Full length articleRobust polarization switching in self-assembled BiFeO3 nanoislands with quad-domain structures
Graphical abstract
Introduction
Multiferroic materials possess abundant intriguing properties because of the interplay among different ferroic orders including (anti)ferromagnetism, ferroelectricity and ferroelasticity [1,2]. The prototype devices based on this kind of materials like magnetoelectric heterostructures [3] and spin valves [4] have been considered as next generation memories owing to the merits such as low power consumption and small leakage current. As a well-known room temperature multiferroic with large remnant polarization Pr (∼90 μC/cm2), high ferroelectric Curie temperature (∼1100 K) and relatively lower crystallization temperature (823 K) [5], BiFeO3 (BFO) has been intensively studied and is expected to be applied in ferroelectric random-access memory (FeRAM) or magnetoelectric devices [[6], [7], [8]]. For BFO thin film, since the lattice, charge, spin and orbital are strongly correlated, the crystalline structure [9], domain pattern [10,11], and spin configuration [12] can be readily manipulated via strain and interface engineering by choosing proper substrates and controlling thickness [[9], [10], [11], [12]]. Besides, high quality BFO thin film can also be obtained on Si substrate with proper bottom electrode [13], signifying its compatibility in semiconductor electronic devices. Therefore, single-phase multiferroic BFO provides a playground to study intrinsic physics, and multifunctionality for applications can also be accomplish in this system.
In the meantime, miniaturization is extremely important to increase the storage density in nanoelectronics. For example, for FeRAM, even it owns the advantages of nonvolatile, low power consumption, fast read/write time and high endurance [14], commercial FeRAM still possess relatively lower storage density compared with other memories like magnetic RAM (MRAM) or resistance RAM (RRAM) [15]. Although the recently proposed 3D multilayer memories by stacking a series of ferroelectric sheets can help to increase the density, this structure would inevitably compromise the writing speed and cycle performance [16,17]. Therefore, reducing the size of memory cell as small as possible is considered to be an alternative way to meet the requirement [18].
In addition, nanoscale topological defects such as domain wall or vortices in ferroic materials also present novel electron transport properties [[19], [20], [21], [22]], which are considered to be good low-dimensional structures applied in high-density memory. Recently, BFO nanoarrays with topological ferroelectric domain structures have been successfully prepared by anodic alumina template and focused ion beam milling [20,23,24]. Other convenient approach such as vertical interfacial stain engineering [25] can also assist to prepare self-assembled nanostructures. In our previous work [26], self-assembled BFO nanoisland arrays were realized by puled laser deposition with topological vertex-like quad-domains and charged domain walls. Especially, the conductivity of the charged domain walls can also be modulated by electric field. Inspired by this self-assembled BFO nanoislands with spontaneous vertex-like quad-domains, we further studied the independent switching of each quarter of quad-domains by electrical field exerted from scanning probe. Exotic domain configurations including vertex and anti-vertex were created via selectively switching. The stability of the domain structure over time and at high temperature, together with resistive switching effect in each quarter of quad-domains indicate the application prospect of the nanoislands in high-density FeRAM.
Section snippets
Experimental and simulation methods
The BFO thin films were grown by pulsed laser deposition on (001) oriented single crystal LaAlO3 (LAO) substrate after 2 nm La0.5Sr0.5MnO3 (LSMO) was first deposited as bottom electrode. An excimer laser with energy of ∼1.5 J cm−2 with wavelength of 248 nm was employed. The laser repetition rate for BFO and LSMO was 2 Hz and 5 Hz, respectively. Both thin films were grown at 700 °C with the oxygen partial pressure at 0.2 mbar. After deposition, they were cooled to room temperature at an oxygen
Results and discussion
BFO thin film was deposited on (001) oriented single crystal LAO substrate with LSMO as bottom electrode. Reciprocal space mapping (RSM) presented in Fig. 1a demonstrates that BFO film comprises a mixture of rhombohedral distorted (R-phase) and tetragonal distorted (T-phase) structures, which can also be verified by the θ-2θ X-ray diffraction pattern (see Supplementary Material Fig. S1). In contrast to previous studies where R and T-phase are combined in the strip-like form [28,29],
Acknowledgement
This work was supported by the Basic Science Center Program of NSFC (grant no. 51788104), the National Basic Research Program of China (grant no. 2016YFA0300103), and the National Natural Science Foundation of China (grant no. 51790494).
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