Photo: Siddharth Kankaria/ Research Matters
Ferroelectric materials carry a spontaneously polarized charge within their crystalline structure that can be reversed by the application of an electric field. As scientists try to shrink them to nanometre sizes, these materials often lose their polarization. Now, a collaborative research team from India and Germany has observed an unexpected effect in the ferroelectric alloy of bismuth ferrite and lead titanate (BiFeO3-PbTiO3). They have found that mechanically grinding this material to smaller sizes actually leads to a different atomic arrangement - a new structural phase that retains the polarization with slight alteration. This discovery opens up interesting possibilities for using this ferroelectric material in a variety of miniaturised devices - computer memory, RFIDs, sensors and actuators.
The team closely studied a ferroelectric alloy of 71% bismuth ferrite and 29% lead titanate, a synthetic compound that belong to the mineral family of calcium titanium oxide (CaTiO3). The team found that around 10 micron (i.e. 10,000 nanometres) sized particles of their alloy show tetragonal crystal structure (a rectangle prism with a square base), with polarization parallel to longer axis of the structure. But when these particles are reduced to 0.5 microns (i.e. 500 nanometres) in size, their crystal structure suddenly flips to rhombohedral (a cube with its faces rhombi instead of squares) with polarization parallel to the body diagonal. Unlike in most situations where size effects come into play as the dimension is reduced to a few tens of a nanometre, the change in ferroelectric property in the present material occurs well above the nanometre range.
"To be frank, it was an unexpected discovery in the first place!", exclaims Prof. Rajeev Ranjan from the Department of Materials Engineering at the Indian Institute of Science (IISc), the corresponding author of this study. "Initially we felt that the system was behaving erratically with regard to the formation of phases", recalls Prof. Ranjan on their early frustration in understanding this phenomenon. He teamed up with researchers from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCSAR), Bangalore and the Technical University of Munich (TUM), Germany, to crack this problem. It took many months of systematic experiments and analyses to eliminate all the external factors and establish the effect of just the size on polarization and magnetization.
Understanding the intrinsic effects of size on ferroelectric materials has been a long standing problem in ferroelectrics research. Scientific literature abounds with various theories on what happens to ferroelectric materials once shrunk below a critical size, typically in the range of nanometres. Many studies observe that the polarization vanishes, and this is undesirable for their use in nanodevices. In the past, size effect studies have used ferroelectric materials either as nanometre-sized thin films or as a collection of nanoparticles made through wet-chemical combinations.
However, both these forms add extrinsic factors that can affect polarization – the behaviour of thin films can be affected by the substrate on which it is grown, while nanoparticles prepared by wet-chemistry based techniques carry residues of chemicals used. To find the true intrinsic effect of size on a ferroelectric material, it was important to adopt a strategy free from the extrinsic factors. The scientists broke the alloy particles by hand grinding them in a mortar-pestle, and were startled to see the drastic change in the ferroelectric property resulting from the rotation of the polarization.
"Most theoretical work has dealt with size effect in ferroelectrics to rationalize the vanishing of ferroelectricity at small sizes. Ours was not vanishing, but changing from one ferroelectric phase to another," reiterates Prof. Ranjan. The team carried out detailed structural evaluation through experiments and simulations to find the underlying reasons for such a transformation. They found that one reason for observing this phenomenon in very large size range of the alloys is the extremely large polarization and the spontaneous strain.
"In miniaturized devices such a multilayer capacitors or micro-electro-mechanical systems (MEMS), ferroelectric materials are used in sub-micron size range. Our results are very significant for understanding the properties of such miniaturized devices," points out Prof. Ranjan.
The team reports that this alloy system can do even more on shrinking. Along with a switch in the structural and polarization state, a magnetic order was also born in the material The material therefore becomes magnetoelectric-multiferroic, which are interesting materials for new kind of memory devices, where information can be written electrically and read magnetically and vice-versa. Though various research groups have studied this alloy since 1960, there was no consensus with regard to the phase stability. "After six decades, our research now explains the exact reason for the differences between different groups. None bothered about size as seriously as we did, and this made the difference!", remarks Prof. Ranjan.
About the author:
Prof. Rajeev Ranjan is an Associate Professor at the Department of Materials Engineering, Indian Institute of Science, Bangalore, India. Email: rajeev@materials.iisc.ernet.in
About the paper:
The paper titled “Interferroelectric transition as another manifestation of intrinsic size effect in ferroelectrics” was recently published in the journal “Physical Review B” by the American Physical Society. This work was primarily funded by the Nanomission Programme of the Department of Science and Technology, Government of India.