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Structural and electromagnetic properties of Ni0.5Zn0.5HoxFe2-xO4 ferrites
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Structural and electromagnetic properties of Ni0.5Zn0.5HoxFe2-xO4 ferrites ⁎
Zhiqing Liua,b, Zhijian Penga, , Xiuli Fub, a b
⁎
School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ni-Zn ferrite Ho3+ ions doping Magnetic properties Dielectric properties
Ni-Zn ferrites with a nominal composition of Ni0.5Zn0.5HoxFe2-xO4 (x = 0–0.06) were prepared by conventional solid state reaction through using analytical-grade metal oxides powders as raw materials. The phase composition, microstructure, magnetic properties and dielectric performance of the as-prepared samples were investigated. The doped Ho3+ ions could enter into the crystal lattice of the resultant spinel ferrites, causing the expansion of the unit cell, reaching a saturated state when x = 0.015; and the additional Ho3+ ions would form a foreign HoFeO3 phase at the grain boundary. The grain size and densification of the samples initially decreased after a small amount of Ho3+ ions was doped, but then increased with more Ho3+ ions added. The saturation magnetization decreased gradually with increasing substitution level of Ho3+ ions. The Curie temperature and coercivity raised initially and declined later with increasing content of Ho3+ ions in the samples, reaching their maximums of 305 °C with x = 0.015 and 2.99 Oe with x = 0.03, respectively. The variation of complex permeability versus Ho3+ ions substitution level presented an opposite trend to that of coercivity. The dielectric loss increased slightly after the introduction of a small amount of Ho3+ ions, but reduced significantly with more Ho3+ ions doped.
1. Introduction Ni-Zn ferrites are one of the most widely used soft magnetic materials in modern communication, internet, house appliance, computer circuitry and so forth, because of their excellent high-frequency performance, high temperature stability and simple production process [1–3]. However, with the development of anti-electromagnetic disturbing technology, surface mounting technology and high-frequency broad-band device, more challenges have to be addressed, such as higher operating frequencies, enhanced temperature stability and reduced costs [4]. Ni-Zn ferrite polycrystalline materials are of spinel structure and belong to the Fd3m space group. They, commonly written as (ZnxFe1-x) [Ni1-xFe1+x]O4, possess two crystallographically distinct locations, tetrahedral A (the parentheses) and octahedral B (the square brackets) sites, in their crystal structures. The spinel crystal structure involves 64 of tetrahedral A and 32 of octahedral B sites, but only 8 of A and 16 of B sites are occupied. Such crystal structure is beneficial for the doping of other cations to change their properties [5]. In the past decades, the searching of additives for Ni-Zn ferrite has attracted great attention from the scientists worldwide, and today, it is still a hot topic in this field to prepare multi-component ferrites with excellent electromagnetic properties.
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With regard to the specific additive, many chemicals presented an influence on the magnetic and dielectric properties of Ni-Zn ferrites. For example, some cations could enter into the spinel crystal structure and locate in tetrahedral A or octahedral B sites, such as Mg2+ [6], W5+ [7], Al3+ [8], Sn4+ [9] and Co3+ [10], which have a significant effect on the electromagnetic properties of the ferrites. Some would not enter into the crystal lattice, such as Ca2+ [11] and Si4+ [12], but they could effectively decrease the eddy current loss of Ni-Zn ferrites, because these additives would form an insulating intergranular layer at the grain boundaries. And some additives could decrease the sintering temperature of Ni-Zn ferrites due to their lower melting point, like V2O5 [13]. In particular, Rezlescu et al. [14,15] first reported the doping effects of rare earth ions on Ni-Zn ferrites. It was revealed that, the substitution by Yb, Er, Dy, Tb, Gd, Sm and Ce ions could obtain an important modification on both magnetic and electrical properties of the ferrites due to their larger ionic radius. Since then, the magnetic and electrical properties of rare-earth ions doped Ni-Zn ferrites have been addressed by many investigations [16–20]. Moreover, the additionally doped rare-earth ions may form thin layers at the grain boundaries, improving the resistivity of Ni-Zn ferrites. Among the various rare-earth additives, Abdul et al. ever indicated [16] that the doping of Ho3+ ions could increase the DC electrical resistivity of Ni0.7Zn0.3Fe2O4 ferrites, which is of significance in the
Corresponding authors. E-mail addresses: [email protected] (Z. Peng), [email protected] (X. Fu).
http://dx.doi.org/10.1016/j.ceramint.2017.08.011 Received 6 May 2017; Received in revised form 3 July 2017; Accepted 1 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Z., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.08.011
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and 1 mm in thickness, or 12 mm in diameter and 1 mm in thickness) and toroidal rings (20 mm in outer diameter, 10 mm in inner diameter and 3 mm in thickness) by a hydraulic press under a pressure of 40 MPa. Finally, the samples were sintered in a muffle furance at 1250 °C for 4 h with a heating rate of 2 °C/min.
applications of high frequency devices. Unfortunately, their discussion on the magnetic and electrical properties of the samples was performed without providing any microstructure proofs. As a matter of fact, however, there is a close relationship between the grain size and coercivity, and the electrical properties of electronic ceramics also have a strong dependence on the sample microstructure, such as pores and defects. In addition, there may exist some uncertainties in their results. (1) The doped Ho3+ ions are much larger than all the ions in Ni0.7Zn0.3Fe2O4, but after the doping of Ho3+ ions, the lattice constant of their samples (Ni0.7Zn0.3Ho2xFe2-2xO4, 0.0 ≤ x ≤ 0.07) decreased, which is completely adverse against the observation on the lattice constant of similar spinel Co ferrites after the doping of Ho3+ ions (CoFe1.9Ho0.1O4) in Ref. [21]. (2) The densification of their Ni0.7Zn0.3Fe2O4 ferrite was too low with an astonishingly high value of porosity (20.47%), when compared with those of Ni0.5Zn0.5Fe2O4 and Ni0.75Zn0.25Fe2O4 ferrites prepared by the same method (generally with a porosity lower than 10%) [7,8]. More regrettably, no literature has been reported on the temperature stability and frequency dependence of electromagnetic properties of Ho3+ doped Ni-Zn ferrites, which are very important properties of a ferrite for high-frequency applications. Therefore, in this work, Ni-Zn ferrites doped with different amounts of Ho3+ ions were prepared by conventional solid state reaction through using analytical-grade metal oxides powders as raw materials. The compositional, microstructural, magnetic and dielectric properties of the Ni-Zn ferrites were systematically investigated as a function of the content of Ho3+ ions. Resultantly, the Ho3+-substituted Ni-Zn ferrites present excellent dielectric properties with the lowest value of dielectric loss tangent of 0.017 at 100 MHz, which is much lower than those of many transition metal doped Ni0.5Zn0.5Fe2O4 ferrites as reported in Refs. [7,22,23]. Moreover, it was known that the Curie temperature of Ni-Zn ferrites could also be enhanced by the doping of other positive ions, such as W6+ [7], Pr3+ [17], Al3+ [22] and so on, in which the doping of W6+ could lead to the highest increase of 70 °C in Curie temperature compared with pure Ni-Zn ferrites. But the present work reveals that, the doping of Ho3+ into Ni-Zn ferrites also has a significant influence on the temperature stability of Ni-Zn ferrites, resulting in an even higher increase in Curie temperature of 85 °C in comparision with pure Ni-Zn ferrites. Therefore, the doping of Ho3+ into Ni-Zn ferrites is also an effective way to improve the temperature stability of Ni-Zn ferrites.
2.2. Materials characterization The phase of the samples was identified by X-ray diffractometer (XRD, D/max-RB, Cu Kα, λ = 1.5418 Å) in a continuous scanning mode at a rate of 8°/min. The lattice constants were calculated from the XRD data by a software Jade 6 PC. The content of the foreign phase (here HoFeO3) was calucated by
WA =
IA IA +
IB KAB
(1)
where WA is the mass percentage of HoFeO3; IA and IB are the integrated intensities of the strongest diffraction peaks of HoFeO3 and Ni-Zn ferrite, respectively; and KAB is a specific value determined by JCPDS card nos. 52-0278 for the Ni-Zn ferrite and 46-0115 for HoFeO3. The microstructure was examined by a scanning electron microscope (SEM, SU8020). The diameter of the hexagonal grains identified clearly in the SEM images and the grain size distribution were evaluated by using a software Nano Measurer 1.2 PC. The reported grain size for each sample was the mean value of all the grains identified from the whole image. The apparent density was measured using Archimedean method according to the international standard ISO18754; the theoretical density of the samples was calculated by the density of unit cell, which was modified by the corresponding content of the foreign phase HoFeO3 in the samples on the basis of mixture rule; and the relative density was defined by the percentage of the apparent density to the theoretical density. The saturation magnetization (Ms), coercivity (HC) and Curie temperature (TC) were measured by a vibrating sample magnetometer (VSM, LakeShore 7307) with a maximum magnetic field of 10 kOe. By employing an impedance analyzer (Agilent E4991A) within the frequency from 1 to 1000 MHz, the measurements of complex permeability (μ = μ′-jμ′′) and complex dielectric constant (ε = ε′-jε′′) were carried out. All these measurements except TC were performed at room temperature.
2. Experimental procedures
3. Results and discussion
2.1. Samples preparation
3.1. Compositional and structural properties
The investigated Ni-Zn ferrites have a nominal composition of Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06). The samples were perpared by conventional two-steps solid state reaction. The applied raw materials were NiO (99.0 wt%), ZnO (99.0 wt%), α-Fe2O3 (99.0 wt%), and Ho2O3 (99.9 wt%) powders, supplyed by Sinopharm Chemical Reagent Co. Ltd. All the chemicals were directly used without any further purification. During the processing, all the raw powders were first mixed and milled in a planetary mill (YXQM-1L, Changsha Mitr Instruments Co. Ltd., China) for 6 h with deionized water as the dispersion media, 0.5 wt% Davon C as the dispersant, and TZP ZrO2 balls as the grinding media. For the milling, the weight ratio of the raw powders, grinding media and dispersion media is 1:4:5, the volume of the jar is 1 L, and the rotation speed of the jar is 250 rpm. After milling, the resultant slurries were dried in an open oven at 120 °C, and the dried powder chuncks were then grinded in an agate mortar. After that, the obtained fine powders were calcined in a Muffle furnace at 800 °C for 3 h. The calcined powder chunks were then planetarily grinded into fine powders again under the same conditions as those for the raw powder mixing, and granulated with 0.5 wt% PVA as the binder. After that, the obtained powders were pressed into pellets (6 mm in diameter
Fig. 1 shows the XRD patterns of the as-prepared Ni-Zn ferrites doped with different contents of Ho3+ ions, which were taken on the assintered samples (6 mm in diameter and 1 mm in thickness). As is seen from this figure, the main phase of the present Ho3+ ions doped Ni-Zn ferrites is of a typical spinel structure (JCPDS card no. 52-0278). But, when x = 0.03, a foreign phase could be detected, which was identified as HoFeO3 (JCPDS card no. 46-0115); and the content of the foreign phase increased with increasing doping amount of Ho3+ ions (see Table 1). The lattice constants of the as-prepared Ni-Zn ferrites were calculated from the XRD data, and the results are presented in Table 1. It can be seen that, the lattice constant increased from 8.383 to 8.401 Å initially after Ho3+ ions were doped (x = 0.015), which is similar with the results reported by other researchers [21]. But the lattice constants did not increase continuously, which kept a constant of about 8.40 Å with further increasing doping amount of Ho3+ ions (x > 0.015) instead. For this phenomenon, it can be explained as follows. Because Ho3+ ion (1.04 Å) is much larger than Fe3+ ion (0.67 Å), Ni2+ ion (0.69 Å) and Zn2+ ion (0.74 Å), so when the doped Ho3+ ions entered into the crystal lattice of the spinel ferrites, they would 2
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with little amount of Ho3+ ions is expected. And the formation of pores could limit the abnormal growth of some grains, which is of help for the homogeneous growth of grains. However, when further increasing amount of Ho3+ ions was added, a foreign phase HoFeO3 would be formed, diffusing at the grain boundaries. The Ho3+ ions with high electronic valance in the grain boundary region increased, so as to balance the electrical charges, and the metallic ion vacancies could increase the speed of the grain boundary movement [25]. As a result, the grain size increased and the porosity decreased. The apparent and relative densities of the prepared Ni-Zn ferrites are also listed in Table 1. It can be seen that, both of them initially decreased and then increased with increasing doping level of Ho3+ ions, reaching their lowest values of 4.94 g/cm3 and 89.32%, respectively, when x = 0.03. It is well-known that, the relative density of a ceramic sample is directly determined by its porosity, and the porosity of the present samples increased first and then decreased, so the above mentioned change for the relative density of the samples can be expected. Moreover, the apparent density of a sample is affected by both the densification and theoretical density of its components. Because the densification of the present samples rapidly decreased when a small amount of Ho3+ ions was added, therefore the apparent density decreased first after the doping of Ho3+ ions. However, because the densities of the formed foreign phase HoFeO3 (7.95 g/cm3) and the additive Ho2O3 (8.41 g/cm3) are much higher than that of pure Ni-Zn ferrites (5.36 g/cm3), so the additional Ho3+ ions could result in an increase in the apparent density of the samples. These results are different from those reported by Abdul et al. [16], in which the samples presented a monotonous increase in apparent density and thus a monotonous decrease in porosity, due to the much higher doping level of Ho3+ ions into their ferrites, and thus higher contents of the formed foreign phase HoFeO3 and the added, residual Ho2O3 in them.
Fig. 1. XRD patterns of the as-sintered Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
substitute for the positive ions, expanding the crystal lattice and thus resulting in the increased lattice constant [24]. However, the Ho3+ ions in the crystal lattice of the spinel ferrites would reach a saturated state when more and more Ho3+ ions were added, because the Ho3+ ion is too large to enter facilely into the ferrite lattice. Therefore, the doped Ho3+ ions would present a saturated state in the crystal lattice of the resultant spinel ferrites, and the additional Ho3+ ions would form the foreign phase HoFeO3 in the samples, when the content of Ho3+, x, is more than 0.015. As a result, a maximum of lattice constant of the Ho3+ions doped Ni-Zn ferrites could be expected. Fig. 2 displays the SEM micrographs of the prepared Ni-Zn ferrites doped different contents of Ho3+ ions, which were taken on the polished and ultrasonically cleaned surfaces of the sintered samples. To examine the grain growth of the samples, the grain size distribution of each sample was statistically analyzed, and the results are presented in the inset in each micrograph. It can be seen from the insets, the grain size of all the samples follows a Gaussian distribution, generally in a narrow range, indicating that the grains in the samples grew homogeneously. Moreover, the grain sizes of the samples doped with different amounts of Ho3+ ions are calculated, and the results are listed in Table 1. As can be seen from this table, the grain size of the samples decreased initially after the doping of Ho3+ ions, but increased with additional Ho3+ ions doped, resulting in the smallest grain size when the doping level of Ho3+ ions is x = 0.03. And the porosity presents an opposite trend to the mean grain size. This phenomenon might be due to that the added Ho3+ ions would enter into the octahedral B sites in the lattice [16], replacing the Fe3+ ions, which would lead to a small portion of Fe3+ and Ni2+ ions emigrate to A sites. Such a rearrangement of ions would lead to the volatilization of Zn2+ ions, contributing to the formation of pores during the sintering process [16]. Meanwhile, the pores would impede the displacement of grain boundaries. So, the decrease in grain size of the samples doped
3.2. Magnetic performance The hysteresis loops and temperature dependence of magnetization for the prepared ferrite are shown in Figs. 3 and 4, respectively. And the corresponding MS, HC and TC of the samples were all calculated from these two figures (see Table 2). As can be seen from Table 2, the MS of the samples decreased gradually as the content of Ho3+ ions, x, increased from 0 to 0.06. As is known, the Ms of Ni-Zn ferrites can be expressed by the following equation [22],
MS = MB − MA ,
(2)
where MB is the magnetic moment in B sites and MA is that in A sites. The chemical formula of the present ferrites can be written as (Zn0.5Fe) [Ni0.5HoxFe1-x]O4, where the parentheses and square brackets denote the tetrahedral A and octahedral B sites, respectively. Because the doped Ho3+ ions will prefer to occupy octahedral B sites due to its much larger ion radius than the other ions in the ferrites [16], the substitution of Ho3+ ions for Fe3+ ions would lead to the migration of small fraction of Ni2+ and Fe3+ ions from octahedral sites to tetrahedral ones. Such a rearrangement of ions along with zinc loss would increase the value of MA. Hence, the net magnetization would decrease. Moreover, the foreign phase HoFeO3 would form during the sintering process, with a feeble saturation magnetic moment of about 0.05 μB per
Table 1 Basic structural parameters of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06). Nominal composition
Apparent density (g/cm3)
Relative density (%)
Content of foreign phase (wt%)
Lattice constant (Å)
Grain size (μm)
Ni0.5Zn0.5Fe2O4 Ni0.5Zn0.5Ho0.015Fe1.985O4 Ni0.5Zn0.5Ho0.03Fe1.97O4 Ni0.5Zn0.5Ho0.045Fe1.955O4 Ni0.5Zn0.5Ho0.06Fe1.94O4
5.16 5.09 4.94 5.06 5.12
96.25 94.74 91.00 92.27 92.42
0 0 3.39 5.01 6.26
8.383 8.401 8.399 8.404 8.402
3.11 2.94 1.85 1.93 2.35
3
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Fig. 2. Typical SEM micrographs on the polished and ultrasonically cleaned surfaces of the prepared Ni0.5Zn0.5HoxFe2-xO4 with x = (a) 0, (b) 0.015, (c) 0.03, (d) 0.045 and (e) 0.06. The inset of each micrograph displays the grain size distribution of the corresponding sample.
formula unit [26]. And, when the doping level of Ho3+ ions increased, the content of foreign phase HoFeO3 increased. Finally, the Ms of the samples decreased with increasing doping amount of Ho3+ ions in the ferrites. The HC of the present samples increased at first and decline later when Ho3+ ions were doped, reaching the highest value when the content of Ho3+ ions x = 0.03. Generally, HC is mainly affected by the sample microstructure. In literature, it was reported that, there was a close relationship between HC and porosity of a ferrite, and larger pores would affect the HC of the samples more heavily [23]. In soft ferrites, there are two mechanisms of magnetization: the first one is domain wall movement, and the other is domain rotation; and the domain wall movement requires less energy. When the porosity increases, a higher anisotropy field is needed to push the domain wall. In addition, the grain size and distribution also affect the mechanism of magnetization;
and the smaller the grains, the harder the domain wall movement. So, the decrease in the grain size as well as the presence of the foreign phase HoFeO3 are not beneficial for the displacement of domain wall. And because the sample with x = 0.03 presented the smallest grain size, distributed in a narrow range from 0.5 to 2.0 µm as shown in Fig. 2c, thus the Ni-Zn ferrites would have the highest HC. On the other hand, the metallic ion vacancies could promote the growth of ferrite grains, when additional Ho3+ ions were doped as stated in the above text. Therefore, the HC decreased when further increasing amount of Ho3+ ions was doped. The TC of a ferrite could be determined by the temperature at the point where the curve of temperature dependence of magnetization just collapses on heating [27]. The TC of the present samples rose first after the doping of Ho3+ ions, and then dropped down with further increased content of Ho3+ ions. As is well known, the TC of a ferrite depends on
4
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Fig. 5. Frequency dependence of the complex permeability of the as-prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
Fig. 3. Hysteresis loops of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
exchange effect of A-B became weaker, finally resulting in decreased TC. The initial permeability (μi = μ′-jμ′′) versus frequency for the asprepared Ni-Zn ferrites is plotted in Fig. 5. The real part of the permeability (μ′) describes the stored energy, and the imaginary part (μ′′) is the permeability loss [28]. So, the imaginary part (μ′′) approximately equals to the opposite value of the slope of real part (μ′). As is seen from this figure, both μ′ and μ′′ decreased initially with increasing doping content of Ho3+ ions, but increased slightly with additional Ho3+ ions added, reaching a minimum value when the content of Ho3+ ions x = 0.03. For applications, the frequency dependence of initial permeability for a ferrite is very important. The variation of the initial permeability of Ni-Zn ferrites can be explained by the following equation [27]:
μi ∝ Fig. 4. Temperature dependence of magnetization for the prepared Ni0.5Zn0.5HoxFe2xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
MS (emu/g)
HC (Oe)
TC (°C)
Ni0.5Zn0.5Fe2O4 Ni0.5Zn0.5Ho0.015Fe1.985O4 Ni0.5Zn0.5Ho0.03Fe1.97O4 Ni0.5Zn0.5Ho0.045Fe1.955O4 Ni0.5Zn0.5Ho0.06Fe1.94O4
70.40 66.23 66.89 66.71 62.61
2.01 2.45 2.99 2.85 2.66
220 305 292 257 255
(3)
where μi is the initial permeability, Ms is the saturation magnetization, D is the average grain size, and K1 is the magnetocrystalline anisotropy constant. The Ms of the present samples did not change significantly, fluctuating between 70.39 and 62.61 emu/g. The K1 value depends on the type and concentration of the dopants, but the content of Ho3+ ions entered into the lattice is relatively low (about x = 0.015, at which the content of Ho3+ ions in the ferrite almost reached a saturated state, as mentioned in last section). Thus the Ms and K1 could be treated as a constant in the present case. Therefore, it is safely concluded that the main reason responsible for the permeability performance is the mean grain size in this series of ferrites. This is why the varition of the initial permeability presents a similar trendency with that of the grain size with increasing doping level of Ho3+ ions in the present case [29].
Table 2 Basic magnetic parameters of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06). Nominal composition
MS2D K1
its chemical constitution. There are three kinds of exchange effects in spinel-type ferrites, A-A, B-B and A-B, and the exchange effect of A-B is the strongest one. So, the TC of the as-prepared samples can be determined by the exchange effect of metallic ions between tetrahedral A and octahedral B sites [7]. In Ni-Zn ferrites, the A sites are occupied actually by Zn2+ and Fe3+ ions, and the B sites are filled by Ni2+ and Fe3+ ions. When Ho3+ ions were doped into Ni-Zn ferrites, the Fe3+ ions in B sites were substituted by Ho3+ ions [16]. As a result, some Fe3+ ions and Ni2+ ions were pushed into A sites as stated above, enhancing the quantity of magnetic ions in A sites. Finally, the A-B exchange effect in the ferrites would be enhanced [25]. Consequently, the TC of the ferrites increased with the addition of Ho3+ ions. On the other hand, the reduction of TC with additional Ho3+ ions was caused by the weaker Fe-Ho exchange effect compared to the Fe-Fe one, when Fe3+ ions were substituted by Ho3+ ions at the B sites [26]. Moreover, with increasing doping amount of Ho3+ ions, some Fe3+ ions joined in forming the foreign phase HoFeO3 with the additional Ho3+ ions. As a result, the concentration of Fe3+ ions in the ferrites decreased, and the
3.3. Dielectric performance The frequency dependence of the dielectric constant (ε′) and dielectric loss tangent (tanδ) of the prepared Ni-Zn ferrites doped with different contents of Ho3+ ions are plotted in Figs. 6 and 7, respectively. The complex dielectric constant (ε = ε′-jε′′) was measured within frequency from 1 MHz to 1 GHz, where ε′ is the real part, and ε′′ is the imaginary part of the dielectric constant. The dielectric loss tangent is a very important property for soft ferrites, which is defined as ε′′/ε′. For semiconductor materials, the tanδ is used to estimate the eddy-current loss in the alternating electric field. As can be seen from Figs. 6 and 7, both ε′ and tanδ of the present samples increased slightly after the doping of Ho3+ ions, and then dropped down gradually when more Ho3+ ions were added. It is believed that the electrical conductivity of ferrites depends on the electron exchange interaction between Fe2+ ⇔ Fe3+ [30], and the 5
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maximum when x = 0.03, and presenting little decrease when x > 0.03. In addition, the introduction of Ho3+ ions would block up the Fe2+ ⇔ Fe3+ transformation [17]. Due to the above reasons, the dielectric constant and dielectric loss are expected to decrease with increasing content of Ho3+ ions when x is higher than 0.03. 4. Conclusions Ni-Zn ferrites with a nominal composition of Ni0.5Zn0.5HoxFe2-xO4 (x = 0–0.06) were prepared by conventional solid state reaction through using analytical-grade metal oxides powders as raw materials. The conclusions can be summarized as follows. (1) All the samples doped with Ho3+ ions contain a ferrite spinel phase, and when x > 0.015, a small amount of a foreign HoFeO3 phase could be detected. And the content of the foreign phase increased with increasing addition amount of Ho3+ ions. The lattice constant of the ferrite phases doped with Ho3+ ions increased compared with that of the Ni0.5Zn0.5Fe2O4, approaching a constant with further increasing addition amount of Ho3+ ions. The grain size and relative density of the samples reached their lowest values when x = 0.03. (2) The Ms of the ferrites decreased with increasing doping level of Ho3+ ions. The coercivity presented an opposite change trend to Ms. The HC initially raised with Ho3+ ions doping, and then dropped down with more Ho3+ ions doped, reaching the highest value when x = 0.03. The initial permeability presented an opposite change trend to HC. The TC increased first with Ho3+ ions doping, and then reduced when more Ho3+ ions were added, reaching the highest value when x = 0.015. (3) The dielectric loss tangent (tanδ) of the samples increased slightly with Ho3+ ions doping, and then dropped down gradually when more Ho3+ ions were added. The introduction of Ho3+ ions could reduce the dielectric loss significantly, especially when the doping level of Ho3+ ions, x, is higher than 0.03.
Fig. 6. Frequency dependence of the dielectric constants (ε′) of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
Fig. 7. Frequency dependence of the dielectric loss tangent (tanδ) of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant nos. 11674035, 11274052 and 61274015), and the Fundamental Research Funds for the Central Universities.
dielectric behavior is strongly correlated to the conductivity of the materials. So, the variations of ε′ and tanδ of the present ferrites can be explained as follows. When the Ho3+ ions were doped into Ni-Zn ferrites, the Fe3+ ions in octahedral B sites were substituted by Ho3+ ions, and the Fe3+ ions were partially pushed into tetrahedral A sites accompanied by some Zn2+ ions loss [16]. Owing to the higher electronic valance of Fe3+ ions than that of Zn2+ ions, after the doping of Ho3+ ions, the metallic ions vacancies increased in order to balance the electrical charges, and the cationic vacancies are the charge carriers. Consequently, the concentration increase of charge carriers resulted in the higher conductivity and dielectric loss. Moreover, with the inclusion of Ho3+ ions, the Ni-Zn ferrite lattice was distorted and the Fe(Ho)-O bond lengths at B sites increased, giving rise to an increase in the atomic polarizability and subsequently the dielectric constant [26], as seen from Fig. 6. However, no more Ho3+ ions could enter into the crystal lattice of the ferrites when further increased amount of Ho3+ ions was added, because the content of Ho3+ ions in the crystal lattice of the spinel would reach a saturated state. In fact, the additional Ho3+ ions would form a thin layer of HoFeO3 with high resistivity at the grain boundary. Besides, because the conductivity of the grain boundary is much higher than that of the grains in soft ferrites, the conductivity of a ferrite is mainly determined by the number of grain boundary. And the more the grain boundary, the lower the conductivity. In the present case, the mean grain size of the sample with x = 0.03 present the minimum value, distributing in a narrow range from 0.5 to 2.0 µm, and the number of the grain boundary is in inverse proportion to the average grain size. Thus, the number of grain boundary increased after the doping of Ho3+ ions, reaching the
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Structural and electromagnetic properties of Ni0.5Zn0.5HoxFe2-xO4 ferrites ⁎
Zhiqing Liua,b, Zhijian Penga, , Xiuli Fub, a b
⁎
School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ni-Zn ferrite Ho3+ ions doping Magnetic properties Dielectric properties
Ni-Zn ferrites with a nominal composition of Ni0.5Zn0.5HoxFe2-xO4 (x = 0–0.06) were prepared by conventional solid state reaction through using analytical-grade metal oxides powders as raw materials. The phase composition, microstructure, magnetic properties and dielectric performance of the as-prepared samples were investigated. The doped Ho3+ ions could enter into the crystal lattice of the resultant spinel ferrites, causing the expansion of the unit cell, reaching a saturated state when x = 0.015; and the additional Ho3+ ions would form a foreign HoFeO3 phase at the grain boundary. The grain size and densification of the samples initially decreased after a small amount of Ho3+ ions was doped, but then increased with more Ho3+ ions added. The saturation magnetization decreased gradually with increasing substitution level of Ho3+ ions. The Curie temperature and coercivity raised initially and declined later with increasing content of Ho3+ ions in the samples, reaching their maximums of 305 °C with x = 0.015 and 2.99 Oe with x = 0.03, respectively. The variation of complex permeability versus Ho3+ ions substitution level presented an opposite trend to that of coercivity. The dielectric loss increased slightly after the introduction of a small amount of Ho3+ ions, but reduced significantly with more Ho3+ ions doped.
1. Introduction Ni-Zn ferrites are one of the most widely used soft magnetic materials in modern communication, internet, house appliance, computer circuitry and so forth, because of their excellent high-frequency performance, high temperature stability and simple production process [1–3]. However, with the development of anti-electromagnetic disturbing technology, surface mounting technology and high-frequency broad-band device, more challenges have to be addressed, such as higher operating frequencies, enhanced temperature stability and reduced costs [4]. Ni-Zn ferrite polycrystalline materials are of spinel structure and belong to the Fd3m space group. They, commonly written as (ZnxFe1-x) [Ni1-xFe1+x]O4, possess two crystallographically distinct locations, tetrahedral A (the parentheses) and octahedral B (the square brackets) sites, in their crystal structures. The spinel crystal structure involves 64 of tetrahedral A and 32 of octahedral B sites, but only 8 of A and 16 of B sites are occupied. Such crystal structure is beneficial for the doping of other cations to change their properties [5]. In the past decades, the searching of additives for Ni-Zn ferrite has attracted great attention from the scientists worldwide, and today, it is still a hot topic in this field to prepare multi-component ferrites with excellent electromagnetic properties.
⁎
With regard to the specific additive, many chemicals presented an influence on the magnetic and dielectric properties of Ni-Zn ferrites. For example, some cations could enter into the spinel crystal structure and locate in tetrahedral A or octahedral B sites, such as Mg2+ [6], W5+ [7], Al3+ [8], Sn4+ [9] and Co3+ [10], which have a significant effect on the electromagnetic properties of the ferrites. Some would not enter into the crystal lattice, such as Ca2+ [11] and Si4+ [12], but they could effectively decrease the eddy current loss of Ni-Zn ferrites, because these additives would form an insulating intergranular layer at the grain boundaries. And some additives could decrease the sintering temperature of Ni-Zn ferrites due to their lower melting point, like V2O5 [13]. In particular, Rezlescu et al. [14,15] first reported the doping effects of rare earth ions on Ni-Zn ferrites. It was revealed that, the substitution by Yb, Er, Dy, Tb, Gd, Sm and Ce ions could obtain an important modification on both magnetic and electrical properties of the ferrites due to their larger ionic radius. Since then, the magnetic and electrical properties of rare-earth ions doped Ni-Zn ferrites have been addressed by many investigations [16–20]. Moreover, the additionally doped rare-earth ions may form thin layers at the grain boundaries, improving the resistivity of Ni-Zn ferrites. Among the various rare-earth additives, Abdul et al. ever indicated [16] that the doping of Ho3+ ions could increase the DC electrical resistivity of Ni0.7Zn0.3Fe2O4 ferrites, which is of significance in the
Corresponding authors. E-mail addresses: [email protected] (Z. Peng), [email protected] (X. Fu).
http://dx.doi.org/10.1016/j.ceramint.2017.08.011 Received 6 May 2017; Received in revised form 3 July 2017; Accepted 1 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Z., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.08.011
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and 1 mm in thickness, or 12 mm in diameter and 1 mm in thickness) and toroidal rings (20 mm in outer diameter, 10 mm in inner diameter and 3 mm in thickness) by a hydraulic press under a pressure of 40 MPa. Finally, the samples were sintered in a muffle furance at 1250 °C for 4 h with a heating rate of 2 °C/min.
applications of high frequency devices. Unfortunately, their discussion on the magnetic and electrical properties of the samples was performed without providing any microstructure proofs. As a matter of fact, however, there is a close relationship between the grain size and coercivity, and the electrical properties of electronic ceramics also have a strong dependence on the sample microstructure, such as pores and defects. In addition, there may exist some uncertainties in their results. (1) The doped Ho3+ ions are much larger than all the ions in Ni0.7Zn0.3Fe2O4, but after the doping of Ho3+ ions, the lattice constant of their samples (Ni0.7Zn0.3Ho2xFe2-2xO4, 0.0 ≤ x ≤ 0.07) decreased, which is completely adverse against the observation on the lattice constant of similar spinel Co ferrites after the doping of Ho3+ ions (CoFe1.9Ho0.1O4) in Ref. [21]. (2) The densification of their Ni0.7Zn0.3Fe2O4 ferrite was too low with an astonishingly high value of porosity (20.47%), when compared with those of Ni0.5Zn0.5Fe2O4 and Ni0.75Zn0.25Fe2O4 ferrites prepared by the same method (generally with a porosity lower than 10%) [7,8]. More regrettably, no literature has been reported on the temperature stability and frequency dependence of electromagnetic properties of Ho3+ doped Ni-Zn ferrites, which are very important properties of a ferrite for high-frequency applications. Therefore, in this work, Ni-Zn ferrites doped with different amounts of Ho3+ ions were prepared by conventional solid state reaction through using analytical-grade metal oxides powders as raw materials. The compositional, microstructural, magnetic and dielectric properties of the Ni-Zn ferrites were systematically investigated as a function of the content of Ho3+ ions. Resultantly, the Ho3+-substituted Ni-Zn ferrites present excellent dielectric properties with the lowest value of dielectric loss tangent of 0.017 at 100 MHz, which is much lower than those of many transition metal doped Ni0.5Zn0.5Fe2O4 ferrites as reported in Refs. [7,22,23]. Moreover, it was known that the Curie temperature of Ni-Zn ferrites could also be enhanced by the doping of other positive ions, such as W6+ [7], Pr3+ [17], Al3+ [22] and so on, in which the doping of W6+ could lead to the highest increase of 70 °C in Curie temperature compared with pure Ni-Zn ferrites. But the present work reveals that, the doping of Ho3+ into Ni-Zn ferrites also has a significant influence on the temperature stability of Ni-Zn ferrites, resulting in an even higher increase in Curie temperature of 85 °C in comparision with pure Ni-Zn ferrites. Therefore, the doping of Ho3+ into Ni-Zn ferrites is also an effective way to improve the temperature stability of Ni-Zn ferrites.
2.2. Materials characterization The phase of the samples was identified by X-ray diffractometer (XRD, D/max-RB, Cu Kα, λ = 1.5418 Å) in a continuous scanning mode at a rate of 8°/min. The lattice constants were calculated from the XRD data by a software Jade 6 PC. The content of the foreign phase (here HoFeO3) was calucated by
WA =
IA IA +
IB KAB
(1)
where WA is the mass percentage of HoFeO3; IA and IB are the integrated intensities of the strongest diffraction peaks of HoFeO3 and Ni-Zn ferrite, respectively; and KAB is a specific value determined by JCPDS card nos. 52-0278 for the Ni-Zn ferrite and 46-0115 for HoFeO3. The microstructure was examined by a scanning electron microscope (SEM, SU8020). The diameter of the hexagonal grains identified clearly in the SEM images and the grain size distribution were evaluated by using a software Nano Measurer 1.2 PC. The reported grain size for each sample was the mean value of all the grains identified from the whole image. The apparent density was measured using Archimedean method according to the international standard ISO18754; the theoretical density of the samples was calculated by the density of unit cell, which was modified by the corresponding content of the foreign phase HoFeO3 in the samples on the basis of mixture rule; and the relative density was defined by the percentage of the apparent density to the theoretical density. The saturation magnetization (Ms), coercivity (HC) and Curie temperature (TC) were measured by a vibrating sample magnetometer (VSM, LakeShore 7307) with a maximum magnetic field of 10 kOe. By employing an impedance analyzer (Agilent E4991A) within the frequency from 1 to 1000 MHz, the measurements of complex permeability (μ = μ′-jμ′′) and complex dielectric constant (ε = ε′-jε′′) were carried out. All these measurements except TC were performed at room temperature.
2. Experimental procedures
3. Results and discussion
2.1. Samples preparation
3.1. Compositional and structural properties
The investigated Ni-Zn ferrites have a nominal composition of Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06). The samples were perpared by conventional two-steps solid state reaction. The applied raw materials were NiO (99.0 wt%), ZnO (99.0 wt%), α-Fe2O3 (99.0 wt%), and Ho2O3 (99.9 wt%) powders, supplyed by Sinopharm Chemical Reagent Co. Ltd. All the chemicals were directly used without any further purification. During the processing, all the raw powders were first mixed and milled in a planetary mill (YXQM-1L, Changsha Mitr Instruments Co. Ltd., China) for 6 h with deionized water as the dispersion media, 0.5 wt% Davon C as the dispersant, and TZP ZrO2 balls as the grinding media. For the milling, the weight ratio of the raw powders, grinding media and dispersion media is 1:4:5, the volume of the jar is 1 L, and the rotation speed of the jar is 250 rpm. After milling, the resultant slurries were dried in an open oven at 120 °C, and the dried powder chuncks were then grinded in an agate mortar. After that, the obtained fine powders were calcined in a Muffle furnace at 800 °C for 3 h. The calcined powder chunks were then planetarily grinded into fine powders again under the same conditions as those for the raw powder mixing, and granulated with 0.5 wt% PVA as the binder. After that, the obtained powders were pressed into pellets (6 mm in diameter
Fig. 1 shows the XRD patterns of the as-prepared Ni-Zn ferrites doped with different contents of Ho3+ ions, which were taken on the assintered samples (6 mm in diameter and 1 mm in thickness). As is seen from this figure, the main phase of the present Ho3+ ions doped Ni-Zn ferrites is of a typical spinel structure (JCPDS card no. 52-0278). But, when x = 0.03, a foreign phase could be detected, which was identified as HoFeO3 (JCPDS card no. 46-0115); and the content of the foreign phase increased with increasing doping amount of Ho3+ ions (see Table 1). The lattice constants of the as-prepared Ni-Zn ferrites were calculated from the XRD data, and the results are presented in Table 1. It can be seen that, the lattice constant increased from 8.383 to 8.401 Å initially after Ho3+ ions were doped (x = 0.015), which is similar with the results reported by other researchers [21]. But the lattice constants did not increase continuously, which kept a constant of about 8.40 Å with further increasing doping amount of Ho3+ ions (x > 0.015) instead. For this phenomenon, it can be explained as follows. Because Ho3+ ion (1.04 Å) is much larger than Fe3+ ion (0.67 Å), Ni2+ ion (0.69 Å) and Zn2+ ion (0.74 Å), so when the doped Ho3+ ions entered into the crystal lattice of the spinel ferrites, they would 2
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with little amount of Ho3+ ions is expected. And the formation of pores could limit the abnormal growth of some grains, which is of help for the homogeneous growth of grains. However, when further increasing amount of Ho3+ ions was added, a foreign phase HoFeO3 would be formed, diffusing at the grain boundaries. The Ho3+ ions with high electronic valance in the grain boundary region increased, so as to balance the electrical charges, and the metallic ion vacancies could increase the speed of the grain boundary movement [25]. As a result, the grain size increased and the porosity decreased. The apparent and relative densities of the prepared Ni-Zn ferrites are also listed in Table 1. It can be seen that, both of them initially decreased and then increased with increasing doping level of Ho3+ ions, reaching their lowest values of 4.94 g/cm3 and 89.32%, respectively, when x = 0.03. It is well-known that, the relative density of a ceramic sample is directly determined by its porosity, and the porosity of the present samples increased first and then decreased, so the above mentioned change for the relative density of the samples can be expected. Moreover, the apparent density of a sample is affected by both the densification and theoretical density of its components. Because the densification of the present samples rapidly decreased when a small amount of Ho3+ ions was added, therefore the apparent density decreased first after the doping of Ho3+ ions. However, because the densities of the formed foreign phase HoFeO3 (7.95 g/cm3) and the additive Ho2O3 (8.41 g/cm3) are much higher than that of pure Ni-Zn ferrites (5.36 g/cm3), so the additional Ho3+ ions could result in an increase in the apparent density of the samples. These results are different from those reported by Abdul et al. [16], in which the samples presented a monotonous increase in apparent density and thus a monotonous decrease in porosity, due to the much higher doping level of Ho3+ ions into their ferrites, and thus higher contents of the formed foreign phase HoFeO3 and the added, residual Ho2O3 in them.
Fig. 1. XRD patterns of the as-sintered Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
substitute for the positive ions, expanding the crystal lattice and thus resulting in the increased lattice constant [24]. However, the Ho3+ ions in the crystal lattice of the spinel ferrites would reach a saturated state when more and more Ho3+ ions were added, because the Ho3+ ion is too large to enter facilely into the ferrite lattice. Therefore, the doped Ho3+ ions would present a saturated state in the crystal lattice of the resultant spinel ferrites, and the additional Ho3+ ions would form the foreign phase HoFeO3 in the samples, when the content of Ho3+, x, is more than 0.015. As a result, a maximum of lattice constant of the Ho3+ions doped Ni-Zn ferrites could be expected. Fig. 2 displays the SEM micrographs of the prepared Ni-Zn ferrites doped different contents of Ho3+ ions, which were taken on the polished and ultrasonically cleaned surfaces of the sintered samples. To examine the grain growth of the samples, the grain size distribution of each sample was statistically analyzed, and the results are presented in the inset in each micrograph. It can be seen from the insets, the grain size of all the samples follows a Gaussian distribution, generally in a narrow range, indicating that the grains in the samples grew homogeneously. Moreover, the grain sizes of the samples doped with different amounts of Ho3+ ions are calculated, and the results are listed in Table 1. As can be seen from this table, the grain size of the samples decreased initially after the doping of Ho3+ ions, but increased with additional Ho3+ ions doped, resulting in the smallest grain size when the doping level of Ho3+ ions is x = 0.03. And the porosity presents an opposite trend to the mean grain size. This phenomenon might be due to that the added Ho3+ ions would enter into the octahedral B sites in the lattice [16], replacing the Fe3+ ions, which would lead to a small portion of Fe3+ and Ni2+ ions emigrate to A sites. Such a rearrangement of ions would lead to the volatilization of Zn2+ ions, contributing to the formation of pores during the sintering process [16]. Meanwhile, the pores would impede the displacement of grain boundaries. So, the decrease in grain size of the samples doped
3.2. Magnetic performance The hysteresis loops and temperature dependence of magnetization for the prepared ferrite are shown in Figs. 3 and 4, respectively. And the corresponding MS, HC and TC of the samples were all calculated from these two figures (see Table 2). As can be seen from Table 2, the MS of the samples decreased gradually as the content of Ho3+ ions, x, increased from 0 to 0.06. As is known, the Ms of Ni-Zn ferrites can be expressed by the following equation [22],
MS = MB − MA ,
(2)
where MB is the magnetic moment in B sites and MA is that in A sites. The chemical formula of the present ferrites can be written as (Zn0.5Fe) [Ni0.5HoxFe1-x]O4, where the parentheses and square brackets denote the tetrahedral A and octahedral B sites, respectively. Because the doped Ho3+ ions will prefer to occupy octahedral B sites due to its much larger ion radius than the other ions in the ferrites [16], the substitution of Ho3+ ions for Fe3+ ions would lead to the migration of small fraction of Ni2+ and Fe3+ ions from octahedral sites to tetrahedral ones. Such a rearrangement of ions along with zinc loss would increase the value of MA. Hence, the net magnetization would decrease. Moreover, the foreign phase HoFeO3 would form during the sintering process, with a feeble saturation magnetic moment of about 0.05 μB per
Table 1 Basic structural parameters of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06). Nominal composition
Apparent density (g/cm3)
Relative density (%)
Content of foreign phase (wt%)
Lattice constant (Å)
Grain size (μm)
Ni0.5Zn0.5Fe2O4 Ni0.5Zn0.5Ho0.015Fe1.985O4 Ni0.5Zn0.5Ho0.03Fe1.97O4 Ni0.5Zn0.5Ho0.045Fe1.955O4 Ni0.5Zn0.5Ho0.06Fe1.94O4
5.16 5.09 4.94 5.06 5.12
96.25 94.74 91.00 92.27 92.42
0 0 3.39 5.01 6.26
8.383 8.401 8.399 8.404 8.402
3.11 2.94 1.85 1.93 2.35
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Fig. 2. Typical SEM micrographs on the polished and ultrasonically cleaned surfaces of the prepared Ni0.5Zn0.5HoxFe2-xO4 with x = (a) 0, (b) 0.015, (c) 0.03, (d) 0.045 and (e) 0.06. The inset of each micrograph displays the grain size distribution of the corresponding sample.
formula unit [26]. And, when the doping level of Ho3+ ions increased, the content of foreign phase HoFeO3 increased. Finally, the Ms of the samples decreased with increasing doping amount of Ho3+ ions in the ferrites. The HC of the present samples increased at first and decline later when Ho3+ ions were doped, reaching the highest value when the content of Ho3+ ions x = 0.03. Generally, HC is mainly affected by the sample microstructure. In literature, it was reported that, there was a close relationship between HC and porosity of a ferrite, and larger pores would affect the HC of the samples more heavily [23]. In soft ferrites, there are two mechanisms of magnetization: the first one is domain wall movement, and the other is domain rotation; and the domain wall movement requires less energy. When the porosity increases, a higher anisotropy field is needed to push the domain wall. In addition, the grain size and distribution also affect the mechanism of magnetization;
and the smaller the grains, the harder the domain wall movement. So, the decrease in the grain size as well as the presence of the foreign phase HoFeO3 are not beneficial for the displacement of domain wall. And because the sample with x = 0.03 presented the smallest grain size, distributed in a narrow range from 0.5 to 2.0 µm as shown in Fig. 2c, thus the Ni-Zn ferrites would have the highest HC. On the other hand, the metallic ion vacancies could promote the growth of ferrite grains, when additional Ho3+ ions were doped as stated in the above text. Therefore, the HC decreased when further increasing amount of Ho3+ ions was doped. The TC of a ferrite could be determined by the temperature at the point where the curve of temperature dependence of magnetization just collapses on heating [27]. The TC of the present samples rose first after the doping of Ho3+ ions, and then dropped down with further increased content of Ho3+ ions. As is well known, the TC of a ferrite depends on
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Fig. 5. Frequency dependence of the complex permeability of the as-prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
Fig. 3. Hysteresis loops of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
exchange effect of A-B became weaker, finally resulting in decreased TC. The initial permeability (μi = μ′-jμ′′) versus frequency for the asprepared Ni-Zn ferrites is plotted in Fig. 5. The real part of the permeability (μ′) describes the stored energy, and the imaginary part (μ′′) is the permeability loss [28]. So, the imaginary part (μ′′) approximately equals to the opposite value of the slope of real part (μ′). As is seen from this figure, both μ′ and μ′′ decreased initially with increasing doping content of Ho3+ ions, but increased slightly with additional Ho3+ ions added, reaching a minimum value when the content of Ho3+ ions x = 0.03. For applications, the frequency dependence of initial permeability for a ferrite is very important. The variation of the initial permeability of Ni-Zn ferrites can be explained by the following equation [27]:
μi ∝ Fig. 4. Temperature dependence of magnetization for the prepared Ni0.5Zn0.5HoxFe2xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
MS (emu/g)
HC (Oe)
TC (°C)
Ni0.5Zn0.5Fe2O4 Ni0.5Zn0.5Ho0.015Fe1.985O4 Ni0.5Zn0.5Ho0.03Fe1.97O4 Ni0.5Zn0.5Ho0.045Fe1.955O4 Ni0.5Zn0.5Ho0.06Fe1.94O4
70.40 66.23 66.89 66.71 62.61
2.01 2.45 2.99 2.85 2.66
220 305 292 257 255
(3)
where μi is the initial permeability, Ms is the saturation magnetization, D is the average grain size, and K1 is the magnetocrystalline anisotropy constant. The Ms of the present samples did not change significantly, fluctuating between 70.39 and 62.61 emu/g. The K1 value depends on the type and concentration of the dopants, but the content of Ho3+ ions entered into the lattice is relatively low (about x = 0.015, at which the content of Ho3+ ions in the ferrite almost reached a saturated state, as mentioned in last section). Thus the Ms and K1 could be treated as a constant in the present case. Therefore, it is safely concluded that the main reason responsible for the permeability performance is the mean grain size in this series of ferrites. This is why the varition of the initial permeability presents a similar trendency with that of the grain size with increasing doping level of Ho3+ ions in the present case [29].
Table 2 Basic magnetic parameters of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06). Nominal composition
MS2D K1
its chemical constitution. There are three kinds of exchange effects in spinel-type ferrites, A-A, B-B and A-B, and the exchange effect of A-B is the strongest one. So, the TC of the as-prepared samples can be determined by the exchange effect of metallic ions between tetrahedral A and octahedral B sites [7]. In Ni-Zn ferrites, the A sites are occupied actually by Zn2+ and Fe3+ ions, and the B sites are filled by Ni2+ and Fe3+ ions. When Ho3+ ions were doped into Ni-Zn ferrites, the Fe3+ ions in B sites were substituted by Ho3+ ions [16]. As a result, some Fe3+ ions and Ni2+ ions were pushed into A sites as stated above, enhancing the quantity of magnetic ions in A sites. Finally, the A-B exchange effect in the ferrites would be enhanced [25]. Consequently, the TC of the ferrites increased with the addition of Ho3+ ions. On the other hand, the reduction of TC with additional Ho3+ ions was caused by the weaker Fe-Ho exchange effect compared to the Fe-Fe one, when Fe3+ ions were substituted by Ho3+ ions at the B sites [26]. Moreover, with increasing doping amount of Ho3+ ions, some Fe3+ ions joined in forming the foreign phase HoFeO3 with the additional Ho3+ ions. As a result, the concentration of Fe3+ ions in the ferrites decreased, and the
3.3. Dielectric performance The frequency dependence of the dielectric constant (ε′) and dielectric loss tangent (tanδ) of the prepared Ni-Zn ferrites doped with different contents of Ho3+ ions are plotted in Figs. 6 and 7, respectively. The complex dielectric constant (ε = ε′-jε′′) was measured within frequency from 1 MHz to 1 GHz, where ε′ is the real part, and ε′′ is the imaginary part of the dielectric constant. The dielectric loss tangent is a very important property for soft ferrites, which is defined as ε′′/ε′. For semiconductor materials, the tanδ is used to estimate the eddy-current loss in the alternating electric field. As can be seen from Figs. 6 and 7, both ε′ and tanδ of the present samples increased slightly after the doping of Ho3+ ions, and then dropped down gradually when more Ho3+ ions were added. It is believed that the electrical conductivity of ferrites depends on the electron exchange interaction between Fe2+ ⇔ Fe3+ [30], and the 5
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maximum when x = 0.03, and presenting little decrease when x > 0.03. In addition, the introduction of Ho3+ ions would block up the Fe2+ ⇔ Fe3+ transformation [17]. Due to the above reasons, the dielectric constant and dielectric loss are expected to decrease with increasing content of Ho3+ ions when x is higher than 0.03. 4. Conclusions Ni-Zn ferrites with a nominal composition of Ni0.5Zn0.5HoxFe2-xO4 (x = 0–0.06) were prepared by conventional solid state reaction through using analytical-grade metal oxides powders as raw materials. The conclusions can be summarized as follows. (1) All the samples doped with Ho3+ ions contain a ferrite spinel phase, and when x > 0.015, a small amount of a foreign HoFeO3 phase could be detected. And the content of the foreign phase increased with increasing addition amount of Ho3+ ions. The lattice constant of the ferrite phases doped with Ho3+ ions increased compared with that of the Ni0.5Zn0.5Fe2O4, approaching a constant with further increasing addition amount of Ho3+ ions. The grain size and relative density of the samples reached their lowest values when x = 0.03. (2) The Ms of the ferrites decreased with increasing doping level of Ho3+ ions. The coercivity presented an opposite change trend to Ms. The HC initially raised with Ho3+ ions doping, and then dropped down with more Ho3+ ions doped, reaching the highest value when x = 0.03. The initial permeability presented an opposite change trend to HC. The TC increased first with Ho3+ ions doping, and then reduced when more Ho3+ ions were added, reaching the highest value when x = 0.015. (3) The dielectric loss tangent (tanδ) of the samples increased slightly with Ho3+ ions doping, and then dropped down gradually when more Ho3+ ions were added. The introduction of Ho3+ ions could reduce the dielectric loss significantly, especially when the doping level of Ho3+ ions, x, is higher than 0.03.
Fig. 6. Frequency dependence of the dielectric constants (ε′) of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
Fig. 7. Frequency dependence of the dielectric loss tangent (tanδ) of the prepared Ni0.5Zn0.5HoxFe2-xO4 (x = 0, 0.015, 0.03, 0.045 and 0.06).
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant nos. 11674035, 11274052 and 61274015), and the Fundamental Research Funds for the Central Universities.
dielectric behavior is strongly correlated to the conductivity of the materials. So, the variations of ε′ and tanδ of the present ferrites can be explained as follows. When the Ho3+ ions were doped into Ni-Zn ferrites, the Fe3+ ions in octahedral B sites were substituted by Ho3+ ions, and the Fe3+ ions were partially pushed into tetrahedral A sites accompanied by some Zn2+ ions loss [16]. Owing to the higher electronic valance of Fe3+ ions than that of Zn2+ ions, after the doping of Ho3+ ions, the metallic ions vacancies increased in order to balance the electrical charges, and the cationic vacancies are the charge carriers. Consequently, the concentration increase of charge carriers resulted in the higher conductivity and dielectric loss. Moreover, with the inclusion of Ho3+ ions, the Ni-Zn ferrite lattice was distorted and the Fe(Ho)-O bond lengths at B sites increased, giving rise to an increase in the atomic polarizability and subsequently the dielectric constant [26], as seen from Fig. 6. However, no more Ho3+ ions could enter into the crystal lattice of the ferrites when further increased amount of Ho3+ ions was added, because the content of Ho3+ ions in the crystal lattice of the spinel would reach a saturated state. In fact, the additional Ho3+ ions would form a thin layer of HoFeO3 with high resistivity at the grain boundary. Besides, because the conductivity of the grain boundary is much higher than that of the grains in soft ferrites, the conductivity of a ferrite is mainly determined by the number of grain boundary. And the more the grain boundary, the lower the conductivity. In the present case, the mean grain size of the sample with x = 0.03 present the minimum value, distributing in a narrow range from 0.5 to 2.0 µm, and the number of the grain boundary is in inverse proportion to the average grain size. Thus, the number of grain boundary increased after the doping of Ho3+ ions, reaching the
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