The miniaturization and integration of electronic devices demand the development of more excellent materials with multifunction as well as novel structural designs. In this dissertation, based on spinel ferrite, the low-loss magnetodielectric ceramics with coexisted dielectric and magnetic properties have been systematically investigated through tuning the chemical constituent and microstructure, or optimizing experimental approach. Besides, the epitaxial garnet ferrite thin films have been also fabricated by adopting a new deposition method with a radio frequency magnetron sputtering system. The main research topics in this dissertation¹ include the magnetodielectric properties in (1-y)(Mg_0.95Zn_0.05)₂TiO₄-yMg_0.95Zn_0.05Fe₂O₄(MZT-MZF) ceramics, the magnetic anisotropy in Co_1.1Fe...[ Read more ]

The miniaturization and integration of electronic devices demand the development of more excellent materials with multifunction as well as novel structural designs. In this dissertation, based on spinel ferrite, the low-loss magnetodielectric ceramics with coexisted dielectric and magnetic properties have been systematically investigated through tuning the chemical constituent and microstructure, or optimizing experimental approach. Besides, the epitaxial garnet ferrite thin films have been also fabricated by adopting a new deposition method with a radio frequency magnetron sputtering system. The main research topics in this dissertation¹ include the magnetodielectric properties in (1-y)(Mg_0.95Zn_0.05)₂TiO₄-yMg_0.95Zn_0.05Fe₂O₄(MZT-MZF) ceramics, the magnetic anisotropy in Co_1.1Fe_1.9O₄ (CFO) laminated ceramics, the magnetodielectric properties in mixed cobalt-rich Co_1.1Fe_1.9O₄/Ba(Zr_0.085Ti_0.915)O₃ (CFO/BZT) laminated composites, and the low-loss magnetic properties in epitaxial yttrium iron garnet thin films. By designing new composites constitutes and new fabrication methods, the physical properties of both spinel and garnet materials were immensely improved, providing valuable guiding significance for design and fabrication of novel multifunctional electronic components and devices.

As spinel ferrites, both Mg₂TiO₄ and MgFe₂O₄ have similar cubic structures, which is the precondition to form a solid solution. Also, an appropriate doping of Zn²⁺ ions can considerably improve the physical properties of these two ferrites. By adding 5% molar Zn²⁺ ions, the sintering temperature of the (1-y)(Mg_0.95Zn_0.05)₂TiO₄-yMg_0.95Zn_0.05Fe₂O₄ (y = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) ceramics prepared by traditional solid-state solution has been decreased and many other interesting phenomena have been observed, for example, a stable relative permittivity ?′ (from ~12 to ~20) as well as a low dielectric loss tan ? (< 0.25 at 1300 ℃) are obtained when y ≤ 0.5. Moreover, the highest giant dielectric constant ?′ of 9.98 × 10³ at 30 kHz is observed from 0.1MZT-0.9MZF sample sintered at 1300 ℃ due to its n-type semiconductor property. As for magnetic properties, the highest magnetic squareness ratios of 0.89 and 0.88 are achieved in 0.1MZT-0.9MZF and 0.3MZT-0.7MZF, respectively. And the largest initial permeability ?′ of ~ 70 has also been observed in 0.3MZT-0.7MZF due to its larger grain size and probably the smaller magnetostriction constant ? and internal stress ? caused by lower relative density. All these enhanced properties reveal that the (1-y)MZT-yMZF solid-solutions have great potential to be used as novel electronic devices.

The ferrite materials with remarkable magnetic anisotropies own more advantages in the application of sensors, transformers and spintronics, etc. In this dissertation, the sintering temperature-dependant and thickness-dependant magnetic anisotropies of laminated cobalt-rich Co_1.1Fe_1.9O₄ (CFO) ceramics fabricated by tape-casting method are systematically studied. It is found that the sample sintered at 1200 ℃ owns both high in-plane and out-of-plane residual magnetization, and high magnetic anisotropies ( Δ?_r = 83.89%, Δ?_C = 49.08%), which may result from its high relative density (97.3%) and large grain size. Besides, the magnetic anisotropy of CFO sample enhances with the decrease of the thickness. In addition, compared with the CFO sample prepared by traditional solid-state method, the samples fabricated by tape-casting method own more uniform grain distribution and larger magnetic anisotropies.

Compared with magnetodielectric composites with coexisted magnetic and dielectric properties, the composites owning coupling effect between these two properties have earned more attention based on the combinational mechanism of magnetostrictive effect and piezoelectric effect. Since the piezoelectric constant in textured ceramics can be considerably improved, the strong magnetodielectric coupling in the composites consisting of magnetic phase and textured dielectric phase without lead (Pb) element can be also obtained in theory. In this dissertation, the mixed Co_1.1Fe_1.9O₄/Ba(Zr_0.085Ti_0.915)O₃ composites with different component ratios by adding 5 wt% SrTiO₃ template are fabricated via tape-casting method. It is noted that, the permittivity and dielectric loss of CFO/BZT composite with ST template are smaller than those of CFO/BZT without ST template. In addition, all the four samples show relatively large magnetic anisotropy, especially for the 2CFO-BZT(Y) sample (Δ?_S = 56.6%) sintered at 1250 ℃. Unfortunately, the piezoelectric constants of all the samples are too small to provide distinct magnetodielectric coupling possibly due to the small average size (~ 2.5 μm) of ST template. Therefore, if one can synthesize better ST template with larger size and design different structures such as “sandwich” structure, a strong MDE coupling is still highly expected.

To obtain smaller electronic components with better performance, thin films owning more advantages than ceramic bulk have attracted much more attention recently, for example, thin films can be even used in the application of wearable devices due to their flexibility and easy preparation. As a ferrite used in microwave field, yttrium iron garnet (YIG) is famous for its extremely narrow ferrimagnetic resonance linewidth and low loss. In this dissertation, epitaxial YIG (444) thin films are deposited on (111)-oriented Gadolinium gallium garnet (GGG) substrates by a radio-frequency magnetron sputtering system. A novel preparation method that room-temperature deposition combined with a short time post-annealing is firstly proposed in this dissertation. It is found that the crystallinity and magnetic properties of YIG thin films by this way are comparable with that of YIG thin films deposited at high temperature. This novel method reducing the demands for deposition facility provides a simple and practical approach for mass production of YIG thin films and is highly instructive for the thin-film growth of other garnet materials.

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¹ This project was supported by the National Natural Science Foundation of China (Nos: 61631166004, 61471290, 51390472 and 51202185), the SRFDP-RGC Joint Research Project 2013/14 (20130201140002), National 973 projects of China (No. 2015CB654603), the National Key Research Program (No. 2017YFB0406303), and the Research Grant Council of HKSAR via SRFDP-RGC Joint Research Schemes (Nos: M-HKUST605/13, N_HKUST605/16).