MWN:A Strong Ferroelectric Ferromagnet Representing MRSEC-IRG1 on “Strain-enabled Multiferroics”

1. Venkatraman Gopalan, Pennsylvania State Univ University Park, DMR 0908718 stretched MWN:A Strong Ferroelectric Ferromagnet Representing MRSEC-IRG1 on “Strain-enabled Multiferroics” DMR-0507146, and the MRSEC DMR-0820404 (Penn State), DMR-0520404 (Cornell), and DMR-0820414 (Ohio State). EuTiO3 DyScO3 substrate Ferroelectric Ferromagnets, the holy grail of multiferroic research, are rare. Further, the properties of what few compounds simultaneously exhibit these phenomena pale in comparison to useful ferroelectrics or ferromagnets: their spontaneous polarizations (Ps) or magnetizations (Ms) are smaller by a factor of 1000 or more. We have predicted a new route to ferroelectric ferromagnets—transforming magnetically ordered insulators that are neither ferroelectric nor ferromagnetic, of which there are many, into ferroelectric ferromagnets using a single control parameter: strain. The system targeted, EuTiO3, was predicted to simultaneously exhibit strong ferromagnetism (Ms ~ 7 µB/Eu) and strong ferroelectricity (Ps ~ 10 µC/cm2) under large biaxial compressive strain. These values are orders of magnitude higher than any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. We have confirmed these predictions experimentally, demonstrating the genesis of a strong ferromagnetic ferroelectric and pointing the way to high temperature manifestations of this giant spin-phonon coupling mechanism. This work is a classic example of the highly collaborative and inter-dependent work enabled by the NSF-MRSEC centers. Ferroelectric Loop Ferromagnetic Loop Nature, 466, p. 954-958 (2010)

2. Elementary! Perspectives of a kindergartener Venkatraman Gopalan, Pennsylvania State University, DMR 0908718, 0820404 Teja Gopalan, now 5 years old, assembled this periodic table with her father, starting from when she was 4 years old. Elements have become her best friends. She has nicknames for them, such as “silly” for silicon and “vinni” for vanadium. Her favorite element is “Hydro, the baby”. Teja is fond of learning which elements are in everything around her, like salt, water, and air. She loves singing Tom Lehrer’s song about the periodic table with a guitar in hand. When she joined Park Forest Elementary as a kindergartener this Fall (2010), she asked, “Why is my school called elementary, Daddy?.” That question crystallized for me why periodic table should be introduced to every child early. “Just as elements are the building blocks for everything, you will learn alphabets and numbers in your school, which are the building blocks for language and knowledge” I replied. She thought about it, and observed “Elements are also made of alphabets and numbers”. Indeed!

3. New Insights into Ferroelectrics: Bloch and Néel-like Walls Venkatraman Gopalan, Pennsylvania State University NSF DMR-0820404, 0507146 and 0602986. Ferroelectric 180° domain walls are well-known to be predominantly Ising-like, which are distinct from magnetic walls which are Bloch and Néel type. Or are they? Using density functional theory, and molecular dynamics simulations, we show that the classic 180° domain walls in prototypical ferroelectrics lead titanate PbTiO3 and lithium niobate LiNbO3 (figure right) have mixed Ising, Bloch- and Néel-like character. Phase-field modeling show that such mixed wall character can be dramatically enhanced in nanoscale thin film heterostructures such as BaTiO3 / SrTiO3, where the internal wall structure can form Bloch and Néel-like polarization vortices that are broad and very tunable under fields. Highly tunable dipole-spring ferroelectrics (similar to the well-known exchange spring magnets) are envisioned. Physical Review B 80, 060102(R) (2009)

4. Long Range Influence of a Narrow, Soft Ferroelectric Wall Venkatraman Gopalan, Pennsylvania State University NSF DMR-0820404, 0908718, 0602986. Domain wall dynamics in ferroic materials underpins functionality of data storage and information technology devices. Using localized electric field of a scanning probe microscopy tip, we experimentally demonstrate a surprisingly rich range of polarization reversal behaviors in the vicinity of the initially flat 180° ferroelectric domain wall in LiNbO3. The nucleation bias is found to increase by an order of magnitude from the wall (center of the plot) to the bulk region (edges of the plot). The wall is thus significantly ferroelectrically softer than the bulk. Another surprising aspect is that though the wall itself is sharp on the unit cell scale (1nm), it profoundly affects switching on length scales of the order of 4-5 micrometers, ~1000 times larger than the wall width. These results show that the local properties of a wall are extremely important to understand macroscopic properties of ferroelectrics. The role of a single domain wall in ferroelectric switching is thus similar to the role of a dislocation in mechanical deformation, in that both dramatically lower the threshold field (or stress) to switch (or deform) the material by an order of magnitude. A map of nucleation field to switch domains in the vicinity of a single ferroelectric wall (Center) in a LiNbO3 crystal. Note the dramatically lower threshold at the wall , Vi as compared to bulk Vb. Also note the influence of the narrow wall over +/-4 microns away from the wall (red line). Phys. Rev. B 82, 024111 (2010)

5. Collaboration with Clarion University, an Undergraduate Institution Vasudeva Rao Aravind , Clarion University Venkatraman Gopalan, Pennsylvania State University NSF DMR- 0908718 Clarion University is a primarily undergraduate education institution. Tthrough supplemental funding from NSF for a Penn state project on ferroelectrics, Clarion undergraduates are being exposed to scanning probe microscopy tools and electronic materials. Research projects for undergraduates provide an excellent opportunity to gain hands-on laboratory experience in a cooperative environment. The aim of this project was to resolve the feature size of nanometer sized line patterns of barium titante, patterned using soft-eBL technique. In this work, Miller has demonstrated using atomic force microscopy that the lateral dimensions of these patterned lines are of the order of 150 nanometers and vertical dimensions are of the order of 100 nanometers. Undergraduate student Hope Miller presenting her project ‘Feature size of soft eBL grown barium tianate nano-patterns studied using Atomic Force Microscopy’ at the National Conference on Undergraduate Research’ conference in La Crosse, WI, 2010

6. Outreach to Home School Students in Clarion, PA Vasudeva Rao Aravind , Clarion University Venkatraman Gopalan, Pennsylvania State University NSF DMR- 0908718 ‘Learning by doing’ is the basis of the widely accepted way of learning physics called ‘physics by inquiry’. Students tend to gain deeper understanding of physical concepts when they perform experiments independently, following a few instructor led conceptual questions. To promote this learning technique, Home schooled students of Clarion and Dubois area were invited for a physics class at Clarion University. By performing experiments with optics kits, the students gained first hand knowledge of the laws governing reflection of light from a surface. Home school students of Clarion area conducing experiments to understand laws of reflection using different reflecting surfaces at Clarion University.