TGA-DTA Analysis of SrM powder before annealing
The thermal decomposition behavior of SrM dried sample analyzed by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) graphs are shown in Fig.1. There were two endothermic peaks at 200˚C & 500˚C and two exothermic peaks at 300˚C and 530˚C were observed. These peaks are responsible for the phase transformation phenomenon in the SrM sample. To analyze the phase transition of SrM hexaferrite, the DTA curve was divided into three regions i.e. Region1 (100˚C-305˚C), Region2 (305˚C-500˚C) and Region3 (500˚C -800˚C). Region1 indicates the removal of water and the nitrate contents from the SrM sample. Region2 confirms the evaporation of several carbon containing compounds along with the remaining crystallized water vapours, whereas the Region3 confirms the formation of oxide from hydroxide, monoferrites and the beginning of hexagonal phase. On the other hand, the TGA shows the continuous weight loss from room temperature to 800˚C. Beyond 800˚C no such change in the weight loss was observed which again confirmed the starting of crystallization of SrM. Both the curves meet each other at 910˚C without showing any endothermic and exothermic peaks or any weight loss [18-19]. Therefore 910˚C is confirmed as the temperature SrM phase formation.
XRD analysis of SrM sintered at 800˚C and 910˚C
The XRD pattern of SrM samples sintered at 800˚C and 910˚C are shown in Fig.2. All the diffracting planes (006), (110), (108), (203), (205), (206), (301), (217), (2011), (220) with (107) and (114) planes as most prominent plane demonstrates the formation of M-phase [20]. Some impurities of hematite (Fe2O3) are also observed and found to be decrease as the sintering temperature increases to 910˚C. The average crystallite size for both the samples sintered at 800˚C and 910˚C are 25 nm and 30 nm calculated by well-known Scherer formula. The structural parameters are refined by Rietveld method by using fullprof software. All the important parameters Rwp (weighted residual error), SGOF (goodness of fit) and chi square (χ2) along with cell parameters were calculated and presented in the Table no.1. Both the samples were belonging to the same space group P63/mmc (194). All the refined parameters are well matched with the standard SrM pattern and results in the formation of high quality SrM hexaferrite.
In the Rietveld analysis, the refined parameters were scale factor, background parameters, 2θ zero point, lattice constants, profile half width parameters (u, v, w), the mixing parameters of the Pseudo Voigt function( NA, NB), occupancy and atomic positions.
FESEM
The morphology and grain size distribution of SrM hexaferrite sintered at 800˚C and 910˚C were examined by FESEM and shown in the Fig.3(a) and Fig.3(b). At 800˚C very fine and uniformly distributed particles along with some agglomerates were observed. The tendency of agglomeration of SrM hexaferrite observed in this sample was the direct consequence of smaller and uniform grains as well as enhanced surface energy [21]. The average grain size obtained was approximately 45 nm. As the temperature increased to 910˚C, very fine and well packed grains along with some hexagonal platelets were observed. These platelets also confirmed the existence of M-phase. The average grain size obtained for SrM sintered at 910C was 50 nm. These results were well matched with the obtained XRD pattern of SrM crystal.
3.4 FTIR spectroscopy
FTIR spectra of SrM sintered 800˚C and 910˚C are shown in Fig.4. The whole spectra were recorded in the range 1000-400 cm-1. The positions of all the absorption bands of samples were similar to the reported data of SrM while their relative intensity varied. The frequency bands in the ranges 550-580 and 430-470 are corresponding to the formation of tetrahedral and octahedral sites confirming the presence of Me-O stretching in ferrites [20]. In the observed pattern, the band positions at 440cm-1 and 547cm-1 confirmed the presence of Fe-O6 (octahedral) and Fe-O4 (tetrahedral) stretching vibration [21]. The frequency band at 597cm-1 attributed to the characteristic peak of strong Sr-O, stretching vibration confirmed the formation of M-phase [22-23]. As the sintering temperature increases to 910˚C, all the three characteristics peaks were observed at 445cm-1, 548cm-1 and 600cm-1. There was a very minimal change in the positions observed but the length of the dip increased very sharply indicates the very strong bond formation in the ferrite samples. Apart from these bands, the dips at 802cm-1 and 866cm-1 were corresponding to the (NO3)-2 stretching vibration [24].
Raman Spectroscopy
Raman spectroscopy at room temperature for both the samples was carried out to hold up the information extracted from XRD graphs on the M-type strontium hexaferrite sintered at 800˚C and 910˚C. It is a very powerful and dynamic tool which gives the information of very minute impurities present in the samples by knowing the position of their atom bonds. On the basis of group theory, in the crystal lattice of M-type strontium hexaferrite there are 42 Raman active modes which include 11A1g, 14E1g and 17E2g [25]. Fig.5 shows the Raman spectra of SrM sintered at 800˚C and 910˚C. The peaks observed at 171 cm-1,310 cm-1,407 cm-1, 503 cm-1, 610 cm-1 and 675 cm-1 confirmed the presence of Raman active modes. The peak present at 171 cm-1 gives the evidence of the whole spinel block with E1g symmetry [26]. The strongest peak observed at 675 cm-1 confirmed the presence of A1g mode representing the presence of trigonal bipyramidal site (2b) whereas the second strongest peak at 310 cm-1 represent the E2g mode revealing the mixed octahedral site (2a) [27]. The peaks position at 407 cm-1 and 610 cm-1 represents the A1g mode with dominating octahedral sites (12k and 4f2). These are the five basic modes which ensure the formation of SrM ferrite sample [28]. The peaks observed were well consistent with the magnetoplumbite structure and also found matched with the reported literature [29-30]. Apart from this one peak at 503cm-1 is also observed which confirms the presence of Fe2O3 impurity phase [31]. This impurity phase was also confirmed in the XRD pattern. As the sintering temperature rises from 800˚C to 910˚C, the same modes were observed but differ in the intensity of peaks which confirms the strong Me-O bond formation. This can be attributed due to the increase in the particle size and their homogeneity. Also at higher temperature the presence of impurity phases starts to diminish.
Magnetic properties of SrM hexaferrite sintered at 800˚C and 910˚C
The hysteresis loops of the M-type strontium hexaferrite (SrFe12O19) sintered at 800˚C and 910˚C are shown in Fig.6. Some important magnetic parameters like saturation magnetization (Ms), coericivity (Hc), retentivity (Mr) and squarenes ratio (SQR) are extracted from these loops and tabulated in Table1. From the M-H loops it is seen that the both the sample doesn’t reach its saturation point exactly so the law of approach to saturation (LAS) method has been implemented to calculate the Ms values for each sample shown in Fig.7. The equation represent this method is shown as below
![](https://myfiles.space/user_files/58854_b38fc7f3db2c487f/58854_custom_files/img1628216442.png)
So by applying this method to our original data we calculate the Ms Values for SrM sintered at 800˚C & 910˚C and the values are 80.85 emu/g and 92.19 emu/g. These values are quite higher than the reported Ms values of bulk SrM at low sintering temperatures [32-33]. The reason behind this increase in Ms values can be easily understood from super exchange interaction Fe3+-O2--Fe3+. In the crystal structure of SrFe12O19, Fe3+ ions have been distributed into five interstitial sites. These sites are 2a, 2b, 12k, 4f1 and 4f2 respectively. From all these sites, 2a, 12k and 4f2 belongs to the octahedral site (A-site) having spin up where as 4f1 belongs to tetrahedral site (B-site) with spin down and 2b belongs to trigonal bypiramidal site with spin up. Net magnetic moment can be calculated by taking the difference between spin up and spin down. So this net magnetic moment will give rise to the super exchange interaction Fe3+-O2--Fe3+ which strongly influenced the magnetization of the materials[34]. From all the five interstitial sites only two sites 12k and 2b are to the ferromagnetic direction due which these site exhibit spin up. Because of this increase in magnetic moment, the super exchange interaction comes to play to enhance the magnetization. From the XRD graph of SrM sintered at 800˚C in fig.2 some impurity peaks of Fe2O3 have been depicted. With increase in the sintering temperature to 910˚C, these peaks intensity are found to be in decreasing order. This decrease in the content of Fe2O3 will increase the amount of Fe3+ ions which further expands the region of super exchange interaction. Hence the interaction between Fe3+ and O2- give rise to the transfer of magnetic moment to the adjacent cations. Therefore with increase in the net magnetic moment, saturation value increased [35-36].
Apart from getting the higher values of Ms higher at lower sintering temperatures, very interesting results of coericivity are observed. The value of Hc of SrM sintered at 800˚C and 910˚C were 107.90 Oe and 262.92 Oe respectively. The prepared samples reflect the soft magnetic behavior of SrM hexaferrite. This kind of behavior is only acceptable if the particles of smaller size are shifted from multidomains to single domain [5]. Very few reports are available which shows such kind of behavior of SrM hexaferrite and claimed that this reduction of coericivity value of the order of few hundred of Oe is due to the smaller particle size and non-magnetic impurities present in SrM structure[17-18].