Nanoscale Domain Control in Multiferroic BiFeO₃ Thin Films

Abstract

With an ever-expanding demand for data storage, transducers, and microelectromechanical (MEMS) systems applications, materials with superior ferroelectric and piezoelectric responses are of great interest. The lead zirconate titanate (PZT) family of materials has served as the cornerstone for such applications up until now. A critical drawback of this material, however, is the presence of lead and the recent concerns about the toxicity of lead-containing devices. Recently, the lead-free ferroelectric BiFeO 3 (BFO) has attracted a great deal of attention because of its superior thin-film ferroelectric properties, [1,2] which are comparable to those of the tetragonal, Ti-rich PZT system; therefore, BFO provides an alternate choice as a "green" ferro/piezoelectric material. Another advantage of BFO is its high ferroelectric Curie temperature (T c = 850°C in single crystals), [3,4] which enables it to be used reliably at high temperatures. The ferroelectric domain structure of epitaxial BFO films are typically discussed in the context of the crystallographic model of Kubel and Schmid; [5] however, by suppressing other structural variants in BFO, we can obtain periodic domain structures that may open additional application opportunities for this material. Ferroelectrics with periodic domain structures are of great interest for applications in photonic devices [6] and nanolithography.[7] Such a periodic polarization could be obtained by applying an external electric field while utilizing lithographically defined electrodes or by a direct writing process. [8,9] To obtain sub-micrometer feature sizes, however, domain engineering using a scanning force microscope with an appropriate bias voltage must be used to fabricate the patterned domain structures.[10] Unfortunately, this method works only on small areas and is limited by its slow scanning rate. Theoretical models predict the feasibility of controlling the domain architecture in thin films through suitable control over the heteroepitaxial constraints. [11] In the case of BFO thin films, we have found that such a control is indeed possible, mainly through control over the growth of the underlying SrRuO 3 electrode. Using this approach, we demonstrate the growth of highly ordered 1D ferroelectric domains in 120 nm thick BFO films. On the (001) C perovskite surface there are eight possible ferroelectric polarization directions corresponding to four structural variants of the rhombohedral ferroelectric thin film. (For simplicity, the c and o subscripts refer to the pseudocubic structures for BFO and orthorhombic structures of SrRuO 3 (SRO) and DyScO 3 (110) O (DSO), respectively.) Domain patterns can develop with either {100} C or {101} C boundaries for (001) C -oriented rhombohedral films. [12] In both cases, the individual domains in the patterns are energetically degenerate and thus equal-width stripe patterns are theoretically predicted. When the spontaneous polarization is included in the analysis, the {100} C boundary patterns have no normal component of the net po...

Probing electric field control of magnetism using ferromagnetic resonance

“…Ferromagnetic-multiferroic thin-film heterostructures of Pt (2.5 nm)/Co 0.9 Fe 0.1 (2.5 nm)/BiFeO 3 (200 nm)/SrRuO 3 (SRO; 10 nm) were grown onto (110)-oriented DyScO 3 (DSO) substrates with a SRO layer as the bottom electrode using pulsed laser deposition 33 . The resulting epitaxial BFO layer exhibits a characteristic, two-domain structure consisting of a one-dimensional, quasi-periodic array of 71°domains 31 ; we use this as a model system to probe the coupling to the ferromagnetic CoFe. The Pt (2.5 nm)/Co 0.9 Fe 0.1 (2.5 nm) bilayers were deposited on BFO films under a 20-mT in situ bias magnetic field along the DSO (1-10) direction, leading to a well-defined IP magnetic easy axis in the CoFe layer.…”

“…The room temperature single-phase multiferroic, BiFeO 3 (BFO), has attracted a lot of recent research interest due to the coexistence of robust ferroelectricity (P) and antiferromagnetism (L) [13][14][15][16][17][29][30][31][32][33][34][35][36] and a weak canted magnetic moment (M C ). In bulk BFO, the weak moment results from the canting of the magnetic sublattices due to the Dzyaloshinskii-Moriya interaction 14 as predicted by the density functional theory 14 and confirmed experimentally 30,36 .…”

The dependence of oxygen vacancy distributions in BiFeO3 films on oxygen pressure and substrate

“…2,3,13 Following growth, these samples were then cooled to room temperature at 5°C / min in 760, 0.1, or 0.001 Torr P O 2 in order to induce different ␦ for BiFeO 3−␦ , SrRuO 3−␦ , or SrTiO 3−␦ . Henceforth, we will refer to these samples as 760-, 0.1-, and 0.001-BFO/ SRO/STO; and 760-, 0.1-, and 0.001-BFO/SRO/DSO.…”

“…When there is no epitaxial strain, the pseudocubic crystal lattice is ϳ0.3922 nm for SRO ͑␦ ϳ 0͒ thick film, ϳ0.3906 nm for STO ͑␦ ϳ 0͒ crystal substrate, ϳ0.3948 nm for DSO ͑␦ ϳ 0͒ crystal substrate, and ϳ0.3965 nm for BFO ͑␦ ϳ 0͒ crystal. [13][14][15] The 002 diffraction peaks of the BFO film were observed at larger angles for BFO/SRO/DSO in fraction peaks of BFO films can be fitted by a peak with 2 002 Յ 45.390°, i.e., c axis of Ͼ0.3996 nm, which suggests that the density of V O s is not high enough to effectively relax the compressive strain between the BFO film and the STO substrate in all BFO/STO/STO samples. Besides, the STO layer of 0.001-BFO/SRO/STO shows a little larger 2 002 and FWHM than those of 760-, 0.1-BFO/SRO/STO, suggesting that a large amount of V O s diffused to STO, and then induced the lightly expanded c axis.…”

Magnetoimpedance spectroscopy of epitaxial multiferroic thin films

“…The most prominent example of well-established MEC at elevated temperature may be BiFeO 3 (BFO), 9,17,18 where the ferroelectric and antiferromagnetic order parameters are coupled. 19 To introduce the missing ferromagnetic moment into the BFO system, several magnetic cations have been considered as a potential dopant. 7,20 One of the most intensely investigated candidates may be Mn, [21][22][23] since the parent compound BiMnO 3 (BMO) is ferromagnetic below T C ≈ 100 K and shows strong indication for ferroelectric order.…”