Like many other multiferroic oxides, bismuth ferrite has the crystal structure of perovskite, in which planes of oxygen atoms and heavy atoms (like bismuth) alternate with planes of oxygen atoms and lighter atoms (like iron). An iron atom is at the center of the basic bismuth ferrite cubic cell, and its position whether slightly off center in one direction or another plus the positions of affected bismuth atoms, gives rise to local polarization.
The bismuth ferrite films contained ferroelectric domains between 5 and 10 micrometers (millionths of a meter) in dimension, and, says Seidel, "we can map and even change the domain structure by using different scanning-probe mechanisms."
Bismuth ferrite is an insulator, but each domain has a distinct polarization or orientation of charge, which Seidel mapped using a piezoresponse force microscope (PFM): as the PFM probe moves across a sample, an alternating electric field in the tip gives rise to a detectable mechanical response in the sample, according to its polarization. The same PFM setup can control the nature of the local polarization of the film by applying a large enough voltage to switch it.
To map out the topographic features of the film surface, the scanning probe was used in atomic force microscopy (AFM) mode. In AFM the probe effectively "feels" its way across the surface, like a stylus in a record player feeling the grooves in a record. Moreover, by setting a voltage that is too slight to affect the polarization of the domains, the researchers can probe the electronic properties of the films.
In this mode (conducting AFM), Seidel measured the local conductivity of the sample across the domains and across domain walls. The domains were indeed nonconducting since bismuth ferrite i
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory