Researchers aim to launch the second revolution in the field of spintronics - and significantly increase the amount of memory in our electronic devices
30 years ago, home computers contained a minimal amount of information. On average, a computer was equipped with hard memory in the volume of several single megabytes, and we were constantly required to copy and delete software due to lack of space. Then, almost overnight, humanity threw away the diskettes.
"Suddenly there was a data boom," recalls Dr. Amir Kapua from the Department of Applied Physics at the Hebrew University of Jerusalem. "Hard disks with volumes of gigabytes and then terabytes came onto the market - volumes that were imaginary until then. This revolution in the memory world was born as a result of a revolution in the spintronics industry. As we know, every electron has a spin - it seems to spin around itself, like a top. If an electric current flows in a circuit, it will flow randomly: one electron will be with up spin, one with down spin, one up and one down. Mess. But if we manage to arrange all the spins so that they 'point' in one direction, we can take advantage of another property of the electrons that flow in the circuit, beyond their charge. This property is their spin angular momentum. This way we can extract more information and perform another calculation operation. The hard disk is the first spintronic product: it is based on the enormous magnetic resistance to spin-polarized currents."
What is the question? Why is it useful to move electrons in rails?
Dr. Kapua - who did his post-doctorate in the same group at IBM that developed the hard disk - says that the world of spintronics is experiencing a second revolution today: the currents from the spin poles are beginning to be interwoven in the processors themselves, so that the entire transistor becomes a house of memory.
"This technology that integrates the spintronic component inside the processor has distinct advantages," explains Dr. Kapua. "It's cheap, it's economical, it's non-volatile, you can crowd it and more. But there is one problem here: her writing is slow. Or, in other words, you have to invest a lot of energy to write fast. Therefore, the memory transistor, which sounds ideal on paper, is only used in about 5% of applications. The memory world is waiting for a solution to the speed problem.
For the past 15 years, scientists have been searching for physical effects that can effectively polarize spins to solve the speed problem. One of these effects is the "Spin Hall Effect", which makes it possible to polarize spins by passing a current through metals. But those metals must be heavy, that is, expensive, and the polarization is not efficient enough. Dr. Kapua and his research partners won a grant from the National Science Foundation, with the aim of testing another direction: the Orbital Hall Effect.
Instead of using the rotation of the spin around itself to convey the information about angular momentum, one can use the momentum of the orbit in which the electrons move.
"Instead of using the rotation of the spin around itself to convey the information about angular momentum, I can use the momentum of the orbit in which the electrons move," says Dr. Kapua. "I can attach two atoms to each other, and the electron will move from the track of one to the track of the other and so on - and this will allow the transfer of angular momentum in a different way. This is a breakthrough: the use of rails instead of spins opens up the world of spintronics for simple metals. The spin hall effect requires metals such as platinum, whose price is $28,000 per kg. We were the first in the world to show spin polarization using the Orbital Hall Effect in aluminum and copper. Aluminum today costs 2.5 dollars per kg, and copper - about 8 dollars per kg. The research has already been published in Physical Review B, and other spintronics researchers have already proven that the effect is also possible in other cheap metals."
For the purpose of demonstrating the Orbital Hall Effect, the members of Dr. Kapua's group developed a new measurement method. They called their innovative method Ferris, since it is based on a disk on which strong magnets are attached that rotate at high speed, which allows to increase the sensitivity to spin-polarized currents generated in the chip. The origin of the name is a play on words consisting of the Latin word for iron, Ferrum, and the name of the inventor of the giant wheel in amusement parks, after whom this wheel is named - Ferris Wheel.
"In the technology we have developed, we move the electrons in defined tracks in the simple metals and at the end and adapt them to the spins. The result is a generation of spin-polarized electrons that move in a circle - and we already know this creature. Therefore, the device we developed consists of several layers, the base of which is the layer of the simple metal in which the orbital Hall effect takes place, and on top of that we attach a very thin layer - only two atoms thick - of the heavy metal, such as platinum, which attaches the angular momentum of the rail to the spin. This way we enjoy both worlds: the price of the orbital Hall effect and the control of the spin polarization that the conversion layer allows; Everything is made here with us, in blue-white Israeli production."
Life itself:
Amir is frozen
"I am really fascinated by all the new ideas, the beautiful technologies and the special science that the students discover and demonstrate. It's fun to come to the lab and see and hear our students! But that's just the hobby. The real work is in front of the group coffee machine, countless conversations and a coffee culture that improves every year - and everyone is invited!"
