Low-voltage magnetoelectric coupling in membrane heterostructures

Strain-mediated magnetic coupling in ferroelectric and ferromagnetic heterostructures can offer a unique opportunity for scientific research in low-power multifunctional devices. Ferroelectrics are materials that can maintain spontaneous and reversible electric polarization. Relaxor-ferroelectrics that exhibit high electrostriction are ideal candidates for ferroelectric layer constructs due to their large piezoelectricity. Although the properties of relaxor ferroelectrics are known, their mechanistic origins remain a mystery, giving rise to an enigmatic form of materials. In addition to that, thin films are ineffective from substrate clamping and can substantially reduce piezoelectric in-plane strains. In a new report now published in Science Advances, Shane Lindemann and a research team in materials science, and physics in the U.S. and Korea, displayed low-voltage magnetoelectric coupling in an all-thin-film heterostructure using anisotropic strains induced by the orientation of the material. The team used an ideal ferroelectric layer of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ abbreviated PMN-PT during this work and coupled it with ferromagnetic nickel overlayers to create membrane heterostructures with magnetization. Using scanning transmission electron microscopy and phase-field simulations, they clarified the membrane response to understand the microstructural behavior of PMN-PT thin films, to then employ them in piezo-driven magnetoelectric heterostructures.

Magnetoelectric (ME) coupling

The electric field control of magnetism also known as converse magnetoelectric coupling has potential for next-generation memory storage and sensing technologies. The PMN-PT material is of interest as a relaxor-ferroelectrics material for applications as the ferroelectric layer with a large piezoelectric composition. By coupling the relaxor-ferroelectric with a ferromagnet containing large magnetostriction, converse ME coupling can be achieved by transferring voltage-induced strain from the ferroelectric layer in to the ferromagnetic layer to result in the strain-mediated control of in-plane anisotropy, tunneling magnetoresistance, ferromagnetic resonance and conductivity. The recent drive towards low-power ME devices and the development of micro- and nanoelectromechanical systems has led to further study of relaxor-ferroelectric thin films. Reducing the thin-film dimensions of relaxor-ferroelectrics can induce a large reduction in piezoelectricity due to mechanical clamping, and scientists therefore aim to overcome this challenge successfully to integrate relaxor-ferroelectric thin films into high-performance devices. In this work, Lindemann et al. overcame the clamping issue and demonstrated low-voltage strain-mediated ME coupling in all-thin-film heterostructures. The work highlighted the microscopic nature of relaxor-ferroelectric thin films to present a crucial step toward their applications in low-power piezo-driven magnetoelectric devices.

Developing and characterizing membrane heterostructures

Lindemann et al. measured the strain-induced changes of magnetic anisotropy in the nickel overlayer using longitudinal magneto-optic Kerr effect (MOKE) hysteresis loops, as a function of PMN-PT bias electric fields. They then showed the significance of removing mechanical clamping by the substrate to achieve large anisotropic in-plane strains. To then understand the strain behavior inferred from the magneto-optic Kerr effect hysteresis, Lindeman et al. plotted the calculated magnetic anisotropy energy density, determined from the saturation field of hard axis loops, and the known differential strain based on the known magnetostriction of nickel. They then determined the domain structure of the as-grown PMN-PT membranes using scanning transmission electron microscopy. The single-crystalline material showed a columnar structure with lattice mismatch during the growth of the film. The findings resembled a mixed ferroelectric and relaxor domain structure consistent with the experimental model.

Phase-field simulations of PMN-PT membranes

To then understand the strain behavior of the PMN-PT membrane, the scientists next performed phase-field simulations. To measure the average strain, they calculated the strain contribution of individual spontaneous polarization elements, multiplied by the electrostriction tensor. The starting point of the simulation indicated the expected structure around the ferroelectric imprint of the experimental PMN-PT membrane. The results of the simulation qualitatively agreed with the experimental strain and polarizations measured in the PMN-PT/nickel membrane. While the strains calculated from the experimental MOKE (magneto-optic Kerr effect) loops exhibited a horizontal and vertical shift relative to the calculated strains from simulation, qualitatively, the two curves were similar.

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Outlook

In this way, Shane Lindemann and colleagues showed the low-voltage, strain-mediated, magnetoelectric (ME) effect in an all-thin-film heterostructure. The film only relied on the large anisotropic strains inherent to the PMN-PT thin films. The PMN-PT/nickel membrane used in this work achieved a robust, piezo-driven, 90 degree rotation of the in-plane magnetic anisotropy of the nickel overlayer under a small voltage of bias to result in strain anisotropy, controlled by the in-plane crystal symmetry of the PMN-PT film. Using scanning transmission electron microscopy, the scientists showed the microscopic structure of the PMN-PT membrane. Then using bulk PMN-PT, they showed how the material exhibited permanent switching between in-plane and out-of-plane polarization states; this behavior provided a desirable trait for memory storage. The work provides key insight to the microstructural behavior of PMN-PT thin film membranes to show their applications in magnetoelectric coupling devices, and also predict their use with a variety of other materials to discover previously unknown piezo-driven phenomena.

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