Effect of microstructural evolution from nano to micron grain size regime towards structural, magnetic, electrical and microwave properties of gadolinium iron garnet (Gd₃Fe₅O₁₂)

The influence of microstructural changes from nano to micron grain size regime towards their structural, magnetic, electrical and microwave properties of gadolinium iron garnet (Gd₃Fe₅O₁₂) has been investigated systematically in this research work. Raw materials were milled via high-energy ball milling (HEBM) followed by subsequent sintering (600–1400 °C) process. X-ray diffraction (XRD) analysis had shown that single phase with garnet structure and highest crystallinity of Gd₃Fe₅O₁₂ was formed at 1000 °C. The B–H hysteresis loop unveil the evolution development of magnetic behaviour (paramagnetism–ferrimagnetism) in samples thru variation of different sintering temperature. The values of linewidth (∆H) can be grouped into two groups which are; sample increased from 600 to 1000 °C due to shape and strain induced anisotropy while decreasing from 1100 to 1400 °C influenced by magnetocrystalline anisotropy with the increment of the sintering temperature.

The gadolinium iron garnet (Gd₃Fe₅O₁₂) has been extensively explored due to its technological significance, particularly regarding microwave applications. Gd₃Fe₅O₁₂ has a cubic crystal structure with space group of Ia3d. Its unit cell is composed of eight formula units of {R₃}[Fe₂](Fe₃)O₁₂ molecules consisting of 160 ions on specific lattice positions, i.e., 96 O²⁻ ions on h sites, and three groups of trivalent metal ions distributed on 24 dodecahedral c sites {R³⁺}, 16 octahedral a sites [Fe³⁺] and 24 tetrahedral d sites (Fe³⁺), respectively [1,2,3,4]. Rare earth metal ions (R³⁺) cannot reside in either tetrahedral (d sites) or octahedral (a sites) site due to its large ionic radius, causing dodecahedral (c sites) which have larger space to form (Fig. 1). At the tetrahedral and octahedral sites, the exchange interactions between Fe³⁺ ions facilitate antiparallel alignment of the Fe³⁺ ion moments, with a net magnetic moment antiparallel to that of the rare earth ions on the c sites. Due to their high relative permittivity (ε_r), high electrical resistivity (\(\rho\)), medium saturation magnetization (M_s), low coercivity (H_c) and low dielectric loss (tan \(\delta\)), rare earth iron garnet (REIG) are promising candidates for use in high-performance microwaves and electrochemical devices [5,6,7,8,9]. There are various methods of synthesizing REIG such as sol–gel [1], chemical co-precipitation [2], solid state reaction [3], microemulsion [4] and hydrothermal [10] methods in order to produce a material with different properties depending on the parameters controlled during the process. The mechanical alloying technique is employed in this research work due to higher yield of nanoparticles can be produced apart from less consumption of time. The research question is: What would be happen during the evolutions of morphology phase through various sintering conditions with the physical properties before the final formation of the microstructure? Several research investigations on the characteristics of Gd₃Fe₅O₁₂, such as their structure, microstructure, magnetic, electric, doping effect and microwave properties have been reported in the literature [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Nevertheless, in the literature, to the best of our knowledge, there is no article/report on the effect of various sintering temperatures towards microstructural and physical properties evolution in Gd₃Fe₅O₁₂. Hence, this research work attempt to observe the effect of nano to micron grain size regime by increasing the sintering temperature towards the magnetic, electrical and microwave properties of Gd₃Fe₅O₁₂ along its microstructural changes.

Fragment of the structure of Gd₃Fe₅O₁₂ [28]

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Materials and methods

The raw materials used were gadolinium(III) oxide, Gd₂O₃ (Alfa Aesar; 99.99%) and iron oxide, α-Fe₂O₃ (Alfa Aesar; 99%) in which later were weighed and ground according to the stoichiometry formula as in Eq. 1:

The mixtures were mixed for 3 h using SPEX8000D high-energy ball milling (HEBM) machine. The particle size of milled powder was characterized using LEO 912AB Energy Filter Transmission Electron Microscope (TEM). The milled powder was then granulated with 1 wt% polyvinyl alcohol, PVA (binder) and zinc stearate (lubricant). Next, the granulated powders were pressed into toroidal (22.1 × 13.7 × 6.4 mm) and pellet (17.1 × 6.5 mm) shape under a pressure of 3 t. The samples were sintered in an ambient air environment for 10 h from 600 to 1400 °C with a constant increment of 100 °C. The phase formation of Gd₃Fe₅O₁₂ samples were carried out using X-ray diffraction (Phillips Expert Pro PW3040) with Cu-Kα radiation (λ = 1.54060 Å) in the range of 20°–80°. The microstructures (grain size) of samples were observed using Field emission scanning electron microscopy, FESEM (FEI Nova NanoSEM 230) machine. The micrographs allowed grain-size distributions to be determined by the grain-boundary intercept method, involving more than 200 grains per sample using software called J-Image. The proportion of the grain area (P_i) was calculated using the following Eq. 2 [29,30,31,32]:

where n_i is the number of grains with a given size, S is the total area of all grains in the analyzed image and d_i is the equivalent disk diameter of a given grain. The average grain size of Gd₃Fe₅O₁₂ was determined as the value corresponding to the maximum of the Gaussian function describing the size distribution plots.

Sample in toroidal form was coiled by using both copper wire and wrapped with 90 turns for primary coil (N1) and 50 turns for secondary coil (N2). Then, the B–H hysteresis loop characteristics of toroidal sample wired-wound were investigated via MATS-2010SD Static Hysteresisgraph analyzer under applied magnetic fields of 0–100 Oe. The analyzer is connected to a computer and software used to acquire and record the data (Fig. 2). In order to acquire the value needed from the measurement, electrical resistance was first measured using Keithley 6485 picoammeter at room temperature (Fig. 3). Each pellet sample was coated with silver paint to obtain good ohmic contacts with the parallel probe plate. Current, I values were obtained from the instrument. By getting the value of resistance from Ohm’s law equation, the resistivity was calculated by means of the following Eq. 3:

where A is the cross section area of sample, R is the resistance calculated from measured current and \(l\) is sample thickness.

Schematic diagram of B–H hysteresis loop measurement

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Schematic diagram of resistivity measurement

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Microwave absorbing properties (linewidth broadening, ∆H) of the prepared samples were performed using Vector Network Analyzer, VNA (PNA N5227A) in the range frequency 4–8 GHz (C-band) as shown in Fig. 4c. The spherical sample was prepared from the toroidal sintered sample where the sample was cut into cubic shape with dimension of ~ 3 mm. The cubic sample were shaped into spherical sample using air-driven mill as shown in Fig. 4a where air pressure would be inserted through the small hole of the air-driven mill. The sample would move in circular motion until it became spherical shape with diameter of ~ 2 mm. The spherical sample was stuck onto the end of glass rod which then was placed into the cylindrical cavity (Fig. 4b). The microwave field would be passing from port 1 to port 2 through waveguide that were attached to both sides of cylindrical cavity as shown in Fig. 4c. The angular frequency bandwidth, Δ\(\omega\) was obtained from the transmission coefficient (S₂₁) data and calculated using Eqs. 9, 10 and 11. The Δ\(\omega\) value would be used to calculate the linewidth broadening, ΔH value using the Eq. 12. The line in Fig. 5 was formed from spin motion of magnetic moment after being applied with electromagnetic wave and the peak would only appear if the frequency of the wave is matched with the frequency of the material, causing the spin magnetic moment to resonate.

a Set up of making spherical sample and spherical sample after the process, b placement of spherical sample inside the cylindrical cavity and c Vector Network Analyzer (VNA) with set up of C-band measurement

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Data display of transmission coefficient (S₂₁) as a function of frequency obtained from Vector Network Analyzer (VNA)

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Particle size analysis

Figure 6a show TEM micrographs of Gd₃Fe₅O₁₂ powder after milling process. The particles are highly agglomerated due to the natural consequence from milling with very high energy imparted from the ball to the powder that causes high surface area of particles to be produced [33]. In addition, there are inter-friction forces of the rough surface among the particles which also contributed from milling process [34]. Since the nanoparticle that have large surface to volume ratio which is < 1 µm leads to the Van der Waals interaction also plays an important role in agglomeration of particle [35]. It can also be observed that the particles are prone to be in spherical-like shape due to the tendency to minimize surface tension, especially for materials with cubic crystal structure [36]. We can observe that there is variation in particle size in the range of 11.5–87.5 nm with average particle size of 36.9 nm which contributed to wide particle size distribution as shown in histogram in Fig. 6b. This is owing to the inconsistency of high energy between balls and powder [3, 13]. This high energy impact leads to abundance of lattice imperfection due to accumulation of lattice defects [37] introduced during the process which is vacancies, dislocations, grain-boundaries and anti-phase boundaries.

a TEM image of particle of Gd₃Fe₅O₁₂ after milling and b histogram of particle size distribution

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Structural analysis

Figure 7 shows a multi-plot of XRD spectra of Gd₃Fe₅O₁₂ before sintering and after sintering starting from 600 to 1400 °C with an increment of 100 °C. After milling, we could see that there were no Gd₂O₃ (JCPDS reference code: 01-086-2477) and Fe₂O₃ (JCPDS reference code: 01-079-0007) observed. The sharp and narrow peaks become broadened after the milling process due to fracturing and cold-welding mechanism in which it have generated alternating layer with fresh interfaces which results in defective particle with amorphous phase and lattice strain [33]. Though the energy supplied was also sufficient to initiate diffusion of both precursors that causes the gadolinium orthoferrite phase (GdFeO₃) (JCPDS reference code: 01-078-0451) to form, yet it is not enough to obtain Gd₃Fe₅O₁₂ phase. At 600, 700 and 800 °C, Gd₃Fe₅O₁₂ phase still could not be formed. The samples with these temperatures showed GdFeO₃ phase (space group Pbnm) (Table 1) with orthorhombic structure still retained and no other phases was traced from XRD peak obtained. However, the intensity peaks of GdFeO₃ phase shows increasing trend with increased sintering temperature. More peaks of GdFeO₃ phase were also formed. It suggests that albeit the thermal energy supplied was not enough to complete the reaction of forming Gd₃Fe₅O₁₂ phase, the energy is able to overcome the internal strain and stress from milling. Imperfections introduced from milling process is essential in allowing the diffusional mass transport to take place [34]. Therefore, with sufficient amount of thermal energy, the atoms would diffuse through lattice, surface of particle and grain boundaries which fill in the vacancies. This would improve the crystal structure, thus improving crystallinity of samples. At 900 °C, some Gd₃Fe₅O₁₂ phase was formed and single phase of Gd₃Fe₅O₁₂ was successfully formed at 1000 oC while Ramesh et al. [38] had synthesized nanocystalline gadolinium iron garnets (Gd₃Fe₅O₁₂) via microwave hydrothermal method and it was found that single-phase with garnet structure without any impurity samples were attained at sintering temperatures of 1050 and 1100 °C. Besides, Zanatta et al. [39] had been investigated that the formation of the single-phase gadolinium iron garnets (Gd₃Fe₅O₁₂) was produced thru high-energy ball milling technique followed by heat treatments of 1100 °C. It can be indexed to (321), (400), (420), (422), (521), (532), (444), (640), (522), (642), (800), (840), (842) and (664) planes of cubic structure with space group of Ia3d (Table 1) that matched with the JCPDS reference code: 01-074-1361. A further increase of sintering temperature up to 1400 °C could only decrease the intensity peak. This would suggest that there was stoichiometric change occurred that causing the reduction of the peak intensity. Variations in the observed XRD peaks intensity (Table 1) are related mainly to the scattering intensity of the crystal structure components and their arrangement in the lattice. When there are changes occurred in the composition of samples at high temperature, the number of Gd₃Fe₅O₁₂ components would be disturbed, hence causing the decrease in intensity and consequently reducing the crystallinity of the samples. The other information obtained through XRD pattern was listed in Table 1. The lattice constant, a values show increasing trend with increasing sintering temperature and significant change in value can be observed between 900 and 1000 °C. The change indicates the transformation from GdFeO₃ phase (orthorhombic structure) to Gd₃Fe₅O₁₂ phase (cubic structure).

XRD patterns of Gd₃Fe₅O₁₂ where @ is α-Fe₂O₃, ^ is Gd₂O₃, + is GdFeO₃ and * is Gd₃Fe₅O₁₂

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The density, relative density and porosity of the sintered samples at different sintering temperature are recorded in Table 2. The density of the sample is in the range of 5.55–6.01 g/cm³ with relative density of 72.27–92.60%. The density of the samples increased generally while the amount of porosity decreased as the sintering temperature increased. There are fluctuation in the density values taken was attributed to inconsistency during preparation of the samples (multi-sample sintering). The X-ray density and pore fraction that are recorded in Table 2 were calculated using the relation given by Eq. 4 [40]:

where ρ_xrd is X-ray density, N_a is the Avogadro’s number, M is the molecular weight of a sample and ‘a’ is the lattice constant which was calculated by indexing the XRD pattern.

Relative density, ρ_r is calculated based on Eq. 5 [40]:

The porosity (P) of the samples was calculated by using Eq. 6:

where ρ_exp is the experimental density determined from the Archimedes Principle.

Microstructural analysis

The FESEM micrographs of Gd₃Fe₅O₁₂ were shown in Fig. 8a exposes the changes of the microstructure on sintered samples from 600 to 1400 °C. The micrograph of the sample sintered at 600, 700, 800 and 900 °C indicates the initial stage of sintering. At 600 and 700 °C, it involves rearrangement of powder particles and do not show apparent changes in microstructure where all grains still in small size and agglomerated to each other due to milling process. As observed at 800 and 900 °C, necking process starts to appear between the particles with the formation of dumbbell shape structure. This is due to the high atomic mobility of nanometer size grains surface via high free energy causing surface diffusion in between the nearest particles to occur. Samples sintered from 1000 to 1200 °C show an intermediate stage of sintering. Grain growth would occur due to difference in free energy of two sides of nearest grains where it tends to lower the energy of the system to a more stable state [41]. That would make the boundaries of the grains to move towards its centre of curvature, also by diffusion of atoms from lattice and surface across it. The sharp neck in the initial stage would be wider as more diffusion occur due to higher thermal energy given. Thus, this would cause the grains to move closer, leading to shrinkage of components. The amount of porosity would be reduced when necks formed between grains as interconnected channels of pores were formed along 3-grain edges. Microstructural changes at 1300 and 1400 °C were corresponded to the final stage of sintering. Hexagonal grains are formed at this stage in order to achieve the most stable state of grains. This would minimize the surface free energy. It can also be observed that the number of grain boundaries and grains have decreased with more formation of large grains [42]. In addition, pores have become closed and isolated due to movement of grain boundaries through diffusion of vacancies and grain boundary that have caused the grains to grow over the pores. Grain size distribution for each sample has been plotted in histogram shown in Fig. 8b. This has revealed that all samples have wide distribution grain size which can be pertained to the natural consequence from milling process with inconsistency in energy imparted from the balls to powder. The shaded area signifies on the grains with single domain behaviour. The next region is the region of grains with multi-domain. It can be observed that the shaded region is getting smaller with higher sintering temperature. The critical size, D_c identified from the histogram is 0.66 µm. The distribution also shifted to larger grain size as sintering temperature in samples increased.

a FESEM micrographs and b grain size distribution of Gd₃Fe₅O₁₂ at sintering temperature 600, 700, 800, 900, 1000, 1100, 1200, 1300 and 1400 °C

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Figure 9 shows plotted graph of log D against (1/T). The energy denotes the energy required in grain growth process during sintering with different temperature. This can be proven through Arrhenius equation from Coble’s theory as given by the Eq. 7 [43, 44]:

Plots of log D as a function of the reciprocal of absolute temperature (1/T)

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where Q is the activation energy, T is the absolute temperature (K), R is the ideal gas constant which is 8.314 J/mol and k is the specific reaction rate constant. The value of k can be directly related to grain size based on the Eq. 8 [44, 45]:

where A is the intercept, T is the absolute temperature (K) and D is the grain size.

From Eq. 8, a best fitted straight line was drawn which depicts the value of activation energy, Q from the slope, − Q/2.303R obtained. From the Fig. 9, it can be observed that there are two slopes where the stiffness of slopes is decreasing with smaller grain size. The phenomenon can be explained as follows:

1. (i)

   Slope 1: The activation energy, Q based on the slope value in this region is 6.80 kJ/mol. In this region of samples sintered from 600 to 900 °C, the orthorhombic structure of GdFeO₃ phase is the dominant structure albeit some of Gd₃Fe₅O₁₂ has appeared in sample of 900 °C as unravelled in XRD result (Fig. 7). At this stage, necking process is the dominant through diffusion of atom on the surface and lattice at higher temperature. This can be observed from FESEM micrograph in Fig. 8a. The low value of energy required is due to high surface to volume ratio contributed by milling process. High number of contact points that has been introduced would elevate the diffusion rate which consequently lowers the activation energy needed.

2. (ii)

   Slope 2: The activation energy, Q calculated is higher than the first slope which is 192.63 kJ/mol. This suggests that high energy required to help to form fully crystalline phase of samples with larger grain size sintered from 1000 to 1400 °C as can be seen in XRD result (Fig. 7). In addition, for samples 1300 and 1400 °C, grain growth occurred by virtue of grain boundary shifting or movement involving atomic diffusion since necking process has been completed. Therefore, higher energy is required for this process to occur.

Magnetic analysis

Figure 10 illustrated the B–H hysteresis loops of Gd₃Fe₅O₁₂ sintered at different temperatures from 600 to 1400 °C. The data of saturation induction (B_s) and coercivity (H_c) values were tabulated in Table 3. The evolution of ordered magnetism in the materials can be observed in two stages of development based on the different sintering temperature. The first stage, with lower sintering temperatures varying from 600 to 1000 °C, shows a linear-looking hysteresis loops (superparamagnetism + paramagnetism) with low value of B_s in the samples. This indicates that the strength of magnetism in the sample is very weak which can be attributed to the very minute size of grains where it has large amount of grain boundaries volume. Large grain boundaries volume signifies the amorphousity of the sample phase where it will cause the interaction between magnetic moments to be weak and in random orientation which leads to paramagnetic state. The sample that was sintered at 600 and 700 °C gave the highest value of B_s compared to 800, 900 and 1000 °C. The value of B_s is started to decrease when the temperature increased and this might be affected by having larger grain size, this will increase the amount of crystalline structure which will compensate the spin moment, therefore, leading to the decrease in net moment. A further increase of the sintering temperature (second stage) from 1100 to 1400 °C which denoted as slanted sigmoid shape produced a noticeable increase of B_s in the hysteresis loop, indicating an improvement of ordered magnetism formed in the sample. At this stage, ferrimagnetism became dominant in comparison with the paramagnetism behavior. Judging from the phase and microstructure, we assume that the amount of ferrimagnetism grains has increased and is contributing to the substantial ordered magnetism. The development of the crystallinity of the samples revealed a single phase, which indicating that there was a permutation of superparamagnetism (from small grains), paramagnetism (from amorphous phase) and ferrimagnetism. However, ferrimagnetism was more significant relative to paramagnetism and superparamagnetism.

a–i B–H hysteresis loop formation of Gd₃Fe₅O₁₂ sintered from 600 to 1400 °C

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Figure 11 indicates a graph plotted on coercivity value, H_c with respect to grain size, D. From the graph, it is perceived that there are two regions which are single domain (SD) region and multi-domain (MD) region. The maximum value of H_c at particular grain size indicates the transition of SD to MD where the grain size at the transition occurred is called critical size, D_c. The H_c values were found to increase to a maximum value from 600 to 1100 °C but it falls from 1200 to 1400 °C. Therefore, a range of grain sizes 0.66–1.83 µm was chosen as the range of D_c for single domain particles. The increasing values of H_c values for lower sintering (≤ 1200 °C) were due to the shape, magnetocrystalline and strain-induced anisotropy. For the higher sintering temperatures (> 1200 °C), the grain size exceeded the D_c with vanishing shape and strain-induced anisotropy while the magnetocrystalline anisotropy remains constant in the sample [46].

Graph of H_c as a function of grain size, D of Gd₃Fe₅O₁₂ plotted from Table 3

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Electrical analysis

Figure 12 demonstrate the resistivity (ρ) of Gd₃Fe₅O₁₂ as a function of sintering temperature. As can be seen the ρ values exhibited by Gd₃Fe₅O₁₂ samples were not as high as theoretical value that obtained from pure garnet (10⁹–10¹⁰) (Table 4) [47]. This might be due to the presence of impurities in the samples from the α-Fe₂O₃ precursor used to produce Gd₃Fe₅O₁₂. The values of ρ showed an increasing trend (600–1200 °C) generally and decreasing for sample sintered from 1300 to 1400 °C. By increasing the sintering temperature, the movement of electron will become restricted due to the better arrangement of crystal and stoichiometric stability in which the ions of Gd³⁺ and Fe³⁺ has resided in the respective sublattices. Therefore, this would increase the ρ of sample and achieve the maximum value by reducing the electron hopping. High ρ will minimize the eddy current generated within the grains which is favoured for high frequency applications. However, the decrease in the value of ρ at high sintering temperature (1400 °C) shown can be ascribed to the electron hopping between Fe³⁺ to Fe²⁺. This can be related to the stoichiometric change which is the possible mechanism that would induce the reduction of Fe³⁺ to Fe²⁺ occurred in order to compensate the stoichiometric interrupted due to the loss [48].

Graph of resistivity, ρ of Gd₃Fe₅O₁₂ sintered from 600 to 1400 °C

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Microwave analysis

Microwaves can perform in three mechanisms which obey law of optics: transmission, absorption and reflection where they are dependent on the type of materials. In this research, the mechanism that would be focused on is transmission since the material used is ought to be low loss material, meaning microwave can pass through the material with small loss. The linewidth, ΔH values was plotted in a graph as a function of grain size as shown in Fig. 13 with the values indexed in Table 5. The ΔH is measured using the following Eqs. 9–12:

where f_a is lower minimum frequency, f_b is upper minimum frequency, f_m is maximum frequency, ∆ω is angular frequency bandwidth and γ is gyromagnetic ratio with a value of 1.76 × 10¹¹/T/s [49].

Graph of linewidth, ΔH of Gd₃Fe₅O₁₂ as a function of grain size, D

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It can be observed that the decreasing trend of ΔH could be attributed to the small grain size of samples sintered from 600 (0.039 µm) to 900 °C (0.049 µm) where the ΔH values are governed by three and four magnon scattering. This condition was resulted from the contribution of GdFeO₃ behaviour which exhibit as a canted antiferromagnetic material. At small grain size regime for sample sintered from 600 (0.039 µm) to 800 °C (0.050 µm) with canted antiferromagnetic behavior, there would be uncompensated magnetic moment on the surface of grain which give rise to larger magnetization as compared to that of magnetization in bulk canted antiferromagnet. At sintering temperature of 900 °C (0.049 µm), as can be observed from XRD pattern in Fig. 7, the intensity peak of GdFeO₃ phase has increased significantly with the formation of additional peaks regarding to GdFeO₃ phase. This would reduce the magnetization which consequently reduced the magnetic loss exhibited by the sample. By increasing the sintering temperature to 1000 °C (0.104 µm) would lead to a jump in the ΔH value. This could be related to the formation of single phase ferromagnetic Gd₃Fe₅O₁₂ for the sample sintered at 1000 °C (0.104 µm) which causing the magnetocrystalline anisotropy to increase with the structural transformation from orthorhombic GdFeO₃ to cubic Gd₃Fe₅O₁₂. The contribution of shape anisotropy could be neglected since the sample of 1000 °C (0.104 µm) has small grain size. For samples sintered from 1100 (0.242 µm) to 1400 °C (8.080 µm), the ΔH value for sample sintered at 1100 °C (0.242 µm) was observed to be the same as in the sample at 1000 °C (0.104 µm). The value was observed to be increased in the sample sintered at 1200 °C (0.660 µm) which could be pertained to porosity or the relative density and anisotropy factor as the extrinsic factor is significant in large grains. It can be observed that the porosity increase from the decrease in relative density for the sample sintered at 1200 °C (0.660 µm). The porosity would act as the pinning site to the spin resonance which generates the demagnetizing centre to produce demagnetization effect from inhomogeneous internal fields. In addition, sample sintered at 1200 °C (0.660 µm) has the highest contribution from the three anisotropy field as can be observed in H_c graph in Fig. 11 which leads to the largest ΔH value. By increasing the sintering temperature to 1300 °C (1.830 µm), the ΔH value was observed to be decreased and remain constant for sample sintered at 1400 °C (8.080 µm). The dip in the value can be attributed to the fact that shape anisotropy and strain-induced anisotropy effect has been reduced for large grain size as the shape of grain is achieving the stable form and the porosity also reduced. This results in magnetocrystalline anisotropy to be dominant in the contribution to the ΔH value. Therefore, the values of ΔH can be grouped into two groups based on the factors that influenced the value exhibited; (i) intrinsic factor for samples sintered from 600 (0.039 µm) to 1000 °C (0.104 µm) and (ii) extrinsic factor for samples sintered from 1100 (0.242 µm) to 1400 °C (8.080 µm).

Gd₃Fe₅O₁₂ samples have been successfully prepared by high-energy ball milling (HEBM) technique. The relationship between changes of microstructure through sintering (act as an agent) process with magnetic, electrical and microwave properties have been scrutinized. The XRD pattern exposed a single Gd₃Fe₅O₁₂ phase was observed for samples sintered at 1000 °C and above. FESEM images described that larger grains were formed when the sintering temperature was increased, and the amount of porosity (P) was decreased. The B–H hysteresis loop indicates that there is a development of magnetic behavior occurs in the sample as the sintering temperature increased. The ρ showed an increment at 600–1200 °C and the decrement of ρ in subsequent temperatures due to the better arrangement of crystal and stoichiometric stability where electron movement will be restricted. As for microwave properties, two stages can be allotted; samples on nanometer region have been influenced by magnetic spin rotation, while samples with micron grain size were affected by microstructural changes factor.

This research was funded by Geran Putra Universiti Putra Malaysia 9533300 and Long-Term Research Grant Scheme (LRGS) 5526200 and special mention by for my late supervisor, Assoc. Prof. Dr. Mansor Hashim.

Funding was provided by Universiti Putra Malaysia (Grant No. 9533300), Kementerian Sains, Teknologi dan Inovasi (Grant No. 5526200)

Affiliations

1. Material Synthesis and Characterization Laboratory (MSCL), Institute of Advanced Technology, Universiti Putra Malaysia, 43400, Serdang, Selangor, Malaysia

   Muhammad Syazwan Mustaffa, Ismayadi Ismail, Idza Riati Ibrahim & Mohd Shamsul Ezzad Shafie

2. Functional Devices Laboratory (FDL), Institute of Advanced Technology, Universiti Putra Malaysia, 43400, Serdang, Selangor, Malaysia

   Farah Nabilah Shafiee, Nor Hapishah Abdullah & Mohd Nizar Hamidon

3. Department of Material Technology, Faculty of Industrial Science and Technology, Universiti Malaysia Pahang, Kuantan, Pahang, Malaysia

   Rodziah Nazlan

4. Kolej PERMATA Insan, Universiti Sains Islam Malaysia, 71800, Nilai, Malaysia

   Fadzidah Mohd Idris

Corresponding author

Correspondence to Farah Nabilah Shafiee.

Conflict of interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.

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