Local polarization dynamics in ferroelectric materials

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Rep. Prog. Phys. 73 (2010) 056502 S V Kalinin et al in which the presence and motion of ferroelastic 71 ◦ and 109 ◦ anodic oxidation by AFM [275] and strongly affect SPM domain walls lead to improved electromechanical properties [265]. In ferroelectric–ferromagnetic heterostructures, non- 180 ◦ switching opens up an effective pathway for strainmediated multiferroic coupling [266] and strain-induced metal–insulator transitions in the second component [267]. In materials such as BiFeO 3 (BFO) enhanced conduction at 180 ◦ and 109 ◦ domain walls has been reported, raising the possibility for novel electronic and memory devices [268]. Finally, given that the magnetic ordering in BFO is ferromagnetic in (1 1 1) planes and antiferromagnetic between the planes, control of non-180 ◦ switching is crucial for the development of exchange-coupled devices [266]. In multiferroics, the control of magnetic and strain order parameters can be achieved by non-180 ◦ polarization switching. In addition, this will allow exotic polarization orderings such as vortex states and other topological defects to be created directly. However, simple symmetry considerations, namely the fact that the tip-induced field is rotationally invariant, forbid direct field manipulation of nonferroelectric order parameters. This fundamental limitation is belied by the number of phenomenological observations of non-180 ◦ switching in BFO. Cruz et al have explored domain switching in BFO using a biased SPM tip and have shown that for BFO grown on (1 1 0) STO, 180 ◦ walls are formed at lower voltages and 109 ◦ domain walls form at higher voltages [269]. The stability of otherwise unstable 109 ◦ domains was increased by the creation of a stack of 180 ◦ domain walls between a 71 ◦ domain wall and a 109 Figure 26. (a) Piezoresponse images of a twinned PZT film after domain wall (domain wall architecture). Zavaliche poling. The ‘out-of plane’ PFM signal is shown on the left and the et al have demonstrated that both ferroelectric and ferroelastic ‘in-plane’ signal is shown on the right. The polarization directions switching is possible during area poling of BFO grown on are shown schematically in the sketches below the PFM images. (0 0 1) STO; however, no further control of the switching (b) The expected scenario of polarization switching and relaxation process was reported [270]. The first control of the in-plane in polydomain PZT films: (upper left) the initial c/a/c domain state, (upper right) the domain configuration during tip-induced 180 ◦ domain switching on (0 0 1) oriented BFO was done by Shafer switching, (lower left) forward growth of the c domain following the et al by using an in-plane electrode structure [271]. 90 ◦ rotation application of the field, (lower right) lateral motion of the 180 ◦ of the stripe-like domains at the edges of the in-plane electrodes domain boundaries during backswitching. Reproduced from [161], copyright 2002, American Institute of Physics. where the electric fields are inhomogeneous was observed. However, this approach is limited to switching between two domain patterns, whereas BFO provides eight possible sets [162, 163], in which antiparallel switching was presumably of stripe domain patterns. Overall, the SPM single point, triggered by elastic interactions [262]. Further examples of anomalous nucleation behavior were observed and analyzed theoretically [263]. In tetragonal (1 0 0) films, the formation of a 180 ◦ domain is associated with an elastic strain due to the piezoelectric coupling in the tip field. This strain can be compensated by the SPM area poling, capacitor-based and in-plane electrode-based switching experiments suggest that polarization switching can proceed along many competing pathways in a seemingly uncontrollable fashion. Recent studies by Balke et al [272] have demonstrated that in-plane domain engineering can be achieved using symmetry breaking by a moving SPM tip. formation of 90 ◦ twin domains. This behavior was observed by Chen et al [264] as shown in figure 28. Remarkably, 3.2.5. Anomalous local switching and ambient effects. both the formation of stabilizing 90 ◦ twins and the formation of a surface depression after the bias is off (to compensate One of the fundamental differences between the SPMbased and capacitor-based switching is the effect of the for strain) are observed. These observations illustrate the environment. Even in capacitors, the poor interface quality surprising ease with which non-180 ◦ domains can form (note that observations in figure 28 imply that not only ferroelastic can result in rapid degradation, as well as significantly affect electrostatic field distribution due to the dead layer domains are formed, but also remain stable in the zero-field effect [273, 274]. In comparison, in SPM experiments, the state). tip is contacted with surfaces with water layers resulting Recently, much interest has been directed toward from exposure to an ambient environment. Water layers applications of rhombohedral ferroelectrics and multiferroics, are known to be responsible for phenomena such as 27
Rep. Prog. Phys. 73 (2010) 056502 S V Kalinin et al Figure 27. (a) Piezoresponse image of a PZT sample showing as-grown (light contrast) and reverse-poled (dark contrast) regions. (b)–(f ) Piezoresponse images from the poled region in (a), which was switched into the opposite polarization (with respect to the as-grown state) state by scanning the surface with the tip biased at 210 V. Images taken after wait times of (b) 6.1 × 10 3 ,(c) 9.2 × 10 3 ,(d) 2.4 × 10 4 , (e)1.1 × 10 5 and (f )2.8 × 10 5 s. Reproduced from [255], copyright 2001, American Physical Society. Figure 28. PFM domain structure (a) before and (b) after locally poling a PZT film. (c) Plan view and (d) 3D view of domain structure. (e) PFM domain structure image (e) before and (f ) after switching a PZT film. Reproduced with permission from [264], copyright 2004, American Institute of Physics. surface charge dynamics measurements [276], tip–surface adhesion forces [277], electrostatic tip–surface interactions [278, 279], tip-induced electrochemical reactions [280], etc. Variable temperature measurements of the surface potential above ferroelectric surfaces have established the fact that polarization is screened by mobile charges in air [281] and allow kinetic and thermodynamics parameters of the screening process to be established [282]. These observations have been corroborated by detailed surface studies [283]. For most perovskites, the surfaces are highly reactive [284, 285]. Furthermore, dissociative water adsorption is one of the favored mechanisms for the screening of ferroelectric 28
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