Periodic Reporting for period 1 - SPEAR (Spin-orbit materials, emergent phenomena and related technology training)
Berichtszeitraum: 2021-03-01 bis 2023-02-28
The Nanoelectronics Roadmap for Europe, released in December 2018, identifies promising novel technologies to be developed on the medium and long term for the European nanoelectronics industry, among which magnetic random-access memory (MRAM) is highlighted as a future memory technology. Indeed, the largest electronics companies such as Samsung, TSMC and Intel have started large scale production of spin transfer torque (STT)-MRAM. The Roadmap also identifies spintronics as one of the best strategies to complement and go beyond current CMOS technology. Spintronics was born in Europe, led by the 2007 Nobel laureates Profs. A. Fert and P. Grünberg. Although the spintronics academic community is strong, the number of European companies taking advantage of spintronic technologies is still limited relative to the number companies in the US and Asia.
The SPEAR network combines the excellence of the European spintronics community with strong participation of the private sector. New ground-breaking scientific and technological objectives are targeted in the emerging field of Spin Orbitronics--the use of spin-orbit coupling (SOC) in spintronics--with a special emphasis on the physics of spin-orbit torques (SOTs), spin-to-charge conversion (SCC), 2D magnetic materials, spin Hall nano-oscillators (SHNOs), voltage control of magnetic anisotropy (VCMA) and skyrmions. Our project is developing state-of-the-art technologies and materials, including device nanofabrication, high-resolution microscopies, and theoretical calculations.
We have identified several major challenges in the advancement of Spin Orbitronics towards applications. By addressing these objectives, we will provide both fundamental knowledge and a basis for the development of spin-orbit-based devices:
O1: Explore new materials with large SOC with potential for fundamental effects and applications.
O2: Uncover emergent phenomena induced by SOC.
O3: Demonstrate spin-orbit based devices for logic and computing applications.
O4: Optimize the performance of SOT-based MRAM memories.
SPEAR's methodology relies on a diverse set of experimental and theoretical methods ranging from material growth, nanofabrication, high-frequency signals, scanning probe microscopies, and electron transport measurements, to analytical and numerical calculations. The combination of experimental progress and development of theoretical concepts will be a key for success.
The SPEAR network combines the excellence of the European spintronics community with strong participation of the private sector. New ground-breaking scientific and technological objectives are targeted in the emerging field of Spin Orbitronics--the use of spin-orbit coupling (SOC) in spintronics--with a special emphasis on the physics of spin-orbit torques (SOTs), spin-to-charge conversion (SCC), 2D magnetic materials, spin Hall nano-oscillators (SHNOs), voltage control of magnetic anisotropy (VCMA) and skyrmions. Our project is developing state-of-the-art technologies and materials, including device nanofabrication, high-resolution microscopies, and theoretical calculations.
We have identified several major challenges in the advancement of Spin Orbitronics towards applications. By addressing these objectives, we will provide both fundamental knowledge and a basis for the development of spin-orbit-based devices:
O1: Explore new materials with large SOC with potential for fundamental effects and applications.
O2: Uncover emergent phenomena induced by SOC.
O3: Demonstrate spin-orbit based devices for logic and computing applications.
O4: Optimize the performance of SOT-based MRAM memories.
SPEAR's methodology relies on a diverse set of experimental and theoretical methods ranging from material growth, nanofabrication, high-frequency signals, scanning probe microscopies, and electron transport measurements, to analytical and numerical calculations. The combination of experimental progress and development of theoretical concepts will be a key for success.
The tasks performed and main results achieved so far can be summarized as follows:
O1. Different materials are being studied here. Task 1.1 optimized nanofabrication recipes to create spin-orbit proximitized graphene using transition metal dichalcogenides (TMD) and oxides. Task 1.2 explored different strategies for using 2D magnets (Fe3GeTe2), including changes in magnetization due to natural oxidation and studying the interaction between ferromagnetic Fe3GeTe2 and antiferromagnetic molecular layers. In task 1.3 we measured the magneto-optical Kerr effect of Fe3GeTe2 using a scanning microscope at low temperature and we are investigating current-induced domain wall motion. Task 1.4 investigated the efficiency of SOT switching from a standard SOT-MRAM device and one with a new free layer design. The devices show comparable switching energy but higher retention in the new design. Task 1.5 measured SCC in HgTe with bilinear magnetoresistance and tuned the Fermi level with top gate, observing sign changes due to the interplay between the two surface states. Task 1.6 studied SCC of two SrTiO3-based 2DEGs. We found unprecedented efficiencies and tuned the SCC amplitude and sign by electrical gate. Task 1.7 focused on predicting materials with high SCC and found that Weyl semimetals TaAs and WTe2, and SrTiO3-based 2DEGs are interesting candidates.
O2. Different emerging phenomena are being studied. Task 2.1 investigates the impact of VCMA on SOT switching in magnetic tunnel junctions (MTJs). Results show that the application of VCMA enhances or decreases the required critical switching voltages depending on the type of switching. Task 2.2 focused on producing single and double layers of 2D materials, including MTJs. We used Fe3GeTe2 and successfully stacked it with PDMS-PC dry transfer technique on pre-patterned Hall bar contacts. Task 2.4 is looks for strategies to suppress the skyrmion Hall effect. Results show that the application of gradients of magnetic anisotropy could be applied to keep the skyrmion motion straight in the center of the racetrack. Task 2.5 investigated the growth of Mn on Ir(111) from submonolayer coverage to four atomic layers using scanning tunneling microscopy. The magnetic state of the pseudomorphic Mn monolayer was found to be an in-plane Néel state. Task 2.6 investigates the growth of Fe on a film of Rh on Nb(110) to induce superconductivity in this system. The submonolayer islands of Rh grow well ordered providing a platform to support an Fe layer. Task 2.7 explores methods to obtain full vector information from a single scan using NV tips. By moving to a frequency feedback mode, the NV to sample distance can be stabilized at a much lower value, increasing resolution and signal strength. In task 2.8 circular, voltage-controlled and memristive nano-gates were demonstrated both on top of and at the sides of nano-constriction SHNOs.
O3. In the first part of the project, we focused on spin-orbit devices for logic applications. Task 3.1 is studying graphene-based heterostructures, and found that the best material is CuOx, with a product of the spin diffusion length and spin Hall angle (figure of merit for SCC) of up to 1.8 nm. The effect is gate tunable. In task 3.3 we are now able to fabricate large ensembles of devices and we measured our first SCC signals in SrTiO3-based 2DEGs. The modulation of the signal by the gate voltage is underway.
O4. This objective focuses on SOT-based MRAMs. Task 4.1 shows that non-rectangular pulses can achieve magnetization reversal with significantly reduced energies, and a voltage "spike" applied after a few ns triggers magnetization reversal after a preheating phase, yielding higher total temperature changes in the sample. Task 4.2 is developing new PVD processes for deposition of Co on graphene without damage to the 2D layer. In task 4.3 large spin signals have been observed in STT-FMR studies on HgTe/HgCdTe/NiFe structures, revealing progress in optimizing the SCC. A strain induced by the deposition of the ferromagnet on top of the TI of van der Waals type has been observed in Sb2Te3. Task 4.4 showed that SOT and STT can be efficiently combined to achieve efficient SOT write operation for SOT-MRAM bitcells using only sputtered materials compatible with VLSI integration.
O1. Different materials are being studied here. Task 1.1 optimized nanofabrication recipes to create spin-orbit proximitized graphene using transition metal dichalcogenides (TMD) and oxides. Task 1.2 explored different strategies for using 2D magnets (Fe3GeTe2), including changes in magnetization due to natural oxidation and studying the interaction between ferromagnetic Fe3GeTe2 and antiferromagnetic molecular layers. In task 1.3 we measured the magneto-optical Kerr effect of Fe3GeTe2 using a scanning microscope at low temperature and we are investigating current-induced domain wall motion. Task 1.4 investigated the efficiency of SOT switching from a standard SOT-MRAM device and one with a new free layer design. The devices show comparable switching energy but higher retention in the new design. Task 1.5 measured SCC in HgTe with bilinear magnetoresistance and tuned the Fermi level with top gate, observing sign changes due to the interplay between the two surface states. Task 1.6 studied SCC of two SrTiO3-based 2DEGs. We found unprecedented efficiencies and tuned the SCC amplitude and sign by electrical gate. Task 1.7 focused on predicting materials with high SCC and found that Weyl semimetals TaAs and WTe2, and SrTiO3-based 2DEGs are interesting candidates.
O2. Different emerging phenomena are being studied. Task 2.1 investigates the impact of VCMA on SOT switching in magnetic tunnel junctions (MTJs). Results show that the application of VCMA enhances or decreases the required critical switching voltages depending on the type of switching. Task 2.2 focused on producing single and double layers of 2D materials, including MTJs. We used Fe3GeTe2 and successfully stacked it with PDMS-PC dry transfer technique on pre-patterned Hall bar contacts. Task 2.4 is looks for strategies to suppress the skyrmion Hall effect. Results show that the application of gradients of magnetic anisotropy could be applied to keep the skyrmion motion straight in the center of the racetrack. Task 2.5 investigated the growth of Mn on Ir(111) from submonolayer coverage to four atomic layers using scanning tunneling microscopy. The magnetic state of the pseudomorphic Mn monolayer was found to be an in-plane Néel state. Task 2.6 investigates the growth of Fe on a film of Rh on Nb(110) to induce superconductivity in this system. The submonolayer islands of Rh grow well ordered providing a platform to support an Fe layer. Task 2.7 explores methods to obtain full vector information from a single scan using NV tips. By moving to a frequency feedback mode, the NV to sample distance can be stabilized at a much lower value, increasing resolution and signal strength. In task 2.8 circular, voltage-controlled and memristive nano-gates were demonstrated both on top of and at the sides of nano-constriction SHNOs.
O3. In the first part of the project, we focused on spin-orbit devices for logic applications. Task 3.1 is studying graphene-based heterostructures, and found that the best material is CuOx, with a product of the spin diffusion length and spin Hall angle (figure of merit for SCC) of up to 1.8 nm. The effect is gate tunable. In task 3.3 we are now able to fabricate large ensembles of devices and we measured our first SCC signals in SrTiO3-based 2DEGs. The modulation of the signal by the gate voltage is underway.
O4. This objective focuses on SOT-based MRAMs. Task 4.1 shows that non-rectangular pulses can achieve magnetization reversal with significantly reduced energies, and a voltage "spike" applied after a few ns triggers magnetization reversal after a preheating phase, yielding higher total temperature changes in the sample. Task 4.2 is developing new PVD processes for deposition of Co on graphene without damage to the 2D layer. In task 4.3 large spin signals have been observed in STT-FMR studies on HgTe/HgCdTe/NiFe structures, revealing progress in optimizing the SCC. A strain induced by the deposition of the ferromagnet on top of the TI of van der Waals type has been observed in Sb2Te3. Task 4.4 showed that SOT and STT can be efficiently combined to achieve efficient SOT write operation for SOT-MRAM bitcells using only sputtered materials compatible with VLSI integration.
SPEAR's objectives include original and challenging tasks beyond the state-of-the-art, and open wide perspectives for further research and commercial exploitation. The 3 most important innovative elements of the research program are: i) the coordinated effort between theory, fundamental and applied research to obtain the best materials for spin-orbit-based devices (SOT-MRAM, MESO); ii) the synergetic approach to understanding skyrmion behavior and exploring alternative magnetic objects will be key for the development of novel non-volatile data storage and processing technologies; iii) the integration of VCMA in different technological targets of the program (MRAMs, SHNOs) will be a major step towards low-power computing.