EFFECT OF TRANSITION METAL ION SUBSTITUTION ON MAGNETIC STRUCTURE AND PROPERTIES OF La-Sr HEXAFERRITES AT NANOSCALE

PAWADE S.A.¹*, REWATKAR K.G.², NANOTI V.M.^3
1Department of Applied Physics, Rajiv Gandhi College of Engineering research &Technology, Chandrapur, MS, India.
²Department of Physics, Dr. Ambedkar College, Dikshabhumi, Nagpur, MS, India.
³Department of Applied Physics, Priyadarshini College of Engineering, Nagpur, MS, India.
* Corresponding Author : asheela05@gmail.com

Received : 28-02-2012     Accepted : 06-03-2012     Published : 15-03-2012
Volume : 3     Issue : 1       Pages : 31 - 35
Int J Knowl Eng 3.1 (2012):31-35

Conflict of Interest : None declared

Al-substituted M-type hexaferrite is a highly anisotropic ferromagnetic material. In the present study, microwave assisted Solgel auto-combustion route using urea as a reductant and nitrates as oxidants of synthesis for SrLayAlxFe12-x-yO19 (y=1;0≤x≤8) powders were explored and their microstructure, grain morphology, magnetic properties, examined. X-ray diffraction (XRD), transmission electron microscopy (TEM), a vibrating sample magnetometer (VSM), were used to characterize the powders. The TEM analysis revealed that a powder with a smaller particle size, near single-domain structure, uniform grain morphology, and smaller shape anisotropy.The synthesis technique also significantly affected the magnetic properties. The grain size was found to be a key factor affecting the property of the powder. (XRD) studies reveal that microwave-assisted combustion synthesis route yields materials with higher degree of compositional stability and phase purity. Both a and c lattice parameters calculated and found to decrease with increasing Al content. The effects of Al3+ ions on the Strontium Lanthanum ferrites were reported and discussed in detail. The site preference of Al3+ and magnetic properties of the ferrites have been studied using hysteresis. The results show that the magnetic properties are closely related to the distributions of Al+3 ions on the five crystallographic sites. The saturation magnetization slowly decreases, however, the coercivity decreases with Al concentration. The magnetization results indicate that the Al+3 ions preferentially occupy the 2a, 12k, and 4f2 sites.. This confirms the use of samples in the various applications of digital data devices. Further attempts could lead to investigate their use in data storage devices, such applications favor nano-sized magnetic materials.

Strontium ferrite, X-ray density, porosity, magnetization, coercivity, retentivity

The hexagonal M-type hard hexaferrites have attracted much attention as the most widely used permanent magnets, which account for about 90% of the annual production of permanent magnets. These ferrites are perspective materials for developing such permanent magnets and high density perpendicular recording media, due to their high uniaxial magneto-crystalline anisotropy and chemical stability [1] with proper doping. The properties of this ferrite are largely dependent on the processing routes used for its fabrication [2-3] .
Due to perfect chemical stability, good thermal durability, carrion resistivity, unique electrical and magnetic properties, Strontium hexaferrites have extensive applications in recording devices [4] , telecommunication [5] magneto-optical and microwave devices [6] . These materials cover a wide field of technological applications from microwave to radio frequencies due to its high electrical resistivity.
The properties of ferrites are strongly influenced by composition and synthesis method.
The rare earth substitution was employed to inhibit the grains growth [7] and to promote ferritization reaction [8] . The lanthenides also improve the mechanical materials hardness [9] . The experiments carried out by Dung [10] showed that the La substitutions improve hard magnetic properties of Sr hexaferrites.
In the present investigation we have synthesized M type nano sized Al⁺³ doped Sr hexaferrites by applying microwave assisted Sol-gel auto combustion process and we systematically amount on properties Viz crystal structure, saturation magnetization (M_s), Coercivity (H_c) and retentivity (M_r).

The large number of preprational techniques have been reported earlier. Viz. solid state method, aerosol pyrolisis, co-precipitation method, hydrothermal method, salt method, micro emulsion method. These conventional methods require repeated grinding and calcination which invariably contaminate the powders.
The better method popularly known as microwave assisted ‘Sol-gel Auto combustion’ is employed to synthesize substituted magnetoplumbites by virtue of many advantages of this technique like facile operation, low anneal or calcination temperature, energy efficient and a short reaction rate, etc. In microwave oven heat is generated within the sample volume itself by the interaction of microwaves within the sample volume by the interaction of microwaves with the material on molecular level which leads uniform heating. In addition, the sol-gel combustion route gives ultra fine powder of nano particles with better particle size distribution, excellent chemical homogeneity and more probability of formation of single domain structure [11] .
M-type Al⁺³ substituted Sr hexaferrites were synthesized by sol-gel combustion method. All chemicals used for this synthesis were of analytical reagent grade. Stichometric amount of metal nitrates. Viz Sr(NO₃)₂, La(NO₃)₃.6H₂O, Fe(NO₃)₃.6H₂O Al(NO₃)₃.9H₂O, viz. were dissolved in triple filtered deionised water. Urea [(NH₂)2CO] was also dissolved in deionised water and is used as reducing agent to supply requisite energy to initiate exothermic reaction amongst oxidants. All solutions as prepared were mixed together to form a homogeneous yellow brown transparent aqueous solutions.and also treated by ultrasonic vibrator for half an hour. After that, mixed solution was kept in room temperature for 2 hrs. The yellow brown transparent solution was heated in microwave oven so that it gets concentrated slowly without producing any precipitation but change its viscosity and color until it turns into brown gel. After about 10-15 minutes of irradiation the gel swelled into foam and underwent self propagating combustion reaction changing brown gel into fluffy mass which is further ground into black powder. The resulting black ashes were grounded in an agate pestle and mortar put into alumina crucibles and then calcined at 800oC for 1 hr.
Calcined powders were ground and after adding polyvinyl alcohol, binder mixtures were shaped into pallets using hydraulic press under uniaxial pressure of 75 kNm-2..These pallets of each composition were then sintered at 12000C in electric furnace for 7-8 hours. During sintering process crystallization gave rise to a hexagonal polycrystalline phase.
The phase identification of the gel precursor, the as-burnt and calcined powders was performed using X-ray diffraction (XRD) on a PW-1710 Philips Holland X -ray diffractometer using Cu Kα radiation (λ=1.5405Å). The average crystallite size of the powders was measured by X-ray line-broadening technique employing the Scherrer formula The particle morphology was examined by a transmission electron microscope (TEM). Magnetic properties were measured by a vibrating sample magnetometer (VSM) with a maximum applied field of 15 KOe. While the magnetic susceptibility measurements were carried out on Gouy’s balance in the temperature range 300 -700 K.

The X-ray pattern showed a single crystalline phase without any trace of impurity indexed as hexagonal magneto plumbite structure [12] . By isomorphism of the M compounds the space group is presumed to be D⁴_6h or P6₃/mmc [13] as shown in [Fig-1] . Reported values of bulk Sr-M hexaferrrites are a=5.88Ã…and c=23.03 Ã… [14] .
As shown in the [Table-1] , the lattice parameter ‘a’ and ‘c’ shows slight variation with substitution of Al⁺³ ions in reference to [Table-1] . This is in agreement with the fact that all hexagonal ferrite exhibits constant lattice parameter ‘a’ variable ‘c’.
It also indicates that change of easy magnetized c-axis is larger than a-axis with Al³⁺ ions substitution [15] . A variation in the values of the lattice parameter is observed because of the fact that atomic radii of Al⁺³ (0.53Ã…) is smaller than Fe⁺³ (0.67 Ã…). The decrease in cell parameter agrees well with the result in BaM system [16-18] . The replacement of Fe⁺³ by Al⁺³ ions have been investigated as resemblance of ionic radii of former with substituent fairly matches. It is further noted that the former ions were very easily replaced at substitution ratio 0≤x≤8 without changing crystal symmetry. The radii of Al⁺³, Cr⁺³ and Fe⁺³ are 0.53 Ã…, 0.64 Ã… and 0.67. Ã… respectively The Al⁺³ is easier than Cr⁺³ to substitute for Fe⁺³ to enter the lattice of strontium ferrite.
The Transmission Electron Micrographs (TEM) gave in [Fig-2a] , [Fig-2b] , [Fig-2c] and [Fig-2d] reveal the remarkable changes in the microstructure, grain size, porosity and homogenous particle size distribution. Therefore it could be attributed that these remarkable changes are observed as a perturbation of many dependant parameters like time of synthesis, temperature, process of annealing, intermittent quenching, grinding, etc. [19-20] . It is also come to the notice that the presence of the large crystalline agglomerations is composed of very small discrete crystallites in the samples at the temperature of synthesis. The particle size is determined with the help of TEM studies [21] . The size is found to be on an average 50nm confirming the nanoparticle size of the hexaferrites.
The crystal structure of magnetoplumbite M-type components (SG: P₆₃/mmc) can be described as superimposition of two structural blocks, namely the R-block with composition (SrFe₆O₁₁)^2-and the S-block with composition (Fe₆O₈)²⁺. The metallic cations are distributed within five different crystallographic sites with octahedral and tetrahedral environment in [Table-2] (Gorter spin colinier model), we have summarized the crystallographic characteristics of the five different sublattices together with the spin alignments corresponding to the collinear magnetic structure proposed by [22] .
For hexaferrites testing magnetic properties is of great importace. The stichometry of the samples is so chosen that desired improvement in coercivity, remanace magnetization, saturation magnetization can be expected.(23,24)The coercivity and remanance magnetization of the synthesized samples is found to be from (5000---375 Oe)and (13.1370-0.1491 emu/gm.) respectively so that they can be used in the application described earlier.The Curie temperature of the samples is found to be between (478-578) The temperature is found to be increasing with the increase in composition.of transition metal ion.(Al⁺³) from (2 to 8). As observed from [Table-3] .
The increase in the Curie temperature (Tc) indicates that some intersublattice exchange interaction are strongly diminished The increase in Curie temperature may be attributed to the presence of nonmagnetic ions in the tetrahedral and octahedral sites. It has already been reported Obradors et.al. [25] that Al³⁺ ions occupy 2a, 12k and 4f₂ sites. Hysteresis loops in [Fig-4] indicates that all the samples exhibit steep rise in magnetization at low applied field followed by a slow variation at high field. The large slope of hysteresis curve of undoped sample i.e. x = 0 at high field indicates unsaturated state and this slope falls with substitution of Al⁺³ ions. The reduction in anisotropy field, discussed later, is responsible for this [Fig-4] shows the variation of all the above mentioned magnetic properties with annealing temperature for the pure strontium hexaferrite nanoparticles. The Saturation magnetization (Ms), Remanence (Mr) and Coercivity (Hc) decreases in room temperature and maximum value for each property is obtained at room temperature of 300 K. This behavior can be explained on the basis that the particle size which was explained by Alamholhoda [26] . It is also well known that particle size has a significant effect on the magnetic properties of materials. The particles smaller than the critical single domain size are mainly in single domain. On the other hand, the particle size becomes bigger than the critical value most of the particles exist in multi-domain.
Al⁺³ ions occupy 2a, 12k and 4f₂ sites, when the sites 2a, 2b, 12k and 4f₂ are occupied by non-magnetic ions the exchange interactions like 2a-12k, 2a-4f₁ and 4f₁-12k decreases. Out of this 4f₁-12k interaction is the strongest and has predominant influence on the magnetic parameters Albanese [27] .
The basic magnetic properties originate from Fe⁺³ ions on both tetrahedral sites and octahedral sites. When Al⁺³ ions are substituted the Fe⁺³ ions decrease from Fe₁₂⁺³ to Fe_12-x⁺³. This reduces the 2a-4f₁ and 4f₁-12k exchange interaction and hence the magnetization.
It has been observed from the [Table-2] that as the concentration of the doping element Al⁺³ increases the Ms and Mr decreases. This is because of agglomerisation of particles or wide particle distribution which changes the magnetic properties of the particles. There is change from single domain to multi domain structure as reported by Huang [28] .
Hc is decreasing as grain size is increasing. To explain the dependence of Hc on grain size by Rezlescu [29] proposed that the coercivity is due to pinning of the magnetization at the grain boundaries. Since surface to volume ratio decreases as the grain size increases smaller grains will have more boundaries to pin in the magnetization. We can say that Al⁺³ substitution causes the change of easy axis of magnetization from ‘c’ axis to basal plane. This fall of coercivity causes the change from hard ferrites to soft ferrites.
The reduction in saturation magnetization occurs due to low magnetic moment of Al⁺³ ions. The magnetic moments of Al⁺³ ions are not able to cancel out with spin down moments of Fe⁺³ ions, thereby decreasing Ms. The magnetization are closely related to distribution of Al⁺³ ions on each crystallographic sites and then magnetic dilution or non collinear structure (spin canting) with the substitution of Fe³⁺ ions by lower magnetic moment ions.
It is known for Sr-M ferrites that the magnetic moment of Fe⁺³ ions arranged collinearly due to existence of super exchange interaction that is to say that it is the superexchange interaction to determine the orientation of magnetic moments of Fe⁺³ ions. When Fe⁺³ is replaced by other divalent or trivalent or tetravalent combination the change in magnetic characters i.e. magnetic intensity occurs, this is called canting of spin structure. In addition to this the substitution causes decrease of Bohr mageton according to M-type structure by Lotgering [30] .

The X-ray diffraction studies confirm the formation of hexaferrites and the a and c values of the sample supports this confirmation. Structural studies have confirmed the space group of samples to be p63/mmc. From TEM of the sample, average particle size is found to be in nanorange (≈50 nm). The reduction of particle size of hexaferrites to nanorange helps to improve many magnetic properties mentioned earlier. The hysteresis curve of the sample at different concentration attributes that such sample specially synthesized by microwave induced sol-gel combustion route can hold the data confirming their use in storage devices [31] . The magnetic characters of the samples are observed to be of immense vital improvisation. It is known that the nanohexa ferrites with such special magnetic properties are highly useful in Data storage applications [32,33] .

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	Fig. 1- Experimental set up (peak reaction).
	Fig. 2- XRD patterns for SrLaAlxFe11-xO19.
	Fig. 3- TEM results for sample SrLaAlxFe11-xO19.
	Fig. 4- Hysteresis Loop for SrLaAlxFe11-xO19.
	Table 1- X-Ray Diffraction Calculations for the samples SrLaAlxFe11-xO19.
	Table 2- Crystallographic sites of Fe ion.
	Table 3- VSM Calculations for SrLaAlxFe11-xO19.