Strain-mediated magnetoelectric storage, transmission, and processing: Putting the squeeze on data

Initial strain-mediated magnetoelectric studies

In 1961, Astrov first measured the magnetoelectric (ME) effect in chromium oxide (Cr₂O₃),¹ verifying an effect predicted only a few years prior by Landau, Lifshitz, and Dzyaloshinskii.² Over the next 40 years, several new materials and some particulate composites displaying the ME effect were developed.^3–9 However, they mostly possessed small ME coupling that vanished above cryogenic temperatures, inhibiting their use in applied technologies. The field took a new direction in 2001 after Ryu et al. created a laminate composite sandwiching the piezoelectric material PZT (Pb[Zr_xTi_1–x]O₃) between two layers of magnetoelastic Terfenol-D (Tb_0.3Dy_0.7Fe_1.92).¹⁰ This composite approach worked at room temperature and exhibited ME coupling almost two orders of magnitude stronger than any previous work. Within only a few years, coupling up to seven orders of magnitude stronger than Cr₂O₃ had been reported using strain-mediated ME composites.^11–13

A strong understanding of the physics governing this phenomenon was critical to obtain strong ME coupling. Initial macroscale studies demonstrated the existence of optimal bias strain/field conditions,^14–18 reduced coupling due to shear-lag and demagnetization fields,¹⁹ and demonstrated that the largest coupling strengths were attained by exciting a mechanical resonance.^{11–13,17,20} Optimal bias conditions exist due to the nonlinear nature of the magnetoelastic component, in contrast to the linear piezoelectric material. Shear lag and demagnetization fields lead to inhomogeneous strain and magnetization distributions that prevent the whole composite from being in an optimally biased state. While ME materials by definition couple magnetic and electric fields, these findings indicate that the structural dynamics must be given equal consideration in strain-mediated ME composites. The macroscale findings have proven equally relevant at the micro-/nanoscale, and the strong coupling found in macroscale composites has translated into a highly energy-efficient method of controlling nanoscale magnetism. As a result, strain-mediated ME coupling is now actively being investigated as one of the most energy-efficient approaches to controlling nanoscale magnetism and may enable a paradigm shift in how we use magnets to store, transmit, and process information.

Strain-mediated control of magnetic domains

Initial experiments

The control of micro-/nanoscale magnetic domains has been extensively studied during the past decade....