Ferrites-by-design for Millimeter waves and Terahertz Technologies (FeMiT) The FeMiT project explained in 5 minutes Have you ever experienced connectivity issues with your mobile? What will happen if as expected there are much more mobiles, in your watches, your home appliances, your cars and in the machines that produce them in a factory? The huge data traffic would saturate the narrow spectrum of today’s networks . In fact, we are already experiencing an explosion in the data demand. 1 That’s why with 5G we are shifting to higher frequencies, into the mm-waves, where plenty of bandwidth is available. A big challenge of mm-waves is that these only allow line-of sight propagation, making the transition to higher frequencies a real paradigm change, with the communications relying on myriads of closely scattered small antennas. Thus, besides working at tens to hundreds of GHz, the new communication devices will have to be cheap, low power and miniaturized . For instance, the circulators isolate emitting antennas from one another, acting as a traffic router for the waves only in one direction. This is an example of the ferrite non-reciprocal devices which will gain importance in this new context However, today, these only operate in the first portion of the mm-wave band, using external magnetic fields which makes them bulky and can only work at one frequency . The aim of FeMiT is Making them work at higher frequencies without external magnetic fields And making them reconfigurable to operate at different frequencies Before explaining how we plan to do this it is important to highlight that these devices exploit the phenomenon of ferromagnetic resonance: at a given frequency, the waves propagate through a ferrite or are absorbed, depending on their direction, and the resonance frequency is higher the larger is the magnetic anisotropy of the ferrite. So, for the mm-waves we need ferrites with very high magnetic anisotropy. The FeMiT Now, my idea is developing a new family of ferrites based on ε -Fe 2 O 3 as it has two unique properties highly suitable for implementing innovative non-reciprocal devices in the mm-waves: First, a Japanese group has shown it displays ferromagnetic resonances well into the mm-waves, which can be increased or decreased by chemical doping. 2-3 But the transmission losses, which are key to applications, are not characterized and the possibilities of chemical doping have not been fully explored, with relevant dopants overlooked. On the other hand we discovered that the onset of large magnetic anisotropy is accompanied by a strain of the structure, a thousand times larger than in other magnetic transition metal oxides. 4 This tells us that one can expect controlling the ferromagnetic resonance through strain, obtaining much larger responses than with a standard ferrite.
To demonstrate this hypothesis we will want to make composites of the epsilon Ferrite materials with piezoelectrics to shift the ferromagnetic resonance by applying an external voltage. Based on these properties we will exploit chemical doping and strain engineering to explore the possibilities of this new family of ferrites. The feasibility of these approaches is backed by the remarkable stability of this phase at high pressure. Selecting the best materials we aim at providing proof-of principle demonstrators of A passive mm-wave device in the form of a faraday rotator with figures of merit matching or exceeding those of state of the art ferrites (50 degree/dB) but in this case in the mm-waves and without external fields. An active device with voltage controlled ferromagnetic resonance We will start by preparing materials by wet chemistry methods which are fast, allow an accurate control the chemical doping and provide large amount of material for characterization. Then, structural and property characterizations will give feedback for new synthesis Two PhDs will be in charge of these tasks. An important functional characterization will be studied with a free-air transmissiometer financed by the project, with a dedicated Postdoc. Selected compositions will be prepared as epitaxial films and/or composites and a second Postdoc will join the team to develop the demonstrators. Expected project outcomes are a library of new ferrites for designing mm-wave components A reconfigurable ferromagnetic resonance device, which would constitute a technological breakthrough enabling to use the same device in zones with different standards. Finally, the frontier research of the project will certainly generate knowledge which can be relevant to related fields for instance in -EM absorbers and taggants - Magnonic transceivers - Spin lattice interactions in oxides References 1. Ericsson Mobility Report, June 2018 ; Ericsson 2018. https://www.ericsson.com/en/mobility-report/reports/june-2018 2. Namai, A.; Sakurai, S.; Nakajima, M.; Suemoto, T.; Matsumoto, K.; Goto, M.; Sasaki, S.; Ohkoshi, S.-i., Synthesis of an Electromagnetic Wave Absorber for High-Speed Wireless Communication. Journal of the American Chemical Society 2009, 131 (3), 1170-1173.
3. Namai, A.; Yoshikiyo, M.; Yamada, K.; Sakurai, S.; Goto, T.; Yoshida, T.; Miyazaki, T.; Nakajima, M.; Suemoto, T.; Tokoro, H.; Ohkoshi, S.-i., Hard magnetic ferrite with a gigantic coercivity and high frequency millimetre wave rotation. Nature Communications 2012, 3 . 4. García-Muñoz, J. L.; Romaguera, A.; Fauth, F.; Nogués, J.; Gich, M., Unveiling a New High- Temperature Ordered Magnetic Phase in ε -Fe2O3. Chemistry of Materials 2017, 29 (22), 9705-9713.
Ferrites-by-design for Millimeter-wave & Terahertz Technologies (FeMiT) Spectrum saturation calls for a shift into mm-waves , a new paradigm in wireless communications 0.5 GHz 3 GHz 300 GHz mm-wave wireless Today’s 4G Line-of-sight propagation Ericsson Mobility Report June 2018 myriads of new antennas New mm-wave devices : Small, Cheap & Low Power Non-reciprocal ferrite devices The FeMiT vision Limitations at mm-waves Non-reciprocal devices for 5G & beyond circulator for antenna isolation ● 100 GHz with magnetic fields ● Higher frequencies without magnets ● Passive (fixed frequency) ● Reconfigurable (several frequencies) ~1500 € 2.5 cm ● Bulky & Expensive ● Cheap & Miniaturized Ferromagnetic resonance ω FMR increases with Magnetic Anisotropy, H c Image: http://www.millitech.com, 2018
A new family of ferrites for mm-waves based on high magnetic anisotropy ε -Fe 2 O 3 ε -Fe 2 O 3 mm-wave ferromagnetic resonances Huge spontaneous magnetostriction H c (kOe) 431 22 24 26 28 20 Magnetic anisotropy , H c (kOe) High Anisotropy Epitaxial films x=0 15 430 Unit cell volume (Å 3 ) Gich et al. Chem. Mater ., 2004 Gich et al. Adv. Mater ., 2014 10 429 Doping level (Rh) 5 Sustains large strains 428 ΔV/V ~10 -3 0.04 427 430 x=0.14 0.03 420 426 0.02 410 Unit Cell Volume (Å 3 ) 0.01 425 400 0.00 300 350 400 450 500 550 600 650 700 390 Temperature (K) 380 Gich et al. Chem. Mater ., 2017 Ohkoshi et al. Nature Comm ., 2012 370 360 Strain engineering Chemical doping 350 0 5 10 15 20 25 30 35 40 Pressure (GPa) M. Gich et al. Nature Comm . 2018, in press Piezoelectric novel dopant selection ε– M x Fe 2-x O 3 Selected compositions Voltage control of ferromagnetic resonance ε– M x Fe 2-x O 3 Mediated by strain Passive component s Active components Without Magnetic Field Switchable operation frequency Proof of principle Figure of demonstrators merit: 50°/dB Faraday rotator Voltage-tunable FMR
Towards passive & active mm-wave devices without external magnetic fields Methodology WP1 WP2 WP3 Expected results & impact Synthesis Characterization Demonstrators Free-space Library of ε -M x Fe 2-x O 3 properties for Transmissiometer future wireless applications Breakthrough Proof of principle towards PDoc1 Reconfigurable Nanoparticles by Soft chemistry mm-wave devices PhD2 PDoc2 FMR shift ~ 0.5 GHz/V Epitaxial films Composites Proof-of principle towards low-cost & sustainable mm-wave devices Rotation ~ 50°/dB PhD1 Impact beyond FeMiT Spectroscopic characterization Prof. A. Muhkin (Russian Ac. Sci.) EM absorbers and taggants (RFID) 1st principle calculations Prof. J. O’Callaghan Magnonic transcievers (UPC, Barcelona) Prof. M. Ležaić (Jülich FZ) Prof. R. Mishra (U. St. Louis) Spin-lattice interactions in oxides External collaborators
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