Effect of thickness and frequency of applied field on the switching dynamics of multiferroic bismuth ferrite thin films

Epitaxial films of multiferroic $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ (BFO) with different thicknesses are grown on $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ buffered $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (001) substrates using the pulsed laser deposition technique. The room-temperature polarization-voltage (P-V) hysteresis loop, leakage current and average domain size of films with varying thicknesses have been investigated. The frequency dependence of the coercive voltage and variation of the coercive voltage as a function of film thickness are ascertained to examine the validity of the Ishibashi-Orihara model and the semi-empirical Kay-Dunn scaling relationship, respectively. The results demonstrate that at room temperature the BFO films indeed follow the Ishibashi-Orihara (I-O) model and Kay-Dunn scaling law. The dimensionality of the domains acquired by piezoresponse force microscopy imaging further validates that found from the I-O model scaling exponent and is consistent with Kittel's law regarding dependence of domain width on film thickness. These combined results elucidate the fundamental properties that govern the switching kinetics of BFO and are relevant for BFO-based multiferroic materials in next generation logic and memory devices operating over a wide range of frequencies.

Figure 1

AFM images of 50 nm SRO//STO showing RMS roughness of about 200 pm (left) and 110 nm BFO//SRO//STO with RMS roughness of about 300 pm (right).

Figure 2

$\theta \text{−}2\theta$ XRD patterns for BFO films with different thicknesses grown on SRO//STO. Both BFO and SRO are crystallized as perovskite with a (00l) texture.

Figure 3

Random (red) and aligned (black) RBS spectra for a 110 nm BFO//SRO//STO heterostructure.

Figure 4

P-V hysteresis loops for 110 nm BFO film with applied frequencies of 1−20 kHz.

Figure 5

P-V hysteresis loops for a 110-nm BFO film with applied frequencies of 10–50 kHz.

Figure 6

Coercive voltage vs frequency plots for BFO films with different thicknesses. The solid lines represent power-law fit for the I-O model.

Figure 7

In-plane phase images for different thickness BFO films [10 nm (left) and 110 nm (right)]. Both films show uniform striped domains. The domain wall density decreases whereas width of domains increases with thickness.

Figure 8

Plot of domain width as a function of film thickness. The solid line is a power-law fit described by Kittel's law.

Figure 9

P-V hysteresis loops measured at 20 kHz for BFO films with different thickness. These measurements are performed at room temperature.

Figure 10

Plot of ${\mathrm{V}}_c$ vs thickness for BFO films using two different techniques. The solid lines are nonlinear power-law fit to the observed data described by Kay-Dunn law. The scaling exponents for both fits yield 0.33. The black line corresponds to coercive voltages obtained from phase loop using PFM for relatively thinner films whereas the red one is for thicker films measured at 20 kHz using a ferroelectric tester.

Figure 11

PFM lithograph images of logo of The University of Alabama written on a 40-nm thick BFO film; phase image (left) and amplitude image (right) patterned in 2 $\mu \mathrm{m}$×2 $\mu \mathrm{m}$ area.

Figure 12

Leakage current measured at room temperature for BFO films with different thickness.