Chiral Domains in Cycloidal Multiferroic Thin Films: Switching and Memory Effects

Cycloidal magnetic order occurring in some ${\mathrm{AMnO}}_3$ perovskites is known to induce ferroelectricity. The polarization is perpendicular to the propagation vector direction of the cycloid and its chirality, and therefore it is directly related to the chiral domain structure. We show that the switching process of chiral domains is sensitively dependent on the magnetoelectric history of the sample. Moreover, by appropriate field cycling, magnetic order can display partial chiral memory. We argue that memory results from electric field coupling of cycloidal domain and nucleation and pinning of chiral domain walls, much like the domain structure in other ferroic systems.

Figure 1

(a) Sketches of the $bc$ and $ac$ cycloids (left and right panels); the degenerated polar ($\mathbf{P}$) and chiral ($\mathbf{Q}$) vectors are indicated. (b) Temperature dependence of the magnetic field ($H_f$) required to flop cycloids from $bc$ to $ab$ planes.

Figure 2

(a) Scheme of the polar state after the $E$-ZFC and the $H$-ZFC process and subsequent application of $\mathbf{H}(6\mathrm{T})\parallel \mathbf{c}$. Solid and striped arrows represent the relative fraction of ferroelectric domains measured using the ND and PU signals of (b) and (c), respectively. (b),(c) Switching current versus electric field measured using PUND (squares), ND (down triangles) and PU (up triangles). The meshed area represents the total amount of switchable charge (PUND) and the solid areas the switched charge during PU or ND measurements.

Figure 3

(a) Scheme of the polar state after $E$-ZFC and $H$-ZFC process, $E$ poling ($\mathbf{E}\parallel \mathbf{a}$) and subsequent application of $\mathbf{H}(6\mathrm{T})\parallel \mathbf{c}$. Solid and striped arrows represent the relative fraction of ferroelectric domains measured using the ND and PU signals of (b) and (c), respectively. (b),(c) Switching current versus electric field measured using PUND (squares), ND (down triangles), and PU (up triangles). The meshed area represents the total amount of switchable charge (PUND) and the solid areas the switched charge during PU or ND measurements.

Figure 4

Switching current obtained by PUND (squares, meshed areas), PU and ND (triangles, solid area) measurements, respectively, at 4.2 K, after the $\mathbf{E}\parallel \mathbf{a}$ poling process under various magnetic fields: (a) 5 T and (b) 8 T.