Influence of dislocations and twin walls in ${\mathrm{BaTiO}}_3$ on the voltage-controlled switching of perpendicular magnetization

We investigate the influence of dislocations and twin walls in ${\mathrm{BaTiO}}_3$ on its ferroelectric response and the resulting effect on the perpendicular magnetic anisotropy (PMA) of a strain-coupled ${\left[\mathrm{Co}\setminus \mathrm{Ni}\right]}_{\mathrm{n}}$ film. A dense twinned structure in conjunction with a high dislocation density significantly reduces the converse piezoelectric effect of ${\mathrm{BaTiO}}_3$ by hindering the propagation of newly nucleated domains with an applied electric field. This, in turn, results in a modest reduction of the PMA of the ferromagnetic layer. On the other hand, the ferroelectric polarization reorients from [100] to [001] direction in a dislocation-free ${\mathrm{BaTiO}}_3$, inducing the maximum achievable in-plane compressive strain of 1.1%. A large fraction of this uniaxial strain is transferred to the magnetoelastically coupled ferromagnetic layers whose magnetization switches to in plane via the inverse magnetostriction effect. This work reveals the critical role of the interplay between twin walls and dislocations within a ferroelectric substrate in the performance of multiferroic heterostructures and provides insight into the development of highly energy-efficient magnetoelectric devices.

Figure 1

(a) Orientation of the ${\mathrm{BaTiO}}_3$ tetragonal cell in the ferroelectric $a$ and $c$ domains; the black arrows indicate the direction of the ferroelectric polarization. [(b),(c)] Schematic illustrations of the ferroelectric domain configuration in BTO-1 (b), which is characterized by a dense $a1\text{−}a2$ twinning and random $c$ domains, and in BTO-2 (c), with a predominantly $a$-domain configuration alternated with random $c$ domains.

Figure 2

Laue diffraction patterns of (a) BTO-1 and (b) BTO-2. Each spot in the Laue pattern is a reflection generated by a given crystallographic set of parallel planes which can be also defined by their normal; an arc of spots in the Laue pattern corresponds to a set of plane normals that are coplanar. The peaks marked in yellow frames are zoomed in on the top right and three-dimensional surface plotted on the bottom right.

Figure 3

${\mathrm{BaTiO}}_3$ tetragonal cell orientation with respect to the direction normal to the sample surface (i.e., [001] direction), before and after applying an electric field along the [001] direction of BTO-1 [(a),(b)] and BTO-2 [(c),(d)]. The red frame in (b) indicates a small $c$ domain nucleated at $0.7\mathrm{MV}{\mathrm{m}}^{-1}$. (e) An angle of 90 ° is equivalent to $a$ domains and 0 ° is equivalent to $c$ domains. (f) In-plane lattice structure in $a$ domains (yellow) and $c$ domains (blue). The oxygen atoms have been removed from the scheme for simplicity.

Figure 4

Modulation of the out-of-plane magnetization in ${\left[\mathrm{Co}\setminus \mathrm{Ni}\right]}_4$ multilayers controlled by a voltage applied along the [001] direction of BTO-1 (a) and BTO-2 (b). The inset in (a) shows the dependence of the out-of-plane coercive field on the electric field, which was varied in the order indicated by the arrows.