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Spin interactions across ferromagnetic layers observed

By Frieda Wiley June 28, 2019
ferromagnetic layers
Schematic of the magnetic state at remanence of the synthetic antiferromagnets studied in a macrospin approximation: two ultrathin CoFeB (top) and Co (bottom) layers with Pt at the interfaces, separated by Ru to create antiferromagnetic coupling between both ferromagnetic layers. The two ferromagnetic layers have different proximities to their corresponding spin-reorientation-transition, with Co remaining out-of-plane and CoFeB becoming canted (tilted) with respect to the substrate plane. θ is the (polar) effective macrospin canting angle of this layer. Credit: Nature Materials

The interfacial Dzyaloshinskii-Moriya interaction (DMI) typically occurs in multilayered thin films of magnetic materials and is an antisymmetric exchange interaction, that is, the spins of electrons are aligned perpendicular to each other. This interaction initiates one sense of rotation of spins in the same ferromagnetic layer; in other words, it is a chiral intralayer interaction or coupling of spin states within a layer. A research group has now made the first experimental observations of interlayer DMI between neighboring ferromagnetic layers separated by a Ru spacer. This gives rise to new options and avenues for spin states that can be used for data storage. The results are published in Nature Materials.

“This study shows for the first time the presence of chiral spin interactions between two ferromagnetic layers separated by a thin non-magnetic spacer,” says Amalio Fernández-Pacheco of the Department of Physics at the University of Glasgow and one of the corresponding authors of this study. “So far, this type of interaction—favoring the formation of advanced spintronic textures such as skyrmions, chiral domain walls, and spin spirals—had been only observed between spins of the same ferromagnetic layer (intralayer), but this study expands this type of interaction, becoming now possible to couple neighboring ferromagnetic layers (interlayer) in a chiral fashion.”  

Magnetic layers in the form of thin films such as those used in this study are very similar to the ones currently used in giant magnetoresistance (GMR) sensors used in hard drives and growing random access memory (MRAM). Traditionally, conventional data storage techniques employed up and down ferromagnetic states. Manipulating magnetic interactions is of particular interest to the emerging field of three-dimensional nanomagnetism and spintronics.

Both intra- and interlayer DMI promote the chiral coupling of spins found in the various ferromagnetic layers via paramagnetic atoms located in the interlayers housed between ferromagnetic layers. While the existence of non-negligible interlayer DMI between neighboring FM layers separated by a spacer has been predicted, the ephemeral nature of this interaction along with the need for appropriate crystallographic symmetry has hindered the ability to experimentally observe the phenomenon, until now.

Fernández-Pacheco says that unconventional engineering of their multilayered system allowed for “canted spin configurations”—which is when the spins are tilted rather than parallel to one another—that provided the symmetry needed to observe the novel interlayer chiral spin interaction. The researchers used two ultrathin magnetic layers made of Co and CoFeB and utilized Pt to provide the paramagnetic property between the ferromagnetic layers.

“In my opinion the most intriguing aspect of the Dzyaloshinskii-Moriya interaction (DMI) between layers is the prospect to design three-dimensional arrays of topological objects,” says Kirsten von Bergmann, a senior scientist at the Department of Physics at the University of Hamburg. “The DMI within a layer can stabilize isolated skyrmions in magnetic thin films, which can be moved within the film plane by currents.”

“The key advantage is not to create clockwise or counterclockwise rotation, but rather to be able to create that or another rotation on request,” says Elena Vedmedenko, a professor at the Department of Physics at the University of Hamburg and second author of the study.

The study’s researchers plan to explore the range of systems where the DMI may occur along with methods to increase the strength of the interaction.

Read the abstract in Nature Materials.