A Study on Synthesis, Structural and Magnetic Properties of La 2 O 3 Doped Isotropic Ba-M Ferrites

Present work reports the synthesis, structural and magnetic properties of BaFe 11 O 17.5 and La 2 O 3 doped (Ba La 0.05 Fe 11 O 17.575 , BaLa 0.1 Fe 11 O 17.65 , BaLa 0.2 Fe 11 O 17.80 and BaLa 0.3 Fe 11 O 17.95 ) ferrites produced through powder metallurgy method with escalated magnetic properties. Phase evaluation, crystal structure, microstructure, and magnetic properties thermomagnetic characteristics for the La doped and La free ferrites have been investigated at varying sintered temperatures. XRD studies revealed that BaFe 12 O 19 phase is formed at 900 ̊C through the reaction of Fe 2 O 3 and BaFe 2 O 4 . Electron microscopy studies elucidated that microstructural features depend critically on the heat treatment conditions as well as La content. La addition up to 1.0 mole found to be beneficial to refine particle size close to single domain wall size, which, in turn, led to enhance magnetic properties of the ferrite. Optimum magnetic properties for the La free BaFe 11 O 17.5 are H c =4.4kOe, B r =1.9kG, and (BH) max =0.85MGOe which were improved to H c =4.9kOe, B r =2.4kG and (BH) max =1.2MGOe with La 2 O 3 additive. The La 2 O 3 acts as a grain growth inhibitor and led to enhance high coercivity of 4.9kOe in the ferrite. The dielectric constant and dielectric loss factor decrease monotonically with increasing applied frequency at the constant temperature. Permeability is found to be sensitive to density and microstructural components like grain size and porosity of the sintered ferrites.


Copyright © Zubair Ahmad
RDMS.000825. 13 (5).2020 development and magnetic properties of BaFe 12 O 19 . Ding et al. [10] prepared barium ferrite with high coercivity, 5.6kOe, by coprecipitation technique. Sozeri et al. [11] studied the effect of La doping on the magnetic properties of Ba 1-x La x Fe 12 O 19 prepared by ammonium-citrate sol-gel method and attained H c of 5.73kOe. Studies have shown that ferrite prepared with La 3+ or Pr 3+ doping exhibit higher H c than simple Ba-M ferrite magnets due to higher magneto-crystalline anisotropy [12]. Ghzaiel et al. [13] invoked the effect of Cr 3+ , Mn 3+ , Al 3+ and Bi 3+ on the magnetic properties of hydrothermally synthesized BaFe 11 O 19 and found that Mn 3+ doped magnet has higher M s (61.1emu/g) than that of Cr 3+ (60.3emu/g). Mn doped magnets have attained optimum H c value of 4.2kOe and (BH) max of 0.94MGOe. Yang et al. [14] synthesized Sr 1-x La x Fe 12-x Cu x O 19 magnets by ceramic process and found that B r and (BH) max increased by La doping up to x=0.2 moles. However, many scientific issues related to microstructure and magnetic properties remain unsolved. The present work focuses the development of isotropic barium hexaferrite by the powder metallurgy technique with improved microstructure and magnetic properties. The effects of La content on microstructure and magnetic properties were investigated with varying annealing and sintering temperatures and their results are presented and discussed.

Materials and Methods
Stoichiometric precursor of BaCO 3 99% Alfa Aesar, Fe 2 O 3 99% Merck and La 2 O 3 99% Alfa Aesar in double distilled H 2 O was sealed in 500ml stainless steel jars and homogenized using planetary ball milling for 6 hrs. Pre-milled powders were sieved through 60 mesh stainless steel screen and compacted in cylinders of 13mm in diameter and 12mm in length using stainless steel die. Cylindrical samples of BaFe 11 O 17.5 were calcined at 800 ºC, 850 ºC, 900 ºC and 950 ºC temperatures for 2.5 hours with respect to CO 2 loss. Likewise, La-substituted Ba-M ferrites were calcined at 900 ºC for 2.5 hours. Calcined samples were ground in pistol and mortar, sieved through 60 mesh screens and milled for 8 hours to promote homogeneity. The aqueous PVA (72k, Merck) binder was used to facilitate high pressure compaction and cracked free sintering. For sintering, cylindrical samples of 13mm in diameters and 12mm in length were compacted at 15MPa pressure to obtain green density of 3.1g/cm 3 . Green compacts of La-0.0 (BaFe 11 O 17.5 ) and La substituted Ba-M ferrites were placed on zirconia plate inside a box furnace with debinding rate of 2 ºC/min at 280 ºC for 1.5hrs followed by sintering at 1050 ºC, 1100 ºC, 1150 ºC and 1200 ºC for 1.5hrs at a heating rate of 5 ºC/min and then cooled to room temperature at the natural cooling rate. Dimensional mass densities were measured using standard digital caliper and precise digital weight balance. Crystal structure and magnetic phases were determined by the X-ray diffraction (Siemen D500-XRD) using Cu kα radiation source in the 2θ scan range of 20º to 90º with 0.05º scanning rate. Phases were identified by comparing the peak positions (2θ) and their corresponding intensities (I) with the standard powder diffraction files (PDF) using the Hanawalt method [15]. The observed phase peaks were indexed with standard JCPD cards (JCPD No. 84-757).
The mean crystallite sizes were determined from the X-ray linebroadening analysis using the Deby Scherrer's equation [16] after appropriate correction of instrumental broadening.
Phase compositions and microstructure of ferrites were investigated through field emission scanning electron microscopy (FESEM Quanta FEG 450) equipped with energy dispersive spectrometer (EDS). Magnetic properties were measured by plotting hysteresis loops with pulsed field magnetometer (Metis 40kJ-PFM) using 3T applied field at room temperature. Dielectric properties were measured using LCR meter (7600 Quadtech) at room temperature in the frequency range of 100Hz to 10kHz. The permeability was measured with 3255B Precision Magnetic Analyzer in the frequency regime of 10GHz to 120GHz at room temperature using conventional measurement method.

Calcination temperature and weight loss
The calcination decomposes BaCO 3 to CO 2(g) Figure 1a). Thus, calcination temperature for the ferrite samples was settled as 900 ºC. The calcined samples were subjected to sintering. Figure 1b illustrates XRD patterns for the samples at varying sintering temperature. It seems that Bragg's peak positions and intensities are almost same for all the sintered samples ranging from 1050 to 1200 ºC. The lattice parameters and crystallite sizes for sintered La-0.0 and La doped samples are listed in Table 1. It seems that crystallite size for the La free (La-0.0) sample is higher than La doped samples. The La-0.05 and La-0.1 samples have 16% smaller crystallite sizes compare to La-0.0 sample while La-0.2 and La-0.3 have 7% lower crystallite sizes than La-0.0 sample. The c/a ratio seems to decrease from 3.98 to 3.94 with increasing the La 3+ doping.    Figure 2c. In order to attain homogeneity, fine particle size and accelerated solid state diffusion process, precursor powders were subjected to ball milling for 6hrs prior to calcination. Figure 3a-3d represent FESEM images for La-0.0 samples sintered at 1050 ºC, 1100 ºC, 1150 ºC and 1200 ºC for 1.5 hours. Samples sintered at 1050 ºC shows a highly porous, sharp edge grains ranging in size from 0.9±0.3µm with lower density of 4g/cm 3 . A well define sharp edge containing hexagonal grains ranging from 0.7±0.2µm in size are produced at 1100 ºC where a density increments up to 15% was noticed compared to 1050 ºC treated samples. Furthermore, grains are found to be inter-link with neighboring grains that, in turn, enhance the grain coherency and stacking density due to sintering at 1100 ºC. Sintering at 1150 ºC led to enlarge grain sizes whereby hexagonal grains accompanied with rods are formed which had attained mean grain size of 1.7±0.2µm with 4.83g/cm 3 density (Figure 3c). At 1200 ºC, an excessive grain growth causes grains welding. Thus, optimum sintering temperature for ferrites samples were selected as 1100 ºC by the experimentation.  FESEM images for La doped ferrites sintered at 1100 °C for 1.5hrs are illustrated in Figure 4. It is evident that La-0.05 microstructure is composed of hexagonal grains around 1.0±0.2µm in size with less porosity. The grain morphology of La-0.10 samples resemble with La-0.05 samples but with a density of 4.5g/cm 3 . However, the severity of inter-grain connectivity and degree of grain stacking density is found to be larger for the La-0.1 compared

RDMS.000825. 13(5).2020
Changes in magnetic properties versus sintering temperature for La-0.0 sample are presented in Figure 5a. It shows that magnetic properties are sensitive to the heat treatment conditions. A small variation from the proper heat treatment conditions may form non ideal microstructure leading to decay in the magnetic parameters. An example, La-0.0 ferrite display a high H c of 5.0kOe sintered at 1050 ºC, whereas the ferrite sintered at 1100 ºC exhibits a high B r of 1.9kG. The high H c is related to fine grain size while high B r is ascribed from the positive dipole interactions, high grain stacking density as well as optimal density of ferrite. The maximum magnetic properties such as H c =4.4kOe, B r =1.9kG, B s =2.2kG and (BH) max =0.85MGOe are obtained for the La-0.0 samples sintered at 1100 ºC as confirmed by plotting the hysteresis loop depicted in Figure 5b. The optimum sintering temperature for La doped samples were selected as 1100 ºC by the experimentations. Figure 6a & 6b illustrates changes in magnetic properties and demagnetization curves as a function of La content sintered at 1100 °C. It shows that both H c and B r increases monotonically for La-0.1 and then decreases sharply for the La-0.2 and La-0.3 samples. The best magnetic properties in the series of La doped ferrites were obtained with La-0.1 doped ferrite as H c =4.9kOe, B r =2.4kG, B s =2.26kG and (BH) max =1.2MGOe. These magnetic properties are better in comparison with previously reported isotropic bulk and nanosize barium or strontium hexa-ferrite materials [17,18]. These sound properties in present study are ascribed by the fine grain size, homogenous phase grains distribution, high magnetocrystalline anisotropy, less porosity, positive dipole interactions and morphology of hexa-ferrite BaFe 12 O 19 phase.

Dielectric properties
The dielectric constant for La doped samples were measured as a function of applied frequency ranging 100Hz to 10kHz and defined by the equation [19,20]: Where, Ԑ is dielectric constant, Ԑ o is permittivity of the free space, C is the capacitance (farads), A = area and H = height of the cylindrical disks. decreased to 99% for the La-0.2 sample while it was around 84% for La-0.3 doped samples. The decreasing trend of dielectric constant and dielectric loss with the increasing applied frequency can be well explained by the Maxwell model [21,22]. According to the model, the space charge carriers need a finite time to line up their axes in the direction of an applied field. As the applied frequency increases, the electrons turn around their direction of motion more frequently. This causes to decrease the arrival of electrons at the nearby grain boundary and as a result net polarization in the hard ferrite decreases. This decreasing polarization causes the dielectric constant to fall.

Magnetic permeability and susceptibility
The permeability is influenced by the ferrite composition, density, phase structure. The magnetization process which contributes to the permeability of ferrites are known to be spin rotation, domain wall bulging and domain wall displacement [23]. Figure 8a & 8b shows the real and imaginary parts of permeability for La doped ferrites. It shows that initial permeability increases with La 2 O 3 doping from 0.05 to 1.0 mole percent and then decline with further La 2 O 3 doping. In general, 1.0 mole percent La 2 O 3 doped ferrite has a sharp peak spectra compared to all La 2 O 3 doped ferrites for both real and imaginary parts of the permeability. The increase in permeability at low frequency is related to the contribution of both domain wall bulging and domain wall displacement in accordance with the reported ferrites [24,25]. It is noticed that the La 2 O 3 doped ferrites show a constant permeability behavior over the frequency range 55 to 100GHz. This constant permeability trend suggests compositional stability and equality of fabricated ferrites and known to be the zone of utility of the ferrite. The stability of ferrite is considered as a key parameter for certain applications, such as video recording and broad band pulse transformation, etc. The imaginary part of the permeability curves indicates a peak shift in the frequency range 35 to 55GHz for the ferrites (Figure 8b). This peak shift is related to the intrinsic properties of the grains as well as porosity present in the microstructure. Thus, it can be said that the sintered La 2 O 3 doped ferrites have attained maximum density and fine grain size, which, in turn, promoted the permeability of ferrites through the domain wall bulging and domain wall displacement mechanism. In general, the permeability of a ferrite influences by the compositions, process parameters, grain morphology and the nature of phases present in the microstructure. Magnetic susceptibility is an important magnetic parameter to judge the magnetic stability, Curie temperature and phase transformation of the ferrites. It is established that magnetic susceptibility is affected by the grain size, shape, orientation, distribution of grains of ferrites. Figure 8b depicts [26]. The low Curie temperature in studied ferrites is suspected to the difference in composition and processing method adopted to prepare ferrites.
Thus, based on XRD, SEM, magnetometery, thermomagnetic analysis and dielectric experimental results, it can be said that magnetic properties in isotropic Ba-ferrite depend critically on the microstructural features which become ideal or non-ideal based on the choice of heat treatment temperature and or La content. Magnetic properties owe its origin from the fine grain size, homogenous phase distribution and phase coherency of BaFe 12 O 19 phase present in the microstructure. High coercivity (4.9kOe) originates from the reduction in grain size while high B r from the positive dipole interactions, high grain stacking density as well as density of ferrite. It is worth to mention that magnetic properties in La-0.1 doped samples are better than the reported ferrites produced by electro-spun [27] co-precipitation [28] and powder metallurgy techniques [29]. The magnetic properties in the present alloy can be further enhanced by adding Co content. Isotropic ferrites have similar properties in all directions and can be easily magnetized even at low applied magnetic fields. Moreover, isotropic magnets can be developed with economical process using less cost raw materials which may find their applications in telecommunication and microwave devices. The research work on the development of La doped anisotropic ferrite permanent magnets is in progress and their results will be published in future publication. Research on the development of La doped anisotropic ferrite permanent magnets is in progress and their results will be published in future publication.

Conclusion
Isotropic La-doped ferrites have been synthesized through powder metallurgy technique. Magnetic properties are affected by the alloy constituents elements and sintering or annealing parameters, which in turn, alter the resultant magnet microstructure. Optimum magnetic properties such as H c =4.9kOe, B r =2.4kG and (BH) max =1.2MGOe were obtained with La-0.1 added magnets. Optimum magnet reveals 98% decrease for dielectric constant at 10kHz. High coercivity (4.9kOe) originates from the reduction in grain size while high B r stems from the positive dipole interactions, high grain stacking density as well as density of ferrite. Magnetic properties in present alloy can be further enhanced by employing powder compaction in magnetic field, cold isostatic pressing and spark plasma sintering techniques.