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Over the last decade, with the introduction of increasingly complex artificial intelligence (AI) technologies, the demand for computing power has increased exponentially. New energy-efficient hardware designs could help meet this demand while reducing computing power consumption, supporting faster processing, and enabling AI training within the device itself.
“In my opinion, we’ve already moved from the Internet age to the AI age,” says Shan Wang, Leland T. Edwards Professor in the School of Engineering at Stanford University. “We want to enable edgetraining AI locally on your home computer, phone, or smartwatch for things like heart attack detection or speech recognition. To do that, you need very fast, nonvolatile memory.”
Wang and his colleagues recently found a material that could bring a new type of memory closer to commercialization. In a new article published in Materials of natureThe researchers showed that a thin layer of a metal compound called manganese palladium three had the properties needed to facilitate a form of working memory that stores data in electron spin directions.
This method of memory storage, known as spin-orbit magnetoresistive random access memory or SOT-MRAM, has the potential to store data faster and more efficiently than current methods, which store data using electricity and require continuous power input to maintain that data.
“We have provided a building block for future energy-efficient storage elements,” says Wang. “It’s very fundamental, but it’s a breakthrough.”
Take advantage of the spin of the electron
SOT-MRAM relies on an intrinsic property of electrons called spin. To understand the spin, imagine an electron as a spinning basketball balanced on the tip of a professional athlete’s finger. Since electrons are charged particles, the spin turns the electron into a tiny magnet, polarized along its axis (in this case, a line extending from the finger that balances the sphere). If the electron changes direction of rotation, the north-south poles of the magnet change. Researchers can use the up or down direction of that magnetism known as a magnetic dipole moment to represent the ones and zeros that make up the bits and bytes of computer data.
In SOT-MRAM, a current flowing through a material (the SOT layer) generates specific directions of rotation. The motion of those electrons, together with their spin directions, creates a torque that can change the spin directions and associated magnetic dipole moments of electrons in an adjacent magnetic material. With the right materials, storing magnetic data is as simple as changing the direction of an electric current in the SOT layer.
But finding the right SOT materials is not easy. Because of the way the hardware is designed, data can be stored more densely when the electron spin directions are oriented up or down in the z-direction. (If you imagine a sandwich on a plate, the x and y directions follow the edges of the bread, and the z direction is the toothpick stuck in the middle.) Unfortunately, most materials polarize electron spins in the y direction if current flows in the direction x.
“Conventional materials only generate spin in the y direction, which means we would need an external magnetic field to make the switch happen in the z direction, which requires more energy and space,” says Fen Xue, a postdoctoral researcher in the Wang’s laboratory. “In order to lower energy and have higher memory density, we want to be able to do this switching without an external magnetic field.”
Researchers have found that manganese palladium three has the properties they need. The material is capable of generating spin in any orientation because its internal structure lacks the kind of crystalline symmetry that would force all electrons into any particular orientation. Using manganese palladium three, the researchers were able to demonstrate the switching of magnetization in both y and z directions without the need for an external magnetic field. Although not demonstrated in the manuscript, the x-direction magnetization can also be switched in the absence of an external magnetic field.
“We have the same input current as other conventional materials, but now we have three different directions of rotation,” says Mahendra DC, who led the work as a postdoctoral researcher at Stanford and is the paper’s first author. “Depending on the application, we can control the magnetization in any direction we want.”
DC and Wang credit the multidisciplinary and multi-institutional collaboration that has enabled these advances. “Evgeny Tsymbal’s lab at the University of Nebraska conducted the calculations to predict unexpected rotational directions and motion, and Julie Borchers’ lab at the National Institute of Standards and Technology conducted the measurements and modeling efforts to reveal the intricate microstructures within manganese palladium three,” says Wang. “It really takes a village.”
Possibility of production
In addition to its symmetry breaking structure, manganese palladium three has several other properties that make it a prime candidate for SOT-MRAM applications. It can, for example, survive and maintain its properties through the post-annealing process that electronics go through.
“Post-annealing requires the electronics to be at 400 degrees Celsius for 30 minutes,” DC says. “That’s one of the challenges for new materials in these devices, and manganese palladium three can do it.”
Additionally, the manganese palladium layer three is created using a process called magnetron sputtering, a technique already used in other aspects of memory storage hardware.
“No new tools or new techniques are needed for this type of material,” says Xue. “We don’t need a structured substrate or special conditions to deposit it.”
The result is a material that not only has new properties that could help meet our growing computing requirements, but can be seamlessly adapted to current manufacturing techniques. Researchers are already working on SOT-MRAM prototypes using manganese palladium three that will integrate into real devices.
“We’re hitting a wall with current technology,” DC says. “So we need to figure out what other options we have.”
Mahendra DC et al, Observation of anti-damping spinorbit pairs generated by in-plane and out-of-plane spin polarizations in MnPd3, Materials of nature (2023). DOI: 10.1038/s41563-023-01522-3
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