnetic properties offerrites [3]. Grain growth is closely related to the grain boundary

∗ Corresponding author. Tel.: +91 11 45608599.E-mail address: rkkotnala@mail.nplindia.ernet.in (R.K. Kotanala).

0038-1098/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2008.07.044

mobility. Whereas solute segregation has an added benefit inlowering the grain-boundary energy. The grain growth is enhancedwhen the dopant concentration in the grains is above the solidsolubility limit. The small amount of cations (Nb5+ and Ta5+) aresoluble in lithium ferrite lattice and occupy the regular positions onthe tetrahedral or octahedral sites, wherein these cations changethe properties of base material [4,5]. However many dopantshave been tried as processing aids and they do not directly affectthe magnetic properties even though the composition has beenmodified [6,7]. In the present work the effect of the addition ofNb2O5 and Ta2O5 as a dopants on the structural, electrical andmagnetic properties of lithium ferrite have been investigated.

2. Experimental method

The lithium ferrite samples were synthesized by the conven-tional double sintering ceramic technique, in which oxides orcarbonates were used as precursors. According to the stoichio-metric composition Li0.5Fe2.5O4, specified molar ratio of the ironoxide (Fe2O3) and lithium carbonate (Li2CO3) were milled by wetgrinding in power ball mill (RETCH, PM 400). Ball milled powderwas presintered at 750 ◦C for 10 h. The presintered powder wasmixed with Nb2O5 and Ta2O5 in 0.3, 0.6, 0.9, 1.2 and 1.5 wt%, re-spectively, to form different batches followed by final sintering

118 V. Verma et al. / Solid State Communications 148 (2008) 117–121

Fig. 1. X-ray powder diffraction pattern of pure and Nb2O5 doped lithium ferrite.

was carried out at 1050 ◦C for 5 h. Heating and cooling rate wascontrolled at 5 ◦C/min. The structural characterization of sampleswas carried out by the X-ray diffraction (XRD Rigaku Miniflex II,step size = 0.02◦) technique using Cu Kα radiation (wavelengthλ = 1.5406 Å). A scanning electron microscope (SEM LEO 440)was used to observe themicrostructure details of the samples. Per-meability values in frequency range 106–109 Hz were measuredby network analyser (Agilent 4284). For permeability measure-ments, toroids were used having outer and inner radius 6 mm and3 mm, respectively. Magnetic measurements were performed atroom temperature by plotting M–H curve using vibrating samplemagnetometer (VSM, Lake Shore7304) for all samples. The mag-netic properties such as saturation magnetization (Ms), retentiv-ity (Mr) and coercivity (Hc) were determined from the hysteresiscurves. Resistivity of samples was measured by two probe methodusing KEITHLEY 4200 SCS.

3. Results and discussion

For the specimen of Nb and Ta doped lithium ferrite, thecrystallographic structure was investigated using the X-raydiffraction 2θ angle from 20◦ to 70◦ in step of 2◦/min. Thepowder X-ray diffraction patterns exhibited that all the sampleswere identified as a single phase of cubic spinel structure and anadditional peak [222] was detected with Nb2O5 = 1.5 wt% dopedlithium ferrite. Nb2O5 and Ta2O5 doped lithium ferrites X-raydiffraction patterns are depicted in Figs. 1 and 2, respectively. TheM–H curves of Nb2O5 and Ta2O5 doped lithium ferrite are shownin Figs. 3 and 4, respectively. It is observed from the curves thatthe saturation magnetic moment value of pure lithium ferrite 58.9emu/g changes abruptly with Nb2O5 doping at 1.5 wt% attainingthe value 38 emu/g.

In case of Ta2O5 doped lithium ferrite saturation magneticmoment value changes slowly as depicted in Fig. 4. The decreasein saturation magnetization value due to doping is attributedto occupancy of Nb5+ and Ta5+ ion on B-site of spinel lithiumferrite [5,8]. The value of retentivity (Mr) and coercivity (Hc) forpure lithium ferrite are 0.435 emu/g and 18.93 G, respectively. Thevalues of coercivity (∼14.5 G) and retentivity (∼0.275 emu/g) isfound to decrease due to Ta2O5 doping in lithium ferrite but trendis reverse in the case of Nb2O5 doping. The typical microstructureof lithium ferrite with different amount of Nb2O5 and Ta2O5 areshown in Fig. 5. Doping of Nb2O5 and Ta2O5 in lithium ferrite notonly influence the lattice-constant but also exhibits an impact onthemicrostructure. Themicrographs of Fig. 5 clearly show that theaddition of Nb2O5 and Ta2O5 in lithium ferrite markedly promotes

Fig. 2. X-ray powder diffraction pattern of pure and Ta2O5 doped lithium ferrite.

Fig. 3. Magnetic hysteresis (M–H) loops of Nb2O5 doped lithium ferrites at roomtemperature.

Fig. 4. Magnetic hysteresis (M–H) loops of Ta2O5 doped lithium ferrites at roomtemperature.

the grain growth [9,10]. The average grain size of pure lithiumferrite is about 4.5 µm which is smaller than Nb2O5 and Ta2O5doped samples. From SEM micrographs it is quite evident that theNb2O5 promotes the grain growth aswell as grain connectivity. Butin case of Ta2O5 doped samples the grain connectivity mechanism

V. Verma et al. / Solid State Communications 148 (2008) 117–121 119

(a) Pure. (b) Nb2O5 = 0.6 wt%.

(c) Nb2O5 = 1.2 wt%. (d) Ta2O5 = 0.6 wt%.

(e) Ta2O5 = 1.2 wt%.

Fig. 5. Typical SEM micrographs of pure and doped lithium ferrite.

is better than grain growth. In Ta2O5 = 0.6 wt% doped lithiumferrite micrograph is showing some rods type structure is alsofound. In general there are three types of additives to modify theproperties of ferrite. The first type of additives that segregatesto the grain boundaries and affect the grain boundary resistivity,e.g. SiO2, CaO, Nb2O5 etc [11,12]. The second type of additivesthat dissolve in to grains and alter the magnetic properties,e.g. TiO2, MnO etc [13]. The third type of additives that affectthe microstructure development, e.g. V2O5, PbO, MoO3, nano-SiO2,Sb2O3 etc [14,15]. Dopants with 5+ cation valance, e.g. Nb2O5 andTa2O5 probably generate little excess cation vacancy concentrationin the ferrite lattice [16]. Since the melting temperature of Nb2O5and Ta2O5 is 1485 ◦C and 1800 ◦C, respectively. Therefore, theenhancement mechanism to improve the properties of a ferrite incase of the addition of Nb2O5 and Ta2O5, cannot be explained interm of liquid phase sintering. Since absence of any extra phasein Nb2O5 and Ta2O5 doped lithium ferrites XRD pattern exceptNb2O5 = 1.5% doped sample, it suggests that there is solidsolubility of Nb2O5 and Ta2O5 within their solid solubility limits.

Real and imaginary permeability response curves for Nb2O5 andTa2O5 doped lithium ferrite in frequency range 106–109 Hz areshown in Fig. 6(a), (b) and 7(a), (b), respectively. It is observedthat the value of real part of permeability (25) of pure lithiumferrite increaseswith themolar concentration of Nb2O5 and attainsthe value ∼35 for Nb2O5 = 1.2 wt% doped lithium ferritefollowed by a reduction with Nb2O5 = 1.5 wt% at 1 MHz. Butin case of Ta2O5 doping permeability decreases continuously withincrease in Ta2O5 concentration in lithium ferrite. Ta2O5 doped at0.9wt% lithium ferrite shows a sudden change in permeability. Thechange in permeability for different compositions can be definedon the dependence of permeability, saturation magnetization andaverage grain size expressed by following relation (1) [17]:

µi ∝ µ0Ms2Dm/[K1 + (3/2)λsσ ]β1/3δ (1)

where µi is the initial permeability, Ms and Dm are thesaturation magnetization and average grain size, respectively,K1 is the magneto-crystalline anisotropy, λs is the saturationmagnetostriction constant, σ is the inner stress, β is the volumeconcentration of impurity and δ is the thickness of domain wall.

120 V. Verma et al. / Solid State Communications 148 (2008) 117–121

Fig. 6. Plots of 6(a) real and 6(b) imaginary permeability of pure and Nb2O5 dopedlithium ferrite at room temperature.

The frequency dispersion of permeability of lithium ferritesamples is governed by Snoek’s limit ωµ′

= constant [18], whereµ′ stands for the real part of permeability and ω the resonancefrequency for domain wall motion above which µ′ decreases. Theresistivity of samples is found in the range of 106–107 � cm asshown in Table 1. The resistivity of lithium ferrite (4.2 × 106)first increases with Nb2O5 concentration and shows maximum7.4×107 value for Nb2O5 = 0.9% doped samples. Further increasein niobium concentration it begins to decrease. In the case of Ta2O5doped lithium ferrite, for Ta2O5 = 0.3% doped sample exhibitsmaximum resistivity (3.7 × 107) and it begins to decrease withfurther increase in tantalum concentration. The variation in the

Fig. 7. Plots of 7(a) real and 7(b) imaginary permeability of pure and Ta2O5 dopedlithium ferrite at room temperature.

resistivity can be defined on the basis of grain connectivity andsegregation of dopants on the grain boundaries where they act asinsulating phase.

4. Conclusion

The Nb2O5 and Ta2O5 doped lithium ferrite have beensuccessfully prepared by the solid-state reaction method. Insummary, the magnetic and electric properties of lithium ferritecan be improved by controlling the dopant amount of Nb2O5 orTa2O5 according to desired application. Microstructure properties

Table 1Magnetic and electrical properties of pure and Ta2O5/Ta2O5 doped lithium ferrites

Composition Saturation magnetization (emu/g) Resistivity (� cm) Permeability at resonance frequencyPermeability Resonance-frequency (MHz)

Pure lithium ferrite 58.9 4.2 × 106 28.7 38Nb2O5 = 0.3% 52.7 9.6 × 106 37.4 28Nb2O5 = 0.6% 48.8 1.5 × 107 36.0 23Nb2O5 = 0.9% 45.9 7.4 × 107 38.0 28Nb2O5 = 1.2% 41.9 2.3 × 107 39.5 22Nb2O5 = 1.5% 38.2 2.1 × 107 36.0 21Ta2O5 = 0.3% 56.2 3.7 × 107 25.7 37Ta2O5 = 0.6% 53.3 1.3 × 107 23.9 37Ta2O5 = 0.9% 50.1 9.2 × 106 30.5 26Ta2O5 = 1.2% 46.7 8.1 × 106 24.8 33Ta2O5 = 1.5% 45.6 1.3 × 106 21.4 43

V. Verma et al. / Solid State Communications 148 (2008) 117–121 121

like grain growth and grain connectivity is significantly influencedby Nb2O5 doping.

Acknowledgement

The authors are grateful to Dr. Vikram Kumar, Director‘‘National Physical Laboratory’’ New Delhi for providing constantencouragement and motivation to carry out this work.

References

[1] S.S. Bellad, S.C. Watawe, B.K. Chougule, J. Magn. Magn. Mater. 195 (1999)57–64.

[2] A. Gonchar, S. Katynkina, L. Letyuk, I. Ryabov, J. Magn. Magn. Mater. 215–216(2000) 224–226.

[3] Wenxun Yan, LinWang, Zhiguo Xia, Ming Cheng, Qiang Li, Yiling Zhang,Mater.Rese. Bull. 42 (2007) 1468–1472.

[4] Naoaki Saito, Kazuyoshi Koyama, Mitsumori Tanimoto, Chem. Phys. Lett. 300(1999) 262–268.

[5] V.T. Zaspalis, E. Antoniadis, E. Papazoglou, V. Tsakaloudi, L. Nalbandian, C.A.Sikalidis, J. Magn. Magn. Mater. 250 (2002) 98–109.

[6] H. Shokrollahi, K. Janghorban, Mater. Sci. Eng. B 141 (2007) 91–107.[7] S.-F. Wang, Y.-R. Wang, T.C.K. Yang, P.-J. Wang, C.-A. Lu, J. Magn. Magn. Mater.

217 (2000) 35–43.[8] V. Tsakaloudi, E. Eleftheriou, M. Stoukides, V. Zaspalis, J. Magn. Magn. Mater.

318 (2007) 58–64.[9] M.F. Yan, D.W. Johnson, J. Am. Ceram. Soc. 61 (1978) 342–349.

[10] Wenxun Yan, LinWang, Zhiguo Xia, Ming Cheng, Qiang Li, Yiling Zhang,Mater.Res. Bull. 42 (2007) 1468–1472.

[11] Vivek Verma, Vibhav Pandey, R.K. Kotnala, Hari Kishan, Nitender Kumar, P.C.Kothari, J. Alloy. Comp. 443 (2007) 178–181.

[12] J. Nie, H. Li, Z. Feng, H. He, J. Magn. Magn. Mater. 265 (2003) 172–175.[13] J. Zhu, K.J. Tseng, IEEE Trans. Magn. 40 (2004) 3339–3345.[14] M.F. Yan, D.W. Johnson, J. Am. Ceram. Soc. 61 (1978) 342–349.[15] R.K. Kotnala, Vivek Verma, Vibhav Pandey, V.P.S. Awana, R.P. Aloysius, P.C.

Kothari, Solid State Commun. 143 (2007) 527–531.[16] I.W. Chen, L.A. Xue, J. Am. Ceram. Soc. 73 (1990) 2585–2609.[17] J. Smit, H.P.J. Wijn, ferrites, Philips Technical Library, Eindhoven, Netherland,

1959, p. 244.[18] Gary G. Bush, J. Appl. Phys. 63 (8) (1998) 3765.