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 125009, Russia 3Moscow Institute of Physics and Technology, Dolgoprudny, 141700, Russia

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

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)5O12 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.]

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)

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

MO thin films manufacture (PVD+annealing) Schematic diagram of the co-sputtering system geometry Bi2O3 Y3Fe5O12 Substrate Bi2O3 Y3Fe5O12 0-30 vol.% 70-100 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)

Nanocomposite garnet-type material synthesis pathways Main results (2008-present): Bi2Dy1Fe4Ga1O12:Bi2O3 10.8º/µm @ 532 nm; Q(635nm) ≈ 45º Bi1.8Lu1.2Fe3.6Al1.4O12:Bi2O3 Q(635nm) > 50º 6.25º/µm @ 532 nm Bi3Fe5O12:Dy2O3 13.3º/µm @ 532 nm Mr≈0.6Ms Bi2Dy1Fe4Ga1O12:Bi3Fe5O12 8.2º/µm @ 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)

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 (Bi1.8Lu1.2Fe3.6Al1.4O12 + 20 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 (Bi2Dy1Fe4Ga1O12 +24 vol.% Bi2O3) on glass; Right: 1080 nm-thick Bi2Dy1Fe4.3Ga0.7O12 on GGG.

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. 70 Oe @ 9.1 GHz 0.52 Oe 6.1 Oe @ 9.77 GHz 17.5 Gs @ 6 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 990-994 (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. 624-632 (2013). Substrate T=250° C

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 30-60 minutes.

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

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.

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

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

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

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, 1976-1992)