synthesis and characterization of microwave absorbing srfe12o19/znfe2o4 nanocomposite

ORIGINAL PAPER

Synthesis and Characterization of Microwave AbsorbingSrFe12O19/ZnFe2O4 Nanocomposite

Sachin Tyagi • Himanshu B. Baskey •

Ramesh Chandra Agarwala • Vijaya Agarwala •

Trilok Chand Shami

Received: 28 March 2011 / Accepted: 11 October 2011 / Published online: 1 December 2011

� Indian Institute of Metals 2011

Abstract Zinc ferrite and strontium hexaferrite; SrFe12

O19/ZnFe2O4 (SrFe11.6Zn0.4O19) nanoparticles having

super paramagnetic nature were synthesized by simulta-

neous co-precipitation of iron, zinc and strontium chloride

salts using 5 M sodium hydroxide solution. The resulting

precursors were heat treated (HT) at 850, 950 and 1150�C

for 4 h in nitrogen atmosphere. The hysteresis loops

showed an increase in saturation magnetization from 1.040

to 58.938 emu/g with increasing HT temperatures. The ‘as-

synthesized’ particles have size in the range of 20–25 nm

with spherical and needle shapes. Further, these spherical

and needle shaped nanoparticles tend to change their

morphology to hexagonal plate shape with increase in HT

temperatures. The effect of such a systematic morpholog-

ical transformation of nanoparticles on dielectric (complex

permittivity and permeability) and microwave absorption

properties were estimated in X band (8.2–12.2 GHz). The

maximum reflection loss of the composite reaches -26.51

dB (more than 99% power attenuation) at 10.636 GHz

which suits its application in RADAR absorbing materials.

Keywords Nano materials � Heat treatments �Thermal analysis � Magnetic

1 Introduction

M type strontium hexaferrite (SrFe12O19), which is a hard

hexagonal magnetic material, has been a subject of con-

tinuous interest for several decades due to its applicability

in electronic components, magnetic memories and record-

ing media [1]. The practical application of strontium

hexaferrite as a permanent magnet is well known and it is

also used as a dielectric/magnetic filler in the electromag-

netic filler attenuation materials (EAM). EAM are used to

minimize the electromagnetic interference (EMI), a specific

type of environmental pollution. EMI problems are

attracting more attention due to the extensive use of elec-

tromagnetic (EM) waves in application of wireless com-

munication such as EM waves of 0.8–1.2 GHz are used for

mobile phones, 2.45 GHz for electronic ranges, 5.6–8.2

GHz (G-band) for synthetic aperture radar (SAR) or

microwave communication on the ground, 8.2–12.4 GHz

(X-band) and 12.4–18 (Ku-band) for SAR or electron spin

resonance (ESR) apparatus [2]. To overcome EMI prob-

lems, it is suggested that EM wave absorbing materials with

capability of absorbing unwanted EM signals having large

absorption peak, less coating thickness and wide working

frequency bandwidth are to be used. Single material cannot

meet these demands, so new systems have been evolved

comprising of composite powder including hard and soft

magnetic materials [3–5]. Zinc ferrite (ZnFe2O4) having a

cubic spinel structure is a soft magnetic material and is

widely studied as microwave absorbing material [6–8].

In view of this, in the present research, the microwave

absorption properties of the composite powder consisting

of hard magnetic strontium hexaferrite and soft magnetic

zinc ferrite were studied.

The conventional way of synthesizing hexaferrites in-

volves solid state reaction route having high HT temperature

S. Tyagi (&) � R. C. Agarwala � V. Agarwala

Department of Metallurgical and Materials Engineering,

Indian Institute of Technology, Roorkee 247667, India

e-mail: [email protected]

H. B. Baskey � T. C. Shami

Special Materials Group, DMSRDE (DRDO Lab), Kanpur,

Uttar Pradesh, India

123

Trans Indian Inst Met (December 2011) 64(6):607–614

DOI 10.1007/s12666-011-0068-7

(C1,200�C), which results in powders with large particle

size, limited chemical homogeneity and low sinterability

[9]. So, the preparation of fine and uniform hexaferrite

powder without impurity is a challenging task. There are

several processing routes available in the literature [9–13].

Hydrothermal method employs expensive autoclaves, good

quality seeds of a fair size and the impossibility of

observing the crystal as it grows [14]. Sonochemical syn-

thesis can generate a transient localized hot zone with

extremely high temperature gradient and pressure [15]

which can assist the destruction of the sonochemical pre-

cursor and the formation of nanoparticles. Low temperature

combustion synthesis is complex, takes long time for self

ignition reaction to occur and needs cationic surfactants to

remove the impurity like hematite phase (aFe2O3) [16, 17].

Mechanical alloying leads to impurity and lattice strains [9].

The present investigation deals with the synthesis of zinc

substituted strontium hexaferrite nanoparticles by modified

flux method. This method is economical and is quite suitable

for mass production as compared to that the other mentioned

methods [18]. Further, studies on relative complex permit-

tivity and permeability of zinc substituted strontium hexa-

ferrite and their influence on magnetic, dielectric and

microwave absorption properties were also carried out.

2 Experimental Procedure

In the present investigation, analytical grades of ferric

chloride (FeCl3�6H2O), strontium chloride (SrCl2), zinc

chloride (ZnCl2) and NaOH were used for synthesizing

SrFe11.6Zn0.4O19 (SrFe12O19/ZnFe2O4) nanoparticles by

modified flux method. Stochiometric amounts of strontium

chloride and zinc chloride were dissolved completely into

ultra pure water to make an aqueous solution (I) and ferric

chloride (FeCl3) was separately mixed in ultra pure water to

make an another aqueous solution (II). Both the above

solutions were mixed in 1:1 molar ratio. The brownish col-

ored ferrite particles were precipitated from this mixture by

gradually adding sodium hydroxide, NaOH (5 M) solution at

room temperature (pH 12.0). The aqueous suspension was

stirred gently for 15 min to achieve good homogeneity.

The precipitates so formed were filtered off, washed with

water and dried at 100�C overnight. The precipitated nano-

crystalline powder was mixed thoroughly with NaCl in 1:2

ratios (by weight). Since the melting point of NaCl is about

800�C, so the ‘as-synthesized’ particles were given a heat

treatment of 850, 950 and 1150�C for 4 h in nitrogen

atmosphere to achieve a uniform growth. During annealing,

the particles surrounded by molten NaCl salt make the pro-

cess similar to one taking place in liquid phase sintering with

high diffusion rates. This makes the process fast and hexa-

ferrite particles crystallize out completely with multiple

morphologies after cooling in the furnace. Then cooled

mixture of nanoparticles and NaCl was washed by ultra pure

water so that NaCl is dissolved and, SrFe11.6Zn0.4O19

(SrFe12O19/ZnFe2O4) nanoparticles could be filtered out.

3 Characterization Studies

Thermal study for the formation of zinc substituted stron-

tium hexaferrite nanoparticles was done in nitrogen atmo-

sphere by differential scanning calorimetry, DSC (Perkin

Elmer, Pyris Diamond) at the heating rates 10 K/min. The

crystallization of phases present in different annealed

samples was identified by X-ray diffraction (XRD) using

Bruker AXS D8 diffractometer with Cu-Ka radiation.

Crystallite size of the powder was measured by X-ray line

broadening technique employing Scherrers’ formula.

Morphological study was carried out by field emission

scanning electron microscope, FESEM (QUANTA FEG

200 FEI) and transmission electron microscope, TEM

(Philips, EM 400; TECHNAI 20G2-S-TWIN). Magnetic

measurements were carried out in the applied field range of

-10,000 to ?10,000 Gauss at room temperature (300 K)

by means of vibrating sample magnetometer, VSM (155,

PAR). To study the dielectric properties, all the samples

(80 wt%) were mixed with epoxy resin and 2% hardener.

The ferrite/epoxy composite thus obtained were cast into

rectangular pellet and cured at 75�C for 30 min. The

composite thus prepared was polished and mounted on an

aluminum foil (to obtain a single layer metal-backed

absorber) to exactly fit into the measuring wave guide. The

complex permittivity and permeability measurements were

carried out on Network Analyzer (Agilent E8364B PNA

series) using Material measurement software 85071 in the

frequency range of 8.2–12.2 GHz at room temperature.

The reflection loss (RL) curves were calculated from

complex permittivity and permeability at given frequency

and absorber thickness with following equations:

Zin ¼ Zo lr=erð Þ1=2tanh j 2pfd=cð Þ lrerð Þ1=2

n o

RL ¼ 20 log Zin � Zoð Þ= Zin þ Zoð Þj j

where f is frequency, d is the thickness of absorber

(d = 2.5 mm), c is the velocity of light, Zo is the imped-

ance of air and Zin is the impedance of absorber.

4 Results and Discussion

4.1 Thermal Study

The DSC/DTG/TG traces of the sample synthesized by

modified flux method at the heating rate of 10 K/min are

608 Trans Indian Inst Met (December 2011) 64(6):607–614

123

shown in Fig. 1. In the DSC study of ‘as-synthesized’ zinc

substituted strontium hexaferrite, only endothermic peaks

are observed indicating that SrFe11.6Zn0.4O19 nanoparticles

are formed by the endothermic reaction. The endothermic

peak at about 813�C is attributed to the formation of various

phases like aFe2O3, ZnFe2O4 and SrFe12O19. This is con-

firmed by the XRD analysis of the powder heat treated at

850�C (Fig. 2). The second endothermic peak at 1,128�C is

attributed to endothermic reaction resulting in the formation

of desired phases (SrFe12O19 and ZnFe2O4) with increased

crystallinity, which is also confirmed by the XRD analysis

of the powder heat treated at 1,150�C (Fig. 2). Thermal

Gravimetry, TG analysis of ‘as synthesized’ powder

(Fig. 1) shows a weight loss of *62% in the temperature

range of 31–1,131�C, after that no weight loss is observed.

This is probably due to degassing and loss of moisture

during the heat treatment process. The differential thermo

gravimetric (DTG) curve indicates the derivative of ther-

mogravimetry data which is attributed to the rate of weight

loss during the endothermic reactions occurring at 813 and

1,128�C. The endothermic reaction occurring at 1,128�C

shows the weight loss of about 50% at the rate of 1.041 mg/

min (DTG) after that both TG and DTG curves become

smoother, this indicates the completion of reaction as M

type zinc substituted strontium hexaferrite nanoparticles

(SrFe12O19/ZnFe2O4) are formed. This study forms the

Fig. 1 DSC–DTG–TG traces of

‘as synthesized’ SrFe12O19/

ZnFe2O4 nanoparticles

at 10 K/min in nitrogen

atmosphere

Fig. 2 XRD pattern of

SrFe12O19/ZnFe2O4

nanoparticles in ‘as synthesized’

and heat treated at 850, 950 and

1150�C in nitrogen atmosphere

Trans Indian Inst Met (December 2011) 64(6):607–614 609

123

basis for the selection of heat treatment temperature for the

formation M type zinc substituted strontium hexaferrite.

4.2 XRD Study

The indexed XRD patterns of the SrFe12O19/ZnFe2O4

nanoparticles in ‘as-synthesized’ and after heat treatment at

850, 950 and 1150�C for 4 h in the nitrogen atmosphere are

shown in Fig. 2. From the results, it can be inferred that the

ferrite powder in ‘as-synthesized’ condition was showing

only the peak corresponding to impurity of NaCl (JCPDS

card No. 5-637). When annealed at 950�C, SrFe12O19,

2h = 34.386, d = 2.616 (JCPDS card No. 24-1207) and

ZnFe2O4, 2h = 35.542, d = 2.520 (JCPDS card No.

02-1043) also contained certain other phases, their peaks

correspond to impurity of aFe2O3, 2h = 33.360, d = 2.680

(JCPDS card No. 05-637). As expected, the degree of

crystallinity and amount of SrFe12O19/ZnFe2O4 nanoparti-

cles was further increased by increasing the heat treatment

temperature from 850 to 1,150�C. The crystallite size of

SrFe12O19 phase (2h = 34.386) was found to increase with

increase in heat treatment temperature. It increases from

50 nm at 850�C to 55 nm at 1,150�C. From the XRD and

DSC results, it may be concluded that under the given co-

precipitation reaction, the ferric chloride is converted to

aFe(OH)3 and NaCl, which is then dehydrated to form

aFe2O3. Zinc chloride is converted to Zn(OH)2 and NaCl

which further dehydrated to zinc oxide; and strontium

chloride is also dehydrated to Sr(OH)2 which further con-

verted to strontium oxide. Finally, SrFe12O19/ZnFe2O4

nanoparticles are formed during the post synthesis calci-

nations stage and can be described by the following

chemical reactions.

Neutralization with sodium hydroxide

12FeCl3 þ 36NaOH! 12Fe OHð Þ3þ 36NaCl

ZnCl2 þ 2NaOH! Zn OHð Þ2þ 2NaCl

SrCl2� þ 2NaOH! Sr OHð Þ2þ 2NaCl

Nucleation of nano crystals

12Fe OHð Þ3! 6Fe2O3 þ 18H2O

Zn OHð Þ2! ZnOþ H2O

Sr OHð Þ2! SrOþ H2O

Growth of nano crystals

ZnOþ Fe2O3 ! ZnFe2O4

SrOþ 6Fe2O3 ! SrFe12O19

4.3 Morphological Study

The FESEM micrographs of ‘as-synthesized’ and heat

treated ferrite powder at temperatures of 850, 950 and

1150�C are shown in Fig. 3. In the ‘as-synthesized’ con-

dition, the particles seem to have spherical and needle

shaped morphology with particle size in the range of

20–25 nm (Fig. 3a). With increasing heat treatment tem-

perature, the particles grow and constitute multiple mor-

phologies. Also, with rise in heat treatment temperature,

the systematic growth of particles is observed with sharp

plane of crystals. This process of crystal growth and mor-

phological evolution can be described in terms of Ostwald

ripening. In ‘as-synthesized’ condition, the nanoparticles

with size in the range of 20–25 nm with spherical and

needle shaped morphology are observed. As the HT tem-

perature is increased, these nanoparticles slowly disappear

except for the few that grow larger, at the expense of

smaller ones. Thus particles of small size act as nutrients

for the bigger ones. At 1,150�C, SrFe12O19/ZnFe2O4 nano-

particles (with size in the range of 80–90 nm) having large

hexagonal plate shape are observed (Fig. 3c, d). This is

also evidenced by TEM micrograph of the powder in ‘as-

synthesized’ and heat treated condition (Fig. 4). These

multiple morphologies, possessing large surface areas,

leads to plenty of interfacial polarization to weaken the

energy of EM waves. It is reported that barium and

strontium hexaferrite nanoparticles with hexagonal plate

like morphology are potential materials for the RADAR

absorption applications [19–21].

4.4 Magnetic Study

The magnetic measurements of SrFe12O19/ZnFe2O4 nano-

particles have almost negligible coercivity and remanance

values (Fig. 5) in the ‘as-synthesized’ condition showing

the superparamagnetic behavior of the material. But when

the ‘as-synthesized’ powders are heat treated (at 850, 950

and 1150�C), the particles appear to transform from su-

perparamagnetic to ferromagnetic nature. Saturation mag-

netization is found to be dependent on HT temperature.

It increases from 1.040 to 58.938 emu/g with increase in

HT temperature (Fig. 5). The rise in saturation magneti-

zation with the HT temperature can be attributed to the

increased formation of strontium hexaferrite and zinc fer-

rite which is confirmed by X-ray study of powder HT at

various temperatures (Fig. 2) [18]. The coercivity of

1,671 Gauss is observed for the powder heat treated at

950�C and thereafter decreases to 1,238 Gauss at 1,200�C.

This might be due to the presence of aFe2O3 up to 950�C;

which has a high intrinsic coercive force [22]. In addition,

the change in morphology and particle size may also affect

the magnetic properties [18]. The co-precipitated powder

heat treated with NaCl at 1,150�C having large hexagonal

plate shape achieved maximum saturation magnetization.

While the co-precipitated powder heat treated with NaCl at

950�C that have similar hexagonal plate like structure in


123

Fig. 3 FESEM micrographs showing the effect of heat treatment temperature on the morphology of SrFe12O19/ZnFe2O4 nanoparticles, a ‘as

synthesized’ and heat treated at b 850�C, c 950�C and d 1,150�C in nitrogen atmosphere

Fig. 4 TEM micrographs showing the effect of heat treatment temperature on the morphology of SrFe12O19/ZnFe2O4 nanoparticles, a ‘as

synthesized’ and b heat treated at 1,150�C in nitrogen atmosphere


123

addition to aFe2O3 achieved a maximum coercivity

(Table 1). Thus the synthesis of spinel, ZnFe2O4 along

with hexagonal, SrFe12O19 particles in nano size range

results in higher saturation magnetization and lower coer-

civity than those reported for single phase strontium

hexaferrite and zinc ferrite [6, 8, 23]. Hexaferrite nano-

particles having low coercivity and high saturation mag-

netization find their applications in magnetic recordings in

hard disks, floppy disks, video tapes, etc. [22].

4.5 Dielectric Study

Complex permittivity and permeability values represent the

dielectric and magnetic properties of the materials. The real

parts (e0, l0) of complex permittivity and permeability

symbolize the storage capability of electric and magnetic

energy. The imaginary parts (e00, l00) represent the loss of

electric and magnetic energy. As a microwave absorber,

big imaginary parts of complex permittivity and perme-

ability are expected. The real and imaginary parts of

complex permittivity (Fig. 6a, b) and permeability

(Fig. 7a, b) of nanoparticles are plotted as a function of

frequency in X-band (8.2–12.2 GHz). It is observed that,

with increase in heat treatment temperature from ‘as-syn-

thesized’ condition to 1,150�C, both complex permittivity

and permeability are observed to increase continuously.

The real part of permittivity is found to increase from 5.228

(average value) in ‘as synthesized’ condition to 9.883

(average value) when heat treated at 1,150�C (Fig. 6a). The

Fig. 5 The effect of HT temperature on hysteresis loops of SrFe12

O19/ZnFe2O4 nanoparticles in ‘as synthesized’ condition and heat

treated at 850, 950 and 1150�C in nitrogen atmosphere

Table 1 Effect of heat treatment temperature on the morphology and magnetic parameters of strontium hexaferrites

Temperature (�C) Morphology Coercivity

(Gauss)

Remanance,

Mr (emu/g)

Saturation magnetisation,

Ms (emu/g)

Mr/Ms

92 (As synthesized) Spherical and needle (20–25 nm) 5.000 0.0209 1.040 0.020

850 Small hexagonal (60–65 nm) 1,655 20.267 37.497 0.540

950 Large hexagonal (70–80 nm) 1,671 23.131 43.204 0.535

1,150 Large hexagonal (80–90 nm) 1,238 29.906 58.938 0.507

Fig. 6 The effect of HT temperature on real (e0) and imaginary (e00) part of permittivity of SrFe12O19/ZnFe2O4 nanoparticles in ‘as synthesized’

condition and heat treated at 850, 950 and 1150�C in nitrogen atmosphere


123

same trend is observed for the imaginary part of permit-

tivity. The maximum imaginary permittivity of 3.48

(average value) is observed for the material heat treated at

1,150�C (Fig. 6b). Also, the real permeability increases

from 1.104 (average value) in ‘as-synthesized’ condition to

1.233 (average value) when heat treated at 1,150�C

(Fig. 7a). Similarly imaginary permeability is also

increasing with increase in heat treatment temperature. The

maximum imaginary permeability of 1.111 (average value)

is observed for the powder heat treated at 1,150�C

(Fig. 7b). The increase in complex permittivity and per-

meability with increase in heat treatment temperature in all

the cases is attributed to the increased formation and

growth of SrFe12O19/ZnFe2O4 nanoparticles with increase

in HT temperature. The significance of the results is the

stability in the values of complex permittivity and perme-

ability (obtained for large bandwidth) than those reported

in literature [6–8, 23] for pure strontium hexaferrite and

zinc ferrite.

4.6 Reflection Loss Study

The RL for ‘as-synthesized’ SrFe12O19/ZnFe2O4 nanopar-

ticles are low for all the frequencies between 8.2 and

12.2 GHz (Fig. 8) and minimum to maximum values are

found to be in the range of -5.04 dB (at 8.200 GHz) to

-14.91 dB (at 10.636 GHz). For SrFe12O19/ZnFe2O4

nanoparticles heat treated at 850�C, the RL is evidently

improved to -22.15 dB at 10.636 GHz and has further

enhanced to -26.51 dB for the material heat treated at

1,150�C (Fig. 8). The increment in RL with increasing heat

treatment temperature is attributed to the increased for-

mation of SrFe12O19/ZnFe2O4 nanoparticles. The strongest

RL and the widest bandwidths (for RL [ 10 dB) are given

in Table 2. The improvement of RL originated from the

formation of hard and soft ferrite which can be explained

on the basis of exchange coupling interaction between hard

magnetic (SrFe12O19) and soft magnetic (ZnFe2O4) phases,

which changes the relative complex permeability of the

materials. In this study, the nanocomposite powders were

synthesized by modified flux method so two kinds of grains

can be combined. The composite powder including hexa-

ferrite (SrFe12O19) and spinel ferrite (ZnFe2O4) coupled to

each other by exchange through interface of ferrite parti-

cles. There will be more interfaces if the grain size is

smaller, and there will be stronger exchange coupling

interaction at the interface. As we know the cubic spinal

crystal structure of ZnFe2O4 is similar with the structure of

S block of SrFe12O19, so it is possible that the vacancy of

ZnFe2O4 is combined with Fe3? at the SrFe12O19 surface,

Fig. 7 The effect of HT temperature on real (l0) and imaginary (l00) part of permeability of SrFe12O19/ZnFe2O4 nanoparticles in ‘as synthesized’

condition and heat treated at 850, 950 and 1150�C in nitrogen atmosphere

Fig. 8 The effect of HT temperature on RL of SrFe12O19/ZnFe2O4 in

‘as synthesized’ condition and heat treated at 850, 950 and 1150�C


123

which is another possible reason for strong interface cou-

pling interaction. Thus, exchange coupling interaction

existing between hard and soft magnetic phases improves

the dielectric and microwave absorption properties [4, 5],

which is in agreement with the present study. Moreover,

microwave absorption enhanced when particles size is

reduced from micron to nano size [24]. This can be

explained by quantum size effect in nanocrystallite parti-

cles which makes the electronic energy levels split and the

spacing between adjacent energy states increases inversely

with the volume of the particles. If the particle of absorber

material is small enough and the discrete energy level

spacing is in the energy range of microwave, the electron

can absorb the energy as it leaps from one level to another,

which may lead to increase in attenuation. The -10 dB

absorption bandwidth corresponds to 68% EM wave

amplitude attenuation or to 90% power attenuation,

whereas a -20 dB absorption bandwidth corresponds to

90% amplitude attenuation or to 99% power attenuation

[25]. The epoxy resin is an insulator and nonmagnetic;

thus, it is transparent to EM waves. In the SrFe12O19/

ZnFe2O4/epoxy composite, the epoxy resin only functions

as matrix [25]. The RL of the composite mainly stems from

the contribution of SrFe12O19/ZnFe2O4 magnetic compos-

ite. Thus more than 99% power attenuation is observed for

the composite material heat treated at 1,150�C which suits

its application in Stealth defense in all military platforms.

5 Conclusions

Uniform needle and spherical shaped nanoparticles

(20–25 nm) of SrFe12O19/ZnFe2O4 have been successfully

synthesized by modified flux method. The SrFe12O19/

ZnFe2O4 nanoparticles have higher saturation magnetiza-

tions of 58.938 emu/g and low intrinsic coercivity

(1,238 Gauss) when heat treated at 1,150�C while com-

paring with heat treated powder at 950�C (43.204 emu/g,

1,671 Gauss). The real and imaginary parts of permittivity

and permeability increases with increase in heat treatment

temperature. The maximum RL of -26.5 dB (more than

99% power attenuation) at 10.636 GHz is obtained for the

material heat treated at 1,150�C.

Acknowledgments The authors acknowledge Ministry of Human

Resource Development (MHRD), Government of India for the fel-

lowship granted to first author of this study.

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Table 2 The strongest RL and

the widest bandwidths (for

RL [ 10 dB) in ‘as

synthesized’ and heat treated

conditions

Powder Minimum

(8.200 GHz)

Maximum

(12.200 GHz)

The widest bandwidth

for RL [ 10 dB (in GHz)

The strongest RL (dB)

at 10.636 GHz

As synthesized -5.04 -6.99 1.8 (9.7–11.5) -14.91

HT at 850�C -7.19 -9.23 3.2 (8.8–12.0) -22.15

HT at 950�C -8.06 -10.17 3.5 (8.7–12.2) -24.12

HT at 1,150�C -8.53 -10.64 3.5 (8.7–12.2) -26.51


123

synthesis and characterization of microwave absorbing srfe12o19/znfe2o4 nanocomposite

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