Researchers discover new way to manipulate magnetism

In a pioneering effort to control, measure, and understand magnetism at the atomic level, researchers at the National Institute of Standards and Technology (NIST) have discovered a new method to manipulate the nanometric properties of magnetic materials.

The ability to control these properties has potential applications in the creation and improvement of magnetic memory in consumer electronics devices and in the development of a sensitive detector for magnetic nanoparticles.

The discovery focuses on a quantum mechanical property known as spin, which gives electrons a tiny magnetic field. The spin of electrons can point in either of two directions, “up” or “down,” just like the accompanying magnetic field. Over the years, scientists have become adept at reversing the direction of the spin and, therefore, the direction of the magnetic field. But the new discovery has a new twist.

In some materials, like cobalt, neighboring electron spins interact, causing them all to point in the same direction. If some of the towers are forced to move away from that direction, they pull some of the nearby towers with them. This causes the rotations to gradually twist clockwise or counterclockwise. In some materials, tendrils prefer to twist in one direction.

A team led by NIST researcher Samuel Stavis and Andrew Balk, now at the Los Alamos National Laboratory, found a way to control the direction of this twist in a cobalt film of barely three atomic layers. In addition, they could set this direction to be different in different places on the same cobalt film, regardless of the other magnetic properties of the metal.

The team achieved this new ability by controlling an effect known as the Dzyaloshinskii-Moriya Interaction (DMI), which imposes a preferred direction of torsion on the rotations. DMI typically occurs at the boundary between a thin film of a magnetic metal and a layer of non-magnetic metal. The spins of the electrons in the magnetic film interact with the atoms of the non-magnetic film, creating a preferential twist.

DMI control can augment magnetic memory, which uses the orientation of the rotation to store information. A memory device needs two distinct states, representing either a one or a zero – in the case of a magnetic hard drive, electrons with a spin pointing up or down. To write data, designers need a predictable way to switch from one rotation orientation to another. Controlling the direction and amount of twist could allow the spin flip to occur more efficiently and reliably than if the twist were random, notes Balk.

DMI control also plays a key role in another type of magnetic memory. If the DMI is strong enough, it will warp neighboring spins into a circular vortex pattern and could potentially create exotic magnetic nodes called skyrmions. These particle-like nodes can store information, and their existence or absence in a magnetic thin film could act a lot like ones and zeros in electronic logic circuits. By regulating DMI, researchers can create skyrmions, which are said to require less energy to function than other types of magnetic memory, and should be able to guide their movement through magnetic material.

The researchers describe their work in Physical Review Letters.

In their experiment, the researchers sandwiched a thin film of cobalt between two layers of platinum, a non-magnetic metal. They then bombarded the trilayer with argon ions, which destroyed the upper platinum film and roughened the upper boundary between platinum and cobalt, depending on the energy of the ions. Scientists found that when they used argon ions with higher energy, the DMI was negative, twisting the spins of the cobalt counterclockwise, and when they used argon ions with higher energy. lower, the DMI was positive and twisted the spins clockwise. . When exposed to mid-energy argon ions, the DMI was zero, making it just as likely that the spins would spin clockwise or counterclockwise.

The researchers made their discovery by adjusting the magnetic properties of a cobalt film to develop a sensor for magnetic nanoparticles. In doing so, the team realized that they had found a new way to manipulate DMI.

Because argon ions with different energies could target specific regions of the cobalt, the researchers were able to make films of cobalt with varying IMDs across the surface of the material.

“Six decades after Dzyaloshinskii and Moriya discovered this interaction, our new process to control it spatially, independent of other magnetic properties, will enable further scientific studies of DMI and enable the fabrication of new nanomagnetic devices,” said Balk.

Finally, the scientists found that controlling the DMI actually made the film more sensitive to the magnetic fields of the nanoparticles. At a later date, the team plans to publish work on the film’s application as a nanoparticle sensor for users at the NIST Center for Nanoscale Science and Technology, where the work was performed.