Journal of Magnetism and Magnetic Materials 324 (2012) 2602–2608

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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 b

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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 January 2012 Received in revised form 20 February 2012 Available online 15 March 2012

Highly Al3 þ ion doped nanocrystalline SrFe12 xAlxO19 (0rxr12), were prepared by the auto-combustion method and heat treated in air at 1100 1C for 12 h. The phase identification of the powders performed using x-ray diffraction show presence of high-purity hexaferrite phase and absence of any secondary phases. With Al3 þ doping, the lattice parameters decrease due to smaller Al3 þ ion replacing Fe3 þ ions. Morphological analysis performed using transmission electron microscope show growth of needle shaped ferrites with high aspect ratio at Al3 þ ion content exceeding xZ2. Al3 þ 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 x¼ 4, which is an unprecedented increase ( 321%) in the coercivity as compared to pure Sr-Ferrite. However, at higher Al3 þ 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.

Keywords: Hexaferrite doped hedxaferrite Sr-Hexaferrite Al doped Sr-Ferrite High Coercivity Ferrite

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 resistivity [1–3], have made them popular for industrial application such as microwave device and electromagnetic wave absorber, ferroxdures, perpendicular magnetic recording media [4–7]. The structure of hexagonal ferrite is represented by an alternate stack of spinel and hexagonal layers, Fe6O28 þ and MFe6O211 , respectively. The structure of the hexaferrite is based on a hexagonal lattice in which closely packed sites of oxygen atoms have, in every fifth site, a mixture of Sr and oxygen ions in the proportion of three to one. The 24 Fe3 þ 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 þ in different sites with other suitable ions, such as Cu2 þ [8], Cr3 þ [9,10], Ga3 þ [11], Ti4 þ [12], Al3 þ [13–15] for Fe3 þ ions

n

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

0304-8853/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.jmmm.2012.02.106

of hexaferrite. The studies on Al3 þ substituted SrFe12  xAlxO19 although is limited, but is well known that M-type hexagonal ferrite with low Al3 þ doping for Fe has very large coercivities [16]. In general, the nonmagnetic Al3 þ ions substitute the octahedral sites at low Al3 þ doping level. It seems interesting to investigate further the effect of replacing Fe with increasing Al3 þ substitution. So far efforts in this direction is hampered because of formation of secondary phases at high Al3 þ substitution level exceeding x ¼2. In view of this, present paper focuses on the synthesis of pure phase SrFe12  xAlxO19 (0 rxr12) with complete replacement of iron with Al3 þ . Concomitantly, the study carefully presents ensuing morphological, structural, and magnetic property changes upon Al3 þ substitutions for Fe3 þ in SrFe12O19. The synthesis of pure phase nanocrystalline SrFe12 xAlxO19 nanoparticles is achieved by solution based auto-combustion technique.

2. Experimental The Al3 þ substituted SrFe12O19 particles were prepared via autocombustion method using nitrate salts. According to the compostion of SrFe12 xAlxO19 (x¼0.0,0.5,1.0,1.5,2,4,6,8,10,12), stoichiometric amounts of Sr(NO3)2, Fe(NO3)3  9H2O, Al(NO3)3  9H2O were dissolved in a minimum amount of deionized water (100 ml for 0.1 mol of Fe3 þ ) 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

H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2602–2608

Table 1 Details of weight fraction of chemicals used in the synthesis of SrFe12  xAlxO19 ferrites.

Intensity (a.u)

0 2 4 6 8 10 12

Weight (g) Sr(NO3)2

Fe(NO3)  9H2O

Al(NO3)3  9H2O

Citric acid

0.1284 0.1284 0.1284 0.1284 0.1284 0.1284 0.1284

2.7876 2.3230 1.8584 1.3938 0.9292 0.4646 0

0 0.4313 0.8625 1.2938 1.7250 2.1563 2.5875

1.5750 1.5750 1.5750 1.5750 1.5750 1.5750 1.5750

(203) (008) (200)(201)(108)

30

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 voluminous powder. The resulting ‘‘precursor’’ powder was calcined at 1100 1C for 12 h to obtain pure SrFe12 xAlxO19 hexa-ferrite phase. The complete reaction proceeds as follow:

(114)

(107) (110)

32

34 36 2θ (degrees)

38

Intensity (a.u)

Al content (x)

2603

40

x = 12.0 x = 10.0 x = 8.0 x = 6.0 x =4.0 x = 2.0

SrðNO3 Þ2 þð12xÞFeðNO3 Þ3 þ x AlðNO3 Þ3 þ C 6 H8 O7

x = 0.0

þ NH4 OH-SrAlx Fe12x O19 þ NH4 NO3 þ CO2 þH2 O

30

3. Results and discussion

40

50 2Θ (degrees)

60

70

Fig. 1. XRD pattern of SrFe12  xAlxO19 as a function of Al3 þ content.

y = -0.0266x + 5.8832

a (Å)

5.80 5.70 5.60 23.0 22.8

c (Å)

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 (0rx r1.5) and SQUID (Quantum Design) (2rx r10). To minimize the effect of demagnetizing field, the samples were compacted 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%.

y = -0.0867x + 23.042

22.6 22.4 22.2

Fig. 1 shows the XRD patterns of SrFe12  xAlxO19 with various Al3 þ 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 þ substitution a gradual shift in the peaks to the right as compared to pure strontium ferrite (x¼0) 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 following formula [18]: ! 2 2 2 1=2 4 h þ hk þk l dðhklÞ ¼ þ , ð1Þ 3 a2 c2 where d(hkl) is the crystal face distance and (hkl) is the Miller indices. The grain size D(hkl) was calculated using Scherrer’s formula [19]. Dðh k lÞ ¼ kl=bcosðyÞ,

ð2Þ

where l is the x-ray wavelength, b is the full-width at half-max, y is the Bragg angle, and k¼0.89.

22.0 0

2

4

6 8 x, SrFe12-x Al xO19

10

12

Fig. 2. Lattice parameters a and c of SrFe12  xAlxO19 as a function of Al3 þ content calculated using Eq. (1).

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 c and 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 of a-axis for the substitution of Al3 þ ion. On the contrary, Cr3 þ substitution in SrFe12O19 was found to affect only the c lattice parameter [21]. These change in lattice constant results from the difference in ˚ and Fe3 þ ion (0.645 A) ˚ [22]. ionic radii of Al3 þ ion (0.535 A) 3þ The smaller Al ion, replacing Fe3 þ ion leads to lattice contraction of the unit cell. Fig. 3 is a comparison of initial Al3 þ doping levels and the average levels in as synthesized individual particles measured via

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H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2602–2608

100 Measured Theoretical

Al Atom (%)

80

60

40

20

0 0

2

4

6

8

10

12

(x), SrFe12-x Al xO19

D (107) / D (008)

Fig. 3. EDX elemental analysis of Al content in SrFe12  xAlxO19 samples.

1.2 1.0 0.8 0.6

Grain Size (nm)

120 D(008) D(107) Average Grain Size

100 80 60

0

2

4

6 8 (x), SrFe12-x AlxO19

10

12

Fig. 4. Crystallite size of SrFe12  xAlxO19 samples as a function of Al3 þ content calculated using Scherrer’s Eq. (2).

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 SrFe12  xAlxO19 synthesis is an effective method for the synthesis of single phase herxaferrite materials. The crystallite size of SrFe12  xAlxO19 calculated using Eq. (2) with reflections (0 0 8) and (1 0 7) is shown in Fig. 4. The average crystallite size of SrFe12  xAlxO19 is observed to decrease from 95 nm to 42 nm in going from pure SrFe12O19 to SrFe4Al8O19. The further increase of Al3 þ 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 þ ion doping from x¼0 to 8. Thus, this value suggests that the SrFe12  xAlxO19 crystals tends to grow preferentially along [1 0 1] direction primarily assuming a plate like morphology for up to x ¼8 Al3 þ doping level. With Al3 þ content x4 8, 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 x¼2, and then gradual thinning of disk and rod is observed with the Al3 þ addition. However, at higher Al3 þ 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 critical size 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 TC may 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 ferrimagnetic materials becomes paramagnetic. The plot of TC as a function of Al3 þ 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 þ content. The decrease in exchange interaction between iron sublattice with Al3 þ replacing Fe3 þ ion is the cause for the observed decrease in the TC values. Fig. 7(a)–(c) shows the hysteresis loops of SrAlxFe12 xO19 powder at RT. At RT the SrFe12O19 displays characteristic hard magnetic properties, i.e., large HC value of 4296 Oe and good remanence of Mr ¼38.10 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 þ substitution significantly affects the magnetic property of doped Sr-Ferrites. The magnetic parameters, MS, HC and Mr extracted from the hysteresis loops for samples x o6 are listed in Table 2. Except for HC, MS and Mr values decrease with the increase in Al3 þ content for x o6. Samples with Al3 þ content x ¼6 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 in the hexagonal structure of the ferrite. The magnetic moment in M-type hexaferrite is due to the distribution of iron on five nonequivalent 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 þ replaces Fe3 þ 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 þ with diamagnetic Al3 þ also reduces the super-exchange interaction between Fe3A þ –O–Fe3B þ [29]. This decrease in exchange interaction also leads to a non-collinear spin arrangement [30–32]. Additionally, it

H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2602–2608

2605

Fig. 5. TEM micrographs of SrFe12  xAlxO19 as a function of Al3 þ content (0 o xo 12). (a) x ¼0.0, (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.

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 arrangement 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 þ content exceeding x42 will have, large reduction in Fe3 þ 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 paramagnetic contribution to the hysteresis loops comes from the increased Al3 þ (Pauli paramagnetic metal, wm  16.5  10  6 cm3 mol  1) content in the samples. The magneton number nB (mB) is obtained using the relation nB ¼(molecular weight  MS)/5585, where MS is the saturation magnetization of the sample [36]. The values of magneton number decrease with increase in Al3 þ substitution. This is due to the substitution of non-magnetic Al3 þ ions in place of Fe3 þ ions in the SrFe12O19 hexaferrite matrix. The values of magneton numbers are listed in Table 2. The decrease in saturation magnetization and remanence magnetization with substitution of Al3 þ closely agrees with the observations made for Al3 þ and Al–Ga ion substituted barium and Sr-hexaferrite prepared by solution combustion 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 magnetic sample to zero, after the magnetization of the sample has reached saturation. The obtained value of HC (4.3 kOe) for SrFe12O19 sample 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 [39–41]. 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 þ 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 þ doping. The maximum enhancement of 321% in HC field is observed at x¼4 Al3 þ doping level as compared to that of SrFe12O19. In our knowledge the observed HC value of 18.1 kOe for x¼4 is the highest ever reported HC value for doped ferrite systems. There are two possible reasons for the observed dependence of HC on Al3 þ doping viz. grain size and magnetocrystaline anisotropy. The average grain size of the SrFe12O19 particles in this study was between about 80–100 nm. The critical size of a single-domain particle is estimated using the formula [42,43] Dm ¼ 9sW =ð2pM S 2 Þ,

ð3Þ 1/2

is the wall density energy, 9K19 is the where sW ¼(2kBTC9K19/a) 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

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H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2602–2608

60 x = 4.0 341 °C

Heat Flow

420

Tc (°C)

x = 2.0

348 °C

40 378 °C 409 °C

x = 1.5

20

429 °C x = 1.0

M (emu/g)

Exoth.

440

x = 0.5

400

459 °C

x=0

300

380

298 K SrFe12O19 SrAl0.5Fe11.5O19 SrAl1Fe11O19 SrAl1.5Fe10.54O19

350

400 450 Temperature (°C)

0

-20

500

-40

360 -60 -10

-15x103

340 0

2

4 (x), SrFe12-xAlxO19

6

-5

0 H (Oe)

5

10

15

8 30

Fig. 6. Curie temperature plot as a function of Al3 þ content for SrFe12  xAlxO19 samples. Inset shows few representative DSC curves of samples up to x¼ 2.

298 K SrAl2Fe10O19 SrAl4Fe8O19

20

X

MS (emu/g)

Mr (emu/g)

Mr/MS

HC (Oe)

nB (lB)

0.0 0.5 1 1.5 2 4

59.33 46.68 43.49 40.98 36.50 9.00

38.10 26.85 26.17 26.06 20.00 6.00

0.64 0.57 0.60 0.63 0.55 0.67

4,295.7 2,447.0 3,346.3 6,295.2 7,400.0 18,100.0

11.16 8.66 7.96 7.39 6.49 1.51

10 M (emu/g)

Table 2 RT magnetic parameters viz. saturation magnetization (MS), remanent magnetization (Mr), coercivity (HC) and magneton number (nB) of SrFe12  xAlxO19 samples.

0 -10 -20 -30 -20

-40x103 0.2

Table 3 Single domain particle size estimated using Eq. (3) for SrFe12  xAlxO19 (0rx r 4). Bulk SrFe12O19 magnetocrystalline anisotropy (9K19) constant is used in the calculation. ˚ Lattice constant a (A)

TC (K)

MS (Gs)

Dm (nm)

0.0 0.5 1.0 1.5 2.0 4.0

5.8748 5.8699 5.8566 5.8433 5.8324 5.7829

7327 2 7027 2 6827 2 6517 2 6217 2 6147 2

314.9 246.0 225.8 211.2 186.8 46.08

516 838 974 1094 1354 2213

particles are multi-domain structures, while for DoDm the particles are mono-domain structures. Table 3 lists Dm values calculated using Eq. (3) for SrFe12 xAlxO19 (xr0r4). For SrFe12O19, TC ¼732 K, ˚ 9K19¼3.7  10  6 erg/cm3 [44] and MS ¼314.9 Gs, a¼5.8748 A, the estimated value of Dm is about 516 nm. With the Al3 þ doping, the value of Dm increases to 2213 nm at x¼4, 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

20

40

0 H (Oe)

5000

10000

298 K

0.1 M (emu/g)

X

0 H (Oe)

SrAl 6 Fe 6O 19 SrAl 8 Fe 4O 19 SrAl10 Fe 2O 19

0.0

-0.1

-0.2 -10000

-5000

Fig. 7. (a) Hysteresis loops for SrFe12 xAlxO19 samples at RT forAl3 þ doping level of 0oxr1.5 measured using AGM. (b) Hysteresis loops for SrFe12 xAlxO19 samples at RT forAl3 þ doping level of 2rxr4 measured using SQUID. Only half hysteresis loop is shown. (c) Hysteresis loops for SrFe12 xAlxO19 samples at RT forAl3 þ doping level of 6rxr10 measured using SQUID. Only half hysteresis loop is shown.

nucleating them. Furthermore, with the addition of Al3 þ up to x¼4, the HC increases as expected from Hcj ¼ a(2K/MS) [45], where MS is the magnetic saturation and K is the magnetocrstalline

H. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2602–2608

doping is explained on the basis of weakening of exchange interaction and non-collinear spin arrangement. The dependence of HC on Al3 þ doping level with maximum value of 18.1 kOe attained at x¼4 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.

298 K 15x103

Coercivity, Hc (Oe)

2607

10 Acknowledgment The authors gratefully acknowledge the financial support of the NSF EAGER (0965801).

5 References

0 0

2

4 6 (x), SrAlxFe12-xO19

8

10

Fig. 8. Coercivity plot as a function of Al3 þ content for SrFe12  xAlxO19 samples.

anisotorpy. According this equation, the decrease in the MS upon Al3 þ doping, leads to an increase in the intrinsic HC. It has been reported earlier that the Al3 þ 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 þ substitution in xr4 samples. Conversely, the HC reduces at higher Al3 þ content, x44, primarily due to considerable decrease in anisotropy constant. Mossbauer studies on ferrites have shown that the Fe3 þ 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 spin–orbit interaction in the 3d electronic shell of the Fe3 þ ions. In fact, the uniaxial magnetic anisotropy of the M-type ferriets is interpreted in terms of the anisotropy energy of single Fe3 þ 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 þ ions in trigonal 2b lattice sites are not substituted by Al3 þ ions thus having a very little effect on the magnetiocrystalline anisotropy. However, at higher concentration of Al3 þ doping, number of Fe3 þ 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 þ ion content up to 4.0, means that the effect of the Al3 þ ion substitution on HC is much more significant than that of the particle size.

4. Conclusion Nanocrystalline Al3 þ substituted SrFe12O19 samples have been successfully synthesized by the auto-combustion method. The x-ray diffraction patterns reveal the formation of M-phase hexagonal structure for all level of Al3 þ substitutions without any trace of secondary phases. A decrease in the lattice parameters has been observed with increasing Al3 þ 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 þ doping. Magnetically, samples are ferromagnetic and ferrimagnetic/paramagnetic at low (xr4) and high (x44) Al3 þ doping levels, respectively. This change in magnetic behavior with Al3 þ

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