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 dissertation
1
include the magnetodielectric properties in (1-y)(Mg
0.95Zn
0.05)
2TiO
4-yMg
0.95Zn
0.05Fe
2O
4(MZT-MZF) ceramics, the magnetic anisotropy in Co
1.1Fe
1.9O
4 (CFO) laminated ceramics,
the magnetodielectric properties in mixed cobalt-rich Co
1.1Fe
1.9O
4/Ba(Zr
0.085Ti
0.915)O
3 (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 Mg
2TiO
4 and MgFe
2O
4 have similar cubic structures, which is the
precondition to form a solid solution. Also, an appropriate doping of Zn
2+ ions can considerably improve the physical properties of these two ferrites. By adding 5% molar Zn
2+
ions, the sintering temperature of the (1-y)(Mg
0.95Zn
0.05)
2TiO
4-yMg
0.95Zn
0.05Fe
2O
4 (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 × 10
3 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
1.9O
4 (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
1.9O
4/Ba(Zr
0.085Ti
0.915)O
3 composites with different
component ratios by adding 5 wt% SrTiO
3 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.
__________________________
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).
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