Structure, Ferroelectricity, and Magnetism in Self‐Assembled BiFeO₃–CoFe₂O₄ Nanocomposites on (110)‐LaAlO₃ Substrates

Abstract

grow as a tetragonal structure with a large c/a ratio (≈1.25) when the compressive epitaxial strain exceeds ≈4.5%, e.g., when grown on LaAlO 3 (LAO). [29] In previous work on BFO-CFO nanocomposites, (001)-oriented pseudocubic substrates were typically used, and there is little work on nanocomposites grown with (110) and (111) orientations. [21,31,32] The CFO nanopillars in nanocomposites grown on (110) and (111) substrates appear as tent-like structures and triangular prisms, respectively, in contrast to the rectangular pillars obtained on the (001)-oriented substrates. Moreover, BFO films grown on (110)-oriented LAO exhibit a uniaxial in-plane strain instead of the biaxial strain found in the (001) orientation, [33,34] and the strain along the [110] in-plane direction is relaxed even in BFO films with a thickness as low as ≈10 nm. BFO-CFO nanocomposite films on (110)-oriented LAO also exhibit strain in the out-of-plane direction of both phases that differs from the strain state obtained in single-phase films. A large out-of-plane strain significantly modifies the magnetism, magnetic anisotropy, and magnetotransport properties in vertical nanocomposite films. [10] In this article, we show that the BFO in a BFO-CFO nanocomposite grown on LAO (110) substrates forms different phases, rhombohedral-like (R-like) or tetragonal-like (T-like), depending on the thickness of the film and the effects of the vertical lattice strain between the BFO and CFO. A variety of domain structures were observed in the BFO matrix, and thickness-dependent magnetic properties of the CFO are interpreted in terms of the film strain and the shape of the CFO. These results demonstrate how the properties of nanocomposites including in-plane anisotropy can be engineered on a (110) substrate. Results and DiscussionA series of epitaxial BFO-CFO nanocomposite thin films with thickness of 25 to 75 nm was deposited on single-crystal LAO (110). The LAO is a rhombohedrally distorted perovskite at room temperature, with a pseudocubic lattice parameter of 3.787 Å. The (110)-oriented LAO substrate therefore has a rectangular surface net with dimensions 5.355 Å along the pseudocubic [110] sub direction and 3.787 Å along the [001] sub direction. The films were grown using pulsed laser deposition (PLD) as described in the Experimental Section. A conductive La(Sr 0.33 ,Mn 0.67 )O 3 (LSMO) layer ≈8 nm thick was first epitaxially grown on the substrate, followed by deposition of the A strain-driven BiFeO 3 (BFO) tetragonal to rhombohedral phase transition is demonstrated in self-assembled BiFeO 3 -CoFe 2 O 4 (BFO-CFO) nanocomposites on LaAlO 3 (110)-oriented substrates with varying thickness. The CFO forms parallel nanoscale fin-shaped structures within a BFO matrix. Out-of-plane lattice strain from the BFO/CFO interfaces stabilizes the BFO rhombohedrallike phase. The BFO exhibits a range of ferroelectric textures including stripe domains and centered domain arrangements, and the magnetic anisotropy of CFO shows a thickness-dependent reorientation.

Magnetoelectricity in vertically aligned nanocomposites: Past, present, and future

“…47 Meanwhile, advanced probe microscopes have been employed to directly observe and verify these phase/polarization distributions (with/without fields) of each domain at the micron/nanoscale. [48][49][50] It has been reported that the pillar size plays a critical role in controlling ME coupling in BTO-CFO VANs as it is determined by the competition between the vertical interface coupling effect and the bulk volume conservation effect. 51,52 Moreover, experiments have also been widely performed to improve the phase distribution morphologies and to enhance the properties of ME VANs.…”

Self‐Poled Heteroepitaxial Bi(1−x)DyxFeO3 Films with Promising Pyroelectric Properties

“…[34][35][36] Doing so, BFO structure and properties can be widely modified, adapting and designing the material to different situations/use/working conditions and recent works have been focused on BFO A-site doping. BFO systems have been deposited on various substrates using pulsed laser deposition (PLD), [37,38] metal organic chemical vapor deposition (MOCVD), [39][40][41][42][43] sputtering, [44] sol-gel, [45,46] and chemical solution deposition. [38,47] In this study, we report on the promising pyroelectric properties of pure BFO and Dy-doped BFO (BDFO) films deposited, through a simple, easily scalable MOCVD approach, on Nb doped SrTiO 3 (STO:Nb) (001) single crystal substrates.…”

Interfacial‐Strain‐Controlled Ferroelectricity in Self‐Assembled BiFeO3 Nanostructures

“…Piezoresponse force microscopy (PFM) is an essential tool for studying ferroelectric properties of such systems with nanoscale spatial variations. [1,2,[4][5][6][7][8][9]14,[16][17][18][19][20][21]38,39,43,44] For ferroelectric switching of BFO-CFO VAN systems, however, only one or a few local hysteresis loops are typically measured, usually by "parking" a conductive atomic force microscopy (AFM) probe at single or arrayed points in BFO, sweeping the tip-sample bias, and recording the resulting changes in the piezoresponse. [2,4,11,[15][16][17][18]28,38] Single-point hysteresis measurements can be challenging to interpret, though, due to the superposition of piezoelectric effects and electrostatics [45] or ionic motion.…”

“…[1,2,[4][5][6][7][8][9]14,[16][17][18][19][20][21]38,39,43,44] For ferroelectric switching of BFO-CFO VAN systems, however, only one or a few local hysteresis loops are typically measured, usually by "parking" a conductive atomic force microscopy (AFM) probe at single or arrayed points in BFO, sweeping the tip-sample bias, and recording the resulting changes in the piezoresponse. [2,4,11,[15][16][17][18]28,38] Single-point hysteresis measurements can be challenging to interpret, though, due to the superposition of piezoelectric effects and electrostatics [45] or ionic motion. [46] Topographic crosstalk is another persistent concern due to sensitivities to the tip-sample contact area, especially when the surface morphology exhibits changes in slope or curvature with length scales equivalent to the dimensions of the tip apex.…”