Solid State Phenomena Vol 232 (2015) pp 65-92 © (2015) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.232.65 Effects of Heat Treatment on the Phase Evolution, Structural, and Magnetic Properties of Mo-Zn Doped M-type Hexaferrites Sami H. Mahmood1,a*, Aynour N. Aloqaily1,b, Yazan Maswadeh1,c, Ahmad Awadallah1,d, Ibrahim Bsoul2,e, Mufeed Awawdeh3,f, Hassan Juwhari1,g 1 Physics Department, University of Jordan, Amman 11942, Jordan Physics Department, Al al-Bayt University, Mafraq 13040, Jordan 3 Physics Department, Yarmouk University, Irbid 21163, Jordan 2 a* [email protected] (corresponding author), [email protected], [email protected], [email protected], [email protected], f [email protected], [email protected] c Key Words: M-type Hexaferrite, W-type Hexaferrite, Structural Properties, Magnetic Properties, Mössbauer Spectroscopy Abstract. In this article we report on the structural and magnetic properties of BaFe12-4xMoxZn3xO19 hexaferrites with Mo-Zn substitution for Fe ions. The starting materials were commensurate with the BaM stoichiometry, and the Mo:Zn ratio was 1:3. The powder precursors were prepared by high energy ball milling, and subsequently sintered at temperatures from 1100 to 1300° C. The structural analyses indicated that all samples sintered at 1100° C were dominated by a major M-type hexaferrite phase. The relative abundance of the BaMoO4 and Zn-spinel secondary phases increased with increasing the concentration of the substituents, resulting in a decrease of the saturation magnetization from about 67 emu/g (for x = 0.0) to 55 emu/g (for x = 0.3). The coercivity also decreased from 3275 Oe (for x = 0.0) to 900 Oe (for x = 0.3), demonstrating the ability to tune the coercivity to the range useful for magnetic recording by the substitution process. The saturation magnetization improved significantly with sintering at T > 1100° C, and the coercivity decreased significantly, signaling the transformation of the samples to soft magnetic materials. These magnetic changes were due to the high-temperature reaction of the spinel phase with the BaM phase to produce the W-type hexaferrite phase on the one hand, and to the growth of the particles on the other hand. The magnetic phases were further investigated using Mössbauer spectroscopy and thermomagnetic measurements. Our study indicated that the sample with x = 0.2 has the highest saturation magnetization (74 emu/g at sintering temperature of 1300° C) and a tunable coercivity between 2100 Oe and 450 Oe. Contents of Paper 1. Introduction 2. Properties of Hexaferrites 2.1. M-Type Hexaferrites 2.2. W-Type Hexaferrites 2.3. The MoZn-substituted M-Type Hexaferrites 3. Experimental Techniques 3.1. Sample Preparation 3.2. Sample Characterization 4. Results and Discussion 4.1. Scanning Electron Microscopy 4.2. X-Ray Diffraction 4.3. Mössbauer Spectroscopy 4.4. Magnetic Measurements 4.4.1. Hysteresis Characteristics 4.4.2. Initial Magnetization Curves 4.4.3. Thermomagnetic Measurements All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 18.104.22.168-20/04/15,09:25:02) 66 Ferroic Materials: Synthesis and Applications 5. Conclusions References 1. Introduction The discovery of barium hexaferrites (M-type or BaM hexaferrite) more than six decades ago, and their promising wide range of technological and industrial applications, had generated great interest in the fabrication and characterization of this class of ferrites [1 – 5]. The progress in manufacturing microwave (MW) devices for different applications such as in home appliances, radar and telecommunication systems had driven a great attention to the problem of electromagnetic interference and radiation pollution. Consequently, interest was developed in the production of magnetic materials for MW applications, and different members of the hexagonal ferrites, namely, the W-type, the Y-type, the X-type, the Z-type, and the U-type, were successfully synthesized and characterized by different techniques [1, 3, 6 – 9]. The wide range of applications of hexaferrites is facilitated by the fact that their magnetic properties cover the range from those characteristic of soft magnetic materials to those of hard magnetic materials. Magnetically hard materials are used for the production of permanent magnets for motors, home appliances, and vehicles, while soft magnetic materials are used in transformers, power supplies, and electronic components for telecommunication applications, and hexaferrites with soft and intermediate magnetic hardness are used in magnetic recording as components for read-write heads and data storage [10 – 13]. The major contribution to the coercivity of the hexaferrites is due to magnetocrystalline anisotropy arising from the coupling of the spins of Fe3+ at different sites. According to Stoner-Wohlfarth model for a random assembly of uniaxial single-domain barium hexaferrites particles, the coercivity is given by : = 0.96 1 (1) Here K1 is the first anisotropy constant and Ms is the saturation magnetization. In addition, shape anisotropy plays a secondary role in determining the coercivity of these hexaferrites. The high coercivity of barium hexaferrites (typically in the range of 2000 – 4000 Oe) is prohibitive for use in data storage media as conventional heads are incapable of swithing the particles. Reduced coercivities for high density magnetic recording media or other applications requiring lower coercivities can be achieved by partial substitution of Fe3+ ions by specific combinations of Mn2+, Ni2+, Mg2+, Co2+, Zn2+, Ti2+ or Sn2+ ions with tetravalent ions such as Ti4+, Ru4+, or Sn4+ [15-27]. On the other hand, improvement of the coercivity for applications requiring hard magnetic materials was achieved by substituting Fe3+ ions by a trivalent metal ion such as Mn3+, Al3+, Ga3+, In3+, Sc3+, As3+, and Cr3+ [28 – 31]. In addition, the structural and physical properties of hexaferrite powders were modified by by adopting different physical or chemical preparation methods, as well as varying the experimental conditions such as stoichiometry, heat treatment, and binder additives [31-39]. Mössbauer spectroscopy was used to obtain information on the local chemical and structural environment in hexaferrites. This information was employed to determine local structural symmetries, preferential site ocupation, and valence state of Fe ions in the hexaferrite lattice [5, 15 – 17, 23, 28, 40, 41]. However, clear differences in the reported hyperfine parameters of hexaferrites were found in the literature [41 – 47]. These differences could be due to different nature of samples prepared by different methods and under different experimental conditions. Further, the complexity of Mössbauer spectra and similarity of the hyperfine parameters corresponding to some crystallographic sites could lead to some confusion in interpreting the Mössbauer spectra. Solid State Phenomena Vol. 232 67 2. Properties of Hexaferrites 2.1. M-Type Hexaferrites: M-type barium hexaferrite (BaFe12O19) is a hard ferrimagnetic material with Curie temperature 450° C, melting point of 1390 °C, molecular mass of about 1112 g, and maximum density of ρ = 5.28 g/cm3. It has a hexagonal structure with typical lattice parameters a = b = 5.88 Å and c = 23.2 Å . The unit cell of M-type barium hexaferrite consists of two R blocks and two S blocks in the stacking sequence RSR*S*, where the star denotes a block rotated by 180° about the c-axis of the hexagonal lattice. The S (Fe6O8) block contains two hexagonal layers of oxygen, while the R block (BaFe6O11) consists of three hexagonal oxygen layers, with one Ba cation replacing an oxygen anion substitutionally in the middle layer. The unit cell therefore contains two (BaFe12O19) molecules. In the BaM lattice, there are five different interstitial sites where small metal ions (such as Fe ions) reside. One octahedrally coordinated (2a) site and two tetrahedrally coordinated (4f1) sites exist in each S block, while two octahedrally coordinated (4f2) sites and one five-fold coordinated (2b) bi-pyramidal site exist in each R block. In addition, there are three octahedral (12k) sites in each interface between an S and an R block. BaM is a uniaxial crystal with a c-easy axis. The spins of the magnetic ions at 2a, 2b, and 12k sites are parallel to the c-axis, forming three spin-up sublattices, while the spins of the ions at the 4f1 and 4f2 sites are aligned antiparallel to the c-axis, thus forming two spin-down sublattices in the hexaferrite structure. The characteristics of these sublattices, and their positions in the hexaferrite lattice are shown in Table 1. Table 1. Metallic sub-lattices of M-type hexaferrite Block sublattice coordination ions per unit Spin cell direction S 4f1 Tetrahedral 4 down 2a Octahedral 2 Up 4f2 Octahedral 4 Down 2b Bi-pyramidal 2 Up 12k Octahedral 12 Up R S-R Assuming spin collinear structure in barium hexaferrites (at 0 K), the net magnetic moment per unit cell of BaM is 20 µB, which corresponds to a saturation magnetization of about 100 emu/g . This theoretical value is consistent with low-temperature magnetization measurements [1, 26]. The saturation magnetization of BaM at 20 K is 72 emu/g, and its Curie temperature is 450° C. 2.2. W-Type Hexaferrites: The W-type hexaferrite with chemical formula BaMe2Fe16O27 (Me is a divalent metal ion such as Co, Zn, Ni, Mg, and Mn) is a ferrimagnetic material with Curie temperature ranging from 415° C (for Mn2W) to 455° C (for Fe2W) . ZnFe-W has a Curie temperature of 430° C, a theoretical density of about 5.31 g/cm3, and a saturation magnetization of 108 emu/g at 0 K and 73 emu/g at 20° C [1, 48]. The lattice parameters of the W-type are typically a = b = 5.88 Å and c = 32.8 Å . The unit cell of the W-type hexaferrite is built from double blocks S and blocks R in accordance with the stacking sequence RSSR*S*S*. In this phase there are seven distinct crystallographic sites: two tetrahedral (4e and 4fIV), four octahedral (4fVI, 6g, 4f, and 12k) and one bi-pyramidal (2d) sites (see Table 2). 68 Ferroic Materials: Synthesis and Applications Table 2. Iron sites, their coordination, spin orientation, number of Fe ions per formula, and location in the W-type lattice. Sub-lattice Coordination Location Number of Spin ions orientation 4e tetrahedral S 2 down 4fIV tetrahedral S 2 down 4fVI octahedral R 2 down 4f octahedral S 2 up 2d Bi-pyramidal R 1 up 12k octahedral R-S 6 up 6g octahedral S-S 3 up 2.3. The MoZn-Substituted M-Type Hexaferrite: While BaM hexaferrites with various substitutions for Fe ions occupied a large area of the studies involving this important class of magnetic materials, substitutions involving molybdenum were found to be extremely rare in the literature. In a previous study, Mössbauer spectroscopy was reported on CoMo substituted M-type hexaferrites, where it was found that the substitution occurs at the 12k sites . This study, however, was limited to Mössbauer spectroscopy and did not address the details of the structural and magnetic properties of the ferrites. In a series of recent studies performed in our laboratories, the effects of MoZn substitutions under different experimental conditions were investigated. [50 – 52]. The magnetic properties and hyperfine parameters were reported for BaFe11.6MoxZn0.4-xO19 (x = 0.1, 0.2, 0.4) hexaferrites with Ba/Fe ratio of 1:11 prepared by wet chemical mixture method and sintered at 1100° C, with the focus on the distribution of Mo ions having different valence states (Mo4+ and Mo6+) and its effects on the magnetic properties . In this study, Mo4+ and Mo6+ ions were reported to occupy (4f1 + 12k) and 2b sites, respectively. The sample with x = 0.4 demonstrated preference of the Mo4+ and Mo6+ ions for 2b and 12k sites, with the development of the Fe2+ valence state. These ferrites exhibited an increase in saturation magnetization which culminated at x = 0.2, and then a decrease at x = 0.4. The saturation magnetization for these compounds were relatively high (70 – 75 emu/g), and the coercivity changed from about 1980 Oe to 2600 Oe, which are properties of materials with potential for high density magnetic recording . In addition, the magnetic properties and hyperfine interactions of BaFe12-2xMoxZnxO19 (0.0 < x < 0.3) hexaferrites with Ba/Fe ratio of 1:11 prepared by wet chemical mixture method and sintered at 1100° C, were investigated . The coercivity of this system was found to change from 2600 Oe (for x = 0.0) to about 1060 Oe (for x = 0.3) while the saturation magnetization increased from 71 emu/g for the un-doped sample to 75 emu/g for the sample with x = 0.15. This system also demonstrated tunable magnetic properties for potential high density magnetic recording. On the other hand, BaFe12-4xMoxZn3xO19 (0.0 < x < 0.3) hexaferrites with Ba/Fe ratio of 1:11 prepared by ball milling and sintered at temperatures between 1100° C and 1300° C were studied . The saturation magnetization for the samples sintered at 1100° C decreased from about 70 emu/g for the un-doped sample down to 57 emu/g for the sample with x = 0.3, and the decrease was attributed to the presence of nonmagnetic impurity phases. These samples demonstrated hard magnetic properties with coercivity variations between about 2600 Oe (for x = 0.1) and 3450 Oe (for x = 0.0). Upon increasing the sintering temperature, however, the saturation magnetization of the sample with x = 0.2 improved significantly (58.9 emu/g for the sample sintered at 1100° C, and Solid State Phenomena Vol. 232 69 70.5 emu/g for the sample sintered at 1300° C), and the coercivity decreased significantly (from about 2800 Oe to about 300 Oe). These dramatic changes were associated with the development of the W-type hexaferrite phase at high sintering temperatures. In view of the above discussion, the MoZn-substituted barium hexaferrite system seems to have rich structural and magnetic phase diagram, and provides the opportunity to produce materials with tunable magnetic properties. Therefore, in this article, we report on the structural and magnetic properties of BaFe12-4xMoxZn3xO19 hexaferrite prepared by ball milling and different experimental conditions. The Mo:Zn ratio of 1:3 was chosen to ensure that molybdenum ions in the hexaferrite lattice would have the Mo6+ valence state. The structural properties and the particle morphology of the prepared compounds were investigated by powder x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Also, the effects of Mo6+Zn2+ concentration on the phase evolution, cationic distribution, and the magnetic properties were investigated by Mössbauer spectroscopy, and isothermal and thermomagnetic measurements. 3. Experimental Techniques 3.1. Sample Preparation: Powder precursors of barium hexaferrites with stoichiometry consistent with the chemical formula BaFe12-4xMoxZn3xO19 were prepared from high purity (~99%) powders of BaCO3, Fe2O3, ZnO and MoO2 (Sigma-Aldrich made). The barium to metal molar ratio was 1:11.5, and the mixing and homogenization of the powder precursors was achieved by using a planetary ball mill (Fritch Pulverisette 7). The milling cups and balls were made of zirconia, and the powderto-ball mass ratio was 1:14 to ensure effective grinding and obtain homogenous powders. Wet milling of the powder precursors in an acetone bath (8 ml for each 5 grams of the powder) was performed for 16 hours at a rotational speed of 250 rpm. The wet powder was then left in the container overnight to dry at room temperature, and subsequently collected in clean glass vials. About 1 g of the powder was mixed with an adhesive agent of 2% wt. of polyvinyl alcohol (PVA) and then dry-pressed into disc-shape pellet (1.5 cm in diameter) in a stainless steel die under a 4 ton force. The discs were then sintered at temperatures ranging from 1100° C to 1300° C for 2 h in air. 3.2. Sample Characterization: The prepared samples were characterized using standard structural and magnetic measurements. X-ray diffraction (XRD) was used to determine the structural phases of the sample, and examine their characteristics and abundance. XRD patterns were collected in the angular range 20° ≤ 2θ ≤ 70° in steps of 0.02° using an XRD 7000-Shimadzu diffractometer with Cu-Kα radiation. The patterns were analyzed by the Expert High Score software to identify the structural phases, and then Rietveld refinement was performed to fit the patterns using the FullProf software . The particle morphology and particle size distribution were examined by scanning electron microscopy using FEI-Inspect F50/FEG instrument. Mössbauer spectroscopy was performed using a conventional constant acceleration spectrometer with 57Co/Cr source. The spectrum of metallic iron at room temperature was used to calibrate the energy scale, and the isomer shift was determined with respect to its center. The spectra were collected over 512 channels, and then analyzed using a standard fitting routine based on least square minimization technique. The magnetic measurements were conducted using a vibrating sample magnetometer (VSMMicroMag 3900, Princeton Measurements Corporation). Magnetization measurements were made at different temperatures in applied fields up to 10 kOe. 70 Ferroic Materials: Synthesis and Applications 4. Results and Discussion 4.1. Scanning Electron Microscopy: SEM images for the samples with x = 0.1 sintered at different temperatures are shown in Fig. 1. The figure indicates that the sample sintered at 1100° C consists of thin hexagonal plates of diameters typically < 500 nm. These particles therefore have diameters up to the critical single domain size in hexaferrites. The particles for the sample sintered at 1200° C grew in size, where the typical particle diameter became 0.5 – 1.0 μm. Upon further increasing the sintering temperature up to 1300° C, the particle diameter grew further up to diameters typically > 1 μm. The samples with higher x values (Fig. 2 and Fig. 3) demonstrated similar tendency of particle growth with increasing the sintering temperature. Further, increasing the dopant concentration seems to result in increasing the particle size. The size distribution for the sample with x = 0.2, for example, is dominated by particles with diameters < 0.5 μm, while the sample with x = 0.3 is dominated by particles with diameters > 0.5 μm. It is also evident that samples with increasing x sintered at 1300° C demonstrated uncontrollable growth of particles, leading to the formation of thin plates with diameters ~ 10 μm. This is indicative that the increased level of Mo – Zn addition to the hexaferrites prohibits the formation of distinct small particles with well-defined boundaries. From these results we can conclude that the particles in the samples sintered at temperatures > 1100° C contain multi-domain magnetic structure. Fig. 1. SEM images of the samples with x = 0.1 sintered at different temperatures. Solid State Phenomena Vol. 232 71 Fig. 2. SEM images of the samples with x = 0.2 sintered at different temperatures. Fig. 3. SEM images of the samples with x = 0.3 sintered at different temperatures. 4.2. X-Ray Diffraction: Fig. 4 shows the refined diffraction patterns for all samples sintered at 1100° C. The patterns of all samples indicated the presence of a major BaM hexaferrite phase with structure consistent with the standard pattern JCPDS: 00-043-0002, and lattice parameters around a = 5.89 Å and c = 23.2 Å. In addition, α-Fe2O3, BaMoO4 and ZnFe2O4 oxides were detected as minor 72 Ferroic Materials: Synthesis and Applications phases whose structures were consistent with the standard patterns JCPDS: 00-033-0664, 01-0894570, and 00-022-1012, respectively. These minor phases were detected by other workers in similar hexaferrites [50, 52]. The intensities of the diffraction peaks corresponding to the Zn-spinel (ZnFe2O4) and barium molybdenum oxide phases obviously increased with increasing x. The relative intensity of the main peak of BaMoO4 at about 2θ = 26.7° increased from about 17.5% of the main BaM peak for the sample with x = 0.1 up to about 52.6% (three times bigger) for the sample with x = 0.3. This is a clear indication that Mo ions do not enter the BaM lattice, and are completely incorporated in forming the BaMoO4 phase. Also, we noticed that the α-Fe2O3 disappeared from the sample with x = 0.2, which leads to the conclusion that this sample consists of three phases with relative molar ratios of: 0.2 (BaMoO4):0.8 (BaFe12O19):0.55 (ZnFe2O4) These ratios were derived by fixing the Ba, Mo, and Fe molar ratios to the experimental ratios of 1.0:0.2:10.7. The derived molar ratios of the phases indicate that the ratio of Zn (0.55) is slightly smaller than the starting value of 0.60, which could be attributed to zinc being more volatile than other elements in the sample. Further, the small amount of the α-Fe2O3 phase in the sample with x = 0.3 is due to the excess of Fe ions which remain in the form of unreacted α-Fe2O3 phase. If the molar ratios of the starting materials of Ba:Fe:Mo:Zn = 1.0:10.3:0.3:0.9 were used to derive the molar ratios of the four existing phases, we would obtain: 0.3 (BaMoO4):0.7(BaFe12O19):0.9 (ZnFe2O4):0.05 (α-Fe2O3) This suggests the presence of the α-Fe2O3 phase with a mass ratio of about 0.7% in the sample with x = 0.3. Rietveld refinement of the XRD pattern for this sample, however, indicates the presence of α-Fe2O3 phase with a mass ratio of about 3%. This also could be attributed to Zn-loss in the sintering process. If we assume 8% loss of Zn (as in the sample with x = 0.2), then the molar ratio of α-Fe2O3 becomes about 0.24, which accounts for about 3.5% of the mass of the sample, in good agreement with the experimentally detected relative proportion of this phase. In an attempt to eliminate the secondary phases and promote the production of a hexaferrite phase with higher quality, the samples were sintered at higher temperatures of 1200° C and 1300° C, respectively. It is obvious from Fig. 5 that the Zn-spinel phase disappeared from all samples sintered at 1200° C, except that with x = 0.3. The presence of the M-type phase as the major phase and BaMoO4 and α-Fe2O3 as minor phases in the patterns of the samples with x < 0.2 suggest that at relatively low Zn concentrations (< 0.6), the high-temperature reaction of the Zn-spinel with BaM promotes the substitution of Zn ions in the BaM lattice, which is evidenced by the change in the relative intensities of the main peaks for this phase. The appearance of the α-Fe2O3 phase in these samples is facilitated by the formation of BaMoO4 phase and the consequent reduction in the amount of Ba which is required to react with α-Fe2O3 to form the hexaferrite phase. On the other hand, the persistence of the Zn-spinel phase, the variation of the relative intensities of the BaM peaks, and the observed increase of the α-Fe2O3 phase in the sample with x = 0.3 compared to the sample sintered at 1100° C (Fig. 6) is an indication of the limited solubility of Zn in the BaM hexaferrite lattice. In the sample with x = 0.2, however, W-type hexaferrite phase whose structure is consistent with the standard pattern JCPDS: 00-052-1868 for Zn2W (BaZn2Fe16O27) developed, and the Zn-spinel phase disappeared almost completely. This result is evidence that the reaction of Znspinel and BaM phases is completed to form Zn2W at 1200° C in this sample in accordance with the reaction : 2 (ZnFe2O4) + BaFe12O19 = BaZn2Fe16O27 (2) Solid State Phenomena Vol. 232 73 Accordingly, the relative intensities of the diffraction peaks corresponding to the W-phase increased at the expense of those corresponding to BaM phase (Fig. 7). It should be kept in mind, however, that the W-phase can exist with the general chemical formula BaZn2-yFe16+yO27, where the value of y is determined by the amount Zn ions substituting Fe ions in the BaM lattice, and has a significant effect on the magnetic properties of the compound . Fig. 4. XRD patterns of the BaFe12-4xMoxZn3xO19 samples sintered at 1100° C. 74 Ferroic Materials: Synthesis and Applications Fig. 5. XRD patterns of the BaFe12-4xMoxZn3xO19 samples sintered at 1200° C. Solid State Phenomena Vol. 232 Fig. 6. XRD patterns of BaFe10.8Mo0.3Zn0.9O19 samples sintered at different temperatures. 75 76 Ferroic Materials: Synthesis and Applications Fig. 7. XRD patterns of BaFe11.2Mo0.2Zn0.6O19 samples sintered at different temperatures. Upon sintering the samples at 1300° C, the XRD patterns (Fig. 8) indicate that the Zn-spinel phase disappeared completely from all samples, and the W-phase appeared in the sample with x = 0.3 as a consequence of the reaction of the Zn-spinel and BaM intermediate phases. However, for x < 0.2 the W-type (BaZn2Fe16O27) phase was not observed, which is an indication of the limited solubility of Zn ions in the BaM phase, and that the critical concentration of Zn ions which is required to Solid State Phenomena Vol. 232 77 produce the W-phase is 0.6. Accordingly, in the subsequent sections we will limit our hyperfine and magnetic studies to samples with x = 0.2 and 0.3. Fig. 8. XRD patterns of the BaFe12-4xMoxZn3xO19 samples sintered at 1300° C. 4.3. Mössbauer Spectroscopy: Mössbauer spectroscopy (MS) is based on a local effect which can provide structural as well as magnetic information [53 – 55]. This effect uses 57Fe as a probe. Iron ions residing at different sites with different coordination and chemical environments give rise to different components in Mössbauer spectrum of a sample. Magnetically ordered iron ions in a given sublattice give rise to a six-line pattern (sextet) magnetic component due to Zeeman splitting. However, a magnetically disordered Fe sublattice gives a paramagnetic component: a singlet or a doublet. The doublet typically has a quadrupole splitting which is dependent on the iron valence state, and the distortion of the iron site and deviation from cubic symmetry. This quadrupole splitting arises from the interaction of the nuclear moment of 57Fe with the electric field gradient at the nuclear site. Fig. 9 shows Mössbauer spectrum for the sample with x = 0 sintered at 1100° C. The spectrum was fitted with five magnetic components corresponding to the five iron sites in the lattice (Table 1). The fitting parameters (Table 3) are in very good agreement with previously reported results [28, 51, 56]. The values of isomer shifts (~ 0.3 – 0.4 mm/s) are typical for Fe3+. The results of MS confirm the structural analysis which indicated the presence of a single BaM phase in this sample. 78 Ferroic Materials: Synthesis and Applications Intensity x=0 -15 -10 -5 0 5 10 15 Velocity (mm/s) Fig. 9. Mössbauer spectrum for BaFe12O19 sintered at 1100° C. Table 3. The hyperfine fields (Bhf) in kOe, isomer shifts (CS) in mm/s, quadrupole splitting (QQ) in mm/s, and percentage relative intensities (I) of the components of the spectra for the samples with different values of x sintered at the indicated temperature. Mössbauer parameters Msites x = 0.0 x = 0.2 x = 0.2 W-sites x = 0.3 (1100° C) (1100° C) (1200° C) Bhf1 4f2 518 518 516 4fVI Bhf2 2a 511 504 494 6g Bhf3 4f1 494 493 479 4f 486 Bhf4 12k 418 418 421 4e+4fIV 418 Bhf5 2b 405 411 383 2d 386 Bhf6 - - - 373 12k 369 CS1 0.39 0.40 0.40 CS2 0.38 0.43 0.27 CS3 0.27 0.28 0.41 0.32 CS4 0.38 0.38 0.38 0.38 CS5 0.32 0.33 0.29 0.43 (1100° C) 511 0.42 Solid State Phenomena Vol. 232 79 CS6 - 0.38 0.37 0.38 QQ1 0.23 0.19 0.19 0.12 QQ2 0.12 0.10 0.28 QQ3 0.22 0.21 0.16 0.18 QQ4 0.42 0.41 0.39 0.40 QQ5 2.19 2.24 2.24 2.50 - 0.34 0.34 0.39 I1 19.7 11.8 9.8 18.0 I2 6.8 10.9 15.9 24.7 I3 16.4 14.2 10.1 I4 50.8 47.0 25.9 27.8 I5 6.3 5.4 5.8 2.8 10.8 32.4 26.7 2.70 2.22 14.2 QQ6 I6 χ2 - 2.68 Fig. 10 shows Mössbauer spectrum for the sample with x = 0.2 sintered at 1100° C and 1200° C. Evidently, the magnetic sub-spectrum for the sample sintered at 1100° C is similar to the spectrum for the sample with x = 0.0. In addition, a significant paramagnetic component appeared near the center of the spectrum. Accordingly, the spectrum was fitted with five magnetic components and one paramagnetic doublet. The hyperfine parameters for the magnetic components are similar to those for the sample with x = 0, and these components are therefore associated with the five iron sites in BaM lattice. The paramagnetic doublet is associated with the paramagnetic ZnFe2O4 spinel phase  which was confirmed by our XRD analyses. The spectrum for the sample with x = 0.2 sintered at 1200° C, however, shows a significantly different structure from that of the sample sintered at 1100° C. In addition, the central paramagnetic component associated with the zinc-spinel phase disappeared at this sintering temperature, which is consistent with the XRD data. The spectrum was best fitted with six magnetic components with hyperfine parameters listed in Table 3 above. The hyperfine parameters for the magnetic components are consistent with those of previously reported results on other W-type hexaferrites [57, 58]. This result is also consistent with the presence of BaZn2Fe16O27 phase observed in XRD pattern for this sample. In this phase there are seven distinct crystallographic sites: two tetrahedral (4e and 4fIV), four octahedral (4fVI, 6g, 4f, and 12k) and one bi-pyramidal (2d) sites (see Table 2). The hyperfine parameters for the 4e and 4fIv cannot be distinguished from each other, and one magnetic component is typically assigned to these two sites. In some cases when the spectrum is complex, the magnetic components associated with 6g and 4f sites also cannot be distinguished from each other. The six-component fit for the sample with x = 0.2 sintered at 1200° C gave relative sub-spectral intensities which are in good agreement with the theoretical relative ratios of 2:3:2:4:1:6. 80 Ferroic Materials: Synthesis and Applications Intensity XRD results indicated the coexistence of the Fe-containing BaM and Zn2W phases in this sample. However, due to the similarity of the crystal structures and symmetries of the sites for these two hexaferrites, and the complexity of the Mössbauer spectrum, it was not possible to resolve the components for the two different types. However, the persistence of the strong, sharp component associated with the 12k sublattice of BaM could be evidence of the presence of the two phases in this sample. Intensity x=0.2, 1100C x=0.2, 1200C -15 -10 -5 0 5 10 15 Velocity (mm/s) Fig. 10. Mössbauer spectrum for BaFe11.2 Mo0.2Zn0.6O19 sintered at 1100 and 1200° C. Mössbauer spectrum for the sample with x = 0.3 sintered at 1100° C is shown in Fig. 11. The spectrum for this sample shows a magnetic sextet and a central paramagnetic component, similar to the spectrum for the sample with x = 0.2 sintered at 1100º C. The spectrum was best fitted with five magnetic components and a paramagnetic component. The hyperfine parameters of the magnetic sextets (Table 3) are consistent with the five different sites of BaM. The two magnetic components corresponding to the 2a and 4f1 sites, however, could not be resolved, and appeared as a single component . The relative intensity (24.7%) of this magnetic sub-spectrum is in good agreement with the theoretical value of 25%. In addition, a new component with relatively low hyperfine field (369 kOe) developed, which is associated with the splitting of the 12k component as a consequence of the perturbation of part of the corresponding sublattice by excessive substitution of nonmagnetic ions at neighboring sites . The reduction of the hyperfine fields of the sextets is an indication of the perturbation of the magnetic sublattices by the substitution process. In the context of discussing the structural properties of this sample, it was mentioned that the Zn-spinel phase does not account for the amount of Zn in the precursor powder, and this was associated with the evaporation of Zn Solid State Phenomena Vol. 232 81 during the sintering process. On the basis of the Mössbauer data, however, we could argue that some Zn ions entered the hexaferrite lattice, resulting in the observed variations of the hyperfine parameters. The central paramagnetic component is associated with the Zn-ferrite phase confirmed by the XRD pattern of this sample. Intensity x=0.3 -15 -10 -5 0 5 10 15 Velocity (mm/s) Fig. 11. Mössbauer spectrum for (BaFe11.8 Mo0.3Zn0.9O19) sintered at 1100° C 4.4. Magnetic Measurements: The magnetic properties of the hexaferrites prepared with x = 0.2 and 0.3 are addressed in the following discussion. The hysteresis properties are discussed in light of the structural characteristics and the effects of heat treatment is discussed. Further, the thermomagnetic measurements were used to shed light on the magnetic phases and the magnetic transition temperatures in the samples. 4.4.1. Hysteresis Characteristics: The hysteresis loops for the samples with x = 0.0, 0.2, and 0.3 sintered at 1100° C are shown in Fig. 12. The magnetization did not saturate up to the maximum field applied, and therefore, the saturation magnetization was determined from the law of approach to saturation, according to which the magnetization in the high field region is given by : = (1 − − 2 ) + (3) Here M is the magnetization (in emu/cm3), A is a constant associated with microstress and/or inclusions, B is a constant due to magnetocrystalline anisotropy, and the last term in eq. 3 is the forced magnetization term. A plot of M vs 1/H2 in the high-field region gave a straight line for each sample, indicating that the contributions of the microstress/ inclusions, and the forced magnetization are negligible due to the high magnetocrystalline anisotropy in our samples. The saturation magnetization σs (= Ms divided by the density) was obtained from the intercept of the straight line, and the results are tabulated in Table 4. The decrease in magnetization with increasing x in the samples sintered at 1100° C is mainly due to the increase in the relative proportions of the nonmagnetic phases as confirmed by XRD. 82 Ferroic Materials: Synthesis and Applications The coercivity and remnant magnetization for each sample were determined from the hysteresis loop, and the results are tabulated in Table 4. We observed that the coercivity decreased with increasing x from a hard magnetic state ( HC = 3270 Oe for x = 0.0) to a state of moderate magnetic hardness ( HC = 910 Oe for x = 0.3). Although the magnetic phase in all samples is BaM, the modification of the coercivity could be associated with the different particle size distributions in the samples, and the growth of the mean particle diameter with increasing x . SEM image of the sample with x = 0.3 indicated the presence of large multi-domain particles which should have a significant effect on lowering the coercivity of this sample with respect to that with x = 0.2, whose SEM image demonstrated that the particle size is dominated by single domain particles. Further, the squareness ratio (= σr/ σs) for the samples with x = 0.0 and 0.2 is ~ 0.5, which is characteristic of randomly oriented single-domain particles (with typical diameters < 0.5 µm). However, the squareness ratio for the sample with x = 0.3 is ~ 0.4, which is a further confirmation that this sample consists of multi-domain particles, and thus has lower coercivity due to the dominance of domainwall motion in the magnetization processes. 60 40 T = 1100 C x=0 x = 0.2 x = 0.3 (emu/g) 20 0 -20 -40 -60 -10000 -5000 0 5000 10000 H (Oe) Fig. 12. Hysteresis loops of BaFe12-4xMoxZn3xO19 ferrites sintered at 1100° C. Solid State Phenomena Vol. 232 83 Table 4. Saturation magnetization σs (emu/g), remnant magnetization σr (emu/g) and coercivity HC (Oe) of ferrites samples sintered at 1100° C, 1200° C and 1300° C. x 0 0.2 0.3 1100 σs 67.1 60.1 54.6 σr 35.7 29.6 22.2 Hc 3275 2100 910 1200 σs 71.7 - σr 20.0 - 1300 Hc σs 1050 74.4 66.7 σr 14.6 6.4 Hc 447 180 The hysteresis loops for the sample with x = 0.2 sintered at 1200° C and 1300° C are presented in Fig. 13, and the magnetic parameters derived from these loops are listed in Table 4. It is evident that increasing the sintering temperature improved the saturation magnetization, and reduced the coercivity significantly. The sample sintered at 1200° C exhibited a saturation magnetization of 71.7 emu/g and coercivity of 1050 Oe. While the observed saturation magnetization is in very good agreement with that (71 emu/g) reported for multi-domain Fe2W particles sintered at 1200° C, the coercivity is significantly higher than the reported value of 305 Oe for Fe2W particles . This difference could be due to the coexistence of the hard M-type phase in our sample, although effects of different particle size and chemical stoichiometry cannot be excluded. As the sintering temperature increased up to 1300° C, the saturation magnetization increased up to 74.4 emu/g, and the coercivity dropped down to 447 Oe. These values are in excellent agreement with those of Fe2W particles sintered at 1250° C  and slightly lower than the saturation magnetization (78 emu/g) of bulk Fe2 hexaferrite . The increase in saturation magnetization upon sintering at 1300° C is associated with the increased level of transformation of the BaM phase to W-type phase, and the observed saturation magnetization (with value intermediate between that of the M-type and that of the W-type) could indicate that the two phases coexist with almost equal mass fractions. The secondary barium molybdenum oxide in the sample also should have an effect in reducing the saturation magnetization, but due to its small mass fraction in the sample, its effect appears to be negligible, and the obtained samples could be considered high quality hexaferrite samples with tunable magnetic properties. 80 60 40 x = 0.2 T = 1200 C T = 1300 C (emu/g) 20 0 -20 -40 -60 -80 -10000 -5000 0 5000 10000 H (Oe) Fig. 13. Hysteresis loops of BaFe11.2Mo0.2Zn0.6O19 ferrites sintered at 1200° C and 1300° C. 84 Ferroic Materials: Synthesis and Applications As the optimal sintering temperature for the formation of the W-type phase in the samples with x = 0.2 and 0.3 seems to be 1300° C, the hysteresis loops of these two samples were measured and analyzed. Fig. 14 and the data in Table 4 indicate that the saturation magnetization of the sample with x = 0.3 is lower than that of the sample with x = 0.2. This marginal decrease could be associated with the higher presence of nonmagnetic barium molybdenum oxide as evidenced by the XRD patterns of the two samples. The coercivity is further reduced with increasing x, which could be due to the increased particle diameter. The produced compounds could be suitable for applications requiring soft magnetic materials. 80 60 T = 1300 C x = 0.2 x = 0.3 40 (emu/g) 20 0 -20 -40 -60 -80 -10000 -5000 0 5000 10000 H (Oe) Fig. 14. Hysteresis loops of BaFe12-4xMoxZn3xO19 ferrites sintered at 1300° C. 4.4.2. Initial Magnetization Curves: The magnetization curves for the samples presented in Fig. 15 are clearly different in more than one aspect. The sample with x = 0.2 sintered at 1100° C has lower magnetization than the un-doped BaM sample as mentioned in the above discussion. However, this sample is clearly softer, as its initial susceptibility (σ/H) is about 35% higher than that of BaM. This result is consistent with the growth of the particles upon substitution for Fe ions in this system. At higher sintering temperatures, the samples became progressively softer, and the magnetization increased. The initial susceptibility of the sample with x = 0.2 sintered at 1200° C increased by up to 3.6 times that of BaM, which is also consistent with the previously discussed structural and magnetic properties of this sample. Further, the sample with x = 0.3 sintered at 1300° C is clearly a soft magnetic material with an initial susceptibility ~ 6.9 times greater than that of BaM. The largest increase in susceptibility is induced by the highest degree of transformation of BaM to W-type, and the increase in particle size. Solid State Phenomena Vol. 232 85 70 60 (emu/g) 50 40 30 x = 0.0, T = 1100 C x = 0.2; T = 1100 C x = 0.2; T = 1200 C x = 0.3; T = 1300 C 20 10 0 0 2000 4000 6000 8000 10000 H (Oe) Fig. 15. Magnetization curves of BaFe12-4xMoxZn3xO19 ferrites sintered at the indicated temperatures. 4.4.3. Thermomagnetic Measurements: The thermomagnetic curve of the sample with x = 0 measured at an applied field of 8 kOe is shown in Fig. 16, together with its derivative with respect to temperature. The derivative shows a single dip at 450 °C, indicating that this sample consists of a single BaM phase. 60 Derivative 50 0.00 H = 8 kOe -0.05 (emu/g) -0.15 30 -0.20 20 -0.25 Derivative -0.10 40 -0.30 10 -0.35 0 0 100 200 300 400 500 -0.40 600 T (deg. C) Fig. 16. Thermomagnetic curve of BaFe12O19 ferrites sintered at 1100° C. 86 Ferroic Materials: Synthesis and Applications The thermomagnetic curve of the sample with x = 0.2 sintered at 1100° C (Fig. 17) shows a dip in the derivative at about 455° C, which is associated with the Curie temperature of BaM phase. The closeness of the transition temperature to that of pure BaM may indicate that substitution did not take place in this sample. In addition, a clear dip is observed at about 225° C. This transition temperature could be associated with (ZnxFe1-x)Fe2O4 spinel ferrite phase observed in XRD. The peak at about 115° C, however, could be due to spin-glass transition at this temperature. The shape of the thermomagnetic curve is, however, dominated by BaM behavior, which is consistent with our XRD and Mössbauer studies. (emu/g) Derivative 0.00 H = 8 kOe 40 -0.05 30 -0.10 20 -0.15 10 -0.20 0 -0.25 0 100 200 300 400 500 Derivative 50 600 T (deg. C) Fig. 17. Thermomagnetic curve of BaFe11.2 Mo0.2Zn0.6O19 ferrite sintered at 1100° C. The thermomagnetic curve of the sample with x = 0.2 sintered at 1200° C shows far more rich structure as indicated by Fig. 18. The dip at 410° C is associated with the Curie temperature of the BaM phase. The decrease in the transition temperature (by about 40° C) could be associated with partial substitution of Zn ions at the tetrahedral site of the BaM lattice, resulting in a decrease in the strength of the super-exchange interactions between spin-up and spin-down sublattices. The dip at 350° C is associated with the transition temperature of the W-phase. The fact that this transition temperature is significantly lower than that of ZnFe-W phase (450° C ) may indicate that the majority of the Me2+ ions in the W-phase in our sample are Zn2+. This low transition temperature is, however, consistent with the 300° C – 400° C reported for substituted Co2W hexaferrite . The clear dip at 217° C is associated with the Zn-spinel ferrite, part of which may have remained at this temperature. Due to the complexity of the magnetic and crystallographic structure of this sample, however, we cannot confirm such an assignment of the transition temperature. In light of our structural analysis, which confirmed the presence of BaM and W-type phases, an alternative assignment can be suggested, according to which the transition temperature at 217° C is associated with a secondary W-type phase which is rich in Zn and has lower transition temperature. In a yet second alternative argument, one could suggest that the dips at 410° C and 350° C are associated with the Curie temperatures of the BaM phases with different levels of Zn substitution. These transition temperatures are consistent with reported temperatures in the range 292 – 402° C for substituted BaM hexaferrites . The transition temperature at 217° C is associated with the Znrich (Zn,Fe)-W phase. These arguments seem to be more consistent with the XRD and Mössbauer data. Solid State Phenomena Vol. 232 87 0.05 60 H = 8 kOe Derivative 0.00 50 -0.05 -0.10 30 -0.15 20 Derivative (emu/g) 40 -0.20 10 -0.25 0 0 100 200 300 400 500 -0.30 600 T (deg. C) Fig. 18. Thermomagnetic curve of BaFe11.2 Mo0.2Zn0.6O19 ferrite sintered at 1200° C. The thermomagnetic curve of the sample with x = 0.3 sintered at 1300° C (Fig. 19) clearly indicates two major transitions at 344° C and 225° C. In light of the XRD data of this sample, which indicates the coexistence of BaM and W-type hexaferrite phases, these transition temperatures are associated with the two hexaferrite phases, and seem to support the second alternative argument in the discussion above. 0.05 (emu/g) Derivative H = 8 kOe 0.00 50 -0.05 40 -0.10 -0.15 30 -0.20 20 -0.25 10 -0.30 Derivative 60 -0.35 0 0 100 200 300 400 500 -0.40 600 T (deg. C) Fig. 19. Thermomagnetic curve of BaFe11.2 Mo0.3Zn0.9O19 ferrite sintered at 1300° C. 88 Ferroic Materials: Synthesis and Applications 5. Conclusions Rich structural and magnetic phase diagram was found in Mo–Zn substituted barium hexaferrites with the stoichiometry of BaM phase. The samples were prepared by ball milling and subsequent sintering at temperatures from 1100° C to 1300° C. The samples sintered at 1100° C consisted of a major M-type phase in addition to barium-molybdenum oxide, Zn-spinel, and α-Fe2O3 oxides. The saturation magnetization decreased with the level of substitution due to the increased relative fractions of the nonmagnetic phases. However, the saturation magnetization of the substituted samples remained relatively high (> 54 emu/g). Also, coercivity values of about 2100 and 900 Oe were obtained by substitution with x = 0.2 and 0.3, respectively. Upon increasing the sintering temperature, the W-phase evolved as a major phase, the saturation magnetization improved, and the samples transformed to soft magnetic materials at 1300° C sintering temperature. 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