1. physical and magnetic properties of highly aluminum doped strontium ferrite nanoparticles...

7/29/2019 1. Physical and Magnetic Properties of Highly Aluminum Doped Strontium Ferrite Nanoparticles Prepared by Auto-c

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Physical and magnetic properties of highly aluminum doped strontium

ferrite nanoparticles prepared by auto-combustion route

H. Luo a, B.K. Rai a, S.R. Mishra a,n, V.V. Nguyen b, J.P. Liu b

a Department of Physics, The University of Memphis, Memphis, TN 38152, USAb Department of Physics, The University of Texas, Arlington, TX 76019, USA

a r t i c l e i n f o

Article history:

Received 26 January 2012Received in revised form

20 February 2012Available online 15 March 2012

Keywords:

Hexaferrite

doped hedxaferrite

Sr-Hexaferrite

Al doped Sr-Ferrite

High Coercivity Ferrite

a b s t r a c t

Highly Al3 ion doped nanocrystalline SrFe12xAlxO19 (0rxr12), were prepared by the auto-combus-

tion method and heat treated in air at 11001C for 12 h. The phase identification of the powdersperformed using x-ray diffraction show presence of high-purity hexaferrite phase and absence of

any secondary phases. With Al3 doping, the lattice parameters decrease due to smaller Al3 ion

replacing Fe3 ions. Morphological analysis performed using transmission electron microscope show

growth of needle shaped ferrites with high aspect ratio at Al3 ion content exceeding xZ2.

Al3 substitution modifies saturation magnetization (MS) and coercivity (HC). The room temperature

MS values continuously reduced while HC value increased to a maximum value of 18,100 Oe at x4,

which is an unprecedented increase ($321%) in the coercivity as compared to pure Sr-Ferrite. However,

at higher Al3 content x44, a decline in magnetization and coercivity has been observed. The magnetic

results indicate that the best results for applications of this ferrite will be obtained with an iron deficiency

in the stoichiometric formulation.

Published by Elsevier B.V.

1. Introduction

The M-phase ferrites (Pb, Sr, Ba)Fe12O19 with magnetoplumbite

structure are commonly known as hexagonal ferrites. Their distinct

magnetic properties such as their high magnetization per formula

unit (20 mB at 0 K), high Curie temperature, high coercive force

(large magnetocrystalline anisotropy), high permeability and low

conductive looses, excellent chemical stability and corrosion resis-

tivity [13], have made them popular for industrial application

such as microwave device and electromagnetic wave absorber,

ferroxdures, perpendicular magnetic recording media [47].

The structure of hexagonal ferrite is represented by an alter-

nate stack of spinel and hexagonal layers, Fe6O82 and MFe6O11

2 ,

respectively. The structure of the hexaferrite is based on a

hexagonal lattice in which closely packed sites of oxygen atomshave, in every fifth site, a mixture of Sr and oxygen ions in the

proportion of three to one. The 24 Fe3 ions are arranged in

five different kinds of interstitial sites, as discussed below. These

sites are coupled by superexchange interaction via O2 leading

to ferrimagnetic structure. The intrinsic magnetic properties of

hexaferrite can be significantly improved by substituting Fe3

in different sites with other suitable ions, such as Cu2 [8],

Cr3 [9,10], Ga3 [11], Ti4 [12], Al3 [1315] for Fe3 ions

of hexaferrite. The studies on Al3 substituted SrFe12xAlxO19although is limited, but is well known that M-type hexagonal

ferrite with low Al3 doping for Fe has very large coercivities

[16]. In general, the nonmagnetic Al3 ions substitute the octa-

hedral sites at low Al3 doping level. It seems interesting to

investigate further the effect of replacing Fe with increasing Al3

substitution. So far efforts in this direction is hampered because

of formation of secondary phases at high Al3 substitution level

exceeding x2. In view of this, present paper focuses on the

synthesis of pure phase SrFe12xAlxO19 (0rxr12) with complete

replacement of iron with Al3 . Concomitantly, the study carefully

presents ensuing morphological, structural, and magnetic prop-

erty changes upon Al3 substitutions for Fe3 in SrFe12O19.

The synthesis of pure phase nanocrystalline SrFe12xAlxO19 nano-

particles is achieved by solution based auto-combustion technique.

2. Experimental

The Al3 substituted SrFe12O19 particles were prepared via auto-

combustion method using nitrate salts. According to the compos-

tion of SrFe12xAlxO19 (x0.0,0.5,1.0,1.5,2,4,6,8,10,12), stoichiometric

amounts of Sr(NO3)2, Fe(NO3)3 9H2O, Al(NO3)3 9H2O were dis-

solved in a minimum amount of deionized water (100 ml for

0.1 mol of Fe3) by stirring on a hotplate at 60 1C. It is better to

set up the ratio of Fe and Al to Sr at 11.5 [17]. Table 1 shows the

weight details of the chemical used. Citric acid was dissolved into the

Contents lists available at SciVerse ScienceDirect

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

Journal of Magnetism and Magnetic Materials

0304-8853/$- see front matter Published by Elsevier B.V.

doi:10.1016/j.jmmm.2012.02.106

n Corresponding author.

E-mail address: [email protected] (S.R. Mishra).

Journal of Magnetism and Magnetic Materials 324 (2012) 26022608
http://www.elsevier.com/locate/jmmmhttp://www.elsevier.com/locate/jmmmhttp://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.jmmm.2012.02.106mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.jmmm.2012.02.106http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.jmmm.2012.02.106mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.jmmm.2012.02.106http://www.elsevier.com/locate/jmmmhttp://www.elsevier.com/locate/jmmm


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solutions to give a molar ratio of metal ions to citric acid of 1:1. Then

the solutions were allowed several minutes to cool down to room

temperature (RT). NH4OH was then added dropwise until the pH

was 6.5. Then the solution was heated on a hotplate at 100 1C until a

brown viscous gel was formed. Instantaneously gel ignites with the

formation of large amounts of gas, resulting in lightweight volumi-

nous powder. The resulting precursor powder was calcined at

1100 1C for 12 h to obtain pure SrFe12xAlxO

19hexa-ferrite phase.

The complete reaction proceeds as follow:

SrNO3212xFeNO33x AlNO33C6H8O7

NH4OH-SrAlxFe12xO19NH4NO3CO2H2O

The x-ray diffraction (XRD) patterns were collected using

Bruker D8 Advance x-ray diffractometer using Cu Ka radiation.

Transmission Electron Microscope (JEOL JEM1200EX II, TEM)

and Scanning Electron Microscopy (Philips XL 30 environmental

scanning electron microscope, SEM) equipped with EDX were

employed to analyze the morphology, chemical composition,

and microstructure of the samples. The magnetic properties of

the samples were investigated at RT using AGM magnetometer

(0rxr1.5) and SQUID (Quantum Design) (2rxr10). To mini-

mize the effect of demagnetizing field, the samples were com-pacted at 3000 psi and cut into rectangular parallelepiped with

the ratio of length to width larger than 3 and embedded in epoxy.

To have the zero initial magnetization value, the demagnetization

process was carried out by the field scanning from 10 kOe to zero

in decrement 1%.

3. Results and discussion

Fig. 1 shows the XRD patterns of SrFe12xAlxO19 with various

Al3 ion contents calcined at 1100 1C for 12 h. It can be seen that

the diffraction patterns belong to the M-type strontium ferrite

(ICDD 080-1198) with absence of any impurity phases. With the

Al3 substitution a gradual shift in the peaks to the right as

compared to pure strontium ferrite (x0) is observed.

Fig. 2 shows the structural parameters viz. crystal lattice a and c,

as a function of x. The lattice constants are calculated using follo-

wing formula [18]:

dhkl 4

3

h2hkk

2

a2

l2

c2

!1=2, 1

where d(hkl) is the crystal face distance and ( hkl) is the Miller

indices. The grain size D(hkl) was calculated using Scherrers

formula [19].

Dh k l kl=bcosy, 2

where l is the x-ray wavelength, b is the full-width at half-max, y

is the Bragg angle, and k0.89.

It can be seen that the value of the lattice constant c and a

decreases with the Al content. Overall, 5.4% and 3.9% lattice

contraction is observed in cand a lattice parameters, respectively,

on going from SrFe12O19 to SrAl12O19 [20]. This indicates that the

change of the main axis (c-axis) is larger than that ofa-axis for the

substitution of Al3 ion. On the contrary, Cr3 substitution in

SrFe12O19 was found to affect only the c lattice parameter [21].

These change in lattice constant results from the difference in

ionic radii of Al3 ion (0.535 A) and Fe3 ion (0.645 A) [22].

The smaller Al3 ion, replacing Fe3 ion leads to lattice contrac-

tion of the unit cell.

Fig. 3 is a comparison of initial Al3 doping levels and the

average levels in as synthesized individual particles measured via

Table 1

Details of weight fraction of chemicals used in the synthesis of SrFe12xAlxO19ferrites.

Al content (x) Weight (g)

Sr(NO3)2 Fe(NO3) 9H2O Al(NO3)3 9H2O Citric acid

0 0.1284 2.7876 0 1.5750

2 0.1284 2.3230 0.4313 1.5750

4 0.1284 1.8584 0.8625 1.57506 0.1284 1.3938 1.2938 1.5750

8 0.1284 0.9292 1.7250 1.5750

10 0.1284 0.4646 2.1563 1.5750

12 0.1284 0 2.5875 1.5750

Intensity

(a.u)

403836343230

2 (degrees)

(110)

(008)

(107) (114)

(203)

(201)(108)(200)

Intensity

(a.u)

7060504030

2 (degrees)

x = 0.0

x = 2.0

x =4.0

x = 6.0

x = 8.0

x = 10.0

x = 12.0

Fig. 1. XRD pattern of SrFe12xAlxO19 as a function of Al3 content.

5.80

5.70

5.60

a

()

121086420

x, SrFe12-xAlxO19

23.0

22.8

22.6

22.4

22.2

22.0

c()

y = -0.0266x + 5.8832

y = -0.0867x + 23.042

Fig. 2. Lattice parameters a and cof SrFe12xAlxO19 as a function of Al3 content

calculated using Eq. (1).

H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 26022608 2603


3/7

EDX. This figure shows that the composition of metals in the as

formed materials was close to that of the initial stoichiometric

ratio of metals used for the synthesis. This further shows that the

applied method for SrFe12xAlxO19 synthesis is an effective

method for the synthesis of single phase herxaferrite materials.

The crystallite size of SrFe12xAlxO19 calculated using Eq. (2)with reflections (0 0 8) and (1 0 7) is shown in Fig. 4. The average

crystallite size of SrFe12xAlxO19 is observed to decrease from

95 nm to 42 nm in going from pure SrFe12O19 to SrFe4Al8O19.

The further increase of Al3 content seems to increase the grain

size. The crystallite growth orientation is estimated by taking the

crystallite size ratio of D(1 0 7)/D(0 0 8), where D(1 0 7) and

D(0 0 8) are the crystallite size calculated from the planes parallel

to the c-axis and from the plane (0 0 8) perpendicular to the

c-axis, respectively. It is evident that the ratio D(1 0 7)/D(0 0 8)

increases from 0.6 to 1.2 with Al3 ion doping from x0 to 8.

Thus, this value suggests that the SrFe12xAlxO19 crystals tends to

grow preferentially along [1 0 1] direction primarily assuming

a plate like morphology for up to x8 Al3 doping level. With

Al3

content x48, the average crystallite size tends to increase

assuming more of a rod shape appearance. This variation in the

particle size is also evident from the TEM images as well, Fig. 5,

where particles are turning from spherical shape to disk and rods,

up to x2, and then gradual thinning of disk and rod is observed

with the Al3 addition. However, at higher Al3 concentration

x48, formation of large thin disk and long rod shape particles is

observed. The observed particle size for SrFe12O19 from the TEM

image ($90 nm) is in good agreement with the value estimated

from XRD analysis. This particle size is smaller than the criticalsize value of 460 nm [26] for single domain magnetic particles,

which indicates that all samples consist of single magnetic

domains. Overall average particle size of samples observed via

TEM is larger than the crystallite size measured by the XRD line

broadening.

For thermal analysis, differential scanning calorimetry (DSC) was

used to determine the Curie temperature, TC, of samples. It is

known from the magnetic theory that when heat is added to the

magnetic material, the thermal energy increases phonons and

kinetic energy of the valence electrons. Part of thermal energy

also disorders spins, which contribute to magnetic specific heat.

As temperature increases, a maximum value in the vicinity of the TCmay be obtained using DSC analyzer [23,24]. At this temperature

magnetization decreases rapidly with increasing randomization of

spin alignment. At temperature above TC, ferromagnetic or ferri-

magnetic materials becomes paramagnetic. The plot of TC as a

function of Al3 content is shown in Fig. 6 and TC values are listed

in Table 3. The TC value of 459 1C obtained for SrFe12O19 is in close

agreement with the published values [25]. Marked decrease in

TC is observed with the increase in the Al3 content. The decrease

in exchange interaction between iron sublattice with Al3 replacing

Fe3 ion is the cause for the observed decrease in the TC values.

Fig. 7(a)(c) shows the hysteresis loops of SrAlxFe12xO19 powder

at RT. At RT the SrFe12O19 displays characteristic hard magnetic

properties, i.e., large HC value of 4296 Oe and good remanence of

Mr38.10 emu/g. Since the maximum applied field was at around

14 kOe for 0rxr1.5 sample measurement, the magnetization did

not reach the saturation state, hence the maximum magnetization

value is used in the data analysis. It is clear that the value of

saturation magnetization (MS) comes out to be 59.33 emu/g at RT,

which is smaller than the theoretically predicted value (67.70 emu/

g), but agrees well with other experimental values of samples

prepared by different preparation methods [26,27].

It can be observed from Fig. 7 that the Al3 substitution signi-

ficantly affects the magnetic property of doped Sr-Ferrites. The

magnetic parameters, MS, HC and Mr extracted from the hysteresis

loops for samples xo6 are listed in Table 2. Except for HC, MS and

Mr values decrease with the increase in Al3 content for xo6.

Samples with Al3 content x6 and 8 show weak coercivity of

0.72 and 1.05 kOe, respectively. While samples with x48 show

paramagnetic behavior.

The behavior of these properties can be explained on the

basis of the occupation of doped cations at different sites inthe hexagonal structure of the ferrite. The magnetic moment in

M-type hexaferrite is due to the distribution of iron on five non-

equivalent sublattices of which three are octahedral (2a, 12k, and

4f2), one tetrahedral (4f1) and one trigonal bipyramidal (2b) [28].

Out of these five sites 12k, 2a, and 2b have upward spins and 4f1

and 4f2 have downward spin of electrons. The total magnetic

moment (i.e., 20 mB) is due to uncompensated upward spins.

The nonmagnetic Al3 replaces Fe3 ion (5 mB) from the sites

having spin upward direction, mainly 12k, which is responsible

for the reduction in saturation magnetization and remanence of

the synthesized materials. The replacement of Fe3 with diamag-

netic Al3 also reduces the super-exchange interaction between

FeA3OFeB

3 [29]. This decrease in exchange interaction also

leads to a non-collinear spin arrangement [3032]. Additionally, it

100

80

60

40

20

0

AlAtom(%

)

121086420

(x), SrFe12-xAl xO19

Theoretical

Measured

Fig. 3. EDX elemental analysis of Al content in SrFe12xAlxO19 samples.

120

100

80

60GrainS

ize(nm)

121086420

(x), SrFe12-xAlxO19

1.2

1.0

0.8

0.6D(107)/D(008)

D(008)D(107)

Average Grain Size

Fig. 4. Crystallite size of SrFe12xAlxO19 samples as a function of Al3 content

calculated using Scherrers Eq. (2).

H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 260226082604


4/7

was observed via Mossbauer spectroscopy that surface defects

in nanocrystalline SrFe10.5 Al1.5O19 are also responsible for the

lowering of the exchange interaction. This lowering of the exchange

interaction also leads to the onset of non-collinear spin arrange-

ment with respect to c-axis in the surface layer [33]. The coupling of

non-collinear surface spins with the core spin, aligned along c-axis,

can further lower the net magnetization of the material [34]. Thus,

samples with Al3 content exceeding x42 will have, large reduc-

tion in Fe3 ions from sites with upward spins, non-collinear spin

arrangement, and relative reduction in super-exchange interaction.

These factors result in reduction of magnetization of samples with

x42, as evident from Fig. 7(b) and (c). In essence the hysteresis

loops of samples with x42 is a mixture of ferromagnetic and

paramagnetic component of the sample [35]. The paramagneticcontribution to the hysteresis loops comes from the increased Al3

(Pauli paramagnetic metal, wm$16.5106 cm3 mol1) content in

the samples.

The magneton number nB (mB) is obtained using the relation

nB(molecular weightMS)/5585, where MS is the saturation

magnetization of the sample [36]. The values of magneton

number decrease with increase in Al3 substitution. This is due

to the substitution of non-magnetic Al3 ions in place of Fe3

ions in the SrFe12O19 hexaferrite matrix. The values of magneton

numbers are listed in Table 2. The decrease in saturation magne-

tization and remanence magnetization with substitution of Al3

closely agrees with the observations made for Al3 and AlGa ion

substituted barium and Sr-hexaferrite prepared by solution com-

bustion and co-precipitation techniques [37,38].

The coercivity for a ferromagnet or ferrimagnet can be reflected

by coercivity field HC. The value refers to the intensity of the

magnetic field required to reduce the magnetization of the mag-

netic sample to zero, after the magnetization of the sample has

reached saturation. The obtained value of HC (4.3 kOe) for SrFe12O19sample is lower than those of the single-domain SrFe12O19 with

HC$5.5 kOe obtained by a modified co-precipitation method

and the theoretical limit of 7.5 kOe [3941]. The low value of the

coercive field obtained in the present case can be due to the low

crystalline anisotropy, which arises from crystal imperfection and a

high degree of aggregation. However, HC of Al3 doped samples, as

shown in Fig. 8, show interesting behavior. The HC of samples

increases for x going from 0 to 4, and then decreases with the

further Al3 doping. The maximum enhancement of$321% in HCfield is observed at x4 Al3 doping level as compared to that of

SrFe12O19. In our knowledge the observed HC value of 18.1 kOe for

x4 is the highest ever reported HC value for doped ferrite systems.

There are two possible reasons for the observed dependence of

HC on Al3 doping viz. grain size and magnetocrystaline anisotropy.

The average grain size of the SrFe12O19 particles in this study was

between about 80100 nm. The critical size of a single-domain

particle is estimated using the formula [42,43]

Dm 9sW=2pMS2, 3

where sW(2kBTC9K19/a)1/2 is the wall density energy, 9K19 is the

magnetocrystalline anisotropy constant, TC is the Curie temperature

as obtained from DSC, MS is the saturation magnetization, kB is

the Boltzmann constant and a is the lattice constant. For D4Dm the

Fig. 5. TEM micrographs of SrFe12xAlxO19 as a function of Al3 content (0oxo12). (a) x0.0, (b) 0.5, (c) 1.0, (d)1.5, (e) 2.0, (f) 4.0, (g) 6.0, (h) 8.0, (i) 10.0, and (j) 12.0.

Scale bars are 100 nm length.



5/7

particles are multi-domain structures, while for DoDm the parti-

cles are mono-domain structures. Table 3 lists Dm values calculated

using Eq. (3) for SrFe12xAlxO19 (xr0r4). For SrFe12O19, TC732 K,

a5.8748 A, 9K193.7106 erg/cm3 [44] and MS314.9 Gs,the estimated value of Dm is about 516 nm.

With the Al3 doping, the value of Dm increases to 2213 nm at

x4, which is far greater than the average diameter of as obtained

SrFe10.5Al1.5O19 particles (ref. TEM images Fig. 5). So the grains

exhibit a monodomain behavior. The formation of monodomain

impedes the domain wall motion which result in the increase in

the HC. However, the role of domain walls in determining HC is

complex since defects may pin domain walls in addition to

nucleating them. Furthermore, with the addition of Al3 up to

x4, the HC increases as expected from Hcj a(2K/MS) [45], where

MS is the magnetic saturation and K is the magnetocrstalline

440

420

400

380

360

340

Tc

(C)

86420

(x), SrFe12-xAlxO19

HeatFlow

500450400350300

Temperature (C)

459 C

429 C

409 C378 C

348 CExoth.

x = 0

x = 0.5

x = 1.0

x = 2.0

x = 1.5

x = 4.0 341 C

Fig. 6. Curie temperature plot as a function of Al3 content for SrFe12xAlxO19samples. Inset shows few representative DSC curves of samples up to x2.

Table 3

Single domain particle size estimated using Eq. (3) for SrFe12xAlxO19 (0rxr4).

Bulk SrFe12O19 magnetocrystalline anisotropy (9K19) constant is used in thecalculation.

X Lattice constant a ( A) TC (K) MS (Gs) Dm (nm)

0.0 5.8748 73272 314.9 516

0.5 5.8699 70272 246.0 838

1.0 5.8566 68272 225.8 974

1.5 5.8433 65172 211.2 1094

2.0 5.8324 62172 186.8 1354

4.0 5.7829 61472 46.08 2213

Table 2

RT magnetic parameters viz. saturation magnetization (MS), remanent magnetiza-

tion (Mr), coercivity (HC) and magneton number (nB) of SrFe12xAlxO19 samples.

X MS (emu/g) Mr (emu/g) Mr/MS HC (Oe) nB (lB)

0.0 59.33 38.10 0.64 4,295.7 11.16

0.5 46.68 26.85 0.57 2,447.0 8.66

1 43.49 26.17 0.60 3,346.3 7.96

1.5 40.98 26.06 0.63 6,295.2 7.39

2 36.50 20.00 0.55 7,400.0 6.494 9.00 6.00 0.67 18,100.0 1.51

-60

-40

-20

0

20

40

60

M(

emu/g)

-15x103 -10 -5 0 5 10 15

H (Oe)

298 KSrFe12O19SrAl0.5Fe11.5O19SrAl1Fe11O19SrAl1.5Fe10.54O19

-30

-20

-10

0

10

20

30

M(

emu/g)

-40x103 -20 0 20 40

H (Oe)

298 KSrAl2Fe10O19SrAl4Fe8O19

-0.2

-0.1

0.0

0.1

0.2

M(

emu/g)

-10000 -5000 0 5000 10000

H (Oe)

298 KSrAl6Fe6O19SrAl8Fe4O19SrAl10 Fe2O19

Fig. 7. (a) Hysteresis loops for SrFe12xAlxO19 samples at RT forAl3 doping level of

0oxr1.5 measured using AGM. (b) Hysteresis loops for SrFe12xAlxO19 samples at

RT forAl3 doping level of 2rxr4 measured using SQUID. Only half hysteresis

loop is shown. (c) Hysteresis loops for SrFe12xAlxO19 samples at RT forAl3 doping

level of 6rxr10 measured using SQUID. Only half hysteresis loop is shown.



6/7

anisotorpy. According this equation, the decrease in the MS upon

Al3 doping, leads to an increase in the intrinsic HC. It has been

reported earlier that the Al3 below xo2 occupies 4f2, 4f1, 2a,

and 12k sites [46,43] and weakly affects the anisotropy constant,

while MS value decreases rapidly. Thus, HC value enhances upon

Al3 substitution in xr4 samples. Conversely, the HC reduces at

higher Al3 content, x44, primarily due to considerable decrease

in anisotropy constant. Mossbauer studies on ferrites have shown

that the Fe3 ion in 2b site play an important role in determining

the magnetic anisotropy properties of the M-type ferrites [47,48].

The strong trigonal crystalline field in 2b site gives rise to a

significant contribution to the spinorbit interaction in the 3d

electronic shell of the Fe3 ions. In fact, the uniaxial magnetic

anisotropy of the M-type ferriets is interpreted in terms of the

anisotropy energy of single Fe3 ions at the 2b trigonal sites.

Furthermore, the Mossbauer study of Al and Ga substituted Ba and

Sr ferrites, at low Al doping xo4, show that the Fe3 ions in

trigonal 2b lattice sites are not substituted by Al3 ions thus

having a very little effect on the magnetiocrystalline anisotropy.

However, at higher concentration of Al3 doping, number of Fe3

ions in 2b site decreases rapidly, leading to a greater change in

the magnetic anisotropy. Overall, the observation that the HC is

increased with the Al3 ion content up to 4.0, means that the effect

of the Al3 ion substitution on HC is much more significant than

that of the particle size.

4. Conclusion

Nanocrystalline Al3 substituted SrFe12O19 samples have been

successfully synthesized by the auto-combustion method. The

x-ray diffraction patterns reveal the formation of M-phase hex-

agonal structure for all level of Al3 substitutions without any

trace of secondary phases. A decrease in the lattice parameters

has been observed with increasing Al3 doping level. EDX analysis

confirms that the synthesized samples have attained the nominal

theoretical stoichiometry. A continues change in morphology

of particles, from sphere to disk and rod shape is observed with

Al3 doping. Magnetically, samples are ferromagnetic and ferri-

magnetic/paramagnetic at low (xr4) and high (x44) Al3 doping

levels, respectively. This change in magnetic behavior with Al3

doping is explained on the basis of weakening of exchange inter-

action and non-collinear spin arrangement. The dependence of

HC on Al3 doping level with maximum value of 18.1 kOe attained

at x4 and then rapid decrease in its value for x44 is explained on

the basis of size and magnetocrystaline anisotropy of the particles

mainly arising from 2b sites. The high aspect ratio nanocrystalline

ferrites can find suitable application in electronic industry.

Acknowledgment

The authors gratefully acknowledge the financial support of

the NSF EAGER (0965801).

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15x103

10

5

0

Coercivity,H

c(Oe)

1086420

(x), SrAlxFe12-xO19

298 K

Fig. 8. Coercivity plot as a function of Al3

content for SrFe12xAlxO19 samples.



7/7

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