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Researchers Discover a New Method of Magnet Control

The majority of magnets we come into contact with on a daily basis are made of "ferromagnetic" materials. As a result of the alignment of most atoms' north-south magnetic axes in the same direction, their combined magnetic force is strong enough to produce significant attraction in the material under consideration. In today's high-tech world, these materials serve as the foundation for the vast majority of data storage devices.

Magnets based on ferrimagnetic materials, denoted by the letter I are less common. In these, some of the atoms are aligned in one direction, while others are aligned in the exact opposite direction of that alignment. So the overall magnetic field they produce is determined by the balance between the two types — for example, when there are more atoms pointed one way than the other, the difference produces a net magnetic field in the direction of the more atoms pointed in the other direction.

Due to the fact that ferrimagnetic materials' magnetic properties are strongly influenced by external forces, they should be able to produce data storage or logic circuits that are significantly faster and can store significantly more data in a given amount of space than today's conventional ferromagnets. However, there has been no simple, fast, and reliable method of changing the orientation of these magnets in order to flip from a 0 to a 1 in a data storage device up until now.

Researchers at MIT and other institutions have developed such a method, which allows them to quickly switch the magnetic polarity of a ferrimagnet 180 degrees by applying a very small voltage to the material. According to the researchers, the discovery could herald the beginning of a new era in ferrimagnetic logic and data storage devices.

The findings were published in the journal Nature Nanotechnology in a paper co-authored by postdoc Mantao Huang, MIT professor of materials science and technology Geoffrey Beach, and professor of nuclear science and technology Bilge Yildiz, as well as 15 other researchers from MIT and institutions in Minnesota, Germany, Spain, and Korea. The findings were made possible by the support of the National Science Foundation.

The new system makes use of a thin film of a material called gadolinium cobalt, which is a member of a class of materials known as rare earth transition metal ferrimagnets, which are rare earth transition metals that are ferrimagnetic. In it, the two elements combine to form interlocking lattices of atoms, with the gadolinium atoms preferentially aligning their magnetic axes in one direction and the cobalt atoms aligning their magnetic axes in the opposite direction. The overall magnetization of the alloy is determined by the balance between the two elements in the alloy's chemical composition.

Using a voltage to split water molecules along the film's surface into oxygen and hydrogen, the researchers discovered that the oxygen can be vented away while the hydrogen atoms — or more precisely their nuclei, which are single protons — can penetrate deeply into the material, causing the magnetic orientations to become more evenly balanced again. The change is sufficient to completely reorient the net magnetic field orientation by 180 degrees, which is precisely the kind of complete reversal required by devices such as magnetic memory arrays to function properly.

"We discovered that by loading hydrogen into this structure, we were able to significantly reduce the magnetic moment of gadolinium," Huang explains. The magnetic moment of an atom is a measure of the strength of the field produced by the alignment of the atom's spin axis.

It is highly energy efficient because the change is accomplished solely by a change in voltage rather than an applied electrical current, which would cause heating and thus waste energy through heat dissipation, according to Beach, who is also the co-director of MIT's Materials Research Laboratory.

According to him, the process of pumping hydrogen nuclei into the material turns out to be remarkably safe and harmless. Normally, you would expect that if you take a material and inject some additional atoms or ions into it, the material would expand and crack. The fact that the proton is such a small entity, it can infiltrate the bulk of the material without causing the kind of structural fatigue that leads to failure, as it turns out for these films."

That stability has been demonstrated through a series of rigorous tests. Huang claims that the material has been subjected to 10,000 polarity reversals without showing any signs of degradation.

Beach claims that the material has additional properties that may be useful in a variety of applications. He explains that the magnetic alignment between the individual atoms in the material works in a similar way to that of springs. The spring-like force pulls an atom back into alignment if it begins to move out of alignment with the others around it. When objects are connected by springs, they have a tendency to generate waves that can travel along the material between their points of connection. "In the context of this magnetic material, these are known as spin waves. You get oscillations of magnetization in the material, and the frequencies of these oscillations can be extremely high."

According to him, they are capable of oscillating in the terahertz range and thus are the only devices capable of generating or sensing extremely high-frequency electromagnetic radiation. "There aren't many materials that can do that."

It is possible to develop relatively simple applications of this phenomenon in the form of sensors within a few years, according to Beach, but more complex applications, such as data and logic circuits, will take longer, in part due to the fact that the entire field of ferrimagnet-based technology is still in its early stages.

He believes that, aside from these specific types of magnetic applications, the basic methodology could be applied to a variety of other applications. This is a method of controlling properties within the bulk of a material through the use an electric field, explains the author. "On its own, that's quite remarkable," says the author. Other work has been done on controlling surface properties using applied voltages, but the fact that this hydrogen-pumping approach allows for such deep alteration allows for "control of a broad range of properties," according to Dr. Chen. "This approach allows for control of a broad range of properties," he says.

Hyunsoo Yang, a professor of electrical and computer engineering at the National University of Singapore who was not involved in the research, says that 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." According to him, "this work applied the voltage control concept into a ferrimagnet to toggle the dominant sublattice, resulting in an effective magnetic bit writing." He believes that if the required voltage can be reduced while the speed is increased, this new method has the potential to "revolutionize the field."

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