Bringing energy-efficient MESO technology a step closer to reality
A favorable scaling of the magnetic state readout using the spin Hall effect allows to increase by 10,000 times the output voltage. This achievement, a joint collaboration between nanoGUNE and Intel, is crucial for the “reading” part of the energy-efficient magnetoelectric spin-orbit (MESO) logic.
“Can you achieve 100 millivolt?”.
This is what Dr. Manipatruni from Components Research at Intel asked me when we first met back in 2015, in a Gordon Resarch Conference in Hong Kong on Spin Dynamics in Nanostructures. He wanted to know whether our lateral spin valves, a type of nanodevices that my research group was using to understand and quantify the contributions of the spin Hall effect in different heavy metals, could generate this voltage.
“Well, not really, the voltage measured in our devices is around 10 nanovolt…”, I answered.
The spin Hall effect (SHE), based on spin-orbit coupling, is now a very popular effect that allows charge-to-spin current conversion without the use of a ferromagnet as the spin source. In particular, it is promising for the development of a new generation of non-volatile magnetic RAM, where the torque driven by the SHE-induced spin current is used to write magnetic memories. The reciprocal phenomena, the inverse SHE, can potentially be used to read the magnetic state of a magnetic element, as was recently proposed by Dr. Manipatruni and colleagues at Components Research Intel, for a novel magnetoelectric spin–orbit (MESO) based logic.
“We have a proposal for a spin-based logic that combines spin-orbit and magnetoelectric effects, and could reach an energy efficiency 10 to 30 times better that current CMOS”, he told me. “But we need the reading output to be increased to 100 millivolt, because it needs to match the writing input for the magnetoelectric part”.
That proposal was finally published in Nature last year. For the logic operations to take place, the reading process must generate an electromotive force large enough to switch the next ferromagnetic element in the circuit.
“Right now we are 7 orders of magnitude away…sounds hard. Let’s try!”.
Finding a replacement to current CMOS technology in electronics that can be smaller, faster and, most importantly, with less energy consumption is a global challenge. MESO logic, a new technology that combines memory, interconnections and logic requirements for future computing needs, might allow us to maintain Moore’s law beyond CMOS while being more energy efficient. This was an exciting project with many different components to be improved. And we could contribute with our experience on the SHE.
This conversation in Hong Kong was the starting point of a fruitful collaboration between Intel and my group at CIC nanoGUNE in order to prove the feasibility of this disruptive technology.
Now, the first results of this joint effort are being published in Nature Electronics.
We have been able to increase by 4 orders of magnitude the output voltage, by simply using a better design but with the standard material for the SHE, platinum. Although we still have not reached the final value for this technology to work, we have unveiled different paths on how to achieve it. First, the output voltage given by the device we have designed scales when the dimensions are reduced, which is a requirement for any technology to be introduced in the market (otherwise miniaturization would not be possible). Second, we identify the exact role of the materials in the device, and estimate that certain materials (such as topological insulators or some two-dimensional systems) have the necessary properties that should allow us to bridge the remaining gap of 3 orders of magnitude enhancement for the 100 millivolt goal. These necessary properties are a large spin-to-charge conversion efficiency (which is not simply the spin Hall angle, but its product with the spin diffusion length) and a high resistivity. These are different from the materials properties needed for spin-orbit torques in MRAM.
Our results, thus, bring the MESO technology a step closer (4 orders of magnitude closer!) to reality and motivates the spintronics community to keep on searching for new spin-to-charge conversion materials with these special properties.
CIC nanoGUNE. The CIC nanoGUNE Nanoscience Cooperative Research Center, located in Donostia-San Sebastian, Basque Country, is a research center set up with the mission to conduct excellence research in nanoscience and nanotechnology with the aim of increasing the Basque Country’s business competitiveness and economic development. NanoGUNE is also a member of the Basque Research and Technology Alliance and is recognized by the Spanish Research Agency as a "María de Maeztu" Unit of Excellence (2017-2021).