Monday, May 16, 2022

A novel way to switch antiferromagnetism on and off

The findings could lead to faster, more secure memory storage, in the form of antiferromagnetic bits.

With the increasing demand for data, scientists are searching beyond silicon for materials that can push memory devices to higher densities, speeds, and security.

Now MIT scientists have discovered a new way to store data. They have shown that data might be stored as faster, denser, and more secure bits made from antiferromagnets.

Antiferromagnetic or AFM materials are conventional magnetic materials. Unlike ferromagnets, where electrons spin in synchrony, electrons in an antiferromagnet prefer the opposite spin to their neighbor, in an “antialignment” that effectively quenches magnetization even at the smallest scales.

There is no magnetization in the antiferromagnet, making it impervious to any external magnetic field. If they were made into memory devices, antiferromagnetic bits could protect any encoded data from being magnetically erased. They could also be made into smaller transistors and packed in greater numbers per chip than traditional silicon.

By doping additional electrons into an antiferromagnetic material, scientists could turn its collective antialigned arrangement on and off. They found this magnetic transition is reversible and sufficiently sharp, similar to switching a transistor’s state from 0 to 1.

The study’s lead author Riccardo Comin, assistant professor of physics at MIT, said, “An AFM memory could enable scaling up the data storage capacity of current devices — same volume, but more data.”

An essential step toward encodable AFM bits is the ability to switch antiferromagnetism on and off. For example, there are several ways to do so, using electric current to switch material from its orderly antialignment to a random disorder of spins.

These approaches help in fast switching. But there is a drawback, these approaches require a lot of energy per operation. When things get very small, the energy and heat generated by running currents are significant.

In this study, scientists achieved antiferromagnetic switching more efficiently. To do so, they worked with neodymium nickelate. This material exhibits nanodomains that consist of nickel atoms with an opposite spin to that of its neighbor and held together by oxygen and neodymium atoms.

Scientists determined if they could manipulate the material’s antiferromagnetism via doping. They doped neodymium nickel oxide by stripping the material of its oxygen atoms.

Removing oxygen atoms left two electrons, which are redistributed among the other nickel and oxygen atoms. Scientists wondered whether stripping away many oxygen atoms would result in a domino effect of disorder that would switch off the material’s orderly antialignment.

Scientists tested their theory by growing 100-nanometer-thin films of neodymium nickel oxide and placed them in an oxygen-starved chamber. They then heated the samples to temperatures of 400 degrees Celsius to encourage oxygen to escape from the films and into the chamber’s atmosphere.

After removing more oxygen, scientists used X-ray crystallography techniques to analyze the films to determine whether the material’s magnetic structure was intact, implying that its atomic spins remained in their orderly antialignment, and therefore retained antiferromagnetism. If the outcomes showed a lack of an ordered magnetic structure, it means that the material’s antiferromagnetism had switched off due to sufficient doping.

The study’s lead author Riccardo Comin, assistant professor of physics at MIT, said“This could present an opportunity to develop a magnetic memory storage device that works similarly to silicon-based chips, with the added benefit that you can store information in AFM domains that are very robust and can be packed at high densities. That’s key to addressing the challenges of a data-driven world.”

The results, published today in Physical Review Letters.

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