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Domain Walls as Nanoscale Functional Elements Jan Seidel* School of Materials Science and Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia ABSTRACT: Domain walls in complex oxides have been the focus of intense research over the past few years. The fact that they can be electrically conducting opens new pathways for a number of possible applications. This paper provides a perspective on general concepts involving domain walls and their impact on material functionality.
orrelated oxides are a growing research field with a rich fundamental physics and phenomena for an increasing number of applications.1 These systems present a great variety of different properties, such as superconductivity, magnetism, and ferroelectricity, to name just the more popular ones. Many so-called smart materials exhibit ferroelectric, ferroelastic, and ferromagnetic transitions. These involve the emergence of spontaneous polarization, spontaneous strain, or spontaneous magnetization and are commonly referred to as primary or firstorder ferroics. Materials that have at least one of these order parameters can change their properties in a preconceived manner through the application of external fields. Thus, given their intrinsic structure, ferroic materials are ideally suited for functionality on the nanoscale. Because ferroic phases can arise in two or more distinct orientations of the order parameter, they can form domains, separated by domain walls (Figure 1). Domains are a representation of long-range order with respect to at least one macroscopic tensor property of the material (order parameter). When orientation states are changed, the interfaces (domain walls) move; thus, the domain structure can be manipulated by external fields, which is a central feature of ferroic materials. In general, in the vicinity of such a transition, one or more macroscopic properties of the material associated with the order parameter can become large and very susceptible to external fields. Field-induced phase transitions around the transition temperature are a common feature. If more than one ferroic order parameter is present, we refer to them as multiferroics, which are, for example, interesting candidates for magnetoelectric and spintronic applications at room temperature.2,3 Current Research. Natural and artificial interfaces in such materials are of special interest because local changes in symmetry and structure at the boundary can lead to additional functionality not present in the parent material.4−6 Domain walls in ferroic materials are one possible type of interface. They separate regions of different orientations of the specific
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order in such a material.7,8 Natural interfaces such as domain walls are intrinsically very small, on the order of 1 nm in ferroelectrics and ferroelastics, making them very attractive as the ultimate functional nanofeature. In addition, the fundamental understanding of electronic properties of such boundaries presents a very interesting new aspect of oxide interface functionality after electronic properties of ferroelectric domain walls, as “natural” oxide interfaces have recently received considerable interest.5−18 For such investigations, high-spatial-resolution probe techniques are needed. Typical examples are combinations of scanning probe techniques, such as piezoforce microscopy (PFM), temperature-dependent conductive atomic force microscopy (c-AFM), scanning tunneling microscopy (STM),19 and high-resolution electron microscopy (TEM,20,21 LEEM (unpublished results)) and photoemission (PEEM),13 as well as ab initio methods.22,23
Natural interfaces such as domain walls are intrinsically very small, on the order of 1 nm in ferroelectrics and ferroelastics, making them very attractive as the ultimate functional nanofeature. The influence of domain walls in terms of the basic properties of the parent material often focuses on their important (and often unwanted) effect on the performance in electronic devices.24 However, identifying and developing their additional intrinsic functionality and using it in tailored applications could actually open a new venue for research. Received: August 6, 2012 Accepted: September 21, 2012 Published: September 25, 2012 2905
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Figure 1. Domain wall types. Arrows indicate the direction and amplitude of the order parameter, (a) Ising wall, (b) Bloch wall, (c) Néel wall, and (d) mixed Ising−Néel wall. A mixed Ising−Bloch wall is also possible. Reproduced from ref 4, Copyright (2009) by The American Physical Society.
on demand. An example is shown in Figure 2. An important requirement for using domain walls for such applications is the precise control of their nucleation position and movement. In future devices, this level of control could be achieved using massively parallel arrangements of nanoscale probes on a field of device elements. In addition, concepts used for magnetic domain walls, such as the racetrack memory concept,27 can be adapted to ferroelectric walls. If this can be achieved, then preconfigured domain wall patterns carrying information in their conductive state could be used as a design platform for domain wall shift registers (Figure 3a). Ferroelectric wall motion could be induced by the application of a suitable electric field. In addition, the unique properties of the wall can be envisioned as interaction-enabling mechanisms with nanoscale particles and objects. Exploiting this concept, functional “nanorails” based on domain walls for material transport on the nanoscale can be imagined; see Figure 3b. In order to achieve these challenging goals, novel interdisciplinary approaches have to be explored. Another important aspect of domain wall research is the interaction with defects,28 for example, point defects, which can broaden the wall.29,30 The width of twin walls in PbTiO3, for example, can be strongly modified by the presence of point defects within the wall. The intrinsic wall width of PbTiO3 is about 0.5 nm, but clusters of point defects can increase the size of the twin wall up to 15 nm.31 Trapped defects at the domain boundary, for example, play a significant role in the spatial variation of the antiparallel polarization width in BaMgF4 single crystals, as seen by PFM.32 Interaction between the order parameter and the point defect concentration causes point defects to accumulate within twin walls;31 conversely, such defects contribute to the twin-wall kinetics and hysteresis33 as they tend to clamp the walls. Oxygen vacancies in particular have been shown to have a lower formation energy in the domain wall than those in the bulk, thereby confirming the tendency of these defects to migrate to, and pin, the domain walls.34 This leads to a mechanism for the domain wall to have a memory of its location during annealing.28 Domain wall (super)conductivity was studied by Aird and Salje.35 By exposing WO3 to sodium vapor, they observed preferential doping along the ferroelastic domain walls.
The changes in structure (and, as a consequence, electronic structure) that occur at ferroelectric (multiferroic) domain walls can lead to changes in transport behavior. Recent reports by several groups demonstrate an important advance in this direction, one example being the electrical conductivity of ferroelectric domain walls in multiferroic systems such as BiFeO3516 and RMnO315 (R = Y, Er, Ho, ...) as well as “conventional ” ferroelectrics like lead zirconium titanate,8 lithium niobate,25 and other oxides like WO3.35 Domain wall conductivity has been shown in these ferroic materials, although with different transport behavior; the domain walls of BiFeO3 were found to be more conductive than the domains,5 while those of YMnO3 were found to be more insulating or conductive depending on their orientation.1516 In YMnO3, a so-called improper ferroelectric multiferroic, in which ferroelectricity is induced by structural trimerization coexisting with magnetism, domain walls are found to be charged and stable. This material exhibits a conductive cloverleaf pattern of six domains emerging from one point, and the ferroelectric state has been reported to be more conducting than the paraelectric state. Tuning and controlling the physical properties of such domain walls provides a new playground for research and offers a new nanoelectronics platform for future nanotechnology.26
These findings open intriguing possibilities for potential device applications of domain walls. Future Prospects. These findings open intriguing possibilities for potential device applications of domain walls. The conductive response, for example, should be very sensitive to changes in applied strain to the material. Detailed investigations of the domain walls in BiFeO3 films on different substrates, with different lattice mismatch and strain, would therefore be desirable. Possible applications include nanoscale strain sensors with direct electric readout. Furthermore, because domain walls can be created and erased almost freely, this opens a way for flexible nanoscale design of electronic circuits that can be reconfigured depending 2906
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Figure 2. Flexible domain wall nanocircuits. (A) c-AFM image of two conductive 180° wall segments in BiFeO3. (B) Reconfiguration by connecting wall segments (writing of an additional 180° domain and the two resulting conductive 180° walls). The schematic connection diagram is overlaid (black and blue lines). The scale bar is 300 nm.
In both cases, it has been shown that the antiferromagnetic domain walls are significantly wider (by ∼1−2 orders of magnitude) compared to the ferroelectric walls. This is also in agreement with the phenomenological predictions of Daraktchiev et al. for coupling-mediated wall broadening.42 Another important aspect of multiferroic domain walls concerns the true state of magnetism at such a wall. Temperature-dependent transport measurements are a possible route to follow to understand the actual spin structure and whether it exhibits a glasslike or ordered ferromagnetic state.43 Of interest is the effect of extra carriers introduced into the system, for example, by doping or electric gating, on magnetism. In addition, electrical and magnetoelectrical transport properties of domain walls need to be investigated in other materials systems and compared to known ones, such as manganites.4445 Such domain walls are potentially interesting for spintronic applications.
Domain walls as well as artificially engineered oxide interfaces may pave the way to novel tailored states of matter with a wide range of electronic properties.
Figure 3. Domain walls as information and nano-object carriers. (A) Schematic ferroelectric domain wall shift register or ferroelectric domain wall “racetrack” (analogous to magnetic racetrack memory by Parkin et al.).27 Walls are moved by electric fields, and information is read out by detection electrodes. (B) Domain walls as “nanorails”. A unique interaction mechanism with the wall (conductivity, permittivity, magnetic property, etc.) keeps nano-objects on track and allows for movement along the wall.
In summary, this Perspective provides an outlook on nanoscale phenomena related to domain walls in complex oxides. Domain walls as well as artificially engineered oxide interfaces may pave the way to novel tailored states of matter with a wide range of electronic properties. Domain wall electronics, particularly with ferroelectrics and multiferroics, may become interesting for nanotechnology17 by identifying, understanding, and designing new material properties that may eventually lead to their incorporation into ever-smaller nanoscale technological devices.
Transport measurements showed superconductivity with a critical temperature of 3 K, while magnetic measurements did not, suggesting that superconductivity was confined to the domain walls only, which provided a percolating superconductive path while occupying a very small volume fraction of the crystal. Later, Bartels et al.36 used c-AFM to show the converse behavior. The domain walls of a calcium-doped lead orthophosphate crystal were found to be more resistive than the domains. In this context, a detailed understanding fo electronic structure at domain walls is highly desirable in order to design new “domain wall materials” with targeted functionality. Considering the multiferroic nature of BiFeO3,37−39 in which ferroelectric and antiferromagnetic domains are magnetoelectrically coupled, possible device geometry based on a BiFeO3 adjacent to a ferromagnetic film are of interest to study interactions of multiferroic domain walls with the ferromagnetic material.40 The exchange coupling between the ferromagnetic layer and BiFeO3 could provide interesting pathways for magnetoelectrical device designs. The interaction between ferroelectric and antiferromagnetic domain walls has been studied in model multiferroics such as YMnO341 and BiFeO3.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biography Jan Seidel is a Senior Lecturer and ARC Future Fellow at the University of New South Wales. He received his doctorate in physics from the University of Technology Dresden in 2005. From 2006 to 2007, he was a Feodor Lynen Fellow (Alexander von Humboldt Foundation) at the University of California, Berkeley. From 2008 to 2011, he worked at Lawrence Berkeley National Laboratory. His main 2907
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interests are in materials physics. www.materials.unsw.edu.au/staff/janseidel
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ACKNOWLEDGMENTS The author acknowledges support by the Australian Research Council through a Future Fellowship (FT110100523). REFERENCES
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