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Physicists are finding a new way to turn antiferromagnetism on and off

When you store an image on your smartphone, that data is written on small transistors that turn on or off electrically in a pattern of “bits” to display and encode that image.

Most transistors today are made of silicon, an element that scientists have been able to switch on an increasingly smaller scale, allowing billions of bits, and thus large libraries of images and other files, to be packed on a single memory chip.

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

MIT physicists have now provided preliminary evidence that data can be stored as faster, denser, and more secure bits made from antiferromagnets.

Antiferromagnetic or AFM materials are the lesser known relatives of ferromagnets or conventional magnetic materials.

Where the electrons in ferromagnets spin synchronously – a property that allows a compass needle to point north and collectively follow the Earth’s magnetic field – electrons in an antiferromagnet prefer the opposite spin to their neighbor, in an ‘antialignment’ that magnetization effectively suppressed even at the smallest scales.

The absence of net magnetization in an antiferromagnet makes it insensitive to any external magnetic field. If made into memory devices, antiferromagnetic bits could protect all encoded data from magnetic removal.

They can also be made into smaller transistors and packaged in greater numbers per chip than traditional silicon.

Now the MIT team has discovered that by doping additional electrons in an antiferromagnetic material, they can turn the collective antialigned setup on and off in a controllable way.

They found that this magnetic transition is reversible, and sufficiently sharp, comparable to switching the state of a transistor from 0 to 1.

The results, published today in Physical Review Letters, demonstrate a possible new path to antiferromagnets as digital switches. to use.

“An AFM memory could make it possible to scale up the data storage capacity of current devices – the same volume, but more data,” said lead author Riccardo Comin, assistant professor of physics at MIT.

Comin MIT co-authors include lead author and graduate student Jiarui Li, along with Zhihai Zhu, Grace Zhang, and Da Zhou; as well as Roberg Green from the University of Saskatchewan;

Zhen Zhang, Yifei Sun, and Shriram Ramanathan from Purdue University; Ronny Sutarto and Feizhou He of the Canadian light source; and Jerzy Sadowski at Brookhaven National Laboratory.

Magnetic memory

To improve data storage, some researchers are looking at MRAM, or magnetoresistive RAM, a type of memory system that stores data as bits made from conventional magnetic materials.

In principle, an MRAM device would be patterned with billions of magnetic bits. To encode data, the direction of a local magnetic domain within the device is reversed, similar to switching a transistor from 0 to 1. MRAM systems may be able to read and write data faster than silicon-based devices and run with less power. to work.

But they can also be vulnerable to external magnetic fields. “The system as a whole follows a magnetic field like a sunflower follows the sun. Therefore, if you place a magnetic data storage device in a moderate magnetic field, the information is completely erased,” says Comin. In contrast, antiferromagnets are not affected by external fields and can therefore be a safer alternative to MRAM designs.

An essential step towards codable AFM bits is the ability to turn antiferromagnetism on and off. Researchers have found several ways to achieve this, usually by using electric current to switch a material from its orderly antialignment to a random disorder of spins.

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

Doped disorder

more efficient antiferromagnetic switching. In their new study, they work with neodymium nickelate, an antiferromagnetic oxide grown in the Ramanathan lab.

This material exhibits nanodomains consisting of nickel atoms with an opposite spin 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, the researchers have looked to manipulate the material’s antiferromagnetism through doping – a process that deliberately introduces impurities into a material to alter its electronic properties.

In their case, the researchers baptized neodymium nickel oxide by stripping the material from its oxygen atoms. When an oxygen atom is removed, it leaves behind two electrons, which are redistributed among the other nickel and oxygen atoms.

The researchers wondered if stripping many oxygen atoms would result in a domino effect of disorder that would turn off the material’s orderly antialignment.

To test their theory, they grew 100 nanometer thin films of neodymium nickel oxide and placed them in an oxygen-poor chamber and then heated the samples to temperatures of 400 degrees Celsius to encourage oxygen to escape from the 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 the atomic spins remained in their ordered antialignment and therefore retained antiferomagnetism.

If their data showed a lack of an ordered magnetic structure, it would be evidence that the material’s antiferromagnetism was turned off due to adequate doping. Their experiments allowed the researchers to turn off the material’s antiferromagnetism at a certain critical doping threshold.

They can also restore antiferromagnetism by adding oxygen back to the material. Now that the team has shown that doping effectively turns the AFM on and off, scientists can use more practical ways to anaesthetize 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 that antiferromagnetic bits can also be switched using suitable voltage gates, which require less energy than other antiferromagnetic switching techniques.

“This could be an opportunity to develop a magnetic memory storage device that works in the same way as silicon-based chips, with the added benefit of being able to store information in AFM domains that are very robust and packaged in high density Comin says. “That’s the key to meeting the challenges of a data-driven world.”