1. Properties of oxide-diluted Yttrium Iron Garnet thin films for magneto-optic and microwave applications
Mikhail Vasiliev1, Mohammad Nur-E-Alam1, Kamal Alameh1, Viacheslav Kotov2, Victor Demidov2 and Dmitry Balabanov3
1Electron Science Research Institute, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia
2Institute of Radio Engineering and Electronics, Russian Academy of Sciences, 11 Mohovaya St, Moscow , Russia
3Moscow Institute of Physics and Technology, Dolgoprudny, , Russia

2. OUTLINE Introduction Material synthesis and characterization
Introduction to bismuth- and rare-earth-substituted iron garnets and application examples
Material synthesis and characterization
RF magnetron sputtering/co-sputtering technologies
Oxide-diluted off-stoichiometric garnets - overview of recent results
YIG and YIG-Bi2O3 films and their properties
oxide-diluted garnet-type nanocomposites with improved magnetic/optical properties – overview of main results achieved
YIG (Y3Fe5O12) and sputtered YIG-Bi2O3 films – characterisation results (FMR, XRD, MO etc.) and comparison to other technologies
Summary and Conclusions

3. Introduction to Rare Earth Iron garnets
Rare-earth iron garnets: R3Fe5O12 where R is a rare earth atom
Commonly known garnet materials are:
Yttrium Iron garnet (YIG = Y3Fe5O12),
Gadolinium Gallium garnet (GGG= Gd3Ga5O12) and
Bismuth Iron garnet (BIG = Bi3Fe5O12 )
Three very important subclasses of garnets for use in magneto-optic (MO) applications are described by:
(Bi, Y)3Fe5O12 , (Bi, Dy)3(Fe, Ga)5O and (Bi, Lu)3(Fe, Al)5O12
- Bismuth-substituted RE iron garnets, are of importance due to the strong Faraday/Kerr effects and a large variety of possible properties adjustable through material composition as well as by designing nano-patterned structures.
Crystal structure of magnetic garnets
Still considered to be the best transparent MO materials for
non-reciprocal optical components
magnetic recording media
magnetic field sensors
MO imaging media
MO planar waveguides and light modulators
magnetically-tunable photonic crystal structures
magneto-plasmonic crystals
[V.I. Belotelov et al, “Plasmon-mediated magneto-optical transparency,”
Nature Communications, 2013.]

4. Applications of garnet films and MPC
Ultrafast magnetic switching and modulation of light intensity
Magneto-optic (MO) imaging/sensing
RF and UHF applications of YIG include:
Magnetically tunable (3-50 GHz) filters, oscillators and resonators for microwave frequencies (YIG spheres)
Waveguide components tunable with magnetic field
1-D Magnetic Photonic Crystal (a schematic diagram; arrows indicate the magnetization direction)

5. Garnet materials synthesis & characterization
RF Magnetron sputtering (a common method & very suitable for integrated-optics devices)
Our system at ESRI, ECU
Optical
characterization
MO characterization
Magnetic and microstructural
characterization
The parameters of importance are: film thickness, absorption spectra, specific Faraday rotation, coercive force, switching field, saturation field, and magnetization direction
Optimization of magnetic properties is crucial for the development of new functional materials and for many emerging technologies in integrated optics and photonics

6. MO thin films manufacture (PVD+annealing)
Schematic diagram of the co-sputtering system geometry
Bi2O3
Y3Fe5O12
Substrate
Bi2O3
Y3Fe5O12
0-30 vol.%
vol.%
Flow chart of garnet nano-composite co-sputtered thin films processing
Optimization of process parameters is crucial for the development of new materials and engineering the material properties for emerging applications and technologies in optics and photonics
The optimum annealing regimes for Bi-substituted garnet materials are strongly dependent on the film composition (stoichiometry)

7. Nanocomposite garnet-type material synthesis pathways
Main results (2008-present):
Bi2Dy1Fe4Ga1O12:Bi2O3
532 nm;
Q(635nm) ≈ 45º
Bi1.8Lu1.2Fe3.6Al1.4O12:Bi2O3
Q(635nm) > 50º
532 nm
Bi3Fe5O12:Dy2O3
532 nm Mr≈0.6Ms
Bi2Dy1Fe4Ga1O12:Bi3Fe5O12
532 nm, square loop
Aims:
to achieve improved transparency and stronger Faraday rotation (larger gyration),or
to synthesize garnets with Bi content approaching 3 f.u. and also be able to crystallize these layers whilst preserving material and surface quality.
Approach:
Stoichiometry variations implemented by co-sputtering from 2 ceramic targets (using either 2 different garnet targets or a garnet-type target codeposited with an extra oxide)

8. Results achieved in other garnet nanocomposites -
significantly improved optical transparency
Left: 650 nm-thick film of Bi1.8Lu1.2Fe3.6Al1.4O12 on a Gd3Ga5O12 (GGG) substrate;
Right: 1150 nm-thick film of (Bi1.8Lu1.2Fe3.6Al1.4O vol.% co-sputtered Bi2O3) on a GGG substrate - both films are as-deposited
After annealing, the transparency is improved and
MFM imaging starts to reveal the domain structure
Annealed (crystallized) films:
Left: 1030 nm-thick (Bi2Dy1Fe4Ga1O vol.% Bi2O3) on glass;
Right: 1080 nm-thick Bi2Dy1Fe4.3Ga0.7O12 on GGG.

9. YIG and YIG-Bi2O3 crystals and films - FMR linewidth comparisons
Technology
YIG
(LPE films)
(PLD films)
YIG (PLD films)
Single-crystal YIG spheres
Sputtered
composites
YIG:Bi2O3
YIG on YAG (PLD)
Sample details
Thickness: 10's to 100's of nm; Ms about 1750 Gs
Epitaxial-quality samples
Polycrystalline films
Polished spheres
RF co-sputtered annealed nanocrystalline films
High-quality films on YAG substrates;
Thickness of about 1 mm
FMR linewidth
about 1 Oe
> 10 Oe
Min GHz
0.52 Oe
GHz
GHz
References
M. P. Horvath, “Microwave applications of soft ferrites,” J. Magn. Magn. Mater., vol. 215–216, pp. 171–183, 2006.
P. C. Dorsey et al, J. Appl.
Phys., vol. 74, pp. 1242–1248, 1993.
H. Buhay et al, IEEE Trans. Magn., vol. 31, no. 6,
pp. 3832–3834, 1995.
B. Bhoi et al, IEEE Trans. Magn, vol. 49, No. 3, pp (2012)
R. C. LeCraw, E. G. Spencer, and C. S. Porter, Phys. Rev. 110, 1311 (1958)
M. Nur-E-Alam et al, in press, Journal of Nanomaterials, 2015
A. Sposito et al, Opt. Mater. Express, vol. 3 No 5, pp (2013).
Substrate T=250° C

10. YIG-Bi2O3 – process parameters and properties overview
Substrate T=250° C
Material composition type
Absorption coefficient (cm-1) at 532 nm
MO figure of merit (deg) at 532 nm
Absorption coefficient (cm-1) at 635 nm
MO figure of merit (deg) at 635 nm
Y3Fe5O12
12342
0.22
7986
0.08
Y3Fe5O12 + Bi2O3 (10 Vol. %)
7965
2.41
3044
1.48
Y3Fe5O12+ Bi2O3 (20 Vol. %)
6607
3.05
2822
1.67
Y3Fe5O12+ Bi2O3 (30 Vol. %)
6072
3.96
2449
2.30
The best oven-annealing durations for these film types were between minutes.

11. YIG-Bi2O3 crystal structure properties
Substrate T=250° C
Material composition type
Average Lattice constant (Å)
Average crystallite size (nm)
Y3Fe5O12
12.37 27
Y3Fe5O12 + Bi2O3 (10 Vol. %)
12.48 35
Y3Fe5O12+ Bi2O3 (20 Vol. %)
12.45 36
Y3Fe5O12+ Bi2O3 (30 Vol. %)
12.49 42
The origins of increasing lattice constant value are related to increasing the bismuth content substitution – the formation of
(Y, Bi)3Fe5O12 garnet phases

12. YIG-Bi2O3 principal FMR characterization results - linewidth
YIG-Bi2O3 films can be useful for the development of a new class of MO ultra-high frequency (UHF) modulators based on the interaction of infrared light waveguide modes with magnetostatic waves due to the small optical absorption of infrared light and better temperature stability of our RF-sputtered films when compared with existing sputtered and also PLD-synthesized Bi3Fe5O12 films
Substrate T=250° C
Measured FMR behaviour and linewidth (ΔHpp= 6.1 Oe at 9.77 GHz) in an annealed Y3Fe5O12-Bi2O3 (20 vol.%) nanocomposite film of 500 nm thickness sputtered onto a GGG substrate.

13. YIG-Bi2O3 FMR characterization results – linewidth vs angle and other data
Angular dependency of FMR linewidth and magnetic-field tunability characteristics have been measured with in-plane directed fields
Substrate T=250° C

14. CONCLUSIONS
A range of Bi-substituted MO garnet thin-film materials (both in-plane and out-of-plane magnetized) with excellent optical and record-high MO quality have been synthesized and characterized
Sputter-deposition and oven annealing processes required for the manufacture of high-quality garnet films have been studied and the process parameters were optimized for various compositions
The combinations of material properties achieved are of interest for the development of different emerging types of reconfigurable nano-photonic and microwave devices
New YIG-Bi2O3 co-sputtered nanocomposite films with very narrow FMR linewidths can be produced industrially, on large-area substrates

15. Electron Science Research Institute, Edith Cowan University
Thank you
Electron Science Research Institute, Edith Cowan University

16. Another interesting recent result with garnet nanocomposites in magnetic trilayer structures
Substrate T=250° C
Our network of collaborating groups:
Moscow State University, General Physics Institute, Moscow Inst. of Physics and Technology, Ioffe Physical-Technical Inst., Image Processing Systems Inst. (Russia), TU Dortmund University (Germany), Tata Inst. of Fundamental Research (India), Royal Inst. of Technology (Sweden), ESRI (Australia).
An interesting new finding – unconventional type of magnetic hysteresis behaviour in exchange-coupled trilayers (Materials 2015, 8, )