Physicists Find New Way to Turn Antiferromagnetism On and Off | MIT News

When you save an image to your smartphone, that data is written to tiny transistors that are electrically turned on or off in a “bit” pattern to represent and encode that image. Most of the current transistors are made from silicon, an element that scientists have managed to switch to smaller and smaller scales, allowing billions of bits to be grouped together, and thus large libraries of images and others. files, on a single memory chip.

But growing demand for data and the means to store it is pushing scientists to look beyond silicon for materials capable of pushing memory devices to higher densities, speeds, and security.

Now physicists at MIT have shown preliminary evidence that data could be stored as faster, denser, and more secure bits made from antiferromagnetics.

Antiferromagnetic or AFM materials are the lesser known cousins ​​of ferromagnetics or conventional magnetic materials. Where electrons in ferromagnets spin in synchrony – a property that allows a compass needle to point north, collectively following Earth’s magnetic field – electrons in an antiferromagnet prefer the opposite spin to their neighbor, in an “anti-ferromagnet”. -alignment ”which effectively extinguishes magnetization even at the smallest scales.

The lack of sharp magnetization in an antiferromagnet makes it impervious to any external magnetic field. If turned into memory devices, the anti-ferromagnetic bits could protect all encoded data from magnetic erasure. They could also be made into smaller transistors and packaged in larger numbers per chip than traditional silicon.

Now, the MIT team have found that by doping extra electrons in an antiferromagnetic material, they can turn its collective anti-aligned arrangement on and off, in a controllable way. They found this magnetic transition to be reversible and sharp enough, similar to switching the state of a transistor from 0 to 1. The results, published today in Physical examination letters, demonstrate a potential new way to use antiferromagnetics as a digital switch.

“AFM memory could increase the data storage capacity of today’s devices – same volume, but more data,” says lead author Riccardo Comin, assistant professor of physics at MIT.

Comin’s MIT co-authors include senior author and graduate student Jiarui Li, as well as Zhihai Zhu, Grace Zhang, and Da Zhou; as well as Roberg Green of the University of Saskatchewan; Zhen Zhang, Yifei Sun and Shriram Ramanathan from Purdue University; Ronny Sutarto and Feizhou He of Canadian Light Source; and Jerzy Sadowski of Brookhaven National Laboratory.

Magnetic memory

To improve data storage, some researchers are turning to MRAM, or magnetoresistive RAM, a type of memory system that stores data in the form of bits made from conventional magnetic materials. In principle, an MRAM device would be structured with billions of magnetic bits. To encode data, the direction of a local magnetic domain inside the device is reversed, much like switching a transistor from 0 to 1.

MRAM systems could potentially read and write data faster than silicon-based devices and could run on less power. But they could also be vulnerable to external magnetic fields.

“The system as a whole follows a magnetic field like a sunflower follows the sun, so if you take a magnetic data storage device and put it in a moderate magnetic field, the information is completely erased,” Comin explains. .

Antiferromagnetics, on the other hand, are unaffected by external fields and therefore could be a safer alternative to MRAM designs. An essential step towards encodable AFM bits is the ability to turn anti-ferromagnetism on and off. Researchers have found various ways to accomplish this, primarily by using electric current to pass material from its ordered anti-alignment sound into a random mess of spins.

“With these approaches, switching is very fast,” Li says. “But the downside is that every time you need current to read or write, it takes a lot of power per operation. When things get very small, the energy and heat generated by current currents is important. “

Doped disorder

Comin and his colleagues wondered if they could achieve anti-ferromagnetic switching in a more efficient manner. In their new study, they are working with neodymium nickelate, an antiferromagnetic oxide grown in Ramanathan’s lab. This material has nanodomains made up of nickel atoms with a spin opposite to that of its neighbor, and held together by oxygen and neodymium atoms. The researchers had previously mapped the fractal properties of the material.

Since then, researchers have looked to see if they can manipulate the material’s antiferromagnetism via doping – a process that intentionally introduces impurities into a material to alter its electronic properties. In their case, the researchers doped neodymium nickel oxide by stripping the material of its oxygen atoms.

When an oxygen atom is removed, it leaves behind two electrons, which are redistributed between the other nickel and oxygen atoms. The researchers wondered if removing many oxygen atoms would result in a domino effect of disorder that would deactivate the material’s orderly anti-alignment.

To test their theory, they grew 100-nanometer thin films of neodymium nickel oxide and placed them in an oxygen-deprived chamber, then heated the samples to temperatures of 400 degrees Celsius to encourage the oxygen to escape films and into the atmosphere of the room. .

As they removed more and more oxygen, they studied the films using advanced x-ray magnetic crystallography techniques to determine if the material’s magnetic structure was intact, implying that its spins atomics remained in their ordered anti-alignment and therefore retained antiferomagnetism. If their data showed an absence of an ordered magnetic structure, this would be proof that the antiferromagnetism of the material had died out, due to sufficient doping.

Thanks to their experiments, the researchers were able to deactivate the antiferromagnetism of the material at a certain critical doping threshold. They could also restore antiferromagnetism by reintroducing oxygen into the material.

Now that the team has shown that doping effectively turns AFM on and off, scientists could use more practical ways to spike similar materials. For example, silicon-based transistors are switched using voltage-activated “gates”, where a small voltage is applied to a bit to change its electrical conductivity. Comin says the antiferromagnetic bits could also be switched using appropriate voltage gates, which would require less power than other antiferromagnetic switching techniques.

“This could present an opportunity to develop a magnetic memory storage device that functions similarly to silicon-based chips, with the added benefit of being able to store information in AFM domains which are very robust and can be packaged at high densities, ”says Comin. “This is the key to meeting the challenges of a data-driven world.”

This research was funded in part by the Young Investigator Program of the Air Force Office of Scientific Research and the Natural Sciences and Engineering Research Council of Canada. This research used resources from the Center for Functional Nanomaterials and the National Synchrotron Light Source II, both located at the Brookhaven National Laboratory.

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