Journal of Magnetism and Magnetic Materials 460 (2018) 171–176
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic and dielectric properties of Nd–Mn substituted Co2Y-hexaferrites Bashir Ahmad a, Saleem Mumtaz a, Muhammad Fahad Ehsan b, Muhammad Najam-ul-Haq a, Afzal Shah c, Muhammad Naeem Ashiq a,⇑ a b c
Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan School of Natural Sciences, National University of Sciences and Technology, H-12, Islamabad 44000, Pakistan Department of Chemistry, Quaid-i-azam University, Islamabad, Pakistan
a r t i c l e
i n f o
Article history: Received 6 September 2017 Received in revised form 26 March 2018 Accepted 2 April 2018 Available online 3 April 2018 Keywords: Hexaferrites Magnetic properties Nd-Mn substitution Saturation magnetization
a b s t r a c t Designing the new multifunctional ferromagnetic materials for multilayer chip inductor has attracted widespread interest in recent years. Co 2 Y strontium hexaferrites with composition of Sr2xNdxCo2Fe12yMnyO22 (for X = 0.00–0.1, Y = 0.00–1.0) were fabricated by the simple economical sol-gel autocombustion method. The surface morphology of the synthesized materials was investigated by using scanning electron microscopy and the particles were in plate-like shape. The magnetic properties calculated through hysteresis loops which were measured by vibrating sample magnetometer showed that the fabricated materials have typical properties related to ferromagnetic materials. The observed values of saturation magnetization were found in the range of 64.43–35.11 (emu/g). The observed decline in the values of magnetic parameters with the substituents (Nd-Mn) resulted due to suppression of magnetic superexchange interactions. The dielectric parameters including impedance and modulus investigation reveal that there are grains where the grain boundaries might play an important role in the conduction mechanism and the same may also be concluded from semicircle shape of Cole-Cole plots. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Since 1950, significant achievements have been made for the oxides ferrimagnetic hexagonal materials owing to their inherent high values of resistivity, magnetization and permeability. Due to these properties, such materials are considered to be excellent candidates for their potential applications in the fields of magnetic recording media, high frequency microwave devices, absorber and high density storage devices [1,2]. Consequently, the hexagonal ferrites have been exfoliated among the researchers on academic as well as industrial scale over the past decade. The hexagonal ferrites are advantageous over the other magnetic materials for their distinct properties i.e., mechanical stability and magnetic anisotropy. The Y-type hexaferrites are classified as important candidates among the other magnetic materials due to their potential applications in the field of high frequency devices in the GHz ranges. The internal restructuring of magnetic materials by the substitution of divalent or trivalent cations may lead to
⇑ Corresponding author. E-mail address: [email protected]
(M.N. Ashiq). https://doi.org/10.1016/j.jmmm.2018.04.002 0304-8853/Ó 2018 Elsevier B.V. All rights reserved.
reorganize the properties of these materials for their potential applications in various devices [3–5]. The Co2Y strontium hexaferrites have distinct properties like remarkable chemical stability, excellent corrosion resistance, high saturation magnetization, high Curie temperature and suitable values of coercivity . The structural characteristics, surface morphology, magnetic properties and polarization of the fabricated hexaferrite nanoparticles eventually rely on fabrication route, chemical composition, annealing temperature and time as well as employed precursors. Specifically, attention has been diverted towards the fabrication of new hexagonal ferrites and effect of various substituents on their magnetic properties. The introduction of rare earth (RE) cations may improve the magnetic and dielectric properties due to interaction of 4f electrons. The electrical polarization of the dielectric materials are manipulated by the dielectric characterization which may be done by the application of applied electric field in terms of frequency. Currently, numerous efforts have been made to improve the dielectric properties of the hexagonal ferrites by combining them with suitable cations and their extant of aggregation. In recent years, a lot of attention has been drawn on the substitution of rare earth cations at Sr and Fe sites in order to improve the magnetic and microwave properties [7,8].
B. Ahmad et al. / Journal of Magnetism and Magnetic Materials 460 (2018) 171–176
Both the magnetic and dielectric parameters (electric permittivity and impedance) gather increasing interest for MLCI. The reason for choosing the Nd-Mn is that firstly our purpose was to reduce the dielectric parameters such as impedance so that the materials can be used for multilayer chip inductors as such devices required highly resistive materials. Secondly, the electronic configuration of Nd3+ showed that it has three unpaired electrons while strontium has zero unpaired electrons which we also expected that it will improve the magnetic properties. The reason for Mn2+ along with Nd is that the manganese ion
has five unpaired electrons as that of ferric ion. The transfer of electrons from Mn2+ to Mn3+ is not so easy as compared to ferrous to ferric ions which also help in reducing the conductivity as well as the dielectric parameters. The ionic size of strontium and neodymium is also comparable. The impact of Nd-Mn co-doping for Co2Y strontium hexaferrites materials synthesized via sol-gel autocombustion method in order to describe variation in impedance and saturation magnetization, remanence and role of grains in coercivity variation. In the present investigation, the decrease in the values of the impedance justify
Fig. 1. (A–F) Images for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00–1.0) samples.
B. Ahmad et al. / Journal of Magnetism and Magnetic Materials 460 (2018) 171–176
that the fabricated materials are appropriate for multilayer chip inductors. 2. Experimental and characterization The Nd-Mn doped Sr2xNdxCo2Fe12yMnyO22 Y-type hexaferrites samples were synthesized via already reported ethylene glycol assisted sol-gel autocombustion method . The raw precursors used were Fe(NO3)39H2O (Aldrich, 98%), Sr(NO3)26H2O (Merck, 99%), CoCl26H2O (Merck, 99%), MnCl26H2O (Merck, 99%) and Nd2O3 (Aldrich, 98%). The phase of the as prepared samples was confirmed by X-ray diffractometer. The diffraction patterns of all the prepared samples were consistent with the standard pattern (ICSD-01-072-0750), which is given in Supplementary materials (Fig. S1). The surface morphology of the prepared samples was investigated by using scanning electron microscopy (FE-SEM Hitachi-SU8000). The impedance was investigated using dielectric data which was measured with the help of RF Impedance/Material Analyzer, Agilent E4991A over the frequency range of 1 MHz to 3 GHz. The room temperature magnetic measurements were carried out by using vibrating sample magnetometer (VSM) Lake Shore-74071.
Fig. 2. The frequency dependence real impedance for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00–1.0) samples at room temperature.
3. Results and discussion 3.1. Morphological analysis The surface morphology and grain size distribution of investigated polycrystalline Nd-Mn doped Y-Type hexaferrites has been investigated by FE-SEM and their images are shown in Fig. 1(a– f). The images clearly indicate that the synthesized materials have closed pack plate like hexagonal morphology. The morphology in the present investigation is in good agreement with the already reported study . It has been reported that such type of morphology is good for microwave absorbing devices . On increasing Nd-Mn concentrations in cobalt strontium y-type hexaferrites (Fig. 1d–f), the grains size become relatively larger and less agglomerated. The appearance of agglomerated particles on surface may be the result of weak Van der Waals forces and magnetic interactions . 3.2. Impedance spectroscopy
Fig. 3. The frequency dependence complex Impedance for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00–1.0) samples at room temperature.
The frequency dependence impedance is an important parameter to explain the transport mechanism. The frequency dependent impedance for the Nd-Mn doped Y-type hexaferrites is shown in Fig. 2. It can be seen from the figure that the values of impedance decrease with increasing frequency. The decline in the values of the real part of the impedance may be due to reduction of electron hopping phenomenon with applied AC frequency field. The decreasing trend of impedance factor with frequency has already been reported by other researchers . It has also been observed that the impedance also increases with increasing Nd-Mn content. The increasing trend of the impedance with Nd-Mn concentration is due to the formation of resistive grains near the grain boundaries, which may increase the resistance. It has been found that the impedance data is in good agreement with DC electrical resistivity and the AC conductivity data published in our previous work . The frequency dependence imaginary part of impedance for NdMn doped Co2Y composition is represented in Fig. 3. The frequency dependence plots for Z at room temperature showed that the impedance decrease with increasing frequency. There is no relaxation peak has been observed which may be due to very weak relaxation phenomenon or freezing of dipoles at room tempera-
Fig. 4. The frequency dependence real electric modulus for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00–1.0) samples at room temperature.
B. Ahmad et al. / Journal of Magnetism and Magnetic Materials 460 (2018) 171–176
ture. As the prepared samples have grain boundaries which occupy larger volume which is inconsistent because the prepared samples show Cole-Cole plot for M0 and M00 while no semicircles appear for Z0 and Z00 [14,15]. In case of heterogeneous system, if the region of continuity of grain boundary occupies a small volume, the spectrum of impedance Z00 versus Z0 provides better visualization of the semicircles in the plan. This behavior was not observed in the Nd-Mn doped Co2Y prepared samples. Figs. 4 and 5 show the frequency dependence plots for real (M0 ) and imaginary modulus (M00 ) at room temperature, respectively. It has been observed that the synthesized materials follow the relaxation phenomenon at higher frequency. The appearance of peaks i.e., relaxation phenomenon at higher frequencies in the synthesized materials was resulted due to the matching frequencies among the hopping ions of various oxidation states and applied frequencies. The real modulus plot against the frequency may provide information about the existence of grain boundaries in the dielectric materials. The Nyquist plots (Cole-Cole plot) are given in Fig. 6. The careful investigations of the Cole-Cole plots for the fabricated materials
Fig. 5. The frequency dependence imaginary electric modulus for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00–1.0) samples at room temperature.
Fig. 6. The Nyquist or Cole-Cole plots of electric modulus for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00–1.0) samples at room temperature.
showed the single semi-circles for all the samples. It has been reported that the dielectric and conduction in the ferrite materials show the similar behavior. The width of the semi-circle shows the resistance of the samples. The left region in the lower frequency values of these plots is showing grain resistance, middle region is meant for grain boundary resistance while that at right depicts the overall resistance of grains as well as grain walls [16,17]. It was usually observed that substituents contents had minor effect on the resistance of grains while the grain boundary resistance was markedly affected. As the Mn2+ content was increased, the grain boundary resistance increased. Regarding the hopping of electrons between iron ions as the primary mechanism of conduction in ferrites, it can be ascribed that with increasing Mn2+ content, electronic exchange interactions between Fe2+ and Fe3+ ions get limited because Mn2+ was doped at the octahedral sites which reduces the iron content as well as the hopping of electrons at that site. We concluded that complex impedance data is in good agreement with AC conductivity data . 3.3. Magnetic properties The magnetic properties mainly depend on the distribution of iron ions on the six magnetic sites i.e., 6c1v, 6c1*v, 3av1, 18hvI, 6cv1 and 3bv1 as well as on the super exchange interactions i.e., Fe3+ M O M Fe2+. The hysteresis loops of the prepared Sr2xNdxCo2Fe12yMnyO22 (where: X = 0.00–0.1, Y = 0.0–1.0) samples are presented in Fig. 7 and the values of remanent, coercivity and saturation magnetization are given in Table 1. The observed values of the saturation magnetization (MS) of the synthesized materials are found in the range of 64.43–35.108 (emu/g), which are higher than that of already reported Y-type hexaferrites materials . It is clear from the table that the saturation magnetization and magnetic remanence are strongly dependent on the Nd-Mn content. It is expected that the magnetic materials will show the similar behavior for both the saturation magnetization and remanence magnetic parameters and the same is true in the present study. The saturation magnetization and remanent magnetization showed decreasing trend with increasing Nd-Mn contents, which can be explained on the basis cations distribution within the unit cell. In the present investigation, the Mn2+ occupied the both octahedral and tetrahedral site . During the annealing of samples, the divalent Mn2+ are oxidized to Mn3+ and substitute Fe3+ ions to maintain the neutrality. This leads to reduce the magnetic
Fig. 7. The hysteresis plots for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.00– 1.0) samples at room temperature.
B. Ahmad et al. / Journal of Magnetism and Magnetic Materials 460 (2018) 171–176 Table 1 Saturation magnetization (MS), coercivity (HC), remanent (Mr), squarence ratio (Mr/MS) for Sr2xNdxCo2Fe12yMnyO22 (X = 0.00–0.1, Y = 0.0–1.0) hexaferrites. Parameters
X = 0.0 Y = 0.0
X = 0.02 Y = 0.2
X = 0.04 Y = 0.4
X = 0.06 Y = 0.6
X = 0.08 Y = 0.8
X = 0.1 Y = 1.0
Magnetic saturation (emu/g) Coercivity (Oe) Remanent magnetization (emu/g) Mr/Ms Magnetic moment (nB)
64.43 1942 31.38 0.48 15.21
61.78 2010 30.91 0.50 14.18
55.66 2030 28.20 0.52 12.45
44.60 2043 23.78 0.53 9.43
39.58 2055 20.98 0.53 7.65
35.11 2060 19.78 0.56 6.269
moment from 5 to 4 mB. This conversion of Fe3+ (5 mB) to low spin Fe2+ (4 mB) might be the reason for the reduction of saturation as well as remanent magnetization. In addition, the substitution of rare earth metal ion (Nd3+) at Sr site may constrain the chemical equilibrium of hexaferrites materials so that the net magnetic moment will be decreased due to Fe3+ (5 mB) M Fe2+ (4 mB) . The Nd3+ ions have spin as well as the orbital magnetization. The net magnetic moment has resulted the differences between the orbital and spin magnetic moments. The Nd3+ ions magnetic moment (1.14 mB) is much smaller than Fe3+ (5 mB) . Furthermore, the spin canting effect is associated with the rare earth ions, which may suppress the exchange interaction as well as saturation and remanence magnetization . It is well known fact that the super exchange interactions may also affect the magnetic order between 3av1 (octahedral) and 6c1v (tetrahedral) sites. Increasing Nd-Mn contents at B-site result in the reduction in super exchange interactions among the sub lattices A and B. Interestingly, the octahedral 3bV1 sub lattice oriented in such a way that it sheared coordination figures both upper and lower faces with neighboring 6CVI sub lattice ions . According to Gorter scheme, the nearly small amount of Mn2+ ions in T-block octahedral site may be able to construct the drastic variation in magnetic coordination. In present case, the Mn2+ ions replace the Fe3+ ions at octahedral sites. Hence, it is believed that Mn2+ ions at octahedral 3bV1 may cause the variation of collinear to non collinear order. As in the ferrimagnetic materials, each sub-lattice is spontaneously magnetized while in the same way the two sub-lattices magnetization opposed each other. Similar, antisymmetric interaction may occur between the couples of ions around the opposite sub-lattices i.e., 3bV1, 6CVI or 18hv. The Mn2+ ions cause the antisymmetric interactions parallel to c-axis. As a result of this interaction, angle differences may be constructed among the variant moment along the basal planes. Due to the formation of angles among the different magnetic moments, spin canting may occur which causes the reduction in saturation magnetization. The coercivity ‘Hc’ of the magnetic materials is related to physical properties like magnetic domain size, particle morphology, porosity, lattice defects, grain size and magneto crystalline anisotropy. But among other factors the domain walls play potential role for the magnetic coercivity variations. During this when these domain walls get magnetized or demagnetized, the domain wall movement requires less energy compared to domain rotation. In contrast with the contribution to magnetization or demagnetization due to domain rotation, the wall movement increases which is responsible for the increase in coercivity. The other reason may be the lattice defects. Such small increment in Hc may be due to an increase in lattice defects with doping. The lattice defects can pin the domain wall motion and lead to an increase in coercivity. The value of coercivity (HC) in present case is higher as compared to previously reported data . The values of magnetic moment (nB) have also been calculated and are also given in Table 1. The values of magnetic moments are found in the range of 15.21–6.27 (emu/g). The observed values of magnetic moment are very close to that of the already reported values by other researchers . The trend of magnetic moment is similar to saturation magnetization as previously explained.
The (Mr/Ms) ratio is termed as squareness ratio and their values are tabulated in Table 1. The values of squareness ratio are found in the range 0.43–0.66 (emu/gm), which are relatively higher than those reported earlier related to Y-type magnetic hexagonal phase . It has been reported that the materials with squareness ratio values less than 1 and greater than 0.50 will be in poly domain. Form the observed values of squareness ratio for the Nd-Mn doped fabricated materials, one can say that these materials are in ploy domain. The materials without dopant content (Sr2Co2Fe12O22) are in multi domain and randomly oriented as their squareness ratio is lower than 0.5. 4. Conclusions The surface morphology, dielectric and magnetic properties for Nd-Mn substituted strontium cobalt Y-type hexaferrites fabricated by economical sol-gel method were discussed in this work. The materials are nucleated followed by their growth into plate like morphology. The increase in semicircle width of the Cole-Cole plot with increasing concentration of Nd-Mn co-doping into Co2Y phase suggested that grains have greater resistance. The decline in saturation magnetization was due to decrease in superexchange interactions among Fe3+ M O M Fe2+ by the Nd-Mn content. The values of squareness ratio indicate that the synthesized materials are in multi domain. Acknowledgement The authors (Bashir Ahmad and Muhammad Naeem Ashiq) are grateful to Higher Education Commission (HEC) of Pakistan and Bahuddin Zakariya University Multan Pakistan for the financial support under the Project No. 20-1515/R&D/09-8049 and DR & EL/D-883. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jmmm.2018.04.002. References  S.H. Mahmood, F.S. Jaradat, A.-F. Lehhlooh, A. Hammoudeh, Structural properties and hyperfine interactions in Co-Zn Y-type hexaferrites prepared by sol-gel method, Ceram. Inter. 40 (2014) 5231–5236.  I. Sadiq, I. Ali, E.V. Rebrov, S. Naseem, M.N. Ashiq, M.U. Islam, Influence of NdCo Substitution on structural, electrical, and dielectric properties of X-type hexagonal nano ferrites, J. Mater. Eng. Perfor. 23 (2014) 622–627.  I. Ali, M.U. Islam, I. Sadiq, N. Karamat, A. Iftikhar, M.A. Khan, A. Shah, M. Athar, I. Shakir, M.N. Ashiq, Synthesis and magnetic properties of (Eu-Ni) substituted Y-type hexaferrites by surfactant assisted co-precipitation method, J. Magn. Magn. Mater. 385 (2015) 386–393.  Z. Haijun, Y. Xi, Z. Liangying, The preparation and microwave properties of Ba2ZnzCo2zFe12O22 hexaferrites, J. Eur. Ceram. Soc. 22 (2002) 835–840.  M. Ahmad, F. Aen, M.U. Islam, S.B. Niazi, M.U. Rana, Structural, physical, magnetic and electrical properties of La-substituted W-type hexagonal ferrites, Ceram. Int. 37 (2011) 3691–3696.  L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, Effect of divalent ions on the electrical and magnetic properties of Ni-Zn M-type ferrites, J. Magn. Magn. Mater. 193 (1999) 288–290.
B. Ahmad et al. / Journal of Magnetism and Magnetic Materials 460 (2018) 171–176
 Y.A. Farzin, O. Mirzaee, A. Ghasemi, Magnetic and dielectric investigations of Mn-doped Ba hexaferrite nanoparticles by hydrothermal approach, J. Magn. Magn. Mater. 371 (2014) 14–19.  N. Adeela, U. Khan, M. Iqbal, S. Riaz, M. Irfan, H. Ali, K. Javed, I. Bukhtiar, K. Maaz, S. Naseem, Structural and magnetic response of Mn substituted Co2Ytype barium hexaferrites, J. Alloy Compd. 686 (2016) 1017–1024.  B. Ahmad, M.N. Ashiq, I. Ali, M.N. Haq, M.S. Khan, M. Osada, Elucidation of structure and conduction mechanism in Nd-Mn substituted Y-type strontium hexaferrite, J. Alloys Compd. 723 (2017) 9–16.  N. Lampis, C. Franchini, G. Satta, A.G. Lehmann, S. Massidad, Electronic structure of PbFe1/2Ta1/2O3 crystallographic ordering and magnetic properties, Phys. Rev. B 69 (2004) 064412–066415.  Y. Salunkhe, D. Kulkarni, Structural, magnetic and microstructural study of Sr2Ni2Fe12O22, J. Magn. Magn. Mater. 279 (2004) 64–68.  I. Ali, M.U. Islam, M.N. Ashiq, M.A. Iqbal, H.M. Khan, G. Murtaza, Role of grain boundaries in the conduction of Eu–Ni substituted Y-type hexaferrites, J. Magn. Magn. Mater. 365 (2014) 115–121.  A. Pradeep, P. Priyadharsini, G. Chandrasekaran, Production of single phase nano size NiFe2O4 particles using sol-gel auto combustion route by optimizing the preparation conditions, Mater. Chem. Phy. 112 (2008) 572–576.  H.O. Rodrigues, G.F.M. Pires Junior, A.J.M. Sales, P.M.O. Silva, B.F.O. Costa, P. Alcantara Jr., S.G.C. Moreira, A.S.B. Sombra, BiFeO3 ceramic matrix with Bi2O3 or PbO added: Mössbauer, Raman and dielectric spectroscopy studies, Physica B: Condensed Matter 406 (2011) 2532–2539.  I. Ali, M.U. Islam, M.N. Ashiq, M.A. Iqbal, Hasan M. Khan, N. Karamat, Effect of Tb-Mn substitution on the DC and AC conductivity of Y-type hexaferrites, J. Alloys Compd. 579 (2013) 576–582.  I. Ali, M.U. Islam, M.N. Ashiq, M. Asif Iqbal, Hasan M. Khan, G. Murtaza, Role of grain boundaries in the conduction of Eu–Ni substituted Y-type hexaferrites, J. Mag. Mag. Mater. 362 (2014) 115–161.
 Y. Bai, J. Zhou, Z. Gui, L. Li, Electrical properties of non-stoichiometric Y-type hexagonal ferrite, J. Magn. Magn. Mat. 278 (2004) 208–213.  M.G. Chourashiya, J.Y. Patil, S.H. Pawar, L.D. Jadhav, Studies on structural, morphological and electrical properties of Ce1xGdxO2(x/2), Mater. Chem. Phys. 109 (2009) 39–44.  G. Murtaza, R. Ahmad, T. Hussain, R. Ayub, I. Ali, M. Azharhan, M.N. Akhtar, Structural and magnetic properties of Nd–Mn substituted Y-type hexaferrites synthesized by micro emulsion method, J. Alloys Compds. 602 (2014) 122–129.  B.X. Gu, Magnetic properties of X-type Ba2Me2Fe28O46 (Me = Fe Co, and Mn) hexagonal ferrites, J. Appl. Phys. 71 (1992) 5103–5106.  R. Przeniosto, I. Sosnowska, P. Fischer, W. Marti, F. Bartolom, J. Bartolom, E. Palacios, R. Sonntag, Magnetic moment ordering of Nd3+ and Fe3+ in NdFeO3 at low temperature, J. Magn. Magn. Mater. 160 (1996) 370–377.  I. Sadiq, S. Naseem, M.N. Ashiq, M. Asif Iqbal, I. Ali, M.A. Khan, S. Niaz, M.U. Rana, Spin canting effect and microwave absorption properties of Sm–Mn substituted nano sized material, J. Magn. Magn. Mater. 365 (2015) 159–165.  I. Ali, M.U. Isalm, M.N. Ashiq, M. Asif, M.S. Awan, S. Naseem, Role of Tb-Mn substitution on the magnetic properties of Y-type hexaferrites, J. Alloys Compd. 599 (2014) 131–138.  U. Topal, Factors influencing the remanent properties of hard magnetic barium ferrites: Impurity phases and grain sizes, J. Magn. Magn. Mater. 320 (2008) 331–335.  S. Kamba, V. Goian, M. Savinov, E. Buixaderas, D. Nuzhnyy, M. Marysko, M. Kempa, V. Bovtun, J. Hlinka, K. Knizek, P. Vanek, P. Novak, J. Bursik, Y. Hiraoka, T. Kimura, K. Kouril, H. Stepankova, Dielectric, magnetic, and lattice dynamics properties of Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22: comparison of ceramics and single crystals, J. Appl. Phys. 107 (2010) 104109–104117.  Y.A. Farzin, O. Mirzaee, A. Ghasemi, Influence of Mg and Ni substitution on structural, microstructural and magnetic properties of Sr2Co2xMgx/2Nix/ 2Fe12O22 (Co2Y) hexaferrite, J. Magn. Magn. Mater. 371 (2014) 14–19.