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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TRANSPIRE (Terahertz RAdio communication using high ANistropy SPIn torque REsonators)

Teaser

Summary

The progress of the current revolution in mobile Information and Communication Technology is driven by three main developments: (i) constant miniaturization, (ii) exponential increase in volumes of data stored and transmitted and (iii) reduced power consumption. To maintain the exponential growth of wireless data rates (wireless capacity has been doubling every 18 months), data rates of 15 Gbps will be needed 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.

TRANSPIRE aims to develop nanoscale THz-oscillators that can breach the terahertz gap, based on low/zero-moment, high-anisotropy-field, highly spin-polarized ferrimagnetic spintronics. The objective is 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 until 2030.

Areas to benefit include remote hospitals, immersive audio-visual systems and 3-D remote meetings, all of which will require technology beyond 5G. Low-cost, low-power mobile communication will be needed in disasters areas. TRANSPIRE will lay 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.

The objectives of the project are: to fabricate, develop, and understand the physical processes in magnetic thin-film devices based on zero-moment half-metals; to engage the public in how this technology can enable a new generation of terahertz ICT and benefit society; but also to engage and seek discussion with the public on the implications of the instant availability of sensitive data.

Work performed

The second yearâ€™s work has further developed the foundations of spintronics in the terahertz range, confirming direct THz emission from magnetisation precession in an ultra-thin film, as well as identifying a new possible route to produce an oscillator.

TCD has optimized thin films of the established manganese-rich, tetragonal Heuslers, while concurrently focussing on the near-cubic zero-moment half-metal Mn2RuxGa (MRG). A diffusion barrier study establishes that there is still room for improving the magnetoresistive properties of high-anisotropy-field materials. The new avenue for exciting spin dynamics, pursued in Y2 by TCD and NTNU, demonstrated clear indications that this strand of single-layer spin-orbit-torque may allow reaching TRANSPIRES over-arching goal.

The HZDR-Deac group has analysed a total of 40 multilayers with different composition, grown by TCD. An extensive set of different combinations of diffusion- and spacer-layers was investigated via SQUID measurements (for some, at different temperatures and angles) and magneto-transport experiments. AFM, TEM and SEM characterization has also been performed. This study allowed for a better understanding of the paths that might be pursued in order to increase the magnetoresistance of the stacks. HZDR-Deac has also demonstrated spin-pumping in synthetic ferrimagnets away from the saturation regime, i.e. in a configuration which has previously not been investigated.

The HZDR-Gensch group has shown that ultra-thin high anisotropy films under laser pulse impact emit THz radiation into free-space, which can be used for characterisation of spin dynamics by means of THz time-domain spectroscopy.

SWISSto12 has developed a post-processing method with the help of analytical and numerical models in order to analyse the reflection and transmission measurements performed with its Material Characterisation Kit (MCK) over Y1. The outcome of this approach has been the detection of a potential spin-off application of TRANSPIRE: the estimation of the conductivity for thin films based on the MCK measurements.

The theory work in TRANSPIRE has focused on establishing models that include quasi-random site-occupancy â€“ crucial in real-life materials-science, as well as finalising a phenomenological model for electric-field-induced self-oscillation by spin-orbit torque in single layers.

Final results

TCD have demonstrated that Mn2RuxGa provides record spin-orbit fields for reasonable current densities. This is the first time the anti-damping-like component has been observed in a magnetically homogenous thin film. Order-of-magnitude-type calculations show that the spin-orbit scattering cross section amounts to 60% of the lattice area (2.2 Gbarn).

In combination with TCD, HZDR-Deac has shown that the tunnel magnetoresistance remains undiminished in MRG when its net magnetization is strictly zero, proving that magnetization and spin polarisation are independent. The high-frequency dynamics of an antiferromagnet are therefore accessible in a zero-moment half metal. This is a critical advance. Downsizing devices to the nanoscale will achieve threshold current densities expected to drive auto-oscillations.

The HZDR-Gensch group has shown that ultra-thin high anisotropy films under laser pulse impact emit THz radiation into free-space, which can be used for characterisation of spin dynamics by means of THz time-domain spectroscopy. SWISSto12 has developed a procedure to estimate the transmission and reflection properties of such thin films by removing the effect of the substrate on top of which they were grown. This approach could be useful in the estimation of the conductivity of such films.

The NTNU group has developed a symmetry-based phenomenological framework for the calculation of spin-orbit torques in antiferromagnets and ferrimagnets. Based on TCDâ€™s measurements, we have demonstrated that field- and antidamping-like spin-orbit torques are present in MRG. In turn, current-induce high-frequency self-oscillations in a single layer is possible for the candidate material studied in TRANSPIRE.

The vision of TRANSPIRE is to have a definitive socio-economic impact ten to fifteen years hence as a result of the technology that will flow from the project. So far the project has shown that integration in consumer electronics is feasible and that spin dynamics appear to offer a viable route for data transmission in the 0.3 - 3.0 THz frequency band. All results are beyond the state-of-the-art, and we anticipate further ground-breaking advances as the consortiumâ€™s efforts converge on the common goal of a nanoscale THz spin-torque oscillator.

All partners have engaged in dissemination and outreach activity in Y2, as they did in Y1.