Periodic Reporting for period 2 - TRANSPIRE (Terahertz RAdio communication using high ANistropy SPIn torque REsonators) Période du rapport: 2019-01-01 au 2021-09-30 Résumé du contexte et des objectifs généraux du projet The progress of the current revolution in mobile Information and Communication Technology is driven by three main processes: (i) constant miniaturization, (ii) exponential increase in volumes of data stored and transmitted and (iii) constant drive for reduction in power consumption. To maintain the exponential growth of wireless data rates (wireless capacity has been doubling every 18 months), data rates in excess of 15 Gbps will likely be demanded by regular users, 10 years from now. Big data creates new societal demands, and to meet them, we need novel scalable technologies to work in the low-terahertz frequency range - currently very much the only non-rigorously allocated frequency space.TRANSPIRE aimed to develop nanoscale THz-oscillators that can breach the terahertz gap, based on low/zero-magnetic moment materials, exhibiting high-anisotropy-field, and conducting highly spin-polarized electronic currents within a modern and accelerating area of research now referred to as ferrimagnetic spintronics. The central objective was to demonstrate their suitability as low-power, chip-based wireless transceivers for THz communication that can overcome the present data rate bottleneck and underpin the information revolution beyond the end of the decade.The areas of social activity to benefit are numerous and include remote hospitals, immersive audio-visual systems and 3-D remote meetings, all of which would require technology development beyond the current 5G deployment. Low-cost, low-power mobile communication would also be needed, as substitutes for damaged or disabled traditional communication networks, in disasters areas. TRANSPIRE focused on laying the foundations of the next revolution in 'big data', while contributing to an informed discussion of the wider social implications, helping to raise awareness among European citizens of both the benefits and the risks.While the primary objective of TRANSPIRE was not achieved, alternative approaches towards the construction of THz oscillators were also pursued. These included the engineering of single material layers and thin-film stacks, which are able to efficiently convert pulsed optical laser power into THz emission to free space. Another discovery is the rather significant amount of torque that can be produced in even the simplest of single-layer devices - the Hall bar. Patenting of some of the possible device concepts was successfully pursued by the consortium. Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent In the final two years of activity within TRANSPIRE, the work was focused on producing thin films and stacks of low-moment, highly polarized materials (including the prototypical Mn2RuGa), but also on confirming direct THz emission from magnetisation precession in ultra-thin and granular films, as well as testing a new possible route to produce a THz oscillator, based on single-layer spin-orbit torque. TCD optimized thin films of the manganese-rich, tetragonal Heuslers. New batches of blanket material were sent to the partners in HZDR for patterning. The new approach to exciting spin dynamics uncovered in Y2 by TCD and NTNU, i.e. the single-layer spin-orbit-torque-based devices, became the primary focus in an attempt to reach TRANSPIRE's over-arching goal of creating on-chip THz oscillators and detectors. The partners HZDR-Deac group, patterned more than 10 different variations of the new STT-MTJ stacks into measurable devices (with contacts suitable for wire-bonding). These were all tested first at HZDR and then at TCD. The overall results showed a minor improvement of room TMR ratio (up to 10 %, at RT) on individual junctions, while the majority of devices were failing to operate properly. The work on THz spectroscopy - using both laser-in/THz out and THz-in/THz-out methods continued at pace in the HZDR-Kovalev group. The THz-driven resonances (for example in Mn3Ga films) were confirmed to be much narrower (by a factor of ~4), down to 7 GHz, when compared to laser-driven dynamics on the same films. Newly prepared samples (from Trifolium Dubium) of Mn2Ga, in both continuous and island (granular) conformation, were also investigated.The theory work in TRANSPIRE, actually benefitted from lockdown time in the final period. It was decided to pursue a different approach: computation of electron transport through ordered models of magnetic tunnel junctions using NEGF-DFT.A number of PhD thesis works are now submitted, which are based on work done in TRANSPIRE. A corresponding set of Manuscript is also being prepared for publication. It is projected that some of the data (both experimental and computational) gathered within TRANSPIRE will be subject to publication for a couple of years past the nominal end of the programme. The materials research findings, not published as research papers, will be made fully openly available on the materials.ie platform, established for TRANSPIRE. The domain transpire.eu will be maintained for a number of years past the end of the programme, to act as a locator for information not available through the EC servers and not committed to open publication databases (such as the TARA archive). Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent) TCD have demonstrated that Mn2RuxGa provides record spin-orbit fields for manageable current densities (~10^8 A/cm^2). This first observation of anti-damping-like current-induced torque components, within magnetically homogenous thin films, has been followed by a thorough characterization of torques in the non-linear regime. The order-of magnitude of the spin-orbit scattering cross section amounts to 60% of the lattice area (>2.2 Gbarn) in the low-current regime and grows further (super-quadratically) at higher current densities.In combination with TCD, HZDR-Deac has shown that the tunnel magnetoresistance remains undiminished in MRG when its net magnetization is strictly zero (in the compensation region). This is one of very few demonstrations, proving that magnetization and spin polarization can be made independent in metallic ferrimagnets. This is a critical enabling advance. Downsizing pillar-type devices to 100-nm (or even smaller) diameters is expected help reach the threshold current densities required to drive STT auto-oscillations. This latter part remains to be demonstrated beyond the end of TRANSPIRE.The HZDR-Kovalev group has provided direct proof that ultra-thin high anisotropy field films and stacks can be excited using laser pulses to emit THz radiation into free-space with high efficiency. This can be utilized for characterization of spin dynamics by means of THz time-domain spectroscopy or even to produce efficient THz table-top sources, in the future.SWISSto12 has developed a procedure to estimate the conductivity of thin metallic layers, based on broadband transmission and reflection measurements within their MCK series of products. This approach could be for the rapid, contactless estimation of the conductivity of such films, and find application in quality control and development tasks in industry. The NTNU group has developed a symmetry-based phenomenological framework for the calculation of spin-orbit torques in antiferromagnets and ferrimagnets. The TCD-based theory group has advanced the state-of-the art of the simulation for these complicated metallic ferrimagnets, using a number of complementary techniques: DFT, NEGF and effective Hamiltonian calculations, on a variety of structures, from bulk-symmetry supercells, to model tunnel junction structures. Narrow-band THz emission from thin low-moment ferrimagnetic films Tunnel magnetoresistance with a compensated electrode. Inset shows TMR is finite at all T. The zero-moment half metal is impervious to external fields and does not create a stray field.
