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The Beauty of an Imperfect Fit

June 13, 2019

Michael Fitzsimmons

While beauty and symmetry are often correlated, there are spaces just a few atoms thick where a little symmetrical mismatch can be a much-desired quality. That’s the case with research recently published in Physical Review Letters, wherein Joint Faculty Professor Michael Fitzsimmons (pictured right) and colleagues describe how a less-than-perfect alignment can influence magnetization in nanoscale materials.

The scientific team studied a thin film of lanthanum, cobalt, and oxygen (LaCoO3, or LCO for short) sandwiched between two strontium oxide layers (SrTiO3, better known as STO layers). LCO is of interest for several reasons. As a ferroelastic it’s a member of the largest class of ferroic materials—those that can switch properties under an external stimulus. These materials are vital for applications like smart mechanical switches or vibration sensors. Cobalt itself has interesting electronic states that transition from low to intermediate or high magnetic spins as the temperature increases, and temperature plays a key role in its magnetic susceptibility. Understanding how magnetism originates in LCO materials is a key step to controlling their properties.

In this work, researchers used polarized neutron reflectometry (PNR) to study tensile-strained LCO films, which exhibit ferromagnetism at low temperatures. The strain, as Fitzsimmons explained, means, "that the atoms are displaced from their equilibrium positions—at least from the positions we expect in the bulk."

This displacement is caused by a symmetry mismatch at the interface where the STO and LCO layers meet. The oxide layers have atoms arranged in a cube-like lattice, whereas the cobalt film’s atoms are structured like a rhomboid. To compensate for this mismatch, the LCO layer has to undergo a bit of distortion at or close to the interface, kind of the way a wooden fence dividing a meandering property line requires shifting a few slats.

As Fitzsimmons said, "Maybe one person starts putting slats from one side and another person from the other side (and) when they meet the last slat probably doesn’t fit, so there’s a gap."

By using PNR they were able to measure the distribution of atomic density and the magnetization in the cobalt films, where they found that there was less magnetism but greater atomic density near the interface with the oxide layers, while the films’ interior showed the opposite. Further, when hydrostatic pressure was applied to the system, magnetism was dramatically reduced both in the film’s interior and at the interface with the oxides. Once the pressure was removed, the magnetization was fully recovered.

The research show that in these LCO films, strain and the resulting distortion are strongly correlated with the material’s magnetization. They also show that the change in magnetization in relation to pressure is a key property of the strained LCO film. Revealing the underlying properties of cobaltite thin films shows promise for how these materials can be engineered and controlled for applications. Studies like these also expand the use of scientific tools: for example, Fitzsimmons explained this is the first time polarized neutron reflectometry has been combined with studies of high pressure cells.

The results were published last month in "Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure". Among the authors is Zac Ward (UT Physics PhD, 2008), who is with the Oak Ridge National Laboratory Materials Science and Technology Division.

The paper is an extension of research published in Science Advances earlier this year where Fitzsimmons and colleagues presented results showing strained LCO films form twin domains that open possibilities for fine control of magnetism in these materials. (Read the University of Tennessee press release.

The more researchers learn about how magnetism emerges, disappears, or can be controlled and directed in nanoscale systems like thin films, the greater the opportunities for applying those findings to areas like data storage and sensor development—just one example of how imperfection can be a thing of beauty.

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