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Sensors and Actuators A 137 (2007) 256261

Enhanced magnetostrictive properties of CoFe2O4synthesized by an autocombustion method

S.D. Bhame, P.A. Joy

Physical and Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India

Received 20 September 2006; received in revised form 25 February 2007; accepted 18 March 2007

Available online 23 March 2007

Abstract

The magnetostrictive properties of sintered cobalt ferrite derived from nanocrystalline powders synthesized by three different low-temperaturemethods (citrate, coprecipitation and autocombustion) and the high-temperature ceramic method have been compared. A strong dependence of

the magnetostrictive strain is observed on the initial morphology of the ferrite particles and the microstructure of the final sintered product. The

nanoparticles synthesized by the autocombustion method, with a flaky and porousmorphology, give a sintered material with small grains. Maximum

magnetostriction is found to be decreased with increasing grain size. However, no correlation is observed between the initial particle size of the

powders or the sintered density and magnetostriction. Maximum magnetostriction values of 197, 184, 159, and 135 ppm are obtained for the

sintered samples with average grain sizes of 8 m (combustion), 17m (citrate), 23m (coprecipitation), and >25m (ceramic), respectively.

2007 Elsevier B.V. All rights reserved.

Keywords: Magnetostriction; CoFe2O4; Nanocrystalline materials

1. Introduction

The identification of new materials with enhanced proper-

ties or new processing techniques to improve the performance

of existing materials, along with the economical advantages, is

always a matter of interest to researchers. The desire to produce

novel smart materials is strongly dependent on the availability

of suitable materials with enhanced properties. Magnetostrictive

smart materials can convert energy between the elastic and the

magnetic states. Due to this bidirectional nature, magnetostric-

tive materials can be used as both sensors as well as actuators.

This phenomenon was discovered in 1842 by Joule [1], and

Ni metal was found to be a good magnetostrictive element for

applications in many devices [2]. In 1972, Clark and Belson

discovered giant magnetostriction in alloys of Fe and rare earth

metals [3], and currently, Terfenol-D, an alloy of Tb, Fe and

Dy, is used in many applications such as for stress sensing, con-

trolled fuel injection, vibration control, magnetostrictive filters,

ultrasonic generation, etc [46]. Recently, it has been shown

that oxide-based materials, especially cobalt ferrite, could over-

Corresponding author. Tel.: +91 20 2590 2273; fax: +91 20 2590 2636.

E-mail address: [email protected] (P.A. Joy).

come some of the drawbacks of the alloy based magnetostrictive

materials [7]. The important factors are the high corrosion resis-tance, better mechanical properties, higher magnetostriction in

the polycrystalline form and low cost as compared to the alloy

based sensors. Although the magnetostrictive strains obtained

at saturation are less for the polycrystalline cobalt ferrite when

compared to singlecrystalsof Terfenol, high values of thestrains

at low field strengths along with enhanced magnetomechanical

coupling factor have been identified as the advantages of cobalt

ferrite [7]. It is theslopeof themagnetostriction or thepiezomag-

netic coefficient (d/dH) which determines the stress sensitivity

of a material [8,9]. Apart from obtaining high magnitudeof mag-

netostriction, the control of magnetostrictive hysteresis and the

mechanical strength of the ferrite material are the two important

concerns. Metal-bonded cobalt ferrite [10] and Mn-substituted

cobalt ferrite [11], have been studied in order to enhance the cor-

rosion resistance and the stress sensing properties. It has been

shown that the substitution of Mn for Co as well as Fe can sig-

nificantly alter the magnitude and enhance the low field slope of

the magnetostriction of cobalt ferrite [12,13]. Also, Mn substi-

tution is effective in reducing the magnetomechanical hysteresis

as compared to metal-bonded cobalt ferrite [11].

It is widely known that the sintered products derived from

nanocrystalline ferrite powders exhibit improved magnetic

0924-4247/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.sna.2007.03.016
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S.D. Bhame, P.A. Joy / Sensors and Actuators A 137 (2007) 256261 257

permeability which depends on the microstructure, density,

porosity, grain size, etc., as compared to the materials sintered

from the bulk counterparts [14,15]. However, so far not much

efforts have been invested to understand this very important and

fundamental issue in the case of such effects on the magne-

tostrictive properties of ferrites. To address this issue, we have

synthesized nanocrystalline cobalt ferrite by different low tem-

perature methods and studied the magnetostrictive properties of

the sintered products. It is shown that the material synthesized

by an autocombustion method exhibits better magnetostrictive

properties when compared to the samples synthesized by other

two low-temperature and the high-temperature ceramic meth-

ods. The changes in the properties of the different sintered

samples are correlated with their microstructures.

2. Experimental

2.1. Sample preparation

Nanocrystalline CoFe2O4 samples were synthesized bythree commonly used methods; coprecipitation, combustion

and citrate gel. AR grade metal nitrates, Co(NO3)26H2O and

Fe(NO3)39H2O were used for all three syntheses. For compar-

ison of the properties, the bulk material was synthesized by the

ceramic method also.

2.1.1. Combustion method

The synthesis was carried out using the glycine-nitrate auto-

combustion method [16]. Stoichiometric amounts of cobalt and

iron nitrates were dissolved in distilled water to which a water

solution of glycine was added. Two moles of glycine per mole

of metal ion was used for the synthesis. The mixed solution wasevaporated on a hot plate at 200 C. After the evaporation of

water, the resulting thick mass burned spontaneously to give the

CoFe2O4 powder.

2.1.2. Citrate method

In the citrate precursor method, stoichiometric amounts of

cobalt and iron nitrates were dissolved in distilled water. Water

solution of citricacid was added to themetalion solution keeping

the metal to citric acid ratio as 1:2. The solution was evaporated

on a water bath andfinally a thick gelwas formed. This precursor

was dried overnight in an oven at 100 C and the dried precursor

was calcined at 500 C for 4 h to get the cobalt ferrite powder.

2.1.3. Coprecipitation method

In the coprecipitation method, cobalt and iron nitrates were

taken in the stoichiometric ratio and dissolved in distilled water.

20% KOH solution was added drop wise to this solution under

constant magnetic stirring. A precipitate formed wasfiltered and

washed several times with distilled until the pH of the filtrate

was around 7.0. The precipitate was dried overnight in an oven

at 100 C which eventually converted to a black powder.

2.1.4. Ceramic method

Stoichiometric amounts of CoCO3 and Fe2O3 were weighed

and mixed together in an agate mortar. The mixture was first

heated at 1000 C for 12 h and again for 24 h at the same tem-

perature after an intermediate grinding. The resulting powder

was further heated at 1100 C for 72 h with two intermediate

grindings.

2.2. Measurements

All the four powder samples were pressed in to the form

of circular disks (pellets) and sintered at 1450 C for 10 min.

All the pellets were sintered under identical conditions, with

a heating rate of 4 C/min and cooling rate of 20 C/min. The

phase purity of the powder samples was confirmed by powder X-

ray diffraction studies using a Philips PW-1830 diffractometer.

Magnetic measurements were performed using an EG&G PAR

4500 vibrating sample magnetometer. The sintering behavior of

the powder samples in air were studied using a Perkin-Elmer

Pyris Diamond thermal mechanical analyzer, at a heating rate

of 10 C/min. Magnetostriction, =l/l, which is the relative

change in the length of the sample in an applied magnetic field,

was measured on the sintered pellets at room temperature using350 resistive strain gages. The magnetostrictionwas measured

in the direction of the applied magnetic field. Scanning electron

micrographs were obtained using a Leica Cambridge 440 scan-

ning electron microscope (SEM). The powder morphology was

studied using a JEOL model 1200 EX transmission electron

microscope (TEM).

3. Results and discussion

All the samples were initially characterized by powder X-ray

diffraction (XRD) studies. The XRD patterns of the powders

synthesized by thefour differentmethods arecomparedin Fig.1.A single phase spinel ferrite is obtained by the low-temperature

methods, as evidenced from the absence of any additional reflec-

tions in the XRD patterns, when compared with that of the bulk

(ceramic) material. The XRD reflections of the samples syn-

thesized by the citrate gel and coprecipitation methods are very

broad and those of the sample synthesized by the combustion

Fig. 1. Powder XRD patterns of the cobalt ferrite samples synthesized by the

different methods, as indicated.


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258 S.D. Bhame, P.A. Joy / Sensors and Actuators A 137 (2007) 256261

method are broader than that of the ceramic sample, indicat-

ing smaller crystallite sizes of the low-temperature synthesized

powders. The average crystallite size was calculated using the

Scherrer formula, d=0.9/ cos , where dis the crystallite size

in A, the half maximum line width corrected for the contribu-

tion from instrumental line broadening and is the wavelength

of X-rays [17]. The calculated crystallite sizes are 12, 15 and

45 nm for the samples synthesized by the coprecipitation, cit-

rate and combustion methods, respectively. The larger crystallite

size observed for the sample synthesized by the combustion

method is likely to be due to the higher internal temperatures

achieved during the combustion process. It is known that tem-

peratures above 1000 C is reached for a short duration during

the combustion synthesis using glycine [16].

Fig. 2 shows the TEM photographs of the low-temperature

synthesized samples. The particles of the sample synthesized

by the coprecipitation method are highly agglomerated. The

individual particles are of average size of 10 nm. Similarly, the

average particle size of the citrate sample is obtained as 15 nm.

For the combustion sample, there is a wide distribution of theparticle sizes, with most of the particles of size above 30 nm.

For the three low-temperature synthesized samples, the average

particle sizes obtained from TEM are comparable to the aver-

age crystallite sizes calculated from XRD line broadening. The

average particle size of the sample synthesized by the ceramic

method is obtained from SEM studies as 0.9 m.

Cobalt ferrite is a ferrimagnet with a Curie temperature of

520 C [14]. The Curie temperatures of the samples synthe-

sized by the different methods were found to be comparable to

the reported value. The room temperature magnetic hysteresis

curves of the as-synthesized powder samples are compared in

Fig. 3. Thesaturation magnetization at 15 kOe andthe coercivity

Fig.2. TEMphotographsof thecobalt ferrite samples synthesized by (A) citrate,

(B) coprecipitation and (C) combustion methods and (D) SEM photograph of

the powder synthesized by the ceramic method.

Fig. 3. The MHcurves of the cobalt ferrite powder samples measured at room

temperature. The curve of one of the sintered samples is shown for comparison.

of the samples are compared in Table 1. The highest saturationmagnetization is obtained for the powder sample synthesized by

the combustion method. The room temperature saturation mag-

netization of cobalt ferrite is 80 emu/g. It is observed that the

saturation magnetization of the powder sample synthesized by

the combustion method is almost identical to that of the sin-

tered products. The coercivity and saturation magnetization of

the ceramic and combustion samples are almost comparable.

For magnetostriction measurements, the sample powders

need to be sintered in to specific shapes. Prior to the sinter-

ing of the powders, the sintering characteristics were studied

using a thermo-mechanical analyzer (TMA). The powder sam-

ples were pressed in to the form of small circular disks with flat

surfaces and TMA studies were made on these pressed green

pellets. The TMA curves of the different samples are compared

in Fig. 4. For the powders obtained by the citrate and coprecip-

itation methods, sintering takes place in the temperature range

of 700900 C. This is due to the very small particles of these

two samples. Nanocrystalline ferrite powders can be effectively

sintered at lower temperatures due to the high surface areas the

Fig. 4. TMA curves of the different green samples.


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Table 1

Comparison of the different properties of the powders and sintered samples of cobalt ferrite synthesized by different methods

Synthesis method Powder samples Sintered pellets

dXRD (nm) dTEM (nm) Ms (Am2/kg) Hc (kA/m) D (%) G (m) Ms (Am

2/kg) Hc (kA/m) max (106)

Copptn 12 10 31.9 81.3 92 23 80 12.6 159

Citrate 15 18 66.7 118.5 77 17 81.4 19.2 184

Combustion 45 35 83.5 56.5 84 8 83.5 16.4 197Ceramic 900a 77 47.7 96 >25 83.7 10.7 135

dXRD: average crystallite size from XRD, dTEM: average particle size from TEM (from SEM for ceramic sample), Ms: saturation magnetization, Hc: coercivity, D:

percentage density, G: grain size from SEM, max: maximum magnetostriction.a From SEM.

particles, as observed for other ferrites synthesized by the cit-

rate and coprecipitation methods [18,19]. The ceramic sample

requires sintering temperature of >1100 C and effective sinter-

ing takes place only above 1300 C. Unexpectedly, though the

particle size of the sample synthesized by the autocombustion

method is relatively smaller (45 nm), the sintering conditions

are almost similar to that of the ceramic sample, except for thetemperature at which shrinkage starts. For the combustion sam-

ple, shrinkage starts at a temperature of 800 C whereas for the

ceramic sample this occurs at 1100 C. This unexpected behav-

ior can be explained based on the synthesis conditions. During

the combustion process, the local temperature can be very high.

For the combustion reaction using glycine, the temperature can

be as high as 1500 C, depending on the molar ratio of glycine

and nitrates used. In the present case, when 2 mole of glycine is

used per mole of metal ion, the ratio is closer to 0.5 where the

flame temperature will be very high [16]. This high temperature

generated during a short time makes the particles more sintered

and hence the surface of the particles become non-active, as in

the case of the ceramic samples, although the particle sizes are

smaller. Similar sintering characteristics have been reported for

oxide ceramics synthesized by the combustion technique using

other fuels such as urea [20] or glycine as in the present case

and ceramic samples. For example, Shi et al. have shown that

the difference between the sintering temperature for combustion

and ceramic samples is only 60 C, though the particle size of

the combustion sample is much less than that of the ceramic

sample [21].

For comparison of the properties of the sintered samples,

all the four samples were sintered under identical conditions at

a temperature of 1450 C. All four sintered samples showed

almost similar magnetic behavior, as shown in Table 1. Themagnetostriction curves of the four different sintered samples,

measured at room temperature, are shown in Fig. 5. The sign

of the magnetostrictive strain in the direction of the applied

field was found to be negative and the magnitude of the maxi-

mum magnetostriction of the different samples is compared in

Table 1. The highest magnetostriction is obtained for the sam-

ple synthesized by the autocombustion method and the lowest

value is obtained for the sample synthesized by the ceramic

method. Apart from the difference in the values of maximum

magnetostriction, the field at which maximum magnetostriction

is obtained and the magnetostriction at low fields are also dif-

ferent for the different samples. The samples synthesized by

the autocombustion and coprecipitation methods show higher

magnetostriction at low magnetic fields.

A comparison of the different parameters listed in Table 1

indicates that there is no correlation between the magnetostric-

tion and any other parameters, except the grain size of the

sintered materials. The SEM photographs of the different pow-

der samples are shown in Fig. 6. The powder synthesized bythe autocombustion method is highly porous. This is due to the

liberation of large volume of gases during the combustion reac-

tion occurred in a short time period [16,22]. The ceramic sample

shows the presence of very large particles of almost uniform size

(1m). On the other hand, the powders obtained by the copre-

cipitation and citrate methods are showing the presence of very

large lumps and they are agglomerates of very small spherical

particles. These different powder morphologies are expected to

give different microstructures for the sintered products.

Fig. 7 shows the SEM photographs of the sintered pellets

of the samples synthesized by the different methods. It can be

seen that there are some differences in the microstructures of

the four sintered samples. The micrograph of the sample syn-

thesized by the autocombustion method (micrograph C) shows

smaller grains whereas in the ceramic sample (micrograph D)

no clear grains are visible. The entire micrograph of the ceramic

sample looks like part of a bigger grain. The combustion and

ceramic samples are the ones giving the highest and the low-

Fig. 5. Magnetostriction curves of different samples as a function of magnetic

filed, at room temperature.


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260 S.D. Bhame, P.A. Joy / Sensors and Actuators A 137 (2007) 256261

Fig. 6. SEM photographs of the powder samples synthesized by (A) cit-

rate method, (B) coprecipitation method, (C) autocombustion method and (D)

ceramic method.

est magnetostriction, respectively. For the samples synthesized

by the citrate and coprecipitation methods, clear grains are vis-

ible with different sizes. The grains in the citrate sample are

slightly larger than that in the combustion sample but smaller

than that in the coprecipitation sample. Hence, there is a com-

parison between the magnitude of maximum magnetostriction

and the microstructure of the sintered samples. Sample contain-

ing the smallest grains obtained by the combustion method of

synthesis shows the highest magnetostriction and the magne-tostriction decreases as the grain size is increased. The results

Fig. 7. SEM photographs of the sintered samples synthesized by (A) cit-

rate method, (B) coprecipitation method, (C) autocombustion method and (D)

ceramic method. All micrographs are under the same magnification.

show that the presence of smaller and uniform grains is required

to attain larger values of magnetostriction. Therefore, it can be

concluded that controlling the microstructure is one of the ways

to control the magnetostrictive properties of cobalt ferrite.

4. Conclusions

Polycrystalline cobalt ferrite is synthesized by three

different low-temperature methods and the conventional high-

temperature ceramic method. The effect of the synthesis

conditions, particle sizes of the resulting powders as well as the

densities and grain sizes of the powders sintered under identical

conditions, on the magnetostrictive properties of cobalt ferrite

are studied. Lowest average grain size is obtained for the mate-

rial synthesized by an autocombustion method which shows

highest magnetostriction whereas the material synthesized by

the high-temperature ceramic method and having larger grain

sizes gives the lowest magnetostriction. A correlation is found

between the magnetostrictive properties and the microstructure

of the sintered ferrites, whereas neither the initial particle sizeof the powders nor the sintered density is a deciding factor.

The presence of small but uniform grains and sintering without

substantial grain growth are likely to be one among the factors

controlling the value of maximum magnetostriction. However,

the effect of orientation of the crystallites in the sintered mate-

rial is expected to be one of the important factors, which need

to be studied in detail and is beyond the scope of the present

work. From the present studies, it appears that controlling the

morphology of the powders and microstructure of the sintered

products is essential to obtain high values of saturation mag-

netostriction as well as to tune the low-field magnetostrictive

response of polycrystalline cobalt ferrite.

Acknowledgements

SDB is grateful to CSIR, India, for a Research Fellowship.

Financial assistance from NPSM in the form of a project is

gratefully acknowledged.

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Biographies

Shekhar D. Bhame: He has done MSc in inorganic chemistry from Univer-

sity of Pune, Maharashtra, India, in 2001. Currently he is working for his

doctoral degree in the Materials Chemistry Division of National Chemical Lab-

oratory, Pune. His current research interests are magnetism, magnetostrictive

smart materials and magnetic nanomaterials.

Dr. P. A. Joy: He has done MSc in chemistry from Calicut University, Kerala,India, in 1983; PhD in chemistry from Indian Institute of Science, Bangalore,

India, in 1990. Currently he is working as a senior scientist in the Materials

Chemistry Division of National Chemical Laboratory, Pune, India. His cur-

rent research interests are magnetism and magnetic materials, magnetostrictive

materials, nanomagnetic materials, ferrofluids, structure-property-processing

correlation studies and ceramic oxides.

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