MIT Researchers Develop New Magnet Control Method

Researchers at the Massachusetts Institute of Technology (MIT), have discovered a method where the magnetic polarity  of a ferrimagnet is being switched by 180 degrees with the use of a little applied voltage, in a discovery that has the capacity to usher in a new era of ferrimagnetic logic and data storage devices.

The research published in the journal, Nature Nanotechnology, in a paper by postdoc Mantao Huang, MIT professor of materials science and technology Geoffrey Beach, and professor of nuclear science and technology Bilge Yildiz, along with 15 others at MIT and in Minnesota, Germany, Spain, and Korea, has the new method making use of gadolinium cobalt, a film of material, which is part of a class of materials that are rare earth transition metals known as ferrimagnet.

In this process, two elements are formed form lattices of atoms that are interlocked, with the gadolinium atoms preferentially having their magnetic axes in a direction, while the cobalt atoms aligns the opposite direction. The material’s overall magnetization is then determined by the balance in the two elements alloy composition.

Majority of the known magnets are composed of ‘ferromagnetic’ materials, with the north-south magnetic axes of many of the atoms lining up in the same direction to allow their collective forces produce a very noticeable attraction. The data storage methods and basis in today’s high-tech world formed can be said to be formed from these materials.

Ferrimagnetic materials should ideally be able to produce data storage with logic circuits that are way faster with the capacity to harness data into a given space due to their magnetic properties, which are largely influenced by external forces. But in all these, there is a lack of simple, fast and reliable method of switching the orientations of the magnets and this was what formed the basis of the research of the MIT scholars.

It was discovered by the MIT team that oxygen can be readily vented away from the hydrogen atoms with the use of a voltage to split water molecules along the surface of the film into oxygen and hydrogen, altering the balance of the magnetic orientations, a change sufficient enough to switch by 180 degrees the net magnetic field orientation, akin to a complete reversal needed for devices like magnetic memories.

One of the authors of the paper, Mantao Huang explains the process when he said:

“We found that by loading hydrogen into this structure we can reduce the gadolinium’s magnetic moment by a lot”.

The measure of the field strength that the atom’s spin axis alignment produces is known as the Magnetic moment.

According to MIT’s Materials Research Laboratory co-director, Geoffrey Beach, the process is highly efficient as change is made possible by voltage change instead of an applied electric current that would cause heating, thereby wasting energy through heat dissipation. He also said that the process of pumping hydrogen nuclei into to the material is remarkably benign.

 “You would think that if you take some material and pump some other atoms or ions into that material, you would expand it and crack it. But it turns out for these films, and by virtue of the fact that the proton is such a small entity, it can infiltrate the bulk of this material without causing the kind of structural fatigue that leads to failure”, he said.

Huang on his part said that the stability of the process has been proven through tests as the material was subjected to about 10,000 polarity reversals with no degradation signs.

According to Geoffrey Beach, the material has other properties that will aid the discovery of useful applications, with the magnetic alignment between the individual atoms in the material functioning a bit like springs. If an atom begins to move out of alignment with the others, the spring-like force pulls it back. But when the objects are connected through the use of springs, they begin to generate waves which travel along the material.  “For this magnetic material, these are called spin waves. You get oscillations of magnetization in the material, and they can have very high frequencies.”

He declared that they can oscillate upward of the terahertz range, he says, “which makes them uniquely capable of generating or sensing very high-frequency electromagnetic radiation. Not a lot of materials can do that.”

Relatively simple applications of this phenomenon, in the form of sensors, could be possible within a few years, Beach says, but more complex ones such as data and logic circuits will take longer, partly because the whole field of ferrimagnet-based technology is relatively new.

The basic methodology, apart from these specific kinds of magnetic applications, could have other uses as well, he says. “This is a way to control properties inside the bulk of the material by using an electric field,” he explains. “That by itself is quite remarkable.” Other work has been done on controlling surface properties using applied voltages, but the fact that this hydrogen-pumping approach allows such deep alteration allows “control of a broad range of properties,” he says.

“Voltage-controlled switching has been sought after in order to reduce the power consumption of spin devices, which is the core mechanism of modern silicon technology,” says Hyunsoo Yang, a professor of electrical and computer engineering at the National University of Singapore, who was not associated with this study. “This work applied the voltage control concept into a ferrimagnet to toggle the dominant sublattice, leading to an effective magnetic bit writing,” he adds. If the needed voltage can be reduced and the speed improved, he says, this new method may “potentially revolutionize the field.”

Reference: “Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures” by Mantao Huang, Muhammad Usama Hasan, Konstantin Klyukin, Delin Zhang, Deyuan Lyu, Pierluigi Gargiani, Manuel Valvidares, Sara Sheffels, Alexandra Churikova, Felix Büttner, Jonas Zehner, Lucas Caretta, Ki-Young Lee, Joonyeon Chang, Jian-Ping Wang, Karin Leistner, Bilge Yildiz and Geoffrey S. D. Beach, 29 July 2021, Nature Nanotechnology.
DOI: 10.1038/s41565-021-00940-1