© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 6335 wileyonlinelibrary.com COMMUNICATION Giant Enhancement of Ferroelectric Retention in BiFeO 3 Mixed-Phase Boundary Yen-Chin Huang, Yunya Liu, Yi-Tsu Lin, Heng-Jui Liu, Qing He, Jiangyu Li, Yi-Chun Chen,* and Ying-Hao Chu Y.-C. Huang, Y.-T. Lin, Prof. Y.-C. Chen Department of Physics National Cheng Kung University Tainan 70101, Taiwan E-mail: ycchen93@mail.ncku.edu.tw Prof. Y. Y. Liu School of Materials Science and Engineering and Key Laboratory of Low Dimensional Materials & Application Technology of Ministry of Education Xiangtan University Xiangtan, Hunan 411105, China Dr. H.-J. Liu, Prof. Y.-H. Chu Department of Materials Science and Engineering National Chiao Tung University Hsinchu 30010, Taiwan Prof. Y.-H. Chu Institute of Physics Academia Sinica Taipei 11529, Taiwan Prof. Q. He Department of Physics Durham University Durham DH1 3LE, UK Prof. J. Y. Li Department of Mechanical Engineering University of Washington Seattle, WA 98195–2600, USA DOI: 10.1002/adma.201402442 unstable polarization state. Such an asymmetry is mainly due to different out-of-plane boundary conditions, such as a nano conducting-tip contact and a bottom electrode. Effects of depo- larization fields in the unstable domain become significant when the polarization bound charges are not fully screened. [9] Although efforts on related studies have shown their ways to reduce the energy difference of the polarization double-well by controlling chemical environment, [10] breaking the out-of-plane compositional symmetry, [11–13] or using strain gradient, [14] fer- roelectric retention is still a key issue yet to be solved. In recent studies, researchers have found out that when a BiFeO 3 (BFO) film is under a large compressive strain (>4%), the BFO crystal structure transforms into a mixed-phase state of rhombohedrally (R-) and tetragonally (T-) distorted monoclinics. The T/R phase ratio can be manipulated by external voltage, film thickness, or temperature. [15–18] In the mixed-phase BFO film, the stripe-shaped R-BFO is embedded in T -BFO matrix, forming periodic domain pattern with interesting physical properties, such as the large piezo/ferroelectric response [19–21] and the non-zero magnetic moment. [22–24] These mixed-phase boundaries are formed to minimize the elastic and electrostatic energies. In order to shed a light on the retention problem, we intend to use mixed-phase boundaries in BFO as pinning centers for ferroelectric relaxation. Domain wall motions are usually pinned by different kinds of defects, such as charged vacancies, [25–27] dislocations, [28,29] and transient layers with low-magnitude polarization around ferroelastic domains. [30] In this study, the elastic energy term at phase boundaries as top- ological defects plays an important role to enhance the reten- tion of nano-sized ferroelectric domains. We found out that a stable state of the reversed domain can be achieved when it is pinned by the phase boundaries. The in-plane periodic elastic potentials, instead of out-of-plane electric variables, are used to keep the domain stable. Great improvement on the retention in the mixed-phase system opens a new avenue to ferroelectric retention and the possible application in nanoscale, nonvola- tile memory and spintronics. Moreover, the concept of creating long-retention domains by using periodic elastic potential sug- gests a new way to design strain-mediated FTJs. The relaxation behaviors of switched domains in the strained BFO film vary with domain-located regions. In topo- graphic images of Figure 1a, the flat region is the T-BFO matrix (T-matrix), while the stripe area is the mixed-phase region, where R-BFO is periodically embedded in the T-BFO matrix. [22] The out-of-plane (OP) polarization component of the as-grown film directed downward, i.e. toward the substrate, as shown by the bright contrasts in out-of-plane piezoresponse force micro- scope (OP-PFM) phase images of Figure 1b. As-grown domains Ferroelectric nano-domains, in which the spontaneous polariza- tion can be controlled by external fields, have attracted consid- erable interests as media of nonvolatile functional devices. One of the most promising applications is the ferroelectric tunnel junction (FTJ) where the resistance can be modulated along with the polarization orientation, and the resistive readout scheme is non-destructive. [1,2] A large on/off current ratio due to the tunneling electroresistance (TER) effect has been dem- onstrated in nanoscale FTJs by scanning probe microscopy (SPM); [3–5] moreover, when using ferromagnetic electrodes, spin polarization at the interface can also be affected by the ferroelectric polarization. [6,7] Despite the requirement of an ultrathin tunneling barrier for the TER effect, a recent report has presented the large tunable resistance by either changing the metal conductance of ferroelectric nano-domains [8] or using the tunneling junction between the SPM tip and the ferroelec- tric surface, [3] which makes relatively thick ferroelectric films applicable for the design. A key issue that has to be solved to realize FTJs is the thermodynamic stability of the domain. Asymmetric free energy landscapes between polarizations directed away and toward the substrates result in at least one Adv. Mater. 2014, 26, 6335–6340 www.advmat.de www.MaterialsViews.com