Periodic Reporting for period 4 - FeMiT (Ferrites-by-design for Millimeter-wave and Terahertz Technologies)
Reporting period: 2023-11-01 to 2025-04-30
For future wireless communications, the so-called mm-waves are a promising solution to cope with the ever-increasing data traffic. These waves come from a region of the electromagnetic spectrum where plenty of bandwidth is available, and their high frequencies, of tens to hundreds of GHz, allow faster data transmission rates. However, the challenge of mm-waves is that they are blocked by obstacles such as walls or trees and only allow line-of-sight propagation. This imposes a real change of paradigm, in which the mm-wave communications rely on myriads of closely scattered small antennas. Thus, the new wireless systems require affordable, low-power miniaturized devices.
FeMiT has focused on enabling the mm-wave transition for a key component of wireless communication systems: the circulator, which isolates emitting antennas from one another and allows simultaneous transmission and reception. More specifically, the overarching goal of FeMiT was to develop a new family of ferrites that can be used to fabricate miniaturized mm-wave circulators. The ferrites currently used in circulators can only operate in the first portion of the mm-wave band, using external magnetic fields, which makes the device bulky, expensive and can only work at one frequency. The key functional property of ferrites exploited in circulators is their ferromagnetic resonance (FMR), by which, at a given frequency, the propagation or absorption of electromagnetic waves through a ferrite is non-reciprocal, i.e. it depends on their direction with respect to the magnetization. Since the frequency at which the FMR occurs increases with the magnetic anisotropy of the ferrite, very high magnetic anisotropy ferrites are needed for operating at mm-waves. FeMiT has been developing a new family of ferrites based on epsilon-Fe2O3, a material with a high magnetic anisotropy, which can work in non-reciprocal wireless components at higher frequencies without the need for external magnetic fields. Moreover, we have investigated how the magnetic anisotropy of epsilon-Fe2O3 can be modified by strain, as a way to tune its FMR and the operation frequency of circulators based on this family of ferrites.
FeMiT has focused on enabling the mm-wave transition for a key component of wireless communication systems: the circulator, which isolates emitting antennas from one another and allows simultaneous transmission and reception. More specifically, the overarching goal of FeMiT was to develop a new family of ferrites that can be used to fabricate miniaturized mm-wave circulators. The ferrites currently used in circulators can only operate in the first portion of the mm-wave band, using external magnetic fields, which makes the device bulky, expensive and can only work at one frequency. The key functional property of ferrites exploited in circulators is their ferromagnetic resonance (FMR), by which, at a given frequency, the propagation or absorption of electromagnetic waves through a ferrite is non-reciprocal, i.e. it depends on their direction with respect to the magnetization. Since the frequency at which the FMR occurs increases with the magnetic anisotropy of the ferrite, very high magnetic anisotropy ferrites are needed for operating at mm-waves. FeMiT has been developing a new family of ferrites based on epsilon-Fe2O3, a material with a high magnetic anisotropy, which can work in non-reciprocal wireless components at higher frequencies without the need for external magnetic fields. Moreover, we have investigated how the magnetic anisotropy of epsilon-Fe2O3 can be modified by strain, as a way to tune its FMR and the operation frequency of circulators based on this family of ferrites.
FeMiT has explored the potential of epsilon-Fe2O3 ferrites for applications in mm-wave wireless non-reciprocal devices. For this, we have characterized the functional response of these and other novel high anisotropy ferrites at mm-wave frequencies in a dedicated lab funded by the project. The potential of ferrites based on epsilon-Fe2O3 has been investigated by synthesizing these materials with a broad range of metal substitutions. We have also focused on the scale-up of the materials synthesis and on controlling the size and shape of the synthesized crystals. The study of the magnetic properties and magnetic and structural characterization of the materials in different spectroscopy and diffraction experiments performed at large facilities has provided important clues to understand the origin of the large magnetic anisotropy of these ferrites. Some specific compositions have been prepared in the form of epitaxial thin films on a range of substrates, including flexible mica, and we have investigated the growth mechanism and the potential of controlling the FMR frequency of epsilon-Fe2O3 by means of an applied strain. We also investigated high-anisotropy M-type hexaferrites of composition Sr1-(x/12)Ca(x/12)AlxFe12-xO19, also displaying FMR above 100 GHz. We obtained demonstrators of circulators based on epsilon-Fe2O3 and hexaferites operating above 100 GHz, which open the way for exploiting these materials in the design of miniaturized circulators for the next generation of wireless communications.
We prepared new epsilon-MxFe2-xO3 materials with M=Cr3+, Mn3+, Al3+, In3+, Sc3+, Ga3+, Co2+, Ru3+, Sb3+ and characterized their structural and functional properties. The evolution of the magnetic phase diagrams under the different substitutions has provided clues to understand the origin of the high magnetic anisotropy in epsilon-Fe2O3. The project has also allowed us to investigate how the magnetic anisotropy of epsilon-Fe2O3 is affected by strain. All this knowledge will be relevant for designing new high anisotropy insulting magnets, for which making compatible a high anisotropy and a high magnetization remains a challenge. The demonstrators of non-reciprocal devices based on high anisotropy ferrites operating above 100 GHz, are encouraging and call for further efforts of materials engineering to enhance the responses and develop miniaturized circulators.
We have studied the magnetic relaxation of epsilon-Fe2O3 as a function of crystal size and developed a method for increasing the size of the nanoparticles with batch production in the order of grams. The method also allowed us to discover the role of rare earth silicates as high-temperature surfactants, and provides a new approach for controlling the size and shapes of oxide nanoparticles at high temperatures.
We have studied the magnetic relaxation of epsilon-Fe2O3 as a function of crystal size and developed a method for increasing the size of the nanoparticles with batch production in the order of grams. The method also allowed us to discover the role of rare earth silicates as high-temperature surfactants, and provides a new approach for controlling the size and shapes of oxide nanoparticles at high temperatures.