Materials Letters 63 (2009) 1921–1924

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Materials Letters

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Hysteresis analysis of Co–Ti substituted M-type Ba–Sr hexagonal ferrite

Charanjeet Singh a,⁎, S. Bindra Narang a, I.S. Hudiara b, Yang Bai b, Koledintseva Marina c

a Department of Electronics Technology, Guru Nanak Dev University Amritsar, Indiab School of Materials of Science and Engineering, University of Science and Technology Beijing, Chinac Department of Electrical Engineering, University of Missouri Rolla, USA

⁎ Corresponding author.E-mail address: charanjeet2003@rediffmail.com (C.

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.06.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 April 2009Accepted 2 June 2009Available online 9 June 2009

Keywords:CeramicsMagnetic materialsCoercive forceSaturation magnetizationAnisotropy field

The magnetic, crystallographic properties and grain morphology of synthesized Ba0.5Sr0.5CoxTixFe(12−2x)O19

ferrite have been investigated by XRD, SEM and VSM. XRD and SEM confirm M-type hexagonal crystalstructure. X-ray diffraction indicates expansion of hexagonal unit cell with substitution of Co2+ and Ti4+ ions.The microstructure governs increase in density and intergrain connectivity with substitution. Thepreferential site occupancy of substituted Co2+ and Ti4+ ions results in rapid decline of anisotropy field,hysteresis loops also revealed same effect of substitution. Coercivity and remanence magnetization can beeasily controlled by varying substitution while maintaining high saturation magnetization, making it usefulfor recording media.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Hexagonal ferrites are known for their strong uniaxial magne-tocrystalline anisotropy with ease of magnetization along c-axis[1]. Most substitutions in hexagonal ferrites reduce the anisotropyfield to obtain hysteresis properties suitable for various applica-tions [2–4].

A number of reports are available on substitution of Fe3+ ionsin Ba or Sr hexagonal ferrites, e.g. Co–Zr, Ni–Zr, Co–Ru, Co–Sn [4–7]etc. In this communication, we are reporting hysteresis analysis ofdivalent Co2+ and tetravalent Ti4+ ions substituted Ba–Sr ferrite forthe first time.

2. Experimental

Standard Ceramic Method was used to synthesize required M-typehexagonal ferrite with chemical formula Ba0.5Sr0.5CoxTixFe(12−2x)O19

(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0). The magnetic parameters weremeasured with Lake Shore VSM 7307 at an applied field of ±10 kOeand phase structure was investigated by Philips Expert Diffractometerwith CuKα radiation (λ=1.54 Å). The structure morphology wasidentified using SEM instrument, Hitachi S-4700 FESEM. The Curietemperature was measured by gravity method. The bulk density wasmeasured using Archimedes principle.

Singh).

ll rights reserved.

3. Results and discussion

3.1. Structure characterization

X-ray diffractograms and micrographs of sintered samples (Figs. 1and 2) show the formation of magnetoplumbite structure. In Table 1,lattice constant ‘a’ undergoes slow variation with substitution x.Lattice constant ‘c’ rapidly increases at lower substitution and slowsdown at a higher substitution. It follows the fact that all hexagonalferrites exhibit constant lattice parameter ‘a’ and variable parameter‘c’ [8]. The variation in lattice constants with substitution indicatesthat substitutions are achieved on crystallographic sites. It also impliesthat easy magnetized c-axis undergoes more expansion than a-axiswith Co2+ and Ti4+ ions substitution. It is related to larger ionic radiiof Co2+ ion (0.72 Å) and Ti4+ ion (0.68 Å) than Fe3+ ion (0.64 Å) [9].

Table 1 shows that theoretical variation of cell volume is inaccordance with the experimental variation in bulk density. Thereduction in calculated porosity matches with porosity diminutionreflected in microstructure (grain closeness). The maximum bulkdensity achieved is 93% of theoretical value in sample 1.0 andminimum is 85% in undoped sample 0.0. Thus, substitution acceleratesferrite synthesis reaction.

3.2. Coercivity (Hc), saturation magnetization (Ms) and anisotropyfield (Ha)

Electronegativity is related with attraction of valence electrons andmore electronegative ions tend to occupy octahedral site. This site islarger than the tetrahedral site [5]. According to Ligand theory, ions site

Fig. 1. X-ray diffraction patterns of Ba0.5Sr0.5CoxTixFe(12−2x)O19 ferrite sintered at 1250 °C for 20 h.

Fig. 2. SEM micrographs of Ba0.5Sr0.5CoxTixFe(12−2x)O19 ferrite.

1922 C. Singh et al. / Materials Letters 63 (2009) 1921–1924

Table 1Lattice constant ‘a’ and ‘c’, cell volume, X-ray density, bulk density, porosity, remanence magnetization, Mr/Ms ratio, BHmax., Curie temperature, coercivity, anisotropy field, (grainsize)−1 and saturation magnetization of Ba0.5Sr0.5CoxTixFe(12−2x)O19 ferrite.

x a(Å)

c(Å)

Cell volume(Å)

T.D(g/cm3)

B.D(g/cm3)

Porosity(%)

Mr

(emu/g)Mr/Ms BHmax

(kGOe)Tc(°C)

Hc

(kGOe)Ha

(kOe)(Grain size)−1

(µm)Ms

(emu/g)

0 5.87 23.11 689.5 5.234 4.453 14.91 30 0.5 363 619 2.1 13.36 3.7 64.650.2 5.873 23.12 690.7 5.221 4.554 12.78 14.2 0.21 22 578 0.47 6.5 2.56 72.10.4 5.877 23.15 692.4 5.202 4.618 11.23 11.7 0.17 8 555 0.24 7.2 1.75 71.40.6 5.881 23.16 693.8 5.187 4.692 9.55 9.5 0.14 5 542 0.2 8.1 1.61 70.60.8 5.883 23.18 694.8 5.175 4.733 8.55 5.8 0.09 3 532 0.12 7.59 1.45 70.451 5.884 23.18 695 5.169 4.801 7.11 8.9 0.14 6 523 0.07 7.75 1.35 69.6

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occupancy also depends on d-configuration and nature of otherparticipating cation [10].

The electronegativity for Ti4+ ions is higher than that of Co2+ ions,thus former ions can occupy octahedral site. Co2+ ions prefer tooccupy octahedral sitewith d7 configuration and Ti4+ ions have no sitepreference due to d0 configuration. Besides this, Ti4+ ions (3p6 con-figuration) aremore compressible than Co2+ ions (3d7 configuration),thus can occupy tetrahedral site. It has been reported that the Co2+

ions have preferential occupancy of 4f1–4f2 sites [11]. Zhou et al.reported that Co–Ti ions strongly prefer 2b and 4f2 sites [12] whileBatlle et al. showed Co2+ ions preference for 4f1 site and Ti4+ ions for4f2 site [13].

The saturationmagnetization of samples is derived from the law ofsaturation [14] using the following equation

M = Ms 1− A=H − B =H2� �

+ χpH ð1Þ

where Ms is the saturation magnetization, A is inhomogeneityparameter, χp is the high field susceptibility and B is the anisotropyparameter.

For hexagonal crystal structure, B can be expressed as

B = H2a = 15 = 4K2

1 = 15M2s ð2Þ

where Ha is anisotropy field and K1 is anisotropy constant.Hysteresis loops in Fig. 3(a), (b), (c), (d), (e), and (f) indicate that

all the samples exhibit steep rise in magnetization at low applied fieldfollowed by a slow variation at high field. The large slope of hysteresiscurve of undoped sample 0.0 at high field indicates unsaturated stateand this slope falls with substitution of Co2+ and Ti4+ ions. Thereduction in anisotropy field, discussed later, is responsible for thisvariation.

Table 1 represents the variation of magnetic properties with sub-stitution x. High coercivity (2100Oe) is observed in the undoped sample0.0, which is due to uniaxial magnetocrystalline anisotropy along c-axis.There is a remarkable fall in coercivitywith the substitution of Co2+ andTi4+ ions at x=0.2 (470 Oe). The replacement of Fe3+ ions bysubstituted ions at 4fII and 2b sites underlies the fast reduction incoercivity. These two sites contribute to a large anisotropy field [15].

The second effect accompanying reduction in coercivity isextrinsic, causing increase in grain size with substitution. Samebehavior is observed in microstructure (Fig. 2), this resembles withliterature report about inverse nature of coercivity with grain size [16].Analysis in Table 1 also shows almost linear variation of coercivitywith reciprocal of grain size. Large number of intergranular pores isobserved in Ba–Sr ferrite (x=0.0), which decreases with substitution.The pores act as non-magnetic inclusions and discourage grainconnectivity. The decrease in strength of these pores correspondswith increase in grain size, thereby decreasing coercivity. It alsoreaffirms that porosity strongly affects coercivity [17]. 88% reductionin Hc occurs from sample x=0.0 (2100 Oe) to x=0.4 (240 Oe) andsame decreases by 61% in Ba–Co–Ti ferrite [6].

The saturation magnetization increases (Table 1) with thesubstitution of Co2+ and Ti4+ ions from x=0.0 (63 emu/g) tox=0.2 (72 emu/g). It is due to the replacement of Fe3+ ions (causingmagnetization reduction) in the spin down state by Co2+ and Ti4+

ions. It has been reported elsewhere that magnetization increaseswith substitution of Fe3+ ions by non-magnetic ions [18] and Ti4+ ionsare diamagnetic in nature. However, Ms decreases with furthersubstitution (xN0.2) owing to the magnetic moments of both ionsare not able to cancel out with spin down moments of Fe3+ ions(5 μB). This is further related to low magnetic moment of Co2+ ions(3 μB) and diamagnetic Ti4+ ions (0 μB). More specifically substitutioncauses weakening of superexchange interaction of type FeA3+–O–FeB3+,leading to collapse of magnetic collinearity of the lattice. It isconfirmed from measured values of the Curie temperature. Table 1shows reduction in the Curie temperature from 619 °C to 523 °C withthe substitution.

Comparing Ba0.5Sr0.5CoxTixFe(12−2x)O19 to BaCoxTixFe11.6O19 [12], themagnetization remains high in substituted samples (max. 72 emu/g,sample 0.2) than undoped sample 0.0 in former ferrite while it attainedmaximum value, 62 emu/g, in the latter ferrite (sample 0.8).

Hysteresis graphs reveal linear relationship from8 to10 kOe inall thesamples. Thus A/H and χp terms from Eq. (1) can be eliminated and Bcan be calculated from slope of straight line,M=Ms (1−B/H2) against1/H2. The anisotropy field (Ha) can be calculated from substituting thevalue of B in Eq. (2). Table 1 exhibits rapid fall in Ha by 51% from x=0.0to x=0.2 whereas coercivity lowers down by 78% at the samesubstitution level. Thus, the another reason for Hc reduction is thenature of substituted ions in terms of their large ionic radii than Fe3+

ions. Consequently the uniaxial anisotropy does not becomeplanarwithsubstitution at x=0.2. Further a small increase in coercivity fromx=0.4 to x=1.0 is plausibly due to an increase in in-plane anisotropy.Similar variation was observed by Ali et al. in Ba–Mn–Cu–Ti hexagonalferrite [19].

The analogy betweenHc andHa (Table 1) at low substitution (x=0.2)indicates that anisotropy field (Ha) is the dominant factor affectingmagnetization process of ferrite [20]. However, this variation ofHc andHa

is not same in samples 0.2–1.0. It is certainly due to dependability ofHc ongrain size apart from magnetocrystalline anisotropy.

The Mr/Ms ratio is 0.50 (Table 1) in undoped sample 0.0, samerequired for the non-interacting particles [21]. Table 1 showsmaximum BHmax. in Ba–Sr ferrite (Sample 0.0) due to highest coer-civity. Thus, an undoped Ba–Sr ferrite stores more energy than Co–Tisubstituted Ba–Sr ferrite. Similar explanation can be given for thehighest remanencemagnetization in an undoped sample 0.0 (Table 1).Mr decreases at higher substitution paving for fast recording appli-cations of the synthesized material.

For ferrite application in recordingmedia, lowcoercivity is requiredwhile maintaining high saturation magnetization. In our previousreport of Co–Zr substitution [22], coercivity reduction is almost sameat lower substitution as at Co–Ti substitution butmagnetization is highin the latter at lower substitution. Thus Co–Ti substitution givesadvantage of using small quantities of dopants to produce desirable

Fig. 3. (a), (b), (c), (d), (e), (f). Hysteresis loops of Ba0.5Sr0.5CoxTixFe(12−2x)O19 ferrite (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) at room temperature.

1924 C. Singh et al. / Materials Letters 63 (2009) 1921–1924

recording properties. In another investigation of Co–Ru substitution[23], coercivity decreased rapidly at lower substitution than Co–Tisubstitution while magnetization is still at high value in the latterferrite. Therefore the present composition satisfies the condition for itsuse in recording applications.

Comparing the hysteresis parameters of the Co–Ti ferrite withprevious reports on synthesized ferrites, e.g. Co–Ti [6,12,24], Co–Zr [4],Co–Sn [7] etc., it is evident that the Co2+ and Ti4+ ions enhance Ms

and tune Hc better than above reported substituents.

4. Conclusions

The substitution of Co2+ and Ti4+ ions makes considerable changein extrinsic properties in terms of grain size, porosity and inter-granular pores. The substitution pronouncedly decreases coercivitywhile increasing magnetization at x=0.2. Remanence magnetizationdecreases with the substitution and this change is more apparent atlower substitution, which open up the possibility for fast recordingapplication.

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