Using hydrogen ions to manipulate magnetism at the molecular level

A team of researchers from the Massachusetts Institute of Technology (MIT) and the US Department of Energy’s (DOE) Brookhaven National Laboratory determined how to use hydrogen ions, “pumped” from water in the air. at room temperature, to electrically test the magnetism inside a very thin sample of a magnetic material. This approach to manipulating magnetic properties could accelerate advances in computing, sensors and other technologies.

The research, described in the November 12, 2018 online edition of Natural materials, was carried out in part at Brookhaven’s National Synchrotron Light Source II (NSLS-II), a user facility of the DOE Office of Science. The measurements, taken on the Coherent Soft X-ray Scattering (CSX) beamline of the NSLS-II, were essential in revealing the microscopic mechanism involved, in particular the presence of hydrogen ions in the sample and their role in changes in the magnetic structure of the sample.

Towards mainstream spintronics

Among the many possible applications of this research is its potential to become a new platform for the developing field of spintronics, devices based not only on electronic charge but also on electronic spin, the intrinsic property of ‘an electron that makes it act like a little magnet.

Unlike standard electronics, which rely on complementary metal-oxide semiconductor (CMOS) technology (used to make each of the billions of transistors in a microchip), spintronic devices are built on magnetic materials, which contain atoms. magnetic such as iron or manganese. Spintronic devices can maintain their magnetic properties without a constant power supply, unlike standard microchips, and, because they generate much less heat, are more energy efficient.

“As CMOS technologies near the end of their roadmap, spin-based devices are widely sought after for the era beyond CMOS,” said study principal investigator Geoffrey Beach of MIT. , professor of materials science and engineering and co-director of the MIT Materials Research Laboratory. “One of the requirements for bringing spintronics into the mainstream is an efficient way to electrically control magnetism. Essentially, we’re trying to create a magnetic analog of a transistor.”

One approach to achieve this control is to insert ions into the structure which can move between layers and modulate its electromagnetic behavior. This is called magneto-ionic switching. Researchers have already given promising results, but the types of ions used in previous surveys have caused more problems than they solved. In this study, the team was able to remedy some of these problems by using hydrogen ions (H +), which are relatively harmless and also the smallest possible ions, making them ideal for inducing rapid changes induced by the electric field in solid state structures.

“Magnetoionic switching is an important route to the electrical manipulation of magnetism at low power,” said Wen Hu, principal investigator at Brookhaven, CSX beamline scientist. “Hydrogen-ion migration, controlled by electrical voltages, plays a key role in this research and could potentially lead to new applications of spintronic devices.”

X-rays confirm proton pump

Researchers demonstrated the use of hydrogen ions for reversible magnetoionic switching in a layered structure composed of a base of platinum, cobalt, palladium, gadolinium oxide and a gold contact for to crown it all. Palladium (Pd) is well known for its ability to store hydrogen in the “corners” of its atomic lattice. Placing a voltage across the sample, and alternating between a positive and negative voltage, can pump hydrogen in and out of the Pd layer, flipping the magnetism back and forth in the plane. This is the first time that scientists have demonstrated a reversible “hydriding” of a heavy metal.

To verify that hydrogen was inserted into the Pd layer, the group performed X-ray absorption spectroscopy (XAS) on the CSX beamline. CSX provides researchers with advanced soft X-ray imaging and scattering tools, and was designed to study the electronic texture and behavior of composite materials. With XAS, researchers can determine the local electronic structure around specific elements in their sample – even by detecting very small changes – due to the tunable nature of x-rays.

“We performed XAS measurements with a very small X-ray beam (about 100 microns) to target the active part of the engineered structure. We observed a marked shift in the Pd spectrum when changing the voltage applied to the sample. , which was a sign of the transformation of Pd to PdH, ”said Claudio Mazzoli, CSX beamline lead scientist.“ These measurements provided direct evidence of the microscopic mechanism occurring deep within the sample. Thus, we now know that the insertion of hydrogen into the device is the explanation for the changes in the magnetic properties of the sample detected by laboratory measurements. “

“This is a very new and unique method, and it opens up a whole new way to modulate magnetic fields in semiconductor devices, which could impact spintronics applications,” Hu said.