Atomic‐Scale Evolution of Local Electronic Structure Across Multiferroic Domain Walls

Abstract

In complex, correlated oxides, heterointerfaces have emerged as key focal points of current condensed matter science. [1][2][3] For ferroic oxides, in order to minimize the total energy, domain walls emerge as natural interfaces. Multiferroic materials show a wealth of controllable multiple ferroic order through stress, optical excitation, electric, or magnetic fi elds in the same phase, which in turn suggest potential applications in the realization of oxide-based electronic devices, such as spintronics, information storage devices, or communications. [4][5][6][7] According to the detailed classifi cation given by Mermin in ferroic systems, [ 8 ] domain walls in ferroic systems are considered as two dimensional (2D) topological defects, which play an important role in determining the functionality in materials with long-range order.Recently, several key studies pointed out interesting observations on domain walls in multiferroics. [9][10][11][12][13] For example, Y. Tokunaga et al. showed that ferroelectric polarization and magnetization are successfully controlled by magnetic and electric fi elds in GdFeO 3 , respectively, which is attributed to the unique feature of composite domain wall clamping. [ 14 ] T. Choi et al. observed insulating interlocked ferroelectric and structural antiphase domain walls in a multiferroic YMnO 3 system. [ 15 ] H. Bea et al. pointed out that domain walls are the source of the exchange bias interaction between the ferromagnetic metal layer and multiferroic BiFeO 3 (BFO). [ 16 ] Additionally, L. W. Martin further confi rmed that as-grown 109 ° domain walls in BFO thin fi lms are the contribution for uncompensated spins. [ 17 ] In addition to the above, a very recent work has established electrical conductivity at written multiferroic domain walls in BFO at room temperature, which opens up a pathway by which to manipulate domain walls for next generation nanoelectronics.Exploring details on electronic states of domain polarization reorientations is critical in oxide multiferroic materials. Theoretically, the consideration of the evolution of the polarization across the 109 ° domain walls exhibits a large potential step. The prediction correlates with the enhanced electrical conductivity due to the generation of a space-charge layer for screening the potential discontinuity in the region of the wall. [ 2 ] Experimentally, conductive atomic force microscopy (c-AFM) studies show the occurrence of electrical conduction at 109 ° domain walls within the limited spatial resolution of the experimental technigue. [ 3 ] In spite of the critical importance of these discoveries at such an oxide interface, there have been no effectively direct investigations of the intrinsic evolution of the electronic properties at regions of domain walls specifi cally within the nanoscale.In this work, we explore the subject by measuring the local electronic structure using scanning tunneling microscopy (STM) in a cross-sectional geometry. STM and scanning tunneling spectroscopy (STS) studies provide direct exp...

Local conductivity and the role of vacancies around twin walls of (001)−BiFeO3 thin films

“…Figure 2c shows localized areas of enhanced conduction around both 71 o and 109 o domain walls in the as-grown (001)-BiFeO 3 films described above, as we have previously reported [18]. Similar results have been observed by Chiu et al [17] using scanning tunneling microscopy. However, in our films, unlike in those of ref.…”

“…However, in our films, unlike in those of ref. [17], the magnitude of the current in the 71 o walls is as large as that of the 109 o walls. This difference between the two studies could be due to the nature of the interfaces (out-of-plane twinning versus in-plane twinning) [32,34], which may invoke different conduction mechanisms, as mentioned in the previous section.…”

“…However, it was later reported that also as-grown 71 o walls displayed enhanced conductivity with respect to the domains [17], although that was one order of magnitude smaller than the conductivity found in as-grown 109 o domain walls. This seems to be in agreement with the theoretical calculations that assign the origin of conduction to a decrease in the band gap of the material at the domain walls [14,17]. However, in samples grown under different conditions, we have observed that conductivity at 71 o domain walls can be as large as that in 109 o domain walls and that its main origin is the lowering of the Schottky barrier between the BiFeO 3 n-type semiconducting film and the metallic top electrode [18].…”

“…Initially, conducting behavior was observed only in artificially-written 109 o domain walls [14]. However, it was later reported that also as-grown 71 o walls displayed enhanced conductivity with respect to the domains [17], although that was one order of magnitude smaller than the conductivity found in as-grown 109 o domain walls. This seems to be in agreement with the theoretical calculations that assign the origin of conduction to a decrease in the band gap of the material at the domain walls [14,17].…”

Domain wall nanoelectronics

“…The emergence of combined AFM and STM or SEM (scanning electron microscopy) and STM systems should be a boon in terms of exploring the electronic properties of domain walls in such insulating materials. Research using such combined tools is in its infancy (Wiessner et al, 1997;Yang et al, 2005;García, Huey, and Blendell, 2006;Chiu et al, 2011).…”