Modification of the Magnetic Properties of Co Y …eacademic.ju.edu.jo/s.mahmood/Lists/Published Research...axis) , with typical l attice parameters of a cDQG c c . This hexaferrite

Modification of the Magnetic Properties of Co2Y Hexaferrites by Divalent and Trivalent Metal Substitutions

Sami H. Mahmood1,a*, Muna D. Zaqsaw1,b Osama E. Mohsen1,c,

Ahmad Awadallah1,d, Ibrahim Bsoul2,e, Mufeed Awawdeh3,f, Qassem I. Mohaidat3,g

1Physics Department, The University of Jordan, Amman 11942, Jordan

2Physics Department, Al al-Bayt University, Mafraq 13040, Jordan 3Physics Department, Yarmouk University, Irbid 21163, Jordan

a*[email protected] (corresponding author), [email protected], [email protected], [email protected], [email protected],

[email protected], [email protected]

Key Words: Y-type Hexaferrite, Structural Properties, Magnetic Properties, Mössbauer Spectroscopy

Abstract. The present study is concerned with the fabrication and characterization of Me2Y

substituted hexaferrites, Ba2Me2Fe12-xTxO22 (Me = Co2+

, Mg2+

, and Cr2+

, and T = Fe3+

, and Ga3+

).

The samples were prepared by the conventional ball milling technique and sintering at 1200° C.

The effect of the choices of Me and T ions on the structural and magnetic properties of the

hexaferrites were investigated. XRD patterns, magnetic parameters, and Mössbauer spectra of the

Co2Y were consistent with a single phase Y-type hexaferrite. However, the CoCr-Y sample was

found to be dominated by the Y-type hexaferrite, and M-type and BaCrO4 minority phases were

observed in the XRD pattern of the sample. The small increase in saturation magnetization from

about 34 emu/g up to 37.5 emu/g was therefore attributed to the development of the M-type phase.

On the other hand, XRD pattern of the Cr2Y sample indicated the dominance of the M-type phase in

this sample. The high coercivity (1445 Oe) of this sample is evidence of the transformation of the

material from a typically soft magnetic material (Y-type) to a hard magnet (M-type). The Ga-

substitution for Fe in Co2Y did not affect the saturation magnetization significantly, but the

coercivity was reduced. However, the sample Ba2CoMgFe11GaO22 exhibited a significant reduction

of the saturation magnetization down to a value 26.6 emu/g, which could be due to the attenuation

of the super-exchange interactions induced by the Mg2+

substitution.

Contents of Paper

1. Introduction

2. Classification and Properties of Hexaferrites

2.1. M-Type Hexaferrites

2.2. Y-Type Hexaferrites

3. Experimental Techniques

3.1. Sample Preparation

3.2. Sample Characterization

4. Results and Discussion

4.1. XRD Analysis

4.2. SEM and EDS Analysis

4.2.1. Ba2Co2Fe12O22

4.2.2. Ba2CoCrFe12O22

4.2.3. Ba2Cr2Fe12O22

4.3. Magnetic Measurements

4.3.1. Hysteresis Measurements

Solid State Phenomena Vol. 241 (2016) pp 93-125© (2016) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.241.93

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 TransTech Publications, www.ttp.net. (ID: 149.200.196.22-30/07/15,08:52:46)

http://dx.doi.org/10.4028/www.scientific.net/SSP.241.93

4.3.2. Thermomagnetic Measurements of Cr-Substituted Co2Y

4.4 Mössbauer Spectroscopy

4.4.1. Ga-Substituted Y-type Hexaferrites

4.4.2. Cr-Substituted Y-type Hexaferrites

5. Conclusions

References

1. Introduction

Magnetic materials are widely used as essential components for a large variety of industrial and

technological applications including, and not limited to, automotive industry, consumer electronic,

data storage and processing, and next generation microwave (MW) devices [1-8]. Ferrites with a

hexagonal structure (hexaferrites) were discovered in 1950s [9], and extensive research work

involving the fabrication and control of the physical properties of these ferrites demonstrated their

feasibility of replacing other ferrites and magnetic materials in a large variety of applications [10-

12]. Among these hexaferrites, perhaps M-type hexaferrites are some of the earliest realized and

most widely used for permanent magnet [13-15] and high density magnetic recording media [16-

22].

The advancement in telecommunication technology, and the increased threat of electromagnetic

interference (EMI) effects on the operation of devices call for the need for efficient electromagnetic

absorbers at the operating frequencies of the devices [23]. Accordingly, significant efforts were

made in pursuit of hexaferrite-based absorbing materials [24-32]. In addition, the need for device

miniaturization and minimization of power consumption promoted the search for self-biased

materials to be used in passive microwave devices [1,7, 33-36], where hexaferrites are practically

irreplaceable for this purpose nowadays.

Efficient control of magnetic field by an applied electric field is essential for the production of

materials for the newly evolving spin-electronic technology. Hexaferrites had demonstrated

multiferroic properties and potential for spintronic applications at room temperature [37-40].

Specifically, Y-type hexaferrites were found to exhibit interesting multiferroic properties, and a

significant amount of research work, both theoretical and experimental, was devoted to the

investigation of the electronic and magnetic structure in these materials [41-51].

2. Classification and Properties of Hexaferrites

Barium hexaferrites are usually synthesized by high temperature sintering of a combination of BaO,

MeO (Me = divalent metal ion), and Fe2O3. The type of hexaferrite is obtained by appropriate

choices of the stoichiometric ratios of these oxides. Accordingly, six main types of hexaferrites

were realized, namely, M-type (BaFe12O19), Y-type (Ba2Me2Fe12O22), W-type (BaMe2Fe16O27), X-

type (Ba2Me2Fe28O46), Z-type (Ba3Me2Fe24O41) and U-type (Ba4Me2Fe36O60). These ferrites are

well documented and their properties discussed in the literature [1, 9, 10, 52]. Efforts were made to

optimize the phase purity and physical properties of hexaferrites by adopting several chemical and

physical processes. These include, and are not limited to, sol-gel method [53-55], co-precipitation

[56-58], hydrothermal [59, 60], wet chemical mixing [61, 62], and high-energy ball milling [63-69].

The structure of hexaferrites is determined by the stacking sequence of S, R, and T structural

blocks. The S block with chemical formula Me2Fe4O8 consists of two hexagonal close-packed

oxygen layers, the R block (BaFe6O11) consists of three layers with one oxygen ion in the middle

layer replaced by Ba2+

ion substitutionally, and the T block (Ba2Fe8O14) consists of four layers with

one Ba2+

ion substituting an oxygen ion in each of the two middle layers [9, 52]. The small metal

ions occupy interstitial positions with tetrahedral, octahedral, and bi-pyramidal coordinations with

oxygen cations. The magnetic ions at these sites form the spin-up and spin-down sublattices, and

the imbalance between the net magnetic moments of these sublattices is responsible for the

94 Ferrites and Ceramic Composites II

magnetization of the hexaferrites. These magnetic ions are also responsible for the

magnetocrystalline anisotropy, which in turn has a crucial impact on the coercive fields in these

ferrites.

The two basic hexaferrites are the M-type and Y-type, and the other hexaferrites are obtained by

different combinations of these ferrites and the spinel block. Table 1 summarizes the various types

of hexaferrites with the structural stacking in the unit cell [10].

Table 1. Types of hexagonal ferrites

Hexaferrite type Combination Molecular formula Structural stacking

M M BaFe12O19 RSR*S*

Y Y Ba2Me2Fe12O22 TSTSTS

W M+S BaCo2Fe16O27 RSSR*S*S*

Z M+Y Ba3Co2Fe24O41 RSTSR*S*T*S*

X 2M+S Ba2Co2Fe28O46 RSR*S*S*

U 2M+Y Ba4Co2Fe36O60 RSR*S*T*S*

2.1. M-Type Hexaferrites: The most widely investigated hexaferrite is the M-type, whose unit cell

is composed of the RSR*S* structural stacking (the star indicates rotation by 180° around the c-

axis), with typical lattice parameters of a = 5.88 Å and c = 23.2 Å. This hexaferrite is a uniaxial

hard ferromagnet with easy c-axis, and has a saturation magnetization of about 100 emu/g at zero

temperature, which decreases down to 72 emu/g at 20° C [9]. The physical properties were

modified by the substitution of Ba ions and/or Fe ions by other metal ions. Specifically, Ba ions in

these ferrites were substituted by Sr [8, 70, 71], Pb [72-74], and Ca [21, 75-77]. On the other hand,

the effects of a large variety of substitutions for Fe ions on the structural and physical properties of

hexaferrites were reported. Enhancement of the coercivity of M-type hexaferrite was achieved by

substituting Fe ions by trivalent metal ions such as Al, Ga, or Cr [3, 78-81]. However, the coercivity

was reduced by substitutions of Fe ions by a large variety of metal ion combinations [3, 19, 67-70,

82-87].

2.2. Y-Type Hexaferrites: The unit cell of the Y-type hexaferrites contains three formula units, and

is composed of stacking the S and T structural blocks in the sequence STS′T′S′′T′′, where the

primes indicate rotation by 120° around the c-axis. The lattice parameter a is similar to that of M-

type, while c = 43.56 Å [9, 88]. This kind of ferrite is a soft magnetic material with planar magnetic

anisotropy [10, 89]. The six interstitial crystallographic sites available for the small metal ions in

the Y-type hexaferrite lattice are: two tetrahedral sites ( , ), and four octahedral sites ( ,

, , ). The locations of these sites in the structural blocks, their multiplicities per

formula unit, and spin orientations of magnetic ions in these sites are illustrated in Table 2.

Table 2. Crystallographic sites in Y-type hexaferrites lattice, number of ions per molecule, and spin

orientation of magnetic ions.

Sublattice label Crystallographic

site

Block Number of ions Spin

tetrahedral S 2 ↓

octahedral S 1 ↑

octahedral S-T 6 ↑

octahedral T 2 ↓

tetrahedral T 2 ↓

octahedral T 1 ↑

Solid State Phenomena Vol. 241 95

Although Ba-based Y-type hexaferrites (Me2Y) are the most common among this class of ferrites

[90, 91], a variety of ferrites were synthesized with Ba ions substituted by other ions such as Sr [48,

54, 92, 93], Ca [94], and Pb [95]. Modifications of the properties of Y-type hexaferrites were

achieved by appropriate choice of the Me ion. Specifically, examples of the influence of such

substitutions on the magnetic properties of substituted Y-type hexaferrites are presented in Table 3.

It is worth mentioning here that the coercivity is not well determined for a given stoichiometry,

since in addition, it depends on the shape and size of the particles, and on the fabrication technique.

Large variations in the saturation magnetization, however, could be due to the presence of impurity

phases in some prepared samples. Further, modifications of the properties of Y-type hexaferrites by

substitutions for Fe3+

ions were also reported [12, 35, 49, 51, 94, 96-103]. However, research work

devoted to the effects of such substitutions in Y-type hexaferrites is relatively limited.

Table 3. Magnetic properties of Y-type hexaferrites at room temperature (superscripted values were

obtained at different temperatures).

Ferrite Ms (emu/g) Hc (Oe) Tc (°C) Reference

Ba2Co2-Y

34

39a

35.3b

27.8

21.9

-

-

-

106.3

48.2

340

-

-

-

-

[9]

[9]

[91]

[104]

[95]

Ba2Ni2-Y

24

25a

25.5

45

-

-

205

-

390

-

386.6

-

[9]

[9]

[89]

[105]

Sr2Ni2-Y 40.34 555.44 [93]

Ba2Zn2-Y

42

72a

69.6b

-

-

-

130

-

-

[9]

[9]

[91]

Ba2Mn2-Y 31

42a

-

-

290

-

[9]

[9]

Ba2Mg2-Y

23

29a

22

22.78

30.47c

-

-

100

31.35

545.87

280

-

277

-

-

[9]

[9]

[48]

[106]

[106]

Ba2Cu2-Y 28a

- - [9] a

denotes at 0 K, b

at 5 K, and c at 4.2 K

In this article, the structural and magnetic properties of a series of Y-type hexaferrites with different

choices of the Me divalent ion, and with special substitutions of trivalent metal cations for Fe3+

ions

are addressed. X-ray diffraction (XRD) was used to investigate the phase purity and structural

characteristics of the hexaferrite. Scanning electron microscopy (SEM) was employed to examine

the particle shape and morphology, the homogeneity, and local stoichiometry of the samples.

Mössbauer spectroscopy (MS) was used to investigate the hyperfine interactions of representative

sample, where the hyperfine properties were discussed in light of the structural characteristics.

Finally, the magnetic properties of the samples were investigated using the vibrating sample

magnetometry (VSM).


3. Experimental Techniques

3.1. Sample Preparation: The powder precursors of Ba2Me2Fe12-xTxO22 Y-type hexaferrites were

prepared by high energy ball milling mixtures of stoichiometric ratios of analytical grade barium

carbonate and metal oxides in a Pulvaressitte-7 ball mill equipped with zirconia bowls and balls.

Wet milling was performed in an acetone medium with a powder-ball mass ratio of 1:14. The

mechanical grinding was carried out for 16 hours at a rotational speed of 250 rpm. The resulting wet

mixture was left to dry at room temperature, and then about 1 g of the powder was dry-pressed into

a cylindrical pellet (1.5 mm in diameter) in a stainless steel die under the influence of 4-ton force.

The discs were subsequently sintered at temperatures of 1200 °C for 2 h in air atmosphere.

3.2. Sample Characterization: Rietveld refinement of the XRD patterns of the samples was

carried out to determine the phase purity and structural characteristics of the hexaferrite phases. For

this purpose, FULLPROF software was employed. The patterns were collected in the angular range

20° ≤ 2θ ≤ 70° in steps of 0.01° using XRD 7000-Shimadzu diffractometer equipped with Cu-Kα

tube. The sample for XRD measurement was finely ground powder.

The particle size distribution and particle morphologies, as well as the local chemical stoichiometry

were examined by SEM imaging, and the energy dispersive spectroscopy (EDS) facility in an FEI-

Inspect F50/FEG electron microscope.

The magnetic measurements were performed using a commercial vibrating sample magnetometer

(VSM, MicroMag 3900, Princeton Measurements Corporation) which provides a maximum applied

field of 10 kOe. Hysteresis loops at room temperature were obtained for all samples, from which the

magnetic parameters were derived. Magnetization measurements versus temperature at a constant

applied field of 100 Oe (thermomagnetic curves) were carried out on representative samples. The

samples for magnetic measurements were needle-like pieces cut from the sintered disc.

Finally, the hyperfine interactions in representative samples were investigated using 57Fe Mössbauer

spectroscopy (MS). This spectroscopy is based on the local Mössbauer effect [107], which was

extensively used to investigate the local chemical environment, valence state, and site symmetry of

Fe in alloys [108-112], intermetallic compounds [113-115], geological samples [116, 117], garnets

[118-121], and various types of ferrites [122-125]. Mössbauer spectra for the samples were

collected over 512 channels using a conventional constant acceleration Mössbauer spectrometer

operating with 57Co/Cr source. Room temperature Mössbauer spectrum of a foil of pure metallic

iron was used for velocity calibration, and isomer shifts [108] were measured with respect to the

centroid of the calibration curve. The sample for MS was in the form of a thin powder layer in a

disk-like Teflon holder.

4. Results and Discussion

4.1. XRD Analysis: The patterns of the samples Ba2Co2-xCrxFe12O22 (x = 0, 1, and 2) are shown in

Error! Reference source not found.. The structural analysis indicated that the sample with x = 0

(Co2Y) consists of a major Y-type phase, consistent with the standard (JCPD: 00-044-0206), and a

secondary M-type phase whose pattern is consistent with the standard (JCPD: 00-043-0002). The

presence of a small amount of M-type in this sample is manifested by the appearance (2 0 5) and (2

0 6) reflections of the M-type, between which the (0 2 10) reflection of the Y-type phase lies (see

Fig. 2). Rietveld refinement resulted in lattice constants a = 5.8633 Å, c = 43.522 Å. These

parameters are in agreement with previously reported values [90, 91, 106, 126]. The fraction of M-

type phase in this sample was found to be 10%. The unit cell of Co2Y constructed from the refined

cell parameters is shown in Fig. 3 (top view and side view). The figure shows the hexagonal

symmetry of the lattice, with little distortions.


The sample with x = 1 (CoCr-Y) consists of three phases, the Y-type phase (Y), the M-type phase

(M), and BaCrO4 (Cr) phase whose pattern was found to be consistent with the standard pattern

(JCPD: 00-015-0376). The increase in the fraction of the M-type in this sample is obvious. The

lattice constants for the Y-type in this sample (a = 5.8610 Å, c = 43.4939 Å) are slightly smaller

than those of the previous sample. If this reduction is to be attributed to the substitution of Co2+

ions

by smaller Cr2+

ions, then we conclude that low-spin Cr2+

ions (r = 0.73 Å) substituted high-spin

Co2+

ions (r = 0.745 Å) at octahedral sites [127]. The BaCrO4 reflections are clear in the angular

range below 30° as shown in Fig. 4.

Fig. 1: XRD patterns with Rietveld refinement for Ba2Co2-xCrxFe12O22 (x = 0, 1, and 2)


Fig. 2: Zoomed view of the XRD pattern with Rietveld refinement for Co2Y sample showing

evidence of a small amount of M-type.

Fig. 3: Unit cell as viewed along the c-axis (upper panel) and along the b-axis (lower panel).


Fig. 4: Zoomed view of the diffraction pattern of CoCr-Y sample confirming the presence of the

BaCrO4 phase.

Further, the sample with x = 2, was found to be dominated by the M-type phase, with a strong

evidence of the presence of barium-rich Ba3Fe2O6 (Ba) phase, whose diffraction pattern was

consistent with the standard pattern for this oxide (JCPD: 00-025-1477). Noting that the Ba/(Fe+Cr)

molar ratio of 1:7 used in the starting powders is significantly higher than the molar ratio of 1:12

required for BaM, we conclude that the presence of a Ba-rich phase accompanying the BaM phase

in this sample is natural. This result indicates that the equilibrium phases in this sample under the

prevailing experimental conditions are distinctly different from those of Co2Y or CoCr-Y samples.

Also, strong evidence of ZrO2 (Zr) phase was observed (Fig. 5), with a pattern consistent with the

standard pattern (JCPD: 00-037-1484) for this oxide phase. The zirconium oxide phase in this

sample may have resulted from contamination of the sample in the process of high temperature

sintering in the zirconium oxide crucible.

Fig. 5: Zoomed view of the diffraction pattern of Cr2Y sample confirming the presence of the ZrO2

and Ba3Fe2O6 phases.

The patterns of the samples Ba2Co2Fe11Ga1O22 and Ba2CoMgFe11GaO22 are shown in Fig. 6 and

Fig. 7, respectively. Each of these samples was found to be composed of a major Y-type phase and

a minor barium spinel (BaFe2O4) phase. The lattice parmeters for Ba2Co2Fe11GaO22 decreased

slightly with respect to those of Co2Y sample. However, the lattice parameters for

Ba2CoMgFe11GaO22 increased (Table 4), demonstrating a noticeable rise in the a parameter as


compared with that reported for Mg2Y [48, 106], while the value of c is consitent with that reported

by Koutzarova et al. [106], and higher than that reported by Khanduri et al. [94].

Fig. 6: XRD patterns with Rietveld refinement for Ba2Co2Fe11GaO22.

Fig. 7: XRD patterns with Rietveld refinement for Ba2CoMgFe11GaO22.

Table 4. Structural information obtained by the XRD analysis of all samples.

Sample χ2 a (Å) c (Å) V (Å

3)

Ba2Co2Fe12O22 1.29 5.8633 43.522 1295.8

Ba2CoCrFe12O22

1.10 5.8610 (Y)

5.8909 (M)

43.494 (Y)

23.243 (M)

1293.9 (Y)

698.5 (M)

Ba2Cr2Fe12O22 1.85 5.9179 (M) 23.357 (M) 708.4 (M)

Ba2Co2Fe11GaO22 1.43 5.8585 43.486 1292.6

Ba2CoMgFe11GaO22 1.69 5.8906 43.4917 1306.9204

4.2. SEM and EDS Analysis: The Cr-substitution for Co in Co2Y hexaferrite was found to induce

significant structural changes in the different samples. These samples were consequently examined

by SEM imaging to investigate the grain size and the microstructure of the samples. Further, Energy


Dispersive Spectroscopy (EDS) measurements were carried out to investigate the variations of local

stoichiometry in the samples, in an effort to confirm the presence of the equilibrium phases

observed by XRD.

4.2.1. Ba2Co2Fe12O22: Representative images of the sample Ba2Co2Fe12O22 are shown in Fig. 8

below. The images indicated that the sample consisted of almost hexagonal platelets with a

relatively wide distribution of particle size, and porous structure. The mean size of the plates was

estimated to be in the range of 900-1700 nm. Much larger particles were also observed, with step-

like formations.

Fig. 8: SEM images for the sample Ba2Co2Fe12O22 at different scales

EDS measurements were made on different particles at different positions in the sample to examine

the homogeneity of the sample. The results indicate that the atomic concentrations of Fe and Co

(normalized to the atomic concentration of Ba) at one position are 6.17 and 0.98, respectively.

These ratios are in good agreement with the stoichiometric ratios of 6 and 1 for Co2Y hexaferrite,

which indicates incorporation of all ionic species in forming the Y-type phase at the measured

position. However, measurements of the atomic ratios at a different position revealed lower

concentration for Fe (5.45) and higher concentration for Co (1.63). The normalized atomic ratio of

the two elements combined is 7.08, which is in very good agreement with the theoretical ratio of

7.00 for Y-type hexaferrite. This indicates that while the measured particle stoichiometry is

evidence that the particle is composed of a single Y-type phase, the sample is not locally

homogeneous.

4.2.2. Ba2CoCrFe12O22: SEM images for this sample confirm the formation of well-crystallized

hexagonal plates with a wide range of particle sizes (Fig. 9). The particles in this sample seem to

have two distinct particle size distributions; one with mean particle diameter around 0.5 µm, and

another with mean particle diameter clearly above 1 µm. The hexagonal plates, especially the large

ones, are clearly thin, indicating in-plane preferential crystallization. Generally, the plates are

stacked, with no noticeable porosity. Further, indication of the presence of impurity phases in the

sample is revealed by the presence of regions with distinctly different grey levels (brighter and

darker regions) as indicated by the images obtained in the EDS mode (Fig. 10).

EDS spectra at the darker and brighter regions of the sample are shown in Fig. 11, and the

normalized atomic ratios are presented in Table 5. Fig. 11 shows that the relative intensity of the

peak corresponding to Ba is significantly higher in the lighter regions than in the darker regions.

The normalized atomic ratios of metals in the darker region is close to the stoichiometric ratios

characteristic of Y-type, but generally lower, possibly due to the presence of a small amount of Ba-

rich phase at the measured spot. The atomic ratios in the lighter region are, however, indicative of


the presence Ba-rich phase in this region. The normalized atomic ratio of Cr (0.83) is in satisfactory

agreement with the stoichiometry of BaCrO4 phase detected by XRD. The Fe:Co atomic ratio

(0.93:0.18) is 5.2:1, which is close to the stoichiometric ratio in Co2Y phase, and could indicate the

presence of a small amount of Y-type phase at the measured spot. The Y-phase also contributes to

the Ba spectral area corresponding to Ba atomic fraction similar to that of Co (0.18) in Co2Y.

Indeed, if this fraction is subtracted from the Ba concentration, the resulting Ba:Cr ratio in the Ba-

rich phase would be 0.82:0.83, which is in excellent agreement with the stoichiometry of BaCrO4

phase.

Fig. 9:1 SEM images for the sample Ba2CoCrFe12O22 at different scales as indicated at the bottom

of each image.

Fig. 10: SEM images of the sample Ba2CoCrFe12O22 in the EDS mode.


Table 5. Normalized atomic ratios of metals in Ba2CoCrFe12O22 at darker and lighter regions of the

sample

(a)

Position Element Normalized

Atomic Ratio

Dar

ker

reg

ion

Cr 0.63

Fe 4.12

Co 0.83

Ba 1.00

Lig

hte

r

reg

ion

Cr 0.83

Fe 0.93

Co 0.18

Ba 1.00


(b)

Fig. 11: EDS spectra of the sample Ba2CoCrFe12O22 from (a) darker, and (b) lighter regions.

4.2.3. Ba2Cr2Fe12O22: Representative SEM images of the sample Ba2Cr2Fe12O22 are shown in

Fig.12. Agglomeration of hexagonal-like particles with relatively narrow size distribution and mean

diameter of about 0.5 µm were observed. In addition, smaller, non-hexagonal particles with

diameters typically below 0.5 µm were observed. SEM images indicate the presence of different

phases in this sample as evidenced by the different particle shapes, and the coexistence of darker

and lighter particles in some regions of the sample. This argument is supported by the results of

XRD measurements.

t

Fig. 12: Representative SEM images for the sample Ba2Cr2Fe12O22.

EDS measurment on a hexagonal plate revealed normalized atomic ratios of Fe and Cr of 11.61 and

1.18, respectively. These ratios are consistent with the stoichiometric ratios of BaM phase, and the

EDS results, therefore, confirmed that the hexagonal plates are composed of BaM phase, in


agreement with XRD results. EDS measurements at other positions in the sample revealed lower

normalized metal fractions, which could be associated with the coexistence of BaM and the Ba-rich

(Ba3Fe2O6) phase (which was confirmed by XRD measurements) at the measured spot. EDS

spectrum also showed clear evidence of Zr (Fig. 13), corresponding to the oxide phase detected by

XRD.

Fig. 13: EDS spectrum of the sample Ba2Cr2Fe12O22.

4.3. Magnetic Measurements: Magnetic hysteresis measurements were carried out on three

samples of Co2Y hexaferrites with Co2+

ions partially or fully substituted by Cr2+

ions. Also,

hysteresis measurements were carried out on a sample of Co2Y hexaferrite with Fe3+

ions partially

substituted by Ga3+

ions (Ba2Co2Fe11GaO22) and on a sample with partial co-substitution of Co2+

by

Mg2+

, and Fe3+

by Ga3+

(Ba2CoMgFe11GaO22). The saturation magnetization (Ms) was determined

from the law of approach to saturation [67], while the coercivity (Hc) and remanent magnetization

(Mr) were determined directly from the hysteresis loop. Further, thermomagnetic measurements

were carried out on the Cr-substituted Y-type hexaferrites to investigate the magnetic phases and

magnetic phase transition temperatures in these samples.

4.3.1. Hysteresis Measurements: The effect of the Cr2+

ionic substitution for Co2+

ions on the

magnetic properties of Co2Y was investigated by carrying out magnetic hysteresis measurements on

Ba2Co2Fe12O22, Ba2CoCrFe12O22, and Ba2Cr2Fe12O22 samples (Fig. 14). The magnetic parameters

derived from the hysteresis loops of the measured samples are shown in Table 6.

The saturation magnetization of Co2Y sample was found to be 33.9 emu/g, in very good agreement

with the previously reported value of 34 emu/g [9]. However, this value is significantly higher than

that reported by others [95, 104] (see Table 3). The coercivity of this sample is characteristic of a

soft hexaferrite phase, and is in agreement with the value reported for Co2Y with similar particle

size [104].


The sample Ba2CoCrFe12O22 exhibited a small rise in saturation magnetization up to 37.5 emu/g,

and a significant drop in coercivity down to 23 Oe. The increase in saturation magnetization could

be partially due to the substitution of low-spin Cr2+

ions for high-spin Co2+

ions at spin-down

sublattice, and partially due to the formation of M-type which has a higher characteristic saturation

magnetization than Y-type hexaferrite. The significant drop in coercivity could be due to the

attenuation of the magnetocrystalline anisotropy induced by the substitution of Co2+

ions (which has

a high contribution to magnetocrystalline anisotropy) by Cr2+

ions. This reduction could also be

associated with the differences in particle size and morphology as confirmed by SEM images.

The saturation magnetization of the Ba2Cr2Fe12O22 is also slightly higher than that of Co2Y, and the

relatively high coercivity (1445 Oe) is indicative of transformation to a harder magnetic phase in

this sample. This result is consistent with the XRD patterns which confirmed the dominance of M-

type in this sample. The coercivity in this sample, however, is significantly lower than that for a

typical M-type hexaferrite (few thousand Oe). This could be associated with the attenuation of the

superexchange interactions as a result of the Cr-substitution, which also leads to a reduction in

saturation magnetization. The reduction in saturation magnetization down to 35.1 emu/g in this

sample (with respect to 72 emu/g for BaM) is also associated with the presence of significant

amounts of non-magnetic phases (Ba3Fe2O6 and ZrO4) in this sample.

-10000 -5000 0 5000 10000

-40

-30

-20

-10

0

10

20

30

40

H (Oe)

M (

em

u/g

)

Ba2Co

2Fe

12O

22

Ba2CoCrFe

12O

22

Ba2Cr

2Fe

12O

22

Fig. 14: 2 Hysteresis loops of Cr-substituted Co2Y samples.

Table 6. Magnetic parameters of the hexaferrites

Sample Ms

(emu/g)

Hc

(Oe)

Mr

(emu/g)

Mr/Ms

Ba2Co2Fe12O22 33.9 123 7.90 0.23

Ba2CoCrFe12O22 37.5 23 1.58 0.04

Ba2Cr2Fe12O22 35.1 1445 17.1 0.49

Ba2Co2Fe11GaO22 34.9 88 6.88 0.20

Ba2CoMgFe11GaO22 26.6 81 5.81 0.22

The effect of Ga substitution for Fe, and Mg for Co on the magnetic properties of Co2Y hexaferrite

is demonstrated by Fig. 15. The saturation magnetization (34.9 emu/g) of the sample

Ba2Co2Fe11GaO22 is consistent with the value characteristic of Co2Y hexaferrite [9], which was

confirmed by XRD analysis. The fact that a secondary non-magnetic phase (BaFe2O4) was

confirmed by XRD (which should result in a reduction of the saturation magnetization) makes us


believe that Ga has a preference of substituting Fe at spin-down sites, thus compensating for the

anticipated reduction. Further, the coercivity was lowered down to 88 Oe, which could be

associated with the reduction of the strength of superexchange interactions as a result of substituting

Fe ions by non-magnetic Ga ions. However, this coercivity value remained within the range of

values reported for Co2Y hexaferrite [104].

The effect of Mg-substitution for Co in the sample Ba2CoMgFe11GaO22 was found to reduce the

saturation magnetization significantly (down to 26.6 emu/g). This reduction is consistent with the

reported gradual reduction of the saturation magnetization of Sr-Ni2Y with Mg-substitution for Ni

(down to 32.84 emu/g at x = 0.5) [93]. The saturation magnetization of our sample is higher than

that (~23 emu/g) reported for Mg2Y hexaferrite (see Table 3), and lower than that of Sr-Ni2Y with

0.5 Mg substitution for Co. This result supports the argument that the increase in Mg concentration

results in a decrease in saturation magnetization. Further, the reduction of the coercivity (with

respect to Co2Y) down to 81 Oe could be associated with the reduction of the strength of the

superexchange interactions in this sample [93].

With the exception of the Cr2Y sample, the squareness ratio (Mr/Ms) for all samples was rather low

(below 0.25). This is indication of the multi-domain structure of the magnetic particles, where the

magnetization processes are dominated by magnetic domain-wall motion rather than by

magnetization rotation. This result is consistent with the large particle size observed in these

samples in comparison with the critical single-domain size of less than 1 µm for hexaferrites [10].

The squareness ratio of the Cr2Y sample, however, is very close to the 0.5 value characteristic of an

assembly of randomly orientated single-domain magnetic particles. This is evidence that the

magnetic behavior of this sample is dominated by the small (~ 0.5 µm) BaM particles observed by

SEM in this sample.

-10000 -5000 0 5000 10000

-40

-30

-20

-10

0

10

20

30

40

M (

em

u/g

)

H (kOe)

Ba2Co

2Fe

11GaO

22

Ba2CoMgFe

11GaO

22

Fig. 15: Hysteresis loops of the samples Ba2Co2Fe11GaO22 and Ba2CoMgFe11GaO22.


4.3.2. Thermomagnetic Measurements of Cr-Substituted Co2Y: The critical transition

temperature of a magnetic material can be determined from the temperature dependence of the

magnetization at a fixed applied magnetic field. Consequently, thermomagnetic measurements

provide information on the magnetic phases with different critical temperatures coexisting in the

sample. To investigate the effect of Cr substitution on the phase evolution in Co2Y hexaferrites,

thermomagnetic measurements under an applied field of 100 Oe were carried out on the Ba2Co1-

xCrxFe12O22 (x = 0, 1, 2) samples.

Fig16 shows the temperature dependence of the magnetization for the sample Ba2Co2Fe12O22. The

thermomagnetic curve for this sample shows a sharp ferromagnetic-paramagnetic phase transition

manifested by the rapid drop of the magnetization between 300 and 400° C. The transition

temperature was determined from the derivative of the thermomagnetic curve. The strong (negative)

peak confirmed that the sample was dominated by a single magnetic phase, and the narrow peak

profile indicates magnetic homogeneity of the sample. From this result, we conclude that the

chemical inhomogeneity detected by EDS measurements did not have significant effect on the

broadening of the transition temperature. The Curie temperature for this phase was determined from

the peak position and found to be 342° C, which is consistent with the value of 340 °C reported for

Co2Y hexaferrite [9]. This result is consistent with the XRD results which indicated the presence of

a Y-type hexaferrite in this sample. The small peak at a temperature of 527° C represents magnetic

phase transition at a temperature significantly higher than the transition temperature of 450° C for

pure BaM. A similar high temperature anomaly was observed in Co-Ti doped M-type hexaferrite,

which was associated with the strengthening of the superexchange interactions induced by the

substitution of Co2+

ions in the lattice [127]. The position of this peak is, however, not fixed, and

depends on the Co concentration in the sample. The appearance of this peak in this sample could

therefore be associated with a minor Co-substituted M-type phase which was confirmed by XRD

measurements. Since M-type has a significantly higher saturation magnetization than Co2Y (more

than double), the presence of 10% of this phase in the sample (from XRD data) should lead to a

significant increase in saturation magnetization with respect to Co2Y, contrary to the observed value

which is almost identical to that for Co2Y. However, the magnetization of the high temperature

phase was reported to be small [127], and therefore can hardly affect the magnetic properties of the

sample.

0 100 200 300 400 500 600

0

1

2

3

4

5

6

M

1st derivative of "M"

T (oC)

M (

em

u/g

)

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

De

riva

tive

Fig. 16: Thermomagnetic curve measured at an applied field of 100 Oe, and its derivative for the

sample Ba2Co2Fe12O22.


The thermomagnetic curve for the sample Ba2CoCrFe12O22 clearly shows two transition

temperatures corresponding to two different magnetic phases, in addition to the high-temperature

anomaly at 554° C (Fig17). The positions of the two main dips in the derivative of the

thermomagnetic curve are 339° C and 450° C, corresponding to Co2Y and M-type hexaferrite,

respectively [9]. This result is consistent with the XRD results which confirmed the presence of M-

type hexaferrite phase in this sample. The positive peak (Hopkinson peak) appearing in the

magnetization curve near the Curie temperature of the M-type hexaferrite may indicate the presence

of small particles of this phase which exhibit super-paramagnetic relaxations at this temperature

[65]. The absence of such peak associated with the Y-type magnetic phase transition makes us

believe that the two distinctly different particle distributions observed by SEM imaging correspond

to the two structural and magnetic phases, with the small particles being M-type and the large

particles being Y-type hexaferrites. The high temperature anomaly is again associated with the

second magnetic phase transition in M-type resulting from the enhancement of the super-exchange

interactions [127].

0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

2.5

M


T (oC)

M (

em

u/g

)

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

De

riva

tive

Fig. 17: Thermomagnetic curve and its derivative for the sample Ba2CoCrFe12O22.

The thermomagnetic curve of the sample Ba2Cr2Fe12O22 is shown in Fig. 18. The figure clearly

shows a Hopkinson peak near the critical transition temperature in the M-curve, which is associated

with super-paramagnetic relaxations just below the critical temperature. The enhancement of the

Hopkinson peak in this sample is consistent with the observed reduction in mean particle size as

evidenced by SEM images. In addition, a small shoulder is evident at a temperature above 400° C.

The derivative curve shows a negative peak at 339°, which is significantly less than that of pure

BaM (450° C). This decrease in the transition temperature of the M-phase is correlated with the

reduction of the superexchange interactions as a consequence of the Cr-substitution. This

conclusion is supported by the observed lowering of the coercivity of this sample with respect to

pure M-phase [66]. In addition, a small peak appeared at 425° C, which is within the range of the

transition temperature of lightly-doped M-type phase. The presence of this peak could be evidence

of phase separation in this sample, where islands of almost pure BaM phase are expected to be

distributed in the main Cr-substituted BaM matrix. Further, the absence of the high temperature

anomaly in this sample supports the previous conclusion that this anomaly is associated with the

enhancement of superexchange interactions by Co ions.


0 100 200 300 400 500 600

0.0

0.2

0.4

0.6

0.8

M


T (oC)

M (

em

u/g

)

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

De

riva

tive

Fig. 18: Thermomagnetic curve and its derivative for the sample Ba2CoCrFe12O22.

4.4. Mössbauer Spectroscopy: Although there are six different crystallographic sites which can be

occupied Fe ions in the Y-type hexaferrite lattice, the spectral components corresponding to certain

sites could not be resolved, resulting in a spectrum which can be satisfactorily analyzed in terms of

only three components [54, 92, 124]. These components were variably assigned to different sets of

crystallographic sites, but with a general agreement that the high-field component arises partially

from Fe ions located at a tetrahedral site and partially from those located at the octahedral site.

Also, there is an agreement between the different workers that the middle component is associated

with the octahedral site. In a previous study [54], the high-field component in Mössbauer

spectrum of Co2Y was argued to be associated with Fe ions at the ( + ), while the low-field

component was associated with ( + + ) sites. In this study, Co2+

ions were reported to

occupy octahedral sites in the T block exclusively. The distribution of metal ions over the

different sites is determined from the relative intensities of the sub-spectral components, which, in

principle, are proportional to the number of Fe ions in the corresponding sets of sites [62].

4.4.1. Ga-Substituted Y-type Hexaferrites: Fig. 19 shows Mössbauer spectra of Co2Y with partial

substitution of Co by Mg, and/or Fe by Ga. Generally speaking, the spectrum of the un-substituted

sample is the same as that reported previously for Co2Y prepared by sol–gel method [54]. However,

the substitution of Fe by Ga (in the sample Ba2Co2Fe11GaO22) resulted in a substantial reduction of

the intensity of the high-field component whose outer absorption lines (indicated by the dashed drop

lines) appeared as shoulders on the outer peak profile of the spectrum. This is evidence that at least

partial substitution of Ga took place at sites corresponding to the high-field component, namely,

( + ) [54]. However, a clear and more precise picture of the distribution of Ga ions in the

sublattice can only be obtain by fitting the spectrum and obtaining the relative intensities

corresponding to the different crystallographic sites.

The spectrum of the sample Ba2CoMgFe11GaO22 also demonstrated reduction of the intensity of the

high-field component, consistent with the substitution of Ga at site. Also, the hyperfine fields

(indicated by the separations of the absorption lines) generally shifted to lower values with Mg-

substitution. This reduction in hyperfine field is consistent with the reduction in magnetization for

this sample, which is in agreement with the reported behavior of the hyperfine field with

magnetization in ferromagnetic alloys [109].


-10 -5 0 5 10

(a)

(b)

(c)

Inte

nsity

Velocity (mm/s)

Fig. 19: Mössbauer spectra for (a) Ba2Co2Fe12O22, (b) Ba2Co2Fe11GaO22, and (c)

Ba2CoMgFe11GaO22.

The spectrum of Ba2Co2Fe12O22 sample was fitted with three magnetic components with zero

quadrupole splitting (due to the symmetry of the absorption lines about the center of the spectrum,

Fig. 20). The hyperfine parameters resulting from the fit are listed in Table 7. The three sub-spectral

components with relative intensities of 25%, 50%, and 25% were assigned to the ( ),

, and ( ) sites, respectively [54]. This indicates that 3 Fe3+

ions per molecule

occupy each of the ( ) and ( ) groups of sites, whereas 6 Fe

3+ ions

occupy the site. This is a clear indication that the Co2+

ions occupy the sites corresponding to

the component with the lowest hyperfine field. In light of the fact that ions with lower valence state

should occupy face-sharing polyhedral (in order to reduce the electrostatic energy of the crystal),

we could conclude that Co2+

ions occupy the sites.

-10 -5 0 5 10

Inte

nsity

Velocity (mm/sec)

Ba2Co

2Fe

12O

22

Fig. 20: Fitted Mössbauer spectrum of Ba2Co2Fe12O22


The spectrum of the sample Ba2Co2Fe11GaO22 was also fitted with three magnetic components with

hyperfine parameters shown in Table 7. The relative intensities of 24%, 51%, and 25% of the

( ), , and ( ) components correspond to 2.6, 5.6, and 2.8 Fe

3+ ions

per molecule in these sublattices, respectively. These numbers were obtained by the multiplication

of the relative intensity of a given component by the total number of Fe ions (which is 11 in the

sample). This result suggests that 0.4 Ga ions occupy the spin-down sublattice, and 0.4 Ga ions

occupy the spin-up sublattice, a distribution which does not affect the net magnetic moment

per molecule. Therefore, considering the results of the magnetic measurements on this sample, it is

safe to conclude that the remaining 0.2 Ga ions per molecule in the third set of sites occupied the

spin-down site, since the spin-down site is fully occupied by Co ions [54]. Thus, Ga ions in this

sample are distributed at both spin-up and spin-down sites, with preference for tetrahedral spin-

down sites in the S and T blocks of the Y-type hexaferrite lattice. The hyperfine fields of the three

magnetic components clearly decreased with respect to the Co2Y sample.

-12 -8 -4 0 4 8 12

Inte

nsity

Velocity (mm/sec)

Ba2CO

2Fe

11GaO

22

Fig. 21: Fitted Mössbauer spectrum of Ba2Co2Fe11GaO22

The fitted spectrum of the sample Ba2CoMgFe11GaO22 is shown in Fig. 22. The hyperfine fields

decreased further in this sample, reflecting a larger effect of the substitution of non-magnetic ions

on the net magnetic field sensed by the Mössbauer probe (the 57

Fe nucleus). The relative intensities

of 19%, 55%, and 26% of the ( ), , and ( ) components

correspond to 2.1, 6.0, and 2.9 Fe3+

ions per molecule in these sublattices, respectively. This result

indicates that the substitution of Ga does not occur at site in this sample. The significant

decrease in the number of Fe3+

ions in the ( ) sites indicates higher substitution of non-

magnetic ions at these sites, displacing Fe ions to fill up the site. This substitution for Fe ions

may occur partially at the site, which is responsible for the reduction in the strength of the

superexchange interactions as argued in the context of discussing the magnetic data of this sample.


-12 -8 -4 0 4 8 12

Ba2CO Mg Fe

11Ga O

22

Inte

nsity

Velocity (mm/sec)

Fig. 22: Fitted Mössbauer spectrum of Ba2CoMgFe11GaO22

4.4.2. Cr-Substituted Y-type Hexaferrites: Fig. 23 shows Mössbauer spectra of Co2Y with partial

or total substitution of Co by Cr. The figure clearly indicates that Cr substitution induces significant

changes in the structural and magnetic phases as evidenced by the evolution of new spectral

components. Specifically, the appearance of new components in the high-field region, and the

sharp and strong absorption lines in the Cr2Y sample (indicated by the dashed drop lines), are clear

indications of the structural transformation in the samples. The sharp absorption peaks indicated by

the dashed lines are characteristic of M-type phase [62, 71, 84]. The spectrum of the CoCr-Y

sample, however, seems to be a combination of the spectra characteristic of M-type and Y-type

hexaferrites, with modified hyperfine parameters due to Cr substitution. Since the hyperfine

parameters of different magnetic phases with different structural symmetries are generally different,

full analysis of the spectra of these samples was carried out in order to support the results of the

structural and magnetic data.

The relatively complicated spectrum of the Cr2-Y sample was fitted with five components (Fig. 24).

The development of the first component with high hyperfine field (506 kOe), the strong component

with hyperfine field of 418 kOe (characteristic of the 12k sublattice of the M-type), and the weak

component with high quadrupole splitting (associated with the 2b bi-pyramidal site of the M-type)

are clear indications of the structural transformation in this sample, which accords with the XRD

and magnetic results. The main features of the hyperfine parameters of this sample are consistent

with the parameters of M-type hexaferrite, with a noticeable reduction in the hyperfine fields of the

outer components. A similar reduction was reported in TiRu-substituted SrM hexaferrite [71]. This

reduction is consistent with the previously discussed reduction in saturation magnetization due to

the weakening of the superexchange interactions in this sample.


-10 -5 0 5 10

(c)

(b)

(a)

Inte

nsi

ty

Velocity (mm/s)

Fig. 23: Mössbauer spectra for (a) Ba2Co2Fe12O22, (b) Ba2CoCrFe12O22, and (c) Ba2Cr2Fe12O22

-10 -5 0 5 10

Inte

nsi

ty

Velocity (mm/sec)

Ba2CoCrFe

12O

22

Fig. 24: Fitted Mössbauer spectrum of Ba2Cr2Fe12O22

The spectrum of the sample Ba2CoCrFe12O22 was best fitted with four components (Fig. 25) with

hyperfine parameters shown in Table 7. The general features of the hyperfine parameters suggest

that the spectrum consists of a combination of the Y-type and M-type spectral components with

lowered hyperfine fields (also see Fig. 23), which agrees with the structural refinement for this

sample. The strong overlap between the spectral components, however, makes it difficult to resolve

the individual sub-spectra, which may induce significant uncertainties in the derived hyperfine

parameters. The main achievement of this analysis is therefore limited to the observation of the

clear differences in the spectral characteristics of the samples, which correlate with the coexistence

of different hexaferrite phases with modified magnetic properties in these samples.


-12 -9 -6 -3 0 3 6 9 12

Inte

nsi

ty

Velocity (mm/sec)

Ba2CoCrFe

12O

22

Fig. 25: Fitted Mössbauer spectrum of Ba2CoCrFe12O22.

Table 7. Hyperfine parameters of substituted Co2Y samples: hyperfine field (Bhf) isomer shift (CS),

quadrupole splitting (QQ), and relative sub-spectral intensity (A) of the magnetic components.

Ba2Co2Fe12O22

Bhf (kOe) CS (mm/sec) QQ (mm/sec) A (%)

475 0.14 - 25

446 0.20 - 25

403 0.18 - 50

Ba2Co2Fe11GaO22

543 0.15 - 24

418 0.18 - 51

376 0.20 - 25

Ba2CoMgFe11GaO22

452 0.08 19

397 0.19 55

344 0.22 26

Ba2Cr2Fe12O22

506 0.28 0.17 10.7

478 0.18 0.16 14.7

418 0.26 0.24 68.8

393 0.03 1.83 1.0

348 0.33 0.40 4.8

Ba2CoCrFe12O22

456 0.42 0.27 12

448 0.03 0.11 16

398 0.23 0.37 29

339 0.28 0.52 43

5. Conclusions

Structural refinements and magnetic measurements indicated that the substitution of Fe by Ga

and/or Co by Mg in Co2Y hexaferrites resulted in a major Y-type hexaferrite phase with slight

improvement of the magnetic saturation due to the preferential substitution of Ga at spin-down


sites. The substitution of Co by Mg, however, resulted in a significant drop in saturation

magnetization which was attributed to the attenuation of the superexchange interactions between

spin-up and spin-down sublattices. This reduction in saturation magnetization was accompanied by

a noticeable reduction in the hyperfine fields of the Mg-substituted Y-type hexaferrite. The

substitution of Co by Cr, however, induced major structural and magnetic changes in the samples.

M-type hexaferrite phase, in addition to other nonmagnetic phases, evolved with the increase in Cr

substitution. SEM, XRD, and thermomagnetic measurements on the sample with partial substitution

of Co by Cr indicated phase segregation into M-type and Y-type, separately. The disappearance of

the Y-type phase in the Co-fully substituted sample is an indication that the presence of Co is the

major factor in the success in the production of a Y-type hexaferrite phase. In addition, the

disappearance of the high-temperature peak in the derivative of the thermomagnetic curve of the

Co-fully substituted sample is an indication that Co plays a major role in strengthening the

superexchange interactions, resulting in a magnetic phase transition at elevated temperatures.

Mössbauer spectrum of this (Cr2Y) sample was consistent with that of M-type hexaferrite, while the

thermomagnetic measurements provided evidence of phase separation, indicating signatures of

almost pure BaM islands in a heavily-Cr doped matrix.

Acknowledgments

This work was supported by a generous grant from the Deanship of Scientific Research at the

University of Jordan (Grant # 1404). The technical assistance of Y. Abu Salha and W. Fares (The

University of Jordan) is acknowledged.

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Documents

Modification of the Magnetic Properties of Co Y …eacademic.ju.edu.jo/s.mahmood/Lists/Published Research...axis) , with typical l attice parameters of a cDQG c c . This hexaferrite