Structural and Magnetic Properties of Ba1-xRexCo2ZnxFe16-xO27 W-type Hexaferrite prepared by Ball Milling Method

Rare earth substituted W-type hexaferrites were prepared by mixing and ball milling starting powders with molar ratios consistent with the stoichiometry of Ba1-xRexCo2ZnxFe16-xO27 (Re is a rare-earth element; x = 0.1 and 0.2), pelletizing, and sintering 1300°C. X-ray diffraction patterns showed a pure W-type phase in all samples, except in the Nd-Zn (x = 0.1) substituted sample which revealed the presence of an impurity α-Fe2O3 nonmagnetic phase. The crystallization of the W-type phase in all samples was further confirmed by the characteristic Curie temperature ranging between 476°C and 484°C as revealed by the thermomagnetic measurements. Scanning electron microscopy imaging revealed the variations of the particle size and morphology, and porosity of the prepared samples. The magnetic measurements indicated that the RE–Zn substitution improved the saturation magnetization slightly relative to the un-substituted Co2W hexaferrite, and resulted in a decrease of the coercivity and magnetocrystalline anisotropy field. In addition, the peaks below 300°C in the thermomagnetic curves is an indication of the occurrence of spin reorientation transitions in the prepared hexaferrites. Material Science Research India www.materialsciencejournal.org ISSN: 0973-3469, Vol.17, No.(1) 2020, Pg. 34-46 CONTACT Sami H. Mahmood s.mahmood@ju.edu.jo Physics Department, The University of Jordan, Amman 11942, Jordan. © 2020 The Author(s). Published by Oriental Scientific Publishing Company This is an Open Access article licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License Doi: http://dx.doi.org/10.13005/msri/170106 Article History Received: 7 December 2019 Accepted: 21 January 2020


Introduction
Hexagonal ferrites (hexaferrites) belonging to an important class of magnetic oxides were discovered in the 1950s, and demonstrated high potential for important practical applications with economic feasibility, which led to an increasing interest in the production and characterization of these materials. The major utilization of hexaferrites is in the production of permanent magnets, as well as a wide range of applications including magnetic recording and electrical devices operating in the GHz frequency range. [1][2][3][4][5][6] The employment of hexaferrites in a plethora of magnetic material applications was further encouraged by their favourable properties, chemical stability, relatively high operating temperature, and cost effectiveness. [7][8][9][10][11][12] The various members of the hexagonal ferrite family (M-, Y-, W-, Z-, X-, and U-type) exhibit different magnetic properties, which facilitated their potential use for a variety of applications. 13,14 The molecular formula of the W-type hexaferrite (BaMe 2 Fe 16 O 27 ) can be viewed as a superposition of M-type hexaferrite (BaFe 12 O 19 ) and spinel (S) structural block (W = M + S; S = Me 2 Fe 4 O 8 ). 15,16 In the case of Me = Co, the saturation magnetization of the spinel phase is higher than that of the M-type hexaferrite, 13 which explains the higher saturation magnetization of the W-type hexaferrite compared with the M-type. The unit cell of the W-type structure contains two Co 2 W molecules in the alternate stacking SSRS*S*R* of the R (BaFe 6 O 11 ) and S structural blocks (the star means rotation of the block around the c-axis by 180º). 15,17 W-type hexaferrite was recognized as a soft magnetic material with potential importance for microwave absorption. [18][19][20][21][22][23] The microwave properties of the W-type phase were improved by cationic substitutions, such as the partial substitution of Ba 2+ ions by Er 3+ , Sm 3+ , and Nd 3+ . 15,23,24 Also, partial substitution of Dy 3+ for Fe 3+ in Ni 2 W hexaferrites, 21 and La-Ni substitution in Co 2 W, 25 have shown promising modifications for microwave absorption applications in the GHz range. However, adequate structural and magnetic characterization of these materials remain limited in the literature.
The magnetic structure of Co 2 W hexaferrites was investigated by Mössbauer spectroscopy and neutron diffraction. Mössbauer results revealed spin reorientation transition from planer anisotropy to conical spin structure with vertex angle of 50º to the c-axis at 242ºC; this angle remained constant up to the maximum temperature of 272ºC used in the study, 26 where additional spin reorientation transition to axial spin configuration (parallel to the c-axis) could occur at higher temperatures. The spin reorientation transition temperature was reported to decrease with the decrease of Co content in (Zn,Co) 2 -W hexaferrites, 26 27 , which indicated that the compound is planer up to -68ºC, at which temperature it undergoes spin reorientation to axial spin structure parallel to the c-axis 28 On the other hand, Neutron diffraction study revealed conical spin structure with angle of 70º to the c-axis up to 180ºC, after which the angle decreased and the spin structure becomes axial (parallel to the c-axis) at 280ºC. 29 Among other reasons, the seemingly conflicting reported results could be due to differences in sample preparation, types of measurements adopted, or experimental uncertainties. The present study is concerned with the synthesis of pure RE-substituted Co 2 W hexaferrites. The synthesized hexaferrites were examined by means of structural and magnetic characterization techniques, and the effects of RE substitution were investigated. This article is extracted from the unpublished Ph.D. thesis of M. K. Dmour, where the corresponding section in the thesis was written and organized in a format ready for publication. All samples in this work were prepared by Dmour, but the measurements and data analysis were partly contributed by others, who were included in the list of coauthors.

Experimental Techniques
We prepared Ba 1-x Re x Co 2 Zn x Fe 16-x O 27 (Re = La, Nd, Pr; x = 0.1, 0.2) hexaferrites by mixing stoichiometric ratios of high purity (> 99%) BaCO 3 , Fe 2 O 3 , ZnO, and Re 2 O 3 (Sigma-Aldrich made), and ball milling by Fritch Pulveressette-7 ball mill for 16 h. The milling was performed using zirconia bowls and balls; the powder to ball ratio was 1:12. Then pellets of 12.5 mm in diameter were made from the powder mixtures by compressing under a force of 50 kN, and subsequently sintering in air at 1300°C for 2 h. X-ray powder diffraction (XRD) was employed to examine the structural characteristics of the prepared samples using a 7000-Shimadzu X-Ray Diffractometer (λ = 0.15406 nm). The refined structural parameters were obtained from the results of Rietveld analysis of the diffraction data using FullProf software. The particle size and surface morphology in the samples were investigated by scanning electron microscopy (SEM). The room temperature magnetic data were obtained by performing magnetic measurements on the samples using a commercial vibrating sample magnetometer (VSM).

Results and Discussion XRD Results
The diffraction pattern of BaCo 2 Fe 16 O 27 (x = 0.0) sample was reported in a recent publication, 30 where pure Co 2 W phase was obtained. Fig.1 shows Rietveld analysis of the diffraction patterns of the samples Ba 1-x Re x Co 2 Zn x Fe 16-x O 27 (Re = La, Nd, and Pr; x = 0.1, 0.2), which indicated that all samples, except the (Nd-Zn; x = 0.1) sample, consisted of a pure Co 2 W hexaferrite phase whose peak positions agree with the standard pattern (JCPDS file #: 01-078-0135) shown in Fig. 2.
The relative intensities of the (00l) reflections, however, are clearly enhanced compared with those of the standard pattern. The observed enhancement of the relative intensity of these reflections, especially for the sample with La-Zn substitution (x = 0.1), is consistent with previously reported structural texture along the c-axis in Ga-substituted Co 2 W hexaferrites. 30 Fig. 3 shows a representative plot showing the main structural peaks of the sample with La-Zn substitution (x = 0.1) in the angular range 2θ = 30º to 38º, which demonstrated a perfect match between the experimental peak positions and those of the standard pattern (shown in bars representing the intensities of the reflections). In this figure, the enhancement of the relative intensity of the (0012) reflection is obvious. Further, the pattern of the Nd-Zn substituted sample revealed the presence of a minor α-Fe 2 O 3 oxide phase as indicated by the main structural peak (labeled α) of this phase. Fig. 4 shows a match between the experimental peaks and those of the standard pattern, with a small peak corresponding to the minor α-   Table 1.
The results indicated that the lattice parameter a = 5.90 Å for the W-type phase was the same in all samples and agreed with published data for the unsubstituted Co 2 W. 16,30,35 The observed values of the lattice constant c, however, were in general slightly lower, owing to the smaller ionic radii of the substituted RE ions in comparison with Ba 2+ . 36 The lattice parameter c fluctuated slightly (0.09%) around the average value of 32.89 Å, and the cell volume fluctuated slightly (0.16%) around the average value of 991.6 Å 3 . Evidently, the structural parameters did not increase with the substitution of Zn 2+ ions having higher ionic radius compared with Fe 3+ ions, and the effect of RE substitution was dominant. This could be due to the substitution of a relatively large fraction of Ba 2+ by RE 3+ (10% or 20%) at substitutional positions in the oxygen layers, whereas only 0.625% or 1.25% of the Fe 3+ ions were substituted by Zn 2+ ions at interstitial sites. The X-ray density of the W-type phase was calculated from the molecular mass and the refined cell volume, and the bulk density was measured using Archimedes principle; the results are also listed in Table 1. The constancy of the refined X-ray density (5.29 ± 0.01 g/cm 3 ) could be due to the small changes of the molecular mass and cell volume with the RE-Zn substitution. Comparison between the measured bulk density and theoretical x-ray density indicated that all pure W-type hexaferrites possess relatively high density, reaching 86.3% -91.9% of the theoretical density. The lowest bulk density of 77.2% for the Nd-Zn substituted sample with x = 0.1 is also relatively high.
Stokes and Wilson formula was used to calculate the crystallite size (D) of each sample using the corrected integral breadth (βc) of a given XRD peak, where 33,37,38 D=( λ)/(βc cosθ) ... (1) The instrumental broadening was estimated by the breadth of standard Si reflections, whereas the observed integral breadth of a given reflection in the patterns of the samples was determined from the integrated intensity (area A) and the maximum peak intensity (I 0 ) in accordance with the relation: The average crystallite size was determined from the (0010) reflection at 2θ = 27.11° and the (110) reflection at 2θ = 30.27°, and the results for the size parallel to the c-axis and in the hexagonal plane, respectively, are listed in Table 2. The crystallite size along the c-axis for the sample with (La-Zn; x = 0.1) substitution was 351 nm, which is significantly larger than that for the other samples, where the sizes fluctuated in the range of ~ 110 to 200 nm. This result is consistent with the highest observed bulk density (91.9% of the theoretical density) for this sample. Generally speaking, the crystallite size of all samples is higher than the range of applicability (~ 100 nm) of the technique. 37 However, the results could indicate that the lowest crystallite size (~110 nm) of Nd-Zn substituted sample with x = 0.1 could be associated with the lowest observed bulk density for this sample.  Fig. 5 (a) for the sample with La-Zn substation (x = 0.1) revealed crystallization of large (> 2 μm), perfect hexagonal platelets with step-like formation indicative of topotactical grain growth. 30 The closely-packed plates with no obvious porosity is consistent with the high bulk density of this sample, and the stacking of the hexagonal plates along the c-axis is consistent with the highest degree of structural texture along the c-axis as revealed by the relative peak intensities in the XRD pattern. However, the image of the sample with x = 0.2 revealed the presence of closely-packed, randomly oriented sharp-edged particles with typical size of ~ 2 μm (Fig. 5 (b)).  (Fig.6 (a)), in addition to large non-particulate porous structure with hexagonal pores ( Fig.6 (b)). The hexagonal pores revealed the internal crystallographic hexagonal structure in the large non-particulate mass. Fig. 7 shows representative images of the sample with Pr-Zn substitution (x = 0.2). Some relatively small hexagonal plates few microns in diameter were observed, although the majority of the sample consisted of large, porous non-particulate masses ( Fig.7 (b)). This porosity is associated with the relatively low bulk density of this sample compared to other samples. Further, representative images of the sample with Nd-Zn substitution (Fig. 8) indicated crystallization of large hexagonal plates (> 5 μm) with step-like formation. However, a small fraction of smaller particles was observed in Fig. 8 (b) at x = 0.2.

Magnetic Measurements Isothermal Magnetization
Magnetic hysteresis measurements in the field range ± 10 kOe were carried out on all samples, and revealed characteristics of relatively soft magnetic materials. However, for the sake of clarity, the loops in the field range of ± 2 kOe were presented in Fig. 9. These curves were used to determine the remnant magnetization and coercivity for each sample. On the other hand, the saturation magnetization (M s ) was estimated from the behaviour of the magnetization curve in the high-field range  These values are to be compared with those for Co 2 W hexaferrites prepared by a different method (coprecipitation), which exhibited much higher values. 42,43 The observed differences of the coercivity could be due to differences in the preparation method was dominated by the high magnetocrystalline anisotropy of the ferrites, and that other contributions were negligible. By fitting the linear relation with the best straight line, the saturation magnetization (intercept of the line with the M-axis) was determined. The slope of the straight line was used to determine the the anisotropy field (H a ) using the relation 40 B = ... (4) The results of the analysis of the hysteresis data in Table 3 revealed a 15% to 30% improvement of the saturation magnetization relative to Co 2 W hexaferrites prepared by coprecipitation, [41][42][43] 44 On the other hand, the samples with Nd-Zn substitutions exhibited the lowest values of the saturation magnetization (71.9 to 73.2 emu/g) compared with the other substitutions; these values are still in good agreement with those of Co 2 W. 30,45 The increase of the saturation magnetization with RE-Zn substitution could be due to the preferential substitution of Fe 3+ ions by nonmagnetic Zn 2+ ions at spin-down sites. and experimental conditions, leading to different microstructures which play a key role in the coercivity.
To examine the correctness of the interpretation provided in the last sentence, we must simultaneously investigate the behavior of the anisotropy field (H a ) with RE-Zn substitution, since it is known that changes in H a play an important role in changing the coercivity. Table 3 shows that the values of H a of all samples were in the range of 6.39 -8.64 kOe, in agreement with the values for pure Co 2 W samples 16,30 However, the sample with Nd-Zn substitution (x = 0.1) exhibited the lowest value of 6.39 kOe, whereas this same sample exhibited the highest coercivity. Also, the sample with La-Zn substitution (x = 0.1) exhibited the lowest coercivity of 33 Oe, whereas its anisotropy field is one of the highest in the measured samples. This could be an indication that the observed fluctuations in the coercivity are more likely to be of microstructural origin rather than magnetocrystalline origin.

Thermomagnetic Measurements
The behavior of the magnetization at a constants field of 100 Oe with increasing temperature was investigated for all samples, and the results are shown in Fig. 10. The curves indicated that all samples went through magnetic phase transitions at different temperatures. All thermomagnetic curves exhibited a peak below 300ºC. The initial increase of the magnetization is associated with spin reorientation transition common to Co-containing W-type phase, 16,30 and the peak occurs as a result of competition with thermal effects. At higher temperatures (> 450°C), the magnetization dropped sharply as a consequence of ferromagnetic-toparamagnetic phase transitions. The derivative of the magnetization with respect to temperature (dM/dT) exhibited a sharp (negative) peak at the transition temperature, from which the Curie temperature of the magnetic phase was determined ( Table 4). The transition temperature obtained from the strong peak in the range 476 -484°C is associated with the Curie temperature (T c ) of the W-type phase. These value of Curie did not change appreciably from the values previously reported for Co 2 W hexaferrite. 26,30 A second weak peak in the derivative curves was observed at higher temperatures (534 -539°C). Since the structural analysis did not show secondary phases (except for the sample with Nd-Zn substitution with x = 0.1), this weak peak can be attributed to the enhancement of the superexchange interactions strength in Co-rich regions of the samples. Similar secondary step reduction of the magnetization was observed in the thermomagnetic curves of Co-containing M-type, 46 Y-type, 47 and W-type hexaferrites. 16,30

Conclusion
Highly pure W-type hexaferrites Ba 1-x Re x Co 2 Zn x Fe 16-x O 27 with RE-Zn substitutions were successfully produced by ball milling stoichiometric ratios of the starting powders and sintering at 1300°C. The physical particle size revealed by SEM imaging was found to be large, and the crystallite size determined from analysis of the XRD patterns was found to be > 100 nm for all samples, indicating a high degree of crystallization of highly dense RE-Zn substituted W-type pellets. The saturation magnetization improved with RE-Zn substitution compared with the unsubstituted Co 2 W, while the coercivity remained below 100 Oe for all samples. The thermomagnetic curves confirmed the phase purity, and revealed the Curie temperature of the magnetic phases. The Curie temperature was not influenced significantly by the RE-Zn substitution, and spin reorientation transitions were observed in the temperature range < 300ºC for all samples, rendering these materials of potential importance for magnetic refrigeration applications.