Periodic Reporting for period 1 - SPECTER (Spin-charge conversion in highly resistive spin-orbit materials)
Reporting period: 2020-09-01 to 2022-08-31
After three decades of massive chip innovation, the scaling of CMOS logic devices is reaching its fundamental limits. With voltage and frequency scaling slowing down and power consumption and heating issues increasing, the need for beyond-CMOS solutions has never been bigger. One promising solution for energy-efficient computation are magnetoelectric spin-orbit devices, a new concept where logic operations are performed by electrically switching and reading nanoscale magnets. However, the experimental realization of this device requires major breakthroughs regarding quantum materials and fabrication processes.
SPECTER aims at experimentally benchmarking materials that show potential for spin-based logic devices, that can be used to read magnetic states. Both the physics and experimental approach derived from SPECTER will be a major milestone in spintronics, prompting not only advanced research in the future but also solutions for next-gen device applications.
The project focuses on three interconnected work packages: (1) investigation of the role of tunnel barriers at ferromagnetic/spin-orbit material interfaces, to increase the electrical output readout, and fabrication and experimentally measuring all-electrical spin-charge conversion in (2) topological insulator BiSe and (3) Iridium oxide-based nanodevices for magnetic readout.
SPECTER aims at experimentally benchmarking materials that show potential for spin-based logic devices, that can be used to read magnetic states. Both the physics and experimental approach derived from SPECTER will be a major milestone in spintronics, prompting not only advanced research in the future but also solutions for next-gen device applications.
The project focuses on three interconnected work packages: (1) investigation of the role of tunnel barriers at ferromagnetic/spin-orbit material interfaces, to increase the electrical output readout, and fabrication and experimentally measuring all-electrical spin-charge conversion in (2) topological insulator BiSe and (3) Iridium oxide-based nanodevices for magnetic readout.
The researcher has successfully optimized the nanofabrication of the proposed spin-charge conversion devices, using CoFe as the magnet, Pt as the spin-orbit material, with and without a barrier. Devices without a barrier were achieved using a newly developed subtractive fabrication workflow, which was then adapted and used to realize devices with Al2O3 barriers. The researcher successfully measured spin-charge conversion in multiple device configurations, which exhibited enhanced signal magnitudes in devices with a barrier ((2RSCC>20m)), as theoretically predicted in the initial proposal. Although positive, the results for most devices with an Al2O3 barrier did not show sufficient reproducibility. Consequently, the researcher developed an alternative fabrication and conceptual approach, based on nanopillars and MgO barriers, showing improved reproducibility. Spin-charge conversion experiments using this new approach are still in progress.
To further increase the output readout signal, two highly resistive spin-orbit materials were proposed to replace Pt: BixSe1-x and SrIrO3. BixSe1-x thin films and nanowires were successfully realized using a combination of sputtering and nanofabrication techniques. The samples possessed a polycrystalline structure and high resistivity (at least two orders of magnitude larger than Pt). All-electrical spin-charge conversion was successfully measured. However, given the large electrical noise found in the nanostructures, the devices required an additional thin Ti layer, which reduced the overall resistivity of the material combination. The final spin-charge conversion signal was of similar magnitude (2RSCC=15m) when compared to the CoFe/barrier/Pt combination. We concluded that BixSe1-x is not an appropriate material for the purpose of the project and that extra care should be taken when considering other sputtered topological insulators of similar nature. The work is summarized in a published research article (W. Y. Choi, I. C. Arango, V. T. Pham, D. C. Vaz, et al., Nano Letters (2022) https://doi.org/10.1021/acs.nanolett.2c03429).
Iridium-based oxide films were provided by collaborators at the University of California, Berkeley, including LaSrMnO3 as the magnetic element and SrIrO3 as the spin-orbit material. The researcher successfully fabricated devices with this new all-oxide stack and spin-charge conversion was successfully measured, with a readout output reaching 2RSCC=100m, around one order of magnitude larger than in Pt-based devices. This work provides an important step toward all-electrical spin-charge conversion in all-oxide heterostructures.
To further increase the output readout signal, two highly resistive spin-orbit materials were proposed to replace Pt: BixSe1-x and SrIrO3. BixSe1-x thin films and nanowires were successfully realized using a combination of sputtering and nanofabrication techniques. The samples possessed a polycrystalline structure and high resistivity (at least two orders of magnitude larger than Pt). All-electrical spin-charge conversion was successfully measured. However, given the large electrical noise found in the nanostructures, the devices required an additional thin Ti layer, which reduced the overall resistivity of the material combination. The final spin-charge conversion signal was of similar magnitude (2RSCC=15m) when compared to the CoFe/barrier/Pt combination. We concluded that BixSe1-x is not an appropriate material for the purpose of the project and that extra care should be taken when considering other sputtered topological insulators of similar nature. The work is summarized in a published research article (W. Y. Choi, I. C. Arango, V. T. Pham, D. C. Vaz, et al., Nano Letters (2022) https://doi.org/10.1021/acs.nanolett.2c03429).
Iridium-based oxide films were provided by collaborators at the University of California, Berkeley, including LaSrMnO3 as the magnetic element and SrIrO3 as the spin-orbit material. The researcher successfully fabricated devices with this new all-oxide stack and spin-charge conversion was successfully measured, with a readout output reaching 2RSCC=100m, around one order of magnitude larger than in Pt-based devices. This work provides an important step toward all-electrical spin-charge conversion in all-oxide heterostructures.
On the optimization of the tunneling barriers for efficient spin-charge conversion and magnetization readout, further work is required on the fine-tuning of the barrier quality, through tweaks in the annealing temperature and optimal thickness. The optimized fabrication process should provide a solid platform to test other materials beyond those proposed in the initial proposal, such as Ta, -W, or other Bi-based alloys. Regarding topological insulators, the poor quality and issues imposed by sputtered BixSe1-x suggest that other options grown by molecular beam epitaxy will be more advantageous for highly efficient spin-charge conversion, even though extra care must be taken during the fabrication process to avoid unintentional damage and/or doping of the material. Regarding oxide-based materials, such as the iridates, great emphasis should be put on performing and testing other high spin-orbit coupling correlated oxides within the framework of all-electrical spin-charge conversion, something particularly lacking in the literature. The results of SPECTER regarding all-electrical spin-charge conversion in SrIrO3 should motivate further studies in other materials.
Besides the scientific advances achieved within the proposed work packages of SPECTER, the researcher also studied an additional highly resistive and high spin-orbit material, elemental tellurium, which was found to possess chirality-dependent electrical charge-to-spin conversion. The results were published in F. Calavalle, M. Suárez-Rodríguez, B. Martín-García, A. Johansson, D. C. Vaz, et al., Nature Materials 21, 526 (2022) https://doi.org/10.1038/s41563-022-01211-7). Lastly, as stated in the initial proposal, one of the long-term consequences of SPECTER was to implement the optimized spin-charge conversion nanodevices in magnetoelectric spin-orbit devices, a concept proposed by Intel for energy-efficient computation beyond the current CMOS paradigm. Through direct collaboration with Intel, the researcher fabricated CoFe/Pt nanodevices directly on multiferroic-based heterostructures, to perform both electrical reading of magnetic states using spin-charge conversion and magnetization switching (writing) using the magnetoelectric effect and exchange coupling in BiFeO3/CoFe interfaces. These experiments resulted in the world’s first experimental demonstration of the magnetoelectric spin-orbit device, as well as an important result towards voltage-control of magnetism, crucial for the next generation of energy-efficient non-volatile logic and memory applications. The work was presented and published as a conference paper (D. C. Vaz et al., IEEE International Electron Devices Meeting (IEDM), 32.4. 1-32.4. 4 (2021) https://doi.org/10.1109/IEDM19574.2021.9720677).
Besides the scientific advances achieved within the proposed work packages of SPECTER, the researcher also studied an additional highly resistive and high spin-orbit material, elemental tellurium, which was found to possess chirality-dependent electrical charge-to-spin conversion. The results were published in F. Calavalle, M. Suárez-Rodríguez, B. Martín-García, A. Johansson, D. C. Vaz, et al., Nature Materials 21, 526 (2022) https://doi.org/10.1038/s41563-022-01211-7). Lastly, as stated in the initial proposal, one of the long-term consequences of SPECTER was to implement the optimized spin-charge conversion nanodevices in magnetoelectric spin-orbit devices, a concept proposed by Intel for energy-efficient computation beyond the current CMOS paradigm. Through direct collaboration with Intel, the researcher fabricated CoFe/Pt nanodevices directly on multiferroic-based heterostructures, to perform both electrical reading of magnetic states using spin-charge conversion and magnetization switching (writing) using the magnetoelectric effect and exchange coupling in BiFeO3/CoFe interfaces. These experiments resulted in the world’s first experimental demonstration of the magnetoelectric spin-orbit device, as well as an important result towards voltage-control of magnetism, crucial for the next generation of energy-efficient non-volatile logic and memory applications. The work was presented and published as a conference paper (D. C. Vaz et al., IEEE International Electron Devices Meeting (IEDM), 32.4. 1-32.4. 4 (2021) https://doi.org/10.1109/IEDM19574.2021.9720677).