Elastic strain engineering of ferroic oxides
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The strain game For at least 400 years, humans have studied the effects of pressure (hydrostatic strain) on the properties of materials.1 In the 1950s, it was shown that biaxial strain, where a film is clamped to a substrate but free in the out-of-plane direction, can alter the transition temperatures of superconductors2 (Tc) and ferroelectrics (TC).3 What has changed in recent years is the magnitude of the biaxial strain that can be imparted. Bulk ferroic oxides are brittle and will crack under moderate strains, typically 0.1%. One way around this limitation is the approach of bulk crystal chemists, to apply “chemical pressure” through isovalent cation substitution. A disadvantage of such a bulk approach, however, is the introduction of disorder and potentially unwanted local distortions. Epitaxial strain, the trick of the thinfilm alchemist, provides a potentially disorder-free route to large biaxial strain and has been used to greatly enhance the mobility of transistors4,5 (see the article by Bedell et al. in this issue), increase catalytic activity (see the article by Yildiz in
this issue), alter band structure6 (see the article by Yu et al. in this issue), and significantly increase superconducting,7,8 ferromagnetic,9–11 and ferroelectric12–16 transition temperatures. This approach, which we refer to as the “strain game,” is illustrated in Figure 1 for elastically strained films of oxides with the perovskite structure. Strains of about ±3% are common in epitaxial oxide films today,17–20 with the record to date being a whopping 6.6% compressive strain achieved in thin BiFeO3 films grown on (110) YAlO3.21–24 These strains are an order of magnitude higher than where these materials would crack in bulk.25–27
Strained SrTiO3 and the importance of suitable substrates The strain game for ferroics was ignited by the demonstration that an oxide that normally is not ferroelectric at any temperature can be made ferroelectric at room temperature through the application of biaxial strain.12 Such a gigantic shift in properties and TC had never before been clearly seen in any
Darrell G. Schlom, Department of Materials Science and Engineering, Cornell University and Kavli Institute at Cornell for Nanoscale Science; [email protected] Long-Qing Chen, Millennium Science Complex, Materials Research Institute, Penn State University; [email protected] Craig J. Fennie, School of Applied and Engineering Physics, Cornell University; [email protected] Venkatraman Gopalan, Materials Science and Engineering, Penn State University; [email protected] David A. Muller, School of Applied and Engineering Physics, Cornell University and Kavli Institute at Cornell for Nanoscale Science, Cornell; [email protected] Xiaoqing Pan, Department of Materials Science and Engineering, University of Michigan; [email protected] Ramamoorthy Ramesh, Oak Ridge National Laboratory; [email protected] Reinhard Uecker, Leibniz Institute for Crystal Growth, Berlin; [email protected] DOI: 10.1557/mrs.2014.1
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MRS BULLETIN • VOLUME 39 • FEBRUARY 2014 • www.mrs.org/
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