- Email: [email protected]
Broadened ferromagnetic resonance range in ferrite by gradient composition design
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Broadened ferromagnetic resonance range in ferrite by gradient composition design Chuyang Liua,b, Yufan Caoa, Xiaohan Zhanga, Hao Rena, Yujing Zhangc,∗, Yanting Zhanga, Xinrui Zhaoa, Gang Fanga, Kangsen Penga, Junding Zoub a
School of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027, China c School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic materials Ferromagnetic resonance Barium ferrite Gradient substitution Magnetocrystalline anisotropy
Magnetic materials with broad ferromagnetic resonance (FMR) range have great potential in microwave absorption field. In this work, M type barium ferrites with local chemical gradients were prepared by mixing and heating the ferrite powders with different Zr4+-Ni2+ contents, constructing the locally inhomogeneous magnetocrystalline anisotropy field to widen the FMR range. The results show that ionic diffusion process starts significantly at 1000 °C. The mixed ferrite reheated at 1100 °C finally exhibits the broadest FMR range (7.32 GHz) because of the appropriate gradient composition design, which is much wider than that of the original uniformly substituted barium ferrites.
1. Introduction Ferromagnetic resonance (FMR), as an important phenomenon of ferromagnetic materials, has great applications in various fields [1,2]. For instance, magnetic materials can be used as competitive microwave absorption materials (MAMs) due to the large magnetic loss in specific frequency range induced by FMR [3,4]. Moreover, it has been proved that wide FMR range can help to achieve broad absorption bandwidth in MAMs [5,6]. Therefore, it is of great interest to find a simple and efficient way to widen FMR range of the magnetic materials. As is known, the frequency of FMR is determined by Ha as shown in Eq. (1), where γ is gyromagnetic ratio, g is Landé factor and Ha is magnetocrystalline anisotropy field [7]. Obviously, if Ha is not uniformly distributed in the magnetic materials, FMR could occur simultaneously at different frequency ranges and thus broaden the total FMR range. In fact,
fr =
γ Ha = 1.4gHa 2π
(1)
the Ha of ferrite (one kind of typical magnetic materials) is mainly contributed by the Fe3+ ions in deformed oxygen polyhedra and will decrease gradually with increasing the non-magnetic or weak magnetic ions that substituting for Fe3+ ions [8]. Namely, gradient substitution of Fe3+ ions might cause inhomogeneous distribution of Ha in ferrites, inducing FMRs at different frequency range and eventually broaden the ⁎
FMR range. Herein, we have prepared M type barium ferrite gradiently substituted by Zr4+ and Ni2+ ions. M type barium ferrite is a common ferrite, Zr4+ and Ni2+ ions are typical ions that can replace Fe3+ ions in the ferrite with non-magnetism and weaker magnetism [6,9,10]. In this work, we firstly fabricated the “high concentration” and “low concentration” of Zr4+-Ni2+ uniformly co-doped M type barium ferrite, respectively. Then they are mixed and reheated to establish appropriate gradient substitution via ionic diffusion. The schematic diagram for wide FMR range is illustrated in Fig. 1. 2. Material and methods BaFe12-2xZrxNixO19 (x = 0.6, x = 0.7) ferrites were synthesized by a sol-gel process, using Ba(NO3)2·5H2O、Fe(NO3)3·9H2O、Zr (NO3)4·5H2O、Ni(NO3)2·4H2O as raw materials, C6H8O7·H2O as complexing agent and NH3·H2O as pH regulator, respectively. The specific steps for preparation of the barium ferrite procedures were based on our previous work [6,10]. The procedures were then sintered at 1200 °C for 3 h to obtain BaFe12-2xZrxNixO19 with x = 0.6 and x = 0.7. After that, the obtained barium ferrites with x = 0.6 and x = 0.7 were mixed homogeneously by a planetary ball mill with mass ratio of 1:1 and ultimately heated at 800 °C–1200 °C for 2 h to attain the desired Zr4+Ni2+ ions gradient-substituted M type barium ferrites.
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.107 Received 21 May 2019; Received in revised form 4 August 2019; Accepted 10 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Chuyang Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.107
Ceramics International xxx (xxxx) xxx–xxx
C. Liu, et al.
Fig. 1. The schematic diagram of broadening ferromagnetic resonance range in ferrite by gradient composition design.
plate-like grains appear. The implication here is that reheating below 1000 °C could barely influence the morphology of the barium ferrite while reheating above 1000 °C would promote the grain growth and the formation of plate-like grains. Fig. 3(a) shows hysteresis loops of the mixed barium ferrite powders. As can be seen, the samples with and without secondary heattreatment are all saturated when the external magnetic field reaches to 15 kOe. Coercive field (Hc) of all the samples are deduced from the hysteresis loops and shown as an inset in Fig. 3(a). It is interesting to note that Hc of the mixed barium ferrites almost remain stable at ~1.3 kOe by heating at 800 °C–950 °C, whereas a dramatic drop to ~0.7 kOe occurs when the temperature reaches to 1000 °C and it consistently decreases to ~0.4 kOe with the temperature rising to 1200 °C. To our knowledge, Hc is dominantly controlled by Ha and grain size, and it reduces with the decreasing of Ha or the increasing of grain size [6,11]. As is seen, the grain size shows no distinct change at the temperature range of 800 °C–1000 °C. The stable Hc from unheated condition to 950 °C means that the distribution of Ha in the mixture is not changed, implying that the ionic diffusion barely happens within temperature range. However, a sharp reduction of Hc is seen with the temperature increasing to 1000 °C. It is discussed that the “high concentration” substituted barium ferrite would possess lower Ha while the “low concentration” substituted one would exhibit higher Ha, owing to the substitution amount of magnetic Fe3+ ions by the non-magnetic (Zr4+) ions and the weaker magnetic (Ni2+) ions. Therefore, the “harder phase” with higher Ha would impede the magnetic domain rotation of the adjacent “softer phase” in the mixture by exchange
The phase structures and morphologies of the barium ferrites were identified by X-ray diffraction (XRD, PANaltyical B V Empyrean 200895, Cu Kα radiation) and scanning electron microscopy (SEM, Hitachi S-4800), respectively. The hysteresis loops of the samples were measured by the vibrating sample magnetometer (VSM, YP07-VSM130). The permeability spectra of the ferrites were measured by a vector network analyzer (Agilent, N5244A) in the frequency range of 2–18 GHz.
3. Results and discussion Fig. 2(a) displays the XRD patterns of the samples prepared by mixing BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19 with mass ratio of 1:1 and then reheating at 800 °C–1200 °C for 2 h. It is clearly seen that all the samples only contain M-type barium ferrite phase (BaFe12O19). The intensities of the diffraction peaks are observed to become stronger gradually with the increased temperature to 1200 °C.The results demonstrate that the secondary heat-treatment will not cause the formation of new phases, but it can obviously further improve the crystallinity of the BaFe12O19 phase. The morphologies of the unheated sample and the ones heated at 900 °C, 1000 °C and 1200 °C are observed by SEM and shown in Fig. 2(b). In comparison with the unheated sample, the grains have no obvious change after heating at 900 °C and 1000 °C. In fact, for the three samples, most of the grains possess sharp edges with the size around 200 nm, confirming the formation of crystalline phases. While with the temperature further increasing to 1200 °C, the grains grow larger to ~400 nm and typical
Fig. 2. The (a) XRD and (b) SEM of the mixed BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19 by mass ratio of 1:1 without heat treatment and heated at 800 °C–1200 °C for 2 h. 2
Ceramics International xxx (xxxx) xxx–xxx
C. Liu, et al.
Fig. 3. The (a) hysteresis loops of the mixed BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19 by mass ratio of 1:1 without heat treatment and heated at 800 °C–1200 °C for 2 h and the (b) μ″ spectra over 2–18 GHz for the mixed barium ferrites and the individual BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19.
no obvious influence on morphology at 800–1000 °C, and promotes grain growth and formation of plate-like grains above 1000 °C. The sharp decline in Hc at 1000 °C is because of the reduced Ha difference in local area that diminishes the hindrance effect between “harder” and “softer” magnetic phases, indicating the ionic diffusion starts to occur significantly when the secondary heat-treatment reaches to 1000 °C. Contributed by proper gradient composition, the sample reheated at 1100 °C shows broad FMR range ~7.32 GHz, which is much wider than that of the individual BaFe10.6Zr0.7Ni0.7O19 and BaFe10.8Zr0.6Ni0.6O19.
coupling, contributing relatively high value of the Hc [12]. But once the ionic diffusion emerges, the concentration difference of the doping ions in local area will decrease, and so will the difference of Ha. As a consequence, the hindrance effect would diminish and result in lower values of the Hc. Overall, the dramatic decrease in Hc indicates that ionic diffusion might start to occur significantly at 1000 °C. In addition, the continuous reduction of Hc from 1000 °C to 1200 °C might be contributed by two reasons. One is the sustained ionic diffusion and the other is the grain growth at 1200 °C as shown in Fig. 2(b). The imaginary part of permeability (μ″) spectra over 2–18 GHz for the individual BaFe12-2xZrxNixO19 with x = 0.6 and x = 0.7 and the mixed specimens reheated at 800 °C–1200 °C are summarized in Fig. 3(b). The maximum value approaches to ~0.3 for both two individual samples, and it emerges at ~11.5 GHz for BaFe10.8Zr0.6Ni0.6O19 and ~8.0 GHz for BaFe10.6Zr0.7Ni0.7O19, respectively. Therefore, in the mixed samples reheated at 800 °C–1000 °C, the two resonance regions at 6–9 GHz and 10–14 GHz are attributed to the BaFe10.6Zr0.7Ni0.7O19 and BaFe10.8Zr0.6Ni0.6O19 phase, separately. While due to the decreased content for both two crystalline phases, the resonance intensities of the two regions are lower than 0.2 in the samples heated below 1000 °C. Nevertheless, with the heat treatment temperature rising to 1000 °C, the crystallinity of barium ferrite phase is greatly improved as shown in Fig. 2(a), which strengthens the resonance intensity to just ~0.4. Moreover, as the ionic diffusion occurs dramatically since 1000 °C, gradient substitution with small concentration intervals could be established at 1000 °C. Consequently, the μ″ value higher than 0.2 covers 7.68–14.56 GHz (6.88 GHz) for the sample sintered at 1000 °C, which partly combines the frequency range of individual BaFe10.6Zr0.7Ni0.7O19 (6.24–12.52 GHz) and BaFe10.8Zr0.6Ni0.6O19 (8.52–14.96 GHz), and contributes broader total range than that of both the two individual samples. By consistently improving the temperature to 1100 °C, the frequency range with μ″ value higher than 0.2 is further widened to 6.56–13.88 GHz (7.32 GHz). However, the range is narrowed down to 7.20–14.04 GHz (6.84 GHz) at 1200 °C. It is probably because that the excessive ionic diffusion at high temperature further reduces the concentration intervals and makes the composition tend to be homogeneous again. Overall, in this situation, 1100 °C is the appropriate secondary heat-treatment temperature to broaden the FMR range. And the corresponding sample could be highly competitive in the microwave absorption area.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51802155, 51801103, 51471150), the Natural Science Foundation of Jiangsu Province (No. BK20180443), the “Shuangchuang Doctor” Foundation of Jiangsu Province, the Aeronautical Science Foundation of China (No.2018ZF52078), the visiting scholar fund of state key laboratory of silicon materials (No. SKL2019-09) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] O.N. Martyanov, S.N. Trukhan, V.F. Yudanov, Ferromagnetic resonance fine structure of dispersed magnets: physical origin and applications, Appl. Magn. Reson. 33 (2008) 57–71. [2] H. Suhl, Origin and use of instabilities in ferromagnetic resonance, J. Appl. Phys. 29 (1958) 416–421. [3] K. Zhang, J.H. Luo, N. Yu, M.M. Gu, X.K. Sun, Synthesis and excellent electromagnetic absorption properties of reduced graphene oxide/PANI/ BaNd0.2Sm0.2Fe11.6O19 nanocomposites, J. Alloy. Comp. 779 (2019) 270–279. [4] J.H. Luo, L. Yue, H.R. Ji, K. Zhang, N. Yu, Investigation on the optimization, design and microwave absorption properties of BaTb0.2Eu0.2Fe11.6O19/PANI decorated on reduced graphene oxide nanocomposites, J. Mater. Sci. 54 (2019) 6332–6346. [5] C.Y. Liu, Q.K. Xu, Y. Tang, Z.R. Wang, R.Y. Ma, N. Ma, P.Y. Du, Zr4+ dopingcontrolled permittivity and permeability of BaFe12-xZrxO19 and the extraordinary EM absorption power in the millimeter wavelength frequency range, J. Mater. Chem. C 4 (2016) 9532–9543. [6] J.G. Jia, C.Y. Liu, N. Ma, G.R. Han, W.J. Weng, P.Y. Du, Exchange coupling controlled ferrite with dual magnetic resonance and broad frequency bandwidth in microwave absorption, Sci. Technol. Adv. Mater. 14 (2013) 045002. [7] S.P. Gairola, V. Verma, A. Singh, L.P. Purohit, R.K. Kotnala, Modified composition of barium ferrite to act as a microwave absorber in X-band frequencies, Solid State Commun. 150 (2010) 147–151. [8] Y. Xu, G.L. Yang, D.P. Chu, H.R. Zhai, Magnetic anisotropy of BaM ferrites, J. Magn. Magn. Mater. 31–34 (1983) 815–816. [9] Y. Zhao, J.X. Li, Y.H. Ding, L.H. Guan, Enhancing the lithium storage performance of iron oxide composites through partial substitution with Ni2+ or Co2+, J. Mater. Chem. 21 (2011) 19101–19105. [10] C.Y. Liu, Y.J. Zhang, Y. Tang, Z.R. Wang, N. Ma, P.Y. Du, The tunable magnetic and microwave absorption properties of the Nb5+-Ni2+ co-doped M-type barium ferrite, J. Mater. Chem. C 5 (2017) 3461–3472. [11] J. Dho, E.K. Lee, J.Y. Park, N.H. Hur, Effects of the grain boundary on the coercivity of barium ferrite BaFe12O19, J. Magn. Magn. Mater. 285 (2005) 164–168. [12] D. Roy, P.S.A. Kumar, Enhancement of (BH)max in a hard-soft-ferrite nanocomposite using exchange spring mechanism, J. Appl. Phys. 106 (2009) 073902.
4. Conclusions The Zr4+-Ni2+ ions gradient-substituted M type barium ferrite are prepared by mixing BaFe12-2xZrxNixO19 with x = 0.6 and x = 0.7 with the mass ratio of 1:1 and then reheating at 800 °C–1200 °C for 2 h. The secondary heat-treatment further improves the crystallinity of the barium ferrite phase gradually with the temperature rising. While it has 3
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Broadened ferromagnetic resonance range in ferrite by gradient composition design Chuyang Liua,b, Yufan Caoa, Xiaohan Zhanga, Hao Rena, Yujing Zhangc,∗, Yanting Zhanga, Xinrui Zhaoa, Gang Fanga, Kangsen Penga, Junding Zoub a
School of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027, China c School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic materials Ferromagnetic resonance Barium ferrite Gradient substitution Magnetocrystalline anisotropy
Magnetic materials with broad ferromagnetic resonance (FMR) range have great potential in microwave absorption field. In this work, M type barium ferrites with local chemical gradients were prepared by mixing and heating the ferrite powders with different Zr4+-Ni2+ contents, constructing the locally inhomogeneous magnetocrystalline anisotropy field to widen the FMR range. The results show that ionic diffusion process starts significantly at 1000 °C. The mixed ferrite reheated at 1100 °C finally exhibits the broadest FMR range (7.32 GHz) because of the appropriate gradient composition design, which is much wider than that of the original uniformly substituted barium ferrites.
1. Introduction Ferromagnetic resonance (FMR), as an important phenomenon of ferromagnetic materials, has great applications in various fields [1,2]. For instance, magnetic materials can be used as competitive microwave absorption materials (MAMs) due to the large magnetic loss in specific frequency range induced by FMR [3,4]. Moreover, it has been proved that wide FMR range can help to achieve broad absorption bandwidth in MAMs [5,6]. Therefore, it is of great interest to find a simple and efficient way to widen FMR range of the magnetic materials. As is known, the frequency of FMR is determined by Ha as shown in Eq. (1), where γ is gyromagnetic ratio, g is Landé factor and Ha is magnetocrystalline anisotropy field [7]. Obviously, if Ha is not uniformly distributed in the magnetic materials, FMR could occur simultaneously at different frequency ranges and thus broaden the total FMR range. In fact,
fr =
γ Ha = 1.4gHa 2π
(1)
the Ha of ferrite (one kind of typical magnetic materials) is mainly contributed by the Fe3+ ions in deformed oxygen polyhedra and will decrease gradually with increasing the non-magnetic or weak magnetic ions that substituting for Fe3+ ions [8]. Namely, gradient substitution of Fe3+ ions might cause inhomogeneous distribution of Ha in ferrites, inducing FMRs at different frequency range and eventually broaden the ⁎
FMR range. Herein, we have prepared M type barium ferrite gradiently substituted by Zr4+ and Ni2+ ions. M type barium ferrite is a common ferrite, Zr4+ and Ni2+ ions are typical ions that can replace Fe3+ ions in the ferrite with non-magnetism and weaker magnetism [6,9,10]. In this work, we firstly fabricated the “high concentration” and “low concentration” of Zr4+-Ni2+ uniformly co-doped M type barium ferrite, respectively. Then they are mixed and reheated to establish appropriate gradient substitution via ionic diffusion. The schematic diagram for wide FMR range is illustrated in Fig. 1. 2. Material and methods BaFe12-2xZrxNixO19 (x = 0.6, x = 0.7) ferrites were synthesized by a sol-gel process, using Ba(NO3)2·5H2O、Fe(NO3)3·9H2O、Zr (NO3)4·5H2O、Ni(NO3)2·4H2O as raw materials, C6H8O7·H2O as complexing agent and NH3·H2O as pH regulator, respectively. The specific steps for preparation of the barium ferrite procedures were based on our previous work [6,10]. The procedures were then sintered at 1200 °C for 3 h to obtain BaFe12-2xZrxNixO19 with x = 0.6 and x = 0.7. After that, the obtained barium ferrites with x = 0.6 and x = 0.7 were mixed homogeneously by a planetary ball mill with mass ratio of 1:1 and ultimately heated at 800 °C–1200 °C for 2 h to attain the desired Zr4+Ni2+ ions gradient-substituted M type barium ferrites.
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.107 Received 21 May 2019; Received in revised form 4 August 2019; Accepted 10 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Chuyang Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.107
Ceramics International xxx (xxxx) xxx–xxx
C. Liu, et al.
Fig. 1. The schematic diagram of broadening ferromagnetic resonance range in ferrite by gradient composition design.
plate-like grains appear. The implication here is that reheating below 1000 °C could barely influence the morphology of the barium ferrite while reheating above 1000 °C would promote the grain growth and the formation of plate-like grains. Fig. 3(a) shows hysteresis loops of the mixed barium ferrite powders. As can be seen, the samples with and without secondary heattreatment are all saturated when the external magnetic field reaches to 15 kOe. Coercive field (Hc) of all the samples are deduced from the hysteresis loops and shown as an inset in Fig. 3(a). It is interesting to note that Hc of the mixed barium ferrites almost remain stable at ~1.3 kOe by heating at 800 °C–950 °C, whereas a dramatic drop to ~0.7 kOe occurs when the temperature reaches to 1000 °C and it consistently decreases to ~0.4 kOe with the temperature rising to 1200 °C. To our knowledge, Hc is dominantly controlled by Ha and grain size, and it reduces with the decreasing of Ha or the increasing of grain size [6,11]. As is seen, the grain size shows no distinct change at the temperature range of 800 °C–1000 °C. The stable Hc from unheated condition to 950 °C means that the distribution of Ha in the mixture is not changed, implying that the ionic diffusion barely happens within temperature range. However, a sharp reduction of Hc is seen with the temperature increasing to 1000 °C. It is discussed that the “high concentration” substituted barium ferrite would possess lower Ha while the “low concentration” substituted one would exhibit higher Ha, owing to the substitution amount of magnetic Fe3+ ions by the non-magnetic (Zr4+) ions and the weaker magnetic (Ni2+) ions. Therefore, the “harder phase” with higher Ha would impede the magnetic domain rotation of the adjacent “softer phase” in the mixture by exchange
The phase structures and morphologies of the barium ferrites were identified by X-ray diffraction (XRD, PANaltyical B V Empyrean 200895, Cu Kα radiation) and scanning electron microscopy (SEM, Hitachi S-4800), respectively. The hysteresis loops of the samples were measured by the vibrating sample magnetometer (VSM, YP07-VSM130). The permeability spectra of the ferrites were measured by a vector network analyzer (Agilent, N5244A) in the frequency range of 2–18 GHz.
3. Results and discussion Fig. 2(a) displays the XRD patterns of the samples prepared by mixing BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19 with mass ratio of 1:1 and then reheating at 800 °C–1200 °C for 2 h. It is clearly seen that all the samples only contain M-type barium ferrite phase (BaFe12O19). The intensities of the diffraction peaks are observed to become stronger gradually with the increased temperature to 1200 °C.The results demonstrate that the secondary heat-treatment will not cause the formation of new phases, but it can obviously further improve the crystallinity of the BaFe12O19 phase. The morphologies of the unheated sample and the ones heated at 900 °C, 1000 °C and 1200 °C are observed by SEM and shown in Fig. 2(b). In comparison with the unheated sample, the grains have no obvious change after heating at 900 °C and 1000 °C. In fact, for the three samples, most of the grains possess sharp edges with the size around 200 nm, confirming the formation of crystalline phases. While with the temperature further increasing to 1200 °C, the grains grow larger to ~400 nm and typical
Fig. 2. The (a) XRD and (b) SEM of the mixed BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19 by mass ratio of 1:1 without heat treatment and heated at 800 °C–1200 °C for 2 h. 2
Ceramics International xxx (xxxx) xxx–xxx
C. Liu, et al.
Fig. 3. The (a) hysteresis loops of the mixed BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19 by mass ratio of 1:1 without heat treatment and heated at 800 °C–1200 °C for 2 h and the (b) μ″ spectra over 2–18 GHz for the mixed barium ferrites and the individual BaFe10.8Zr0.6Ni0.6O19 and BaFe10.6Zr0.7Ni0.7O19.
no obvious influence on morphology at 800–1000 °C, and promotes grain growth and formation of plate-like grains above 1000 °C. The sharp decline in Hc at 1000 °C is because of the reduced Ha difference in local area that diminishes the hindrance effect between “harder” and “softer” magnetic phases, indicating the ionic diffusion starts to occur significantly when the secondary heat-treatment reaches to 1000 °C. Contributed by proper gradient composition, the sample reheated at 1100 °C shows broad FMR range ~7.32 GHz, which is much wider than that of the individual BaFe10.6Zr0.7Ni0.7O19 and BaFe10.8Zr0.6Ni0.6O19.
coupling, contributing relatively high value of the Hc [12]. But once the ionic diffusion emerges, the concentration difference of the doping ions in local area will decrease, and so will the difference of Ha. As a consequence, the hindrance effect would diminish and result in lower values of the Hc. Overall, the dramatic decrease in Hc indicates that ionic diffusion might start to occur significantly at 1000 °C. In addition, the continuous reduction of Hc from 1000 °C to 1200 °C might be contributed by two reasons. One is the sustained ionic diffusion and the other is the grain growth at 1200 °C as shown in Fig. 2(b). The imaginary part of permeability (μ″) spectra over 2–18 GHz for the individual BaFe12-2xZrxNixO19 with x = 0.6 and x = 0.7 and the mixed specimens reheated at 800 °C–1200 °C are summarized in Fig. 3(b). The maximum value approaches to ~0.3 for both two individual samples, and it emerges at ~11.5 GHz for BaFe10.8Zr0.6Ni0.6O19 and ~8.0 GHz for BaFe10.6Zr0.7Ni0.7O19, respectively. Therefore, in the mixed samples reheated at 800 °C–1000 °C, the two resonance regions at 6–9 GHz and 10–14 GHz are attributed to the BaFe10.6Zr0.7Ni0.7O19 and BaFe10.8Zr0.6Ni0.6O19 phase, separately. While due to the decreased content for both two crystalline phases, the resonance intensities of the two regions are lower than 0.2 in the samples heated below 1000 °C. Nevertheless, with the heat treatment temperature rising to 1000 °C, the crystallinity of barium ferrite phase is greatly improved as shown in Fig. 2(a), which strengthens the resonance intensity to just ~0.4. Moreover, as the ionic diffusion occurs dramatically since 1000 °C, gradient substitution with small concentration intervals could be established at 1000 °C. Consequently, the μ″ value higher than 0.2 covers 7.68–14.56 GHz (6.88 GHz) for the sample sintered at 1000 °C, which partly combines the frequency range of individual BaFe10.6Zr0.7Ni0.7O19 (6.24–12.52 GHz) and BaFe10.8Zr0.6Ni0.6O19 (8.52–14.96 GHz), and contributes broader total range than that of both the two individual samples. By consistently improving the temperature to 1100 °C, the frequency range with μ″ value higher than 0.2 is further widened to 6.56–13.88 GHz (7.32 GHz). However, the range is narrowed down to 7.20–14.04 GHz (6.84 GHz) at 1200 °C. It is probably because that the excessive ionic diffusion at high temperature further reduces the concentration intervals and makes the composition tend to be homogeneous again. Overall, in this situation, 1100 °C is the appropriate secondary heat-treatment temperature to broaden the FMR range. And the corresponding sample could be highly competitive in the microwave absorption area.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51802155, 51801103, 51471150), the Natural Science Foundation of Jiangsu Province (No. BK20180443), the “Shuangchuang Doctor” Foundation of Jiangsu Province, the Aeronautical Science Foundation of China (No.2018ZF52078), the visiting scholar fund of state key laboratory of silicon materials (No. SKL2019-09) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] O.N. Martyanov, S.N. Trukhan, V.F. Yudanov, Ferromagnetic resonance fine structure of dispersed magnets: physical origin and applications, Appl. Magn. Reson. 33 (2008) 57–71. [2] H. Suhl, Origin and use of instabilities in ferromagnetic resonance, J. Appl. Phys. 29 (1958) 416–421. [3] K. Zhang, J.H. Luo, N. Yu, M.M. Gu, X.K. Sun, Synthesis and excellent electromagnetic absorption properties of reduced graphene oxide/PANI/ BaNd0.2Sm0.2Fe11.6O19 nanocomposites, J. Alloy. Comp. 779 (2019) 270–279. [4] J.H. Luo, L. Yue, H.R. Ji, K. Zhang, N. Yu, Investigation on the optimization, design and microwave absorption properties of BaTb0.2Eu0.2Fe11.6O19/PANI decorated on reduced graphene oxide nanocomposites, J. Mater. Sci. 54 (2019) 6332–6346. [5] C.Y. Liu, Q.K. Xu, Y. Tang, Z.R. Wang, R.Y. Ma, N. Ma, P.Y. Du, Zr4+ dopingcontrolled permittivity and permeability of BaFe12-xZrxO19 and the extraordinary EM absorption power in the millimeter wavelength frequency range, J. Mater. Chem. C 4 (2016) 9532–9543. [6] J.G. Jia, C.Y. Liu, N. Ma, G.R. Han, W.J. Weng, P.Y. Du, Exchange coupling controlled ferrite with dual magnetic resonance and broad frequency bandwidth in microwave absorption, Sci. Technol. Adv. Mater. 14 (2013) 045002. [7] S.P. Gairola, V. Verma, A. Singh, L.P. Purohit, R.K. Kotnala, Modified composition of barium ferrite to act as a microwave absorber in X-band frequencies, Solid State Commun. 150 (2010) 147–151. [8] Y. Xu, G.L. Yang, D.P. Chu, H.R. Zhai, Magnetic anisotropy of BaM ferrites, J. Magn. Magn. Mater. 31–34 (1983) 815–816. [9] Y. Zhao, J.X. Li, Y.H. Ding, L.H. Guan, Enhancing the lithium storage performance of iron oxide composites through partial substitution with Ni2+ or Co2+, J. Mater. Chem. 21 (2011) 19101–19105. [10] C.Y. Liu, Y.J. Zhang, Y. Tang, Z.R. Wang, N. Ma, P.Y. Du, The tunable magnetic and microwave absorption properties of the Nb5+-Ni2+ co-doped M-type barium ferrite, J. Mater. Chem. C 5 (2017) 3461–3472. [11] J. Dho, E.K. Lee, J.Y. Park, N.H. Hur, Effects of the grain boundary on the coercivity of barium ferrite BaFe12O19, J. Magn. Magn. Mater. 285 (2005) 164–168. [12] D. Roy, P.S.A. Kumar, Enhancement of (BH)max in a hard-soft-ferrite nanocomposite using exchange spring mechanism, J. Appl. Phys. 106 (2009) 073902.
4. Conclusions The Zr4+-Ni2+ ions gradient-substituted M type barium ferrite are prepared by mixing BaFe12-2xZrxNixO19 with x = 0.6 and x = 0.7 with the mass ratio of 1:1 and then reheating at 800 °C–1200 °C for 2 h. The secondary heat-treatment further improves the crystallinity of the barium ferrite phase gradually with the temperature rising. While it has 3