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Influence of La-Mg on Optical-Dielectric and Magnetic Properties of Barium Hexaferrite

Talwinder Kaura, Riti Sethib, A. K. Srivastavaa*

aDepartment of Physics, Lovely Professional University, Phagwara, (Punjab)- 144411, India

b Department of Physics, Jamia Millia Islamia, Central University, New Delhi-110025 (INDIA)

*Email:srivastava_phy@yahoo.co.in

Abstract
In the present study, Ba1-x Mgx LaxFe12-xO19 (x =0. 1, 0.2, 0.3, 0.4 and 0.5) hexaferrites are prepared by sol-gel auto combustion method and subsequent heat treatment from 5000C to 9000C. The crystal structure, grain size, optical properties, dielectric properties, thermal properties and magnetic properties are studied by means of X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet visible near infrared (UV-Vis-NIR), inductance capacitance resistance meter bridge (LCR meter) , thermo gravimetric analysis (TGA)/ differential thermal analysis (DTA)/differential scanning calorimeter (DSC) and vibrating sample magnetometer (VSM). XRD studies indicate that barium hexaferrite still maintained the hexagonal magneto-plumbite phase after substitution of La-Mg and calcination. DTA studies show that the formation of hexaferrite phase takes place nearly at 8000C. VSM analysis reveals an increase in saturation magnetization, coercivity, retentivity and anisotropy constants with increase in substituent.
Keywords: Magnetic materials; Magnetic properties; Microstructure; Optical properties; Dielectric properties; Fourier transform infrared spectroscopy (FTIR).
1.Introduction
M-type hexaferrites have been investigated for high density recording[1], permanent magnets [2], loading coils [3], biosensor devices and medicine such as targeted drug delivery [4], self-biased microwave and millimeter wave devices like filters, isolators and circulators [5,6] future multiferroics [7] and high frequency applications [8] due to its high coercivity (~6700Oe) [9], high curie temperature (~730 K) [10], large uniaxial magnetic anisotropy (~17 kOe) [2], large saturation magnetization (~72 emu/g) [11] and good chemical stability [12].
Barium hexaferrite belongs to magneto-plumbite group of oxides and having hexagonal symmetry with space group P63/mmc. There are four building blocks like S, S*, R and R* in the structure of barium hexaferrite. S and S* have spinal structure with 2 oxygen layers while R and R* consists of three oxygen layers with hexagonal structure [13, 14]. R* and S* blocks are obtained by rotating R and S blocks by 1800 about the c-axis. There are 38 O2-, 2 Ba2+ ions, 24 Fe3+ ions in a unit cell of hexagonal ferrite. Fe3+ ions are situated in trigonal bipyramidal site (2b), tetrahedral site (4f1) and octahedral sites (12 k, 2a and 4f2).
The structure and magnetic properties are highly influenced by the substitution of elements on crystallographic sites and calcination temperature. The substituted element forms a superexchange interaction with oxygen atoms and occupies iron sites that lead to the enhancement in the properties. The magnetic properties of barium hexaferrite can be significantly changed by substituting cations like Pr, La [15], Ga [16], Er [17], La [18,19] and different cationic combinations like La-Na [20], La-Zn [21], La-Co [22], Mg-Sc [23], Mg-Co [24], Mg-Ti [25]. Saturation Magnetization and coercivity can be increased with the substitution and temperature [15,19,26].
A large number of techniques are available in literature to prepare M-type hexaferrite such as sol-gel method [27] co-precipitation method [28], hydrothermal method [29], ball milling method [16], micro wave induced combustion method [30] and mechanical combustion method [31]. Among the various synthesis ways, the sol-gel is the easiest one to incorporate. It is extensively used in literature because it offers lower calcination temperature that inclines to the formation of ultrafine resultant particles attributed to the amalgamation of the chemical solution at molecular level at the start and low cost [13, 32, 33].
Hexaferrite with a high Curie temperature (Tc), high saturation magnetization and varying coercivity are the main requirement for the applications [34, 35, 36].Very fine particles are needed for high density recording media that must have uniform dispersion (for high areal density), chemically and thermally inactive and can be implemented in a very thin layer for memory storage. It is very difficult task to prepare desired material because of thermal effect, particle size control and structural distribution. These need to be tuned according to the applications. In the present research work, we have investigated the structural, optical, dielectric and magnetic properties of La-Mg substituted barium hexaferrites synthesized via sol gel auto combustion method by varying concentration of substituent and calcination temperature.
2.Experimental Method
2.1 Synthesis of La-Mg barium hexaferrites
The stoichiometric amounts of the powders of analytical grade barium nitrate, magnesium nitrate hexa-hydrate, lanthanum nitrate hexa-hydrate and ferric nitrate are dissolved separately in minimum amounts of distilled water (pH=6.5). Citric acid solution is prepared to retain the molar ratio of metal nitrates to citric acid to 1:2 and added in an aqueous mixture. The pH of the resultant solution is adjusted in between 6.5 to 7 by drop wise adding liquor ammonia (25 %) to the solution to facilitate the reaction. The solution is then heated at 65-800C with continuous stirring for about 2 hours with a help of a magnetic stirrer until a viscous residual is formed. The viscous residual is then dried over a hot plate at 280-3000C to form the precursor material. The precursor is then divided into parts and each part is calcined in a muffle furnace for two hours at different temperatures ranging between 5000C to 9000C. Fig.1 shows the flow chart for the whole procedure of synthesis of La-Mg substituted M- type barium hexaferrites.

Fig.1: Flow Chart for the whole procedure of synthesis of La-Mg substituted barium hexaferrite
2.2 Characterization Techniques
The X-ray diffraction measurements are carried out using CuKα radiation (λ=1.54060 Å) with a Pananalytical X-ray diffraction unit (X’Pert Pro) diffractometer in the range 20-800 to identify phase compositions of different samples. The structural bonds identification in the mid infrared region of the as-prepared samples are performed using FTIR spectra obtained from FTIR interferometer IR prestige-21 FTIR (model-8400S). The spectra are collected in the frequency range 400 to 4000 cm-1using KBr palette. For DTA/TGA/DSC plot of the combustion product of the sample, the instrument used is SDT Q600 V20.9 BUILD 20. Experimental conditions are: flowing synthetic air atmosphere (80% N2, 20% O2), in the temperature range 0-10000C, with a heating rate of 100C/min. The microstructure observation of the specimens are performed using FE-SEM (TESCAN MIRA II) at an operating voltage of 15-25 kV and at a scale of 5 µm. Dielectric properties have been studied with LCR meter (Model:6440B).The band gap study has been carried out with UV-Vis-NIR (Model: Varian Carry 5000 at room temperature with 0.2 nm resolution). The magnetic properties of the synthesized samples have been measured with the VSM (PAR-155 Princeton Applied Research, USA) at room temperature.
3.Results and Discussions
3.1.XRD study 

The X-ray diffraction patterns of Ba(1-x)MgxLaxFe12-xO19 (x =0. 1, 0.2, 0.3, 0.4 and 0.5) hexaferrites are shown in Figs. 2 and 3 respectively. The diffraction patterns consist of peaks corresponding to crystallographic planes (006), (008), (107), (114), (110), (200), (108), (203), (205), (206), (217), (220), (304) and (2011) in samples having composition variations (x=0.1 to x = 0.5). Similarly, the peaks in diffraction patterns corresponding to the enlisted crystallographic planes (107), (110), (114), (203), (205), (206), (217), (220), and (2011) are observed in samples having temperature variation. The observed peaks of different substituted samples peak match with the standard pattern for M-type barium hexaferrite (JCPDS 39-1433) which demonstrate the formation of hexagonal structure (space group P63/mmc) in substituted barium hexaferrite [32, 33]. It can be seen that at low temperatures (< 7000C), the formation of hexaferrites does not take place. Corresponding peaks of hexaferrite are absent in XRD patterns obtained at 5000C and 6000C while peaks of the γ-phase of Fe2O3 (tetragonal space group-P) are observed (JCPDS-25-1402). At high temperatures (800-9000C), the intensity of α-phase of Fe2O3 (JCPDS-84-0306) continuously diminishes and a single phase structure is obtained identical to M-type barium hexaferrite. The intensities of diffraction peaks are somewhat different but approximately appear at same positions. Minor changes in theta value may attribute to the substitution. The grain size ‘D’ is calculated using Scherer formula [10] i.e.

Where, λ is the X-ray wavelength and is equal to 1.54060 Å, β is the half peak width in radian, θ is the Bragg’s angle and ⱪ is the shape factor that is equal to 1 for hexagonal system. The range of average grain size lies between 31 nm to 79 nm (tabulated in tables 1 and 2). The lattice constants ‘a’ and ‘c’ are calculated, for prominent peaks (008) and (110) for specimen having composition variation and the peaks (107) and (110) are considered for samples with temperature variation, using the equation [26]:

Where, dhkl is the inter-atomic spacing. It can be seen that the lattice constants ‘a’ and ‘c’ remain nearly constant. It supports the replacement of host elements (Ba+2- Fe+3) by the substituent without deformation of the hexagonal symmetry of the crystal. The volume of the unit cell for hexagonal is calculated using the relation [26] Vcell = 0.8666 a2c and Vcell = a2c for tetragonal, where, ‘a’ and ‘c’ are the lattice constants. The volume of the unit cell does not show a regular pattern and has minor variations as the temperature increases. This may attribute to the substitution of elements having different ionic radii. The unit cell volume of La-Mg substituted barium hexaferrite is less than the M- type barium hexaferrites (~697 Å3) [37]. The possible reason for the shrinkage of crystal lattice is that lanthanum forms strong bond with oxygen and the binding energy of lanthanide oxygen octahedral (RO6) in rare-earth substituted oxide materials is much higher than the transition metal ion-oxygen octahedral (MO6) where ‘R’ stands for rare earth elements and ‘M’ is the transition metal elements . Surface area has been calculated for the prominent peak (114) and (107) for the samples having composition variations and reported in Table 1.
The density of substituted barium hexaferrite has been calculated using the relation [26]:

Where, M is the molecular weight of the compound, Vcell is the volume of the unit cell and NA is the Avogadro’s number. The density is observed to decrease from 5.392 g/cm3 to 5.295g/cm3 with increasing substituent concentration. The possible reason for this is the substituent element contributes to impede the process of densification of the hexaferrite matrix.

Fig.2:XRD patterns of Ba1-xMgxLaxFe12-xO19 ((a) x=0.1(b) x=0.2 (c) x=0.3 (d) x=0.4 and (e)x=0.5) hexaferrite obtained after calcination for 2 hrs at 8000C

Fig.3:XRD patterns of Ba0.5Mg0.5La0.5Fe11.5O19 nano-hexaferrite at (a) T=5000C (b) T=6000C, (c) T=7000C, (d)T= 8000C and (e) T=9000C nano-hexaferrite

Table 1: Brief of d spacing (d in Å), average grain size (D), lattice parameters (‘a’ and ‘c’), volume of cell (Vcell), X-ray density (Dx),surface area, peak intensity (Ihkl), strain and phase present in Ba1-xMgxLaxFe12-xO19 (x=0.1 to 0.5)

Table 2: Calcination temperature (T), average grain size (D in nm), lattice constants (‘a’ and ‘c’) and volume of cell (Vcell) of Ba0.5Mg0.5La0.5Fe11.5O19



Strain has been calculated from relation [13]:

Thus, strain has been induced due to substitution which effects the magnetic properties of La-Mg substituted barium hexaferrites. Increase in peak intensity reveals the advancement in crystallinity and broadening means size has been affected.

3.2.FTIR Study

The infrared absorption spectra contribute to the information of sample synthesis during the thermal treatment period i.e. about the attained intermediates of the reaction period and present molecular bonds. Figs. 4 and 5 show the FTIR spectra of the synthesized samples (x=0.0 - 0.5, T=5000C - 9000C) in range 400 to 4000 cm-1. The spectra show broad absorption peaks in the range 3200-3700 cm-1 and 1600 cm-1 which are assigned to the hydroxyl (O-H) and carboxyl groups (COOH) of the citric acid respectively. The as-burnt powders show some bands below 600-550 cm-1 (at 580 and 424 cm-1) which assign to magnetite, stretching vibration of Fe-O at tetrahedral and octahedral site confirms the synthesis of hexaferrite. The band at 1384 cm-1 affirms the presence of nitrate ion appears in samples. The absorption edge below 600 cm-1 shows some shift in the peak position above 7000C (Fig. 4). This change attributes to the occupying site of the substituted cations. The spectra for x > 0.1 shows less transmittance area with relatively more prominent peaks. This shows that the cations has been substituted at the ferric sites accompanying strong interaction. The broad but small peak near 1650 cm-1 indicates the presence of water molecule because of moisture absorbed by samples.

Fig. 4: FTIR spectra of samples Ba0.5Mg0.5La0.5Fe11.5O19 calcined for 2 hrs. at (a) T=5000C, (b) T=6000C, (c) T=7000C, (d) T=8000C and (e) T=9000C

Fig. 5: FTIR spectra of samples BaxMg1-xLaxFe12-xO19 calcined at 8000C for 2 hrs. (a) x=0.1, (b) x=0.2, (c) x=0.3, (d) x=0.4 and (e) x=0.5

3.3.TGA/DTA/DSC Analysis

The DSC curve (Fig. 6) demonstrates an endothermic peak at 288.460C which is assignable to the evaporation of water molecules. The succeeding endothermic peak at 405.900C occurs mainly as a result of decomposition of organic matter [38].The exothermal peak in the DTA curve (Fig. 6) at 292.380C, accompanied by a drastic mass loss is as a consequence of autocatalytic oxidation reduction reaction between the metal nitrates and citric acid. Citric acid reacts as chelating agent as well as fuel. The exothermic peak having maxima at 419.450C is accompanied only by a small weight loss. This weight loss is the outcome of the decomposition of the remaining organic matter and also arises due to the decomposition of metal carbonates formed during the reaction [39].Small exothermic peak shoulder in the DTA curve at about 8000C subsequent to which the mass loss becomes nearly steady, confirming the formation of hexaferrite phase. The same confirmation has been given by XRD pattern of the same sample at 8000C.There exists only very small residual amounts of α-Fe2O3. Therefore, it can be concluded that the formation temperature of the hexaferrite phase is 8000C. Also, from the TGA curve (Fig. 6), the reaction interval can be calculated as:

Reaction Interval= Tf -Ti = (967.50-292.98)0C=674.520C

Fig. 6: TGA/DTA/DSC curves for Ba0.6Mg0.4La0.4Fe11.6O19 hexaferrite

3.4. FE-SEM Analysis

The FE-SEM micrographs are shown in the figures 7 (a) and (b). It is evident from the figures that the crystal structure of the synthesized hexaferrites is uniform with a very small variation in the grain size.

Fig. 7: FE-SEM micrographs of (a) Ba0.5Mg0.5La0.5Fe11.5O19 hexaferrite calcined at 8000C at (a) 15kV and (b) 25kV magnification

3.5.Dielectric properties

The dielectric property has been studied with LCR meter in the range (20 Hz-3 MHz). The dielectric properties help to understand electrical behavior of the charge carriers. Dielectric constant has been calculated by using well known parallel plate capacitor equation [40]:

Where K is dielectric constant, C is capacitance of the palette, d is thickness of sample palette and A is cross section area and ε0 is the permittivity of free space (8.85 X 10-12 F/m).Substituted hexaferrite shows a high dielectric constant for low frequency. Maxwell-Wagner type interfacial polarization [41] is responsible for high dielectric constant at low frequency which can be understood using Koop’s phenomological theory [42]. Maxwell and Wagner suggested that inhomogeneous polycrystalline structure has space charge polarization. The inhomogeneity arises due to porosity and grain boundaries in the crystal. The mechanism of the polarization in ferrites is similar to the conduction process.

Interfacial polarization, Fe2+ ions, grain boundaries and oxygen vacancies, etc. are the main factors for large values of dielectric constant at lower frequencies [40]. The Fe2+ ions participate in the electron exchange interaction between Fe2+ and Fe3+ ions and these Fe2+ ions introduce a polarization [41]. Polarization takes place due to local displacement of electrons (produced due to exchange between Fe2+ions and Fe3+ ions) in accordance with applied electric. Ferrites nanoparticles consist of a large number of perfectly conducting grains separated by less conducting grain boundaries. These boundaries appear during the heat treatment process. Due to insulating nature, these grain boundaries do not allow the electrons to flow [43]. The non-uniform distribution of oxygen ion which possesses two loosely bound electrons is responsible for interfacial polarizations at interfaces of grains and grain boundaries. Due to loosely bound electrons, only small electric field is required for polarization. This may be the reason for the observed high values of dielectric [44].

At low frequencies, the large value of dielectric constant can be explained using grain boundary contributions. But the dielectric constant decreases with increase in frequency. Possible reason for this is the lag of hopping response of electrons between Fe2+ and Fe3+ ions against the frequency of the external applied field beyond a certain limit of the external field, dielectric constant becomes independent of applied frequency.

With the increase in temperature and introduction of La-Mg, the strain has been induced (affirm by XRD result) that cause change in structure and enhancement in polarity and density. The resistivity increases at high frequency and due to non-conducting grain boundaries, electrical characteristics reduces. It has been observed that the dielectric constant for T= 9000C is low as compared to 8000C which may be due to decrease in grain boundaries decrease at T= 9000C that produces less resistance and less polarization. The dielectric constant shows a large variation in a small region of frequency (Hz). Above certain region, it shows a static behavior which means ferrites characteristics become independent from frequency variation. The properties like high dielectric constant at room temperature for low frequency can be used for microwave applications [13].

Fig. 8: Variation of dielectric constant with frequency for Ba0.5Mg 0.5 La0.5 Fe 11.5 O 19 at (a) T=6000C and (b) T= 8000C and (c) T= 9000C.

3.6.Optical characteristics of La-Mg substituted BaM particles

The energy band gap of La-Mg substituted barium hexaferrite has been calculated from UV–Vis-NIR absorption spectra. When electromagnetic waves falls on the material, electrons in the valence band absorb the energy and raised its energy level. The energy band gap has been estimated from the spectrum that is based on the following relation [45]:

Where A is a constant and E is defined as the energy band gap. From this relation, the band gap has been obtained from the linear plot of the fundamental absorbance edges. Figure 9 shows the energy band gap spectra and inset shows the absorbance spectra. The band gap values interpreted from graph are: 3.92 eV (x=0 at 9000C), 4 eV (x=0.5 at 9000C), 4.13 eV (x=0.5 at 8000C), 4.24 eV (x=0.5 at 6000C). It is observed that with increase in temperature the band gap decreases. Band gap varies as La-Mg is introduced in hexaferrite. Without substituted sample shows low value of band gap than substituted sample. Obtained band gap is higher than barium hexaferrite (3.18 eV) and barium hexaferrite thin films (2.32eV) [45, 46]. The presence of strong bond between substituted and host cations and quantum confinement at nano scale are responsible for the increase of the band gap. From UV-NIR absorption spectra (inset), it has been concluded that the absorption region for hexaferrites is ~ 214-570 nm.

Fig. 9: Energy band gap (Eg) for Bax Mg1-x Lax Fe12-x O19 (a) x=0 at T=9000C, (b) x=0.5 at T= 9000C, (c) x=0.5 at T= 8000C, (d) x=0.5 at T= 6000C , (Inset) - UV Vis-NIR absorption spectra for (a) x=0 at T=9000C, (b) x=0.5 at T= 9000C, (c) x=0.5 at T= 8000C, (d) x=0.5 at T= 6000C

3.7.VSM study

Magnetic properties of the hexaferrite materials have been studied by plotting hysteresis curves, as illustrated in the Figs.10 and 11.The saturation magnetization (Ms) depends upon the calcination temperature as well as on the amount of substituent. In case of variation of substituted cations concentration, Ms increases as crystallite size decreases even the iron content is decreasing. As calcination temperature increases from 6000C to 9000C, Ms increases as crystallite size increases. This may ascribed to the increment in the number of domains which increases magnetization. According to XRD study, there is a phase change with calcination temperature which may contribute to the increase of Ms value. Magnetic parameters have been affected by various complexities like ionic radii, cationic distribution, lattice distortion, strain, canting angle, valency of magnetic ion. Rare earth elements cause the valence state of Fe to be changed from Fe3+ to Fe2+. La3+ occupies the Fe3+ site and shifts the Fe3+ ion to the octahedral site and changes the valence state of the nearest neighbor to Fe2+ ions that cause the saturation magnetization to be increased. Saturation magnetization is an intrinsic property, independent of particle size but depends on temperature and ordering of magnetic moments whether grain size increases or decreases and the alignment of the atomic spins in the direction of magnetic field is getting better. In case of composition variation (Table 1), the increase in surface area and decrease in size causes more magnetic moments per unit volume.
There is increase in the coercive field strength (Hc) with increase in substitution concentration. This suggests an increase in magneto crystalline anisotropy constant which can be calculated using the relation[47],


Where µ0 is the universal constant of permeability in free space and is equal to 4π x 10-7 H/m. These results indicate that substituted elements enter into hexaferrite solution to inhibit crystallite growth mechanism and enhance magneto crystalline anisotropy constant. Hc is greatly enhanced with La-Mg substitution due to increase in anisotropy constant. The increase in Hc may also attribute to the super exchange interaction between the ions that opposes the reverse magnetic field. The property like high coercivity can be used in rewritable recording devices which require a high coercivity value above 1200 Oe [48]. Prepared hexaferrite samples show coercivity 2500 Oe (at x = 0.3) and 3750 Oe (at T = 9000C). So, the prepared hexaferrite is appropriate for rewritable perpendicular recording media. The value of Mr /Ms for our samples are not close to 0.5, indicating that powders of single magnetic domains were not produced [49]. This shows the presence of impurities in the barium ferrite powder.

TABLE 3:Saturation magnetization (Ms), retentivity (Mr), coercivity (Hc) and magneto crystalline anisotropy constant (K) of Ba0.9Mg0.1La0.1Fe11.9O19 (A) and Ba0.7 Mg0.3 La0.3 Fe11.7 O19 (B)

S. No. x Ms (emu/g) Mr (emu/g) Hc(Oe) K(HA2/kg) Mr/Ms
A 0.1 12.5 5 625 0.3925 0.39
B 0.3 48 38 2500 6.0288 0.79

The energy barrier (EA) for rotation of magnetization orientation in a single domain particle can be calculated by [40] the relation EA=KV sin2θ, where V is the volume of the nanoparticle and θ is the angle between an applied field and the easy axis of the nanoparticle. If magnetization direction remains same, energy barrier (EA) will be proportional to product of KV. By using the value of crystal volume from tables 3 and 4, it can be calculated that the order of energy barrier for samples should be EA(B) > EA(A) and EA(D) > EA(C). Low anisotropy of a material means low activation energy barrier and a lower applied field is required for spin reversal, that is, a lower coercivity.

TABLE 4:Saturation magnetization (Ms), retentivity (Mr), coercivity (Hc) and magneto crystalline anisotropy constant (K) and squareness ratio of Ba0.5 Mg0.5 La0.5 Fe11.5 O19 calcined at 6000C and 9000C.

Sample (T)(0C) Ms (emu/g) Mr (emu/g) Hc(Oe) K(HA2/kg) Mr/Ms
C 600 27 10 312.5 0.4239 0.37
D 900 50 30 3750 9.4200 0.60

Fig. 10: Magnetic hysteresis loop of nanoferrite Ba0.9 Mg0.1 La0.1 Fe11.9O19 and Ba0.7Mg0.3 La0.3 Fe11.7 O19 calcined at 8000C for 2 hrs

Fig. 11: Magnetic hysteresis loop of Ba0.5 Mg0.5 La0.5 Fe11.5 O19 nanoferrite calcined at 6000C and 9000C for 2 hrs.

Conclusions
The formation of hexagonal structure takes place as per XRD study.The morphology of the surface of the as-prepared samples is homogenous. From thermal analysis techniques, it was observed that the hexaferrite phase forms at a temperature of 8000C. VSM results show that saturation magnetization, retentivity, coercivity and anisotropy constants increased with increase in substituent concentration and temperature. Samples show normal ferrite dielectric behavior. UV –Vis-NIR reveals that the band gap of as prepared samples increases with substitution.
Acknowledgements
We are thankful to Panjab University Chandigarh (SAIF) for providing XRD facility, IIT Roorkee (SAIF) for FE-SEM and VSM facilities, Guru Nanak Dev University for TGA/DTA/DSC facility and UV-Vis-NIR for SAIF-STIC-Kochi (Ernakulam).
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