UCLA

Strain-coupled multiferroic heterostructures provide a path to energy-efficient, voltage-controlled magnetic nanoscale devices, a region where current-based methods of magnetic control suffer from Ohmic dissipation. Magnetoelectric coupling behavior in such composite heterostructures has thus been of substantial interest for scientific research and applications. As the dimension of the devices scale down, novel physical phenomenon emerges and thus also requires further understanding of both the magnetization and strain behavior at micro- and nanoscale.

When it comes to the magnetization behavior, there has been a growing interest in highly magnetoelastic materials, such as Terfenol-D, prompting a more accurate understanding of their magnetization behavior. To address this need, we simulate the strain-induced magnetization change with two modeling methods: the commonly used unidirectional model and the recently developed bidirectional model. Unidirectional models account for magnetoelastic effects only, while bidirectional models account for both magnetoelastic and magnetostrictive effects. We found unidirectional models are on par with bidirectional models when describing the magnetic behavior in weakly magnetoelastic materials (e.g., Nickel), but the two models deviate when highly magnetoelastic materials (e.g., Terfenol-D) are introduced. These results suggest that magnetostrictive feedback is critical for modeling highly magnetoelastic materials, as opposed to weaker magnetoelastic materials, where we observe only minor differences between the two methods’ outputs. To our best knowledge, this work represents the first comparison of unidirectional and bidirectional modeling in composite multiferroic systems, demonstrating that back-coupling of magnetization to strain can inhibit formation and rotation of magnetic states, highlighting the need to revisit the assumption that unidirectional modeling always captures the necessary physics in strain-mediated multiferroics.

In terms of the strain behavior, there hasn’t been a system-level work that quantifies the strain distribution as a function of the electric field at these so-called mesocales level (100 nm- 10 um), in the range of the constitutive grain size, etc. To obtain mechanical properties at such length scale, including strain information, we used synchrotron polychromatic scanning x-ray diffraction (micro-diffraction) on beamline 12.3.2 at the Advanced Light Source of the Lawrence Berkeley National Lab.

With given ferromagnetic and ferroelectric components, it is the magnetoelectric coupling between the two that governs the interaction. In this work, we also demonstrate a method to enhance the coupling behavior between two existing components by interposing a polymer layer.