"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 antiferromagnetic domains that are very robust and can be packed at high densities."Riccardo Comin, MIT
When you save an image to your smartphone, the data are written onto tiny transistors that are electrically switched on or off in a pattern of 'bits' to represent and encode the image. Most transistors today are made from silicon, an element that scientists have managed to switch at ever-smaller scales, allowing billions of bits, and therefore large libraries of images and other files, to be packed onto a single memory chip.
But growing demand for data, and the means to store them, is driving scientists to search beyond silicon for materials that can push memory devices to higher densities, speeds and security. Now, in a paper in Physical Review Letters, physicists at Massachusetts Institute of Technology (MIT) and elsewhere report preliminary evidence that data might be stored as faster, denser and more secure bits made from antiferromagnets.
Antiferromagnetic (AFM) materials are the lesser-known cousins to ferromagnets, or conventional magnetic materials. Whereas the electrons in ferromagnets spin in synchrony – a property that allows a compass needle to point north, collectively following the Earth's magnetic field – electrons in an antiferromagnet prefer the opposite spin to their neighbor, in an 'antialignment' that effectively quenches magnetization even at the smallest scales.
The absence of net magnetization in an antiferromagnet makes 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. Antiferromagnets could also be made into smaller transistors that can be packed in greater numbers per chip than traditional silicon.
The MIT team has now found that by doping extra electrons into an antiferromagnetic material, they can turn its collective antialigned arrangement on and off, in a controllable way. This magnetic transition is reversible and sufficiently sharp, similar to switching a transistor's state from 0 to 1. The team's results demonstrate a potential new pathway to using antiferromagnets as a digital switch.
"An AFM memory could enable scaling up the data storage capacity of current devices – same volume, but more data," says the study's lead author Riccardo Comin, assistant professor of physics at MIT.
To improve data storage, some researchers are looking to 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 flipped, similar to switching a transistor from 0 to 1.
MRAM systems could potentially read and write data faster than silicon-based devices and could run with less power. But they would also be vulnerable to external magnetic fields.
"The system as a whole follows a magnetic field like a sunflower follows the Sun, which is why, if you take a magnetic data storage device and put it in a moderate magnetic field, information is completely erased," Comin explains.
Antiferromagnets, in contrast, are unaffected by external fields and could therefore provide a more secure alternative to MRAM designs. An essential step toward encodable AFM bits is the ability to switch antiferromagnetism on and off. Researchers have found various ways to accomplish this, mostly by using electric current to switch an AFM material from its orderly antialignment to a random disorder of spins.
"With these approaches, switching is very fast," says Jiarui Li, a graduate student at MIT and co-author of the paper. "But the downside is every time you need a current to read or write, that requires a lot of energy per operation. When things get very small, the energy and heat generated by running currents are significant."
Comin and his colleagues wondered whether they could achieve antiferromagnetic switching in a more efficient manner. In their new study, they work with neodymium nickelate, an antiferromagnetic oxide. This material exhibits nanodomains consisting of nickel atoms with an opposite spin to that of their neighbors, held together by oxygen and neodymium atoms. The researchers had previously mapped the material's fractal properties.
Since then, the researchers have looked to see if they could manipulate the material's antiferromagnetism via doping – a process that intentionally introduces impurities in 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 among the other nickel and oxygen atoms. The researchers wondered whether stripping away many oxygen atoms would result in a domino effect of disorder that would switch off the material's orderly antialignment.
To test their theory, they grew 100nm-thin films of neodymium nickel oxide and placed them in an oxygen-starved chamber, then heated the samples to temperatures of 400°C to encourage oxygen to escape from the films and into the chamber's atmosphere.
As the researchers removed progressively more oxygen, they studied the films using advanced magnetic X-ray crystallography techniques to determine whether the material's magnetic structure stayed intact, implying that its atomic spins remained in their orderly antialignment and thus retained antiferromagnetism. If their data showed a lack of an ordered magnetic structure, it would be evidence that the material's antiferromagnetism had switched off, due to sufficient doping.
Through their experiments, the researchers were able to switch off the material's antiferromagnetism at a certain critical doping threshold. They could also restore the antiferromagnetism by adding oxygen back into the material.
Now that the team has shown that doping effectively switches AFM on and off, scientists might use more practical ways to dope similar materials. For instance, silicon-based transistors are switched using voltage-activated 'gates', where a small voltage is applied to a bit to alter its electrical conductivity. Comin says that antiferromagnetic bits could also be switched using suitable voltage gates, which would require less energy than other antiferromagnetic switching techniques.
"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," Comin says. "That's key to addressing the challenges of a data-driven world."
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.