Domain Wall Creep in Epitaxial Ferroelectric $\mathrm{P}\mathrm{b}\mathrm{(}\mathrm{Z}{\mathrm{r}}_{\mathrm{0.2}}\mathrm{T}{\mathrm{i}}_{\mathrm{0.8}}\mathrm{)}{\mathrm{O}}_{\mathrm{3}}$ Thin Films

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline $\mathrm{P}\mathrm{b}\mathrm{(}\mathrm{Z}{\mathrm{r}}_{\mathrm{0.2}}\mathrm{T}{\mathrm{i}}_{\mathrm{0.8}}\mathrm{)}{\mathrm{O}}_{\mathrm{3}}$ thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent $\mu$ close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior.

Figure 1

Schematic of a tetragonal perovskite ferroelectric, characterized by two oppositely polarized ground states, separated by an energy barrier $U_0$. For $\mathrm{P}\mathrm{b}\mathrm{(}\mathrm{Z}{\mathrm{r}}_{\mathrm{0.2}}\mathrm{T}{\mathrm{i}}_{\mathrm{0.8}}\mathrm{)}{\mathrm{O}}_{\mathrm{3}}$, the corner Pb ions and the center $\mathrm{T}\mathrm{i}/\mathrm{Z}\mathrm{r}$ ions are positively charged, and the face O ions are negatively charged.

Figure 2

(a) Domain size increases logarithmically with pulse widths longer than $\sim 20\mu \mathrm{s}$, and saturates for shorter times as indicated by the shaded area. In all cases, the domains are homogeneous and no random nucleation is detected, as shown by three piezoelectric images of domain arrays on a $370\AA$ thick film written with $50\mu \mathrm{s}$, 1 ms, and 100 ms voltage pulses. The scale is the same in all images. In (b), a large array shows that domains are centered only where the AFM tip was positioned during writing.

Figure 3

Domain wall speed as a function of the inverse applied electric field for 290, 370, and $810\AA$ thick samples. The data fit well to $v\sim \exp [-\frac{R}{k_{B}T}(\frac{E_0}{E}{)}^{\mu }]$ with $\mu =1$, characteristic of a creep process.

Figure 4

Piezoelectrically imaged high-density ferroelectric domain array at $28\mathrm{G}\mathrm{b}\mathrm{i}\mathrm{t}/\mathrm{c}{\mathrm{m}}^2$, written with 500 ns pulses.