Short communication

Static magnetic properties of Co and Ru substituted Ba–Sr ferrite

Charanjeet Singh a, S. Bindra Narang a,*, I.S. Hudiara a,Yang Bai b, Faride Tabatabaei c

a Department of Electronics Technology, Guru Nanak Dev University, Amritsar, Punjab, Indiab Department of Material Science and Engineering, Tsinghua University, Beijing, China

c Department of Material Science, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

Received 15 April 2007; received in revised form 3 June 2007; accepted 18 June 2007

Available online 22 June 2007

Abstract

M-type hexagonal ferrite powders, Ba0.5Sr0.5CoxRuxFe(12�2x)O19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) have been synthesized by

conventional ceramic method. Magnetic properties have been investigated as a function of substitution of Co and Ru ions at applied

external field of 10 kOe. XRD and SEM revealed hexagonal structure for these ferrites. The Co and Ru ions substitution cause

increase in saturation magnetization and rapid decrease in magnetocrystalline anisotropy at lower substitution. The magnetic

parameters variation has been explained by taking into account preferential site occupancy of sublattice sites by substituted ions.

Curie temperature decreases with substitution due to weakening of superexchange interaction. The obtained hysteresis parameters

suggest that the proposed materials cannot be used for recording applications.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics; D. Magnetic properties

1. Introduction

Ferroxdure MFe12O19, hexagonal ferrites (barium or strontium) are known for their uniaxial magnetocrystaline

anisotropy with ease of magnetization along c-axis [1,2]. These ferrites are being used for magnetic recording

applications due to their good intrinsic properties [3,4]. The intrinsic magnetic properties, i.e. coercivity and saturation

magnetization of hexagonal ferrites can be modified through cationic substitution of Fe3+ ions in the sublattice of the

crystal. The above properties are strongly dependent on the electronic configuration and site preference of substituting

cations. However, it has been reported that substitution causes intrinsic coercivity to decrease effectively but this

considerably decreases saturation magnetization, restricting the use of hexagonal ferrites for magnetic recording [5].

Thus increasing efforts are being made to decrease coercive force and simultaneously increase magnetization with

substitution. The application of hexagonal ferrites in the area of magnetic recording media demands proper control of

homogeneity and morphology.

Detailed research work has been carried out to study magnetic parameters of substituted BaM as well as SrM ferrite.

An et al. [6] have reported magnetic properties of Ba–Sr ferrite synthesized by sol–gel method, while Parkin et al. [7] have

rationalized magnetic properties of same ferrite synthesized by self propagating high temperature synthesis. But

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Materials Research Bulletin 43 (2008) 176–184

* Corresponding author. Tel.: +91 183 2256203; fax: +91 183 2258820.

E-mail addresses: sukhleen2@yahoo.com, sukhleen3@yahoo.com (S. Bindra Narang).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.06.050

magnetic behavior of substituted Ba–Sr ferrite, i.e. Ba–Sr-M has not been looked into. The present work entails magnetic

analysis of divalent Co2+ ion and tetravalent Ru4+ ion substituted Ba–Sr ferrite synthesized by ceramic method.

2. Experimental

Polycrystalline M-type hexagonal ferrites of composition Ba0.5Sr0.5CoxRuxFe(12�2x)O19 (x = 0.0, 0.2, 0.4, 0.6, 0.8,

1.0, 1.2) were prepared using two-route standard ceramic method. The starting analytical grade reagents used for

C. Singh et al. / Materials Research Bulletin 43 (2008) 176–184 177

Fig. 1. X-ray diffraction patterns of Ba0.5Sr0.5CoxRuxFe(12�2x)O19 ferrite calcined at 1250 8C for 20 h.

sample preparation were of high purity (BaCO3, SrCO3, CoCO3, RuO2 and Fe2O3). The compounds were mixed in

stoichiometric ratios to synthesize the ferrite series. The required reactants for each sample are calculated based on

Eq. (1) of chemical reaction.

ð1=2ÞSrCO3þð1=2ÞBaCO3þðxÞCoCO3þ xðRuO2Þ þ ½ð12 � 2xÞ=2�Fe2O3

) Ba0:5Sr0:5CoxRuxFeð12�2xÞO19þð1þ xÞCO2 (1)

The powders were grounded in an agate pestle and mortar for 8 h in distilled water and presintered at 1000 8C for

8 h in electric furnace. The mixtures were then grounded again to a very fine powder under the same conditions.

Sieving was performed after adding polyvinyl alcohol as binder. Mixtures were converted into pellets using hydraulic

press under uniaxial pressure of 75 kN/m2. Final sintering was carried out at 1250 8C for 20 h. Slow heating and

cooling rates were maintained at �2 8C/min.

Magnetic properties were measured by vibrating sample magnetometer (Lake Shore VSM 7307) at applied external

field of �10 kOe. Curie temperature has been measured by gravity method. The phase structure was characterized

using X-ray diffraction (Philips Expert Diffractometer) with Cu Ka radiation (l = 1.54 A) and microstructure was

examined by SEM instrument (Hitachi S-4700 FESEM). Curie temperature of ferrite samples under investigation has

been determined by using gravity method. Ferrite sample is made to attach itself to a bar magnet due to the magnetic

attraction and the combination is suspended inside a electrical furnace the temperature of which can be varied upto

1000 8C. As the temperature of the system is increased, at a particular temperature the sample is found to drop when

the ferrite sample loses its spontaneous magnetization and becomes paramagnetic. This temperature is taken as the

Curie point of the sample. The temperature of the sample is measured by thermocouple inserted in the furnace.

3. Results and discussion

3.1. XRD

X-ray diffraction pattern of samples (Fig. 1) show that magnetoplumbite structure has been formed. The variation in

relative intensities may be related to occupation of lattice sites by substituted ions. From structural parameters (Fig. 2)

characterized by lattice constants ‘a’ and ‘c’, it becomes evident that lattice constant ‘a’ reflects less variation, while

lattice constant ‘c’ initially varies rapidly and then slows down with substitution. This is in agreement with the fact that

all hexagonal types exhibit constant lattice parameter ‘a’ and variable parameter ‘c’ [8]. It indicates that change of easy

magnetized c-axis is larger than a-axis with Co2+ and Ru4+ ions substitution. This is attributed to large ionic radii of

C. Singh et al. / Materials Research Bulletin 43 (2008) 176–184178

Fig. 2. Dependence of lattice constants a and c on substitution x in Ba0.5Sr0.5CoxRuxFe(12�2x)O19 ferrite.

Co2+ ion (0.72 A) and Ru4+ ion (0.67 A) than Fe3+ ion (0.64 A) [9,10]. The change in lattice constants also varies with

the distance between magnetic ions resulting in change of exchange interaction and thus magnetic properties are

altered with substitution.

3.2. SEM

Grain morphologies (Fig. 3) of Ba0.5Sr0.5CoxRuxFe(12�2x)O19 particles indicate improvement in inter-grain

connectivity with Co2+ and Ru4+ ions substitution. The agglomeration of grains with increase in size takes place with

substitution. This is due to ferrite formation reaction promoted by Co2+ and Ru4+ ions.

C. Singh et al. / Materials Research Bulletin 43 (2008) 176–184 179

Fig. 3. SEM micrographs of ferrite samples: (a) Ba0.5Sr0.5Fe12O19, (b) Ba0.5Sr0.5Co0.4Ru0.4Fe11.2O19 and (c) Ba0.5Sr0.5Co1.2Ru1.2Fe9.6O19.

Table 1

Lattice constants a and c, cell volume, X-ray density, bulk density and porosity of Ba0.5Sr0.5CoxRuxFe(12�2x)O19 ferrite

Substitution (x) a (A) c (A) V (A) TD (g/cm3) BD (g/cm3) Porosity (%)

0 5.871 23.106 689.781 5.231 4.403 15.835

0.2 5.874 23.122 690.881 5.270 4.530 14.028

0.4 5.878 23.148 692.589 5.303 4.612 13.028

0.6 5.879 23.161 693.203 5.460 4.834 11.478

0.8 5.881 23.178 694.248 5.383 4.902 8.925

1 5.885 23.180 695.130 5.422 5.049 6.886

1.2 5.886 23.181 695.532 5.133 5.211 6.076

C. Singh et al. / Materials Research Bulletin 43 (2008) 176–184180

Fig. 4. Hysteresis loops of Ba0.5Sr0.5CoxRuxFe(12�2x)O19 ferrite (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) at room temperature.

The increase in cell volume, bulk density and decrease in porosity (Table 1) with substitution of Co2+ and Ru4+ ions

are in agreement with each other. The decrease in calculated porosity matches with observed porosity diminution as

revealed in SEM (grain closeness) of different samples. The density of all the sintered samples is greater than 84% of

the theoretical value.

3.3. Coercivity (Hc), saturation magnetization (Ms) and anisotropy field (Ha)

Ions occupancy depends on d-configuration and more electronegative ions tend to occupy octahedral site which is

larger than tetrahedral site [11]. Electronegativity for Co2+ and Ru4+ ion is 1.88 and 2.28. Ru4+ ions prefer to occupy

tetrahedral site due to d4 configuration and Co2+ ions prefer to occupy octahedral site owing to d7 configuration. Apart

from this, specific site occupancy of substituting ion also depends on sample preparation, ionic radius

(Ru4+ = 0.67 A, Co2+ = 0.72 A) and other substituting ion. Thus Co2+ ions would occupy 12k–2a–4f2 sites and Ru4+

ions 4f1–2b sites. It has been reported also that Ru4+ ions occupy 4f1–2b sites [10] and Co2+ ions occupy 4f1–4f2 sites

[12].

Hysteresis loops graph (Fig. 4) show that doped samples exhibit sharp increase in magnetization at low

applied field which slows down at high field. There is large reduction in hysteresis with substitution. It becomes

clear that sample 0.0 is not saturated whereas all doped samples nearly get saturated state. The occurrence of this

state is due to large drop in anisotropy field as reflected in Hc variation. The saturation magnetization of sample

0.0 is 63 emu/g, which is derived from numerical computation based on the law of saturation [13] using following

equation:

M ¼ Ms

�1� A

H� B

H2

�þ xpH (2)

where Ms is the saturation magnetization, A the inhomogenity parameter, xp the high field susceptibility and B is the

anisotropy parameter.

C. Singh et al. / Materials Research Bulletin 43 (2008) 176–184 181

Fig. 5. Saturation magnetization (Ms) and coercivity (Hc) of Ba0.5Sr0.5CoxRuxFe(12�2x)O19 ferrite as a function of substitution x. Inset in the figure is

for better guide to eye of coercivity variation from samples 0.2 to 1.0.

Furthermore, B can be expressed for hexagonal symmetry crystals as:

B ¼ H2a

15¼ 4K2

1

15M2s

(3)

where Ha is the anisotropy field and K1 is the anisotropy constant.

In M-type hexagonal ferrite, Fe3+ ions occupy seven octahedral sites 12k and 2a, trigonal site 2b with spins in one

direction, and two octahedral sites 4f1, two tetrahedral sites 4f2 with spins in opposite directions. It has been observed

from variation of magnetic properties (Fig. 5) that undoped sample exhibits high coercivity, which is due to strong

uniaxial magnetocrystaline anisotropy field. The Hc of doped samples rapidly decreases with substitution of Co2+ and

Ru4+ ions. This frequent reduction in anisotropy is primarily related to intrinsic effect associated with replacement of

Fe3+ ions at both 4f2 and 2b sites. These two sites contribute to large anisotropy field [14]. Another factor responsible

for decrease in coercivity is extrinsic effect accompanied by increase in grain size with substitution as depicted in SEM

morphology. More specifically, Co2+ and Ru4+ ions substitution causes change of easy axis of magnetization from c-

axis to basal plane. Fast reduction of Hc (�96%) occurs from sample x = 0.0 (2263 Oe) to x = 0.2 (90 Oe), followed by

slight increase (inset in Fig. 5) from x = 0.4 to x = 1.2. The increase in in-plane anisotropy may be the reason of this

small increment. The steep fall in coercivity translates hard ferrite to soft ferrite. Similar trend has also been observed

by Ghasemi et al. [15] in Ba–Mn–Cu–Ti hexagonal ferrite.

Saturation magnetization increases with Co–Ru ions substitution up to x = 0.2 followed by linear reduction with

further substitution. It can be seen (Fig. 5) that Co2+ and Ru4+ ions substitution in sample 0.2 enhances Ms to 70 emu/g

as compared to undoped sample 0.0 (63 emu/g). This can be ascribed to replacement of Fe3+ ions (causing

magnetization reduction) in spin down state by Co2+ and Ru4+ ions, resulting in increase of Ms. The reduction in Ms

beyond x = 0.2 occurs due to low magnetic moment of Co2+and Ru4+ ions as compared to Fe3+ ions. The magnetic

moments of both ions are not able to cancel out with spin down moments of Fe3+ ions, thereby decreasing Ms.

There exist a linear relationship between M and 1/H2 from 8 to 10 kOe in all the samples. Thus, A/H and xp terms in

Eq. (2) can be neglected and value of B can be obtained from slope of the straight line M = Ms(1 � B/H2) against 1/H2.

Further, anisotropy field, Ha, can be calculated by putting value of B in Eq. (3). The obtained graph (Fig. 6) shows rapid

fall in Ha from x = 0.0 to x = 0.2 causing Hc to decrease as discussed above. The similarity between Hc and Ha curves

for low substitution (x = 0.2) indicates that anisotropic field (Ha) is the dominant factor influencing magnetization

mechanism of material [16]. Also trend (inset in Fig. 6) of Hc and Ha is not same from samples 0.2 to 1.2. This is due to

the fact that Hc is also influenced by grain size apart from magnetocrystalline anisotropy, thus making difference

between Hc and Ha.

C. Singh et al. / Materials Research Bulletin 43 (2008) 176–184182

Fig. 6. Variation of Ha and Hc in Ba0.5Sr0.5CoxRuxFe(12�2x)O19 ferrite as a function of substitution x. Inset in the figure is for better guide to eye of Ha

and Hc variation from samples 0.2 to 1.0.

When compared with Co–Ru-doped Ba ferrite (Ms = 65 emu/g, x = 0.1 and Hc = 250 Oe, x = 0.3) [17], magnetic

properties of Co–Ru doped Ba–Sr ferrite are improved with enhancement of Ms = 70 emu/g (x = 0.2) and lower

Hc = 90 Oe (x = 0.2). Similarly, while comparing with other substituents, i.e. Ni–Sn [18] and Sn–Ru [10], it becomes

clear that Ru4+ and Co2+ ions enhance Ms and decrease Hc better than above reported substituents.

3.4. Curie temperature

It can be noted from Curie temperature (Tc) variation (Fig. 7) that it decreases with Co2+ and Ru4+ ions substitution.

This can be explained on the basis of number of magnetic ions present in the two sublattice and their mutual

interactions. As the Fe3+ ions are replaced by Co2+ and Ru4+ ions with low magnetic moment, the number of magnetic

ions begin to decrease on both sides weakening AB superexchange interaction of type FeA3+–O–FeB

3+. Thus spin

alignment can be imbalanced at lower thermal energy, leading to decrease in Tc. Similar reduction of Tc with

substitution has also been observed by Kim et al. [19] in Ba–Cr hexagonal ferrites and by Liu et al. [20] in Sr–La

hexagonal ferrites.

4. Conclusions

To the best of our knowledge the Co–Ru substituted Ba–Sr ferrite is synthesized for the first time using ceramic

method. The coercivity decreases due to microstructural and anisotropy field variation while saturation magnetization

is enhanced due to replacement of Fe3+ ions in spin down state. The transition from hard phase (undoped Ferrite) to

soft phase (doped ferrite) results in reduction of hysteresis loss area per cycle.

Acknowledgement

We would like to thank Prof. Koledintseva Marina for valuable discussions during this work.

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