Strain-induced coupling of electrical polarization and structural defects in SrMnO3 films

Abstract

Local perturbations in complex oxides such as domain walls 1,2 , strain 3,4 and defects 5,6 are of interest because they can modify the conduction or the dielectric and magnetic response and even promote phase transitions. Here we show that the interaction between different types of local perturbations in oxide thin films is an additional source of functionality. Taking SrMnO 3 as a model system, we use nonlinear optics to verify the theoretical prediction that strain induces a polar phase, and density functional theory to show that strain simultaneously increases the concentration of oxygen vacancies. These vacancies couple to the polar domain walls where they establish an electrostatic barrier to electron migration. The result 2 is a state with locally structured room-temperature conductivity consisting of conducting nanosized polar domains encased by insulating domain boundaries, which we resolve using scanning probe microscopy. Our "nanocapacitor" domains can be individually charged, suggesting stable capacitance nanobits with a potential for information storage technology.At first we verify the occurrence of strain-induced polar order in SrMnO 3 thin films.Motivated by the search for novel multiferroic materials, which combine magnetic and ferroelectric orders in the same phase, density functional theory (DFT) predicted the occurrence of ferroelectricity in the perovskite-structure alkaline-earth manganites at larger-than-equilibrium lattice parameters 7,8,9 . For bulk SrMnO 3 this prediction was confirmed by partial substitution of Sr by Ba which induces negative chemical pressure and leads to a polar state 10 . According to DFT, epitaxial SrMnO 3 films should develop a polarisation along one of the pseudocubic <110> axes under >1% epitaxial tensile strain 8 .20-nm films of single-phase SrMnO 3 were grown using pulsed laser deposition on (001)-oriented (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT) with 1.7% tensile strain (see Methods). We characterised the strain state of the films using scanning transmission electron microscopy (STEM) and X-ray and electron diffraction. Figure 1a shows a cross-sectional STEM image evidencing the high quality of the films on the atomic scale with a sharp SrMnO 3 /LSAT (001) interface. The reciprocal space map in Fig. 1a verifies that the films are tetragonal and coherently strained. The electron diffraction In the anisotropy plot in Fig. 1c we present the optical polarisation analysis of the SHG signal obtained on a test area of 0.1 mm 2 . We fitted the angular dependence of the SHG signal by assuming a distribution of four polar domain states denoted as P 1+ , P 1− , P 2+ , P 2− . The indices refer to the orientation of the polar axis according to 1 ± ↔ ±[110] and 2 ± ↔ ±[1 10], see Fig. 1c. The coincidence of the measured data and the fit is excellent with a fitted ratio r = P 1 /P 2 = 0.53 in the population of P 1 -and P 2 -type domain states (r varied between different test areas). In contrast, fits assuming a polarisation along the[100] and [010] directions failed. We co...

Polar-Graded Multiferroic SrMnO3 Thin Films

“…19 . Here, the oxygen-pressure dependence on the perovskite phase stabilization is the direct evidence that the formation of oxygen vacancies during thin film growth plays a central role in its stability, since perovskite SMO is only stabilized under low oxygen pressure 28,36,37 . The formation of oxygen vacancies is charge compensated by a reduction of the formal oxidation state of some Mn 4+ to Mn 3+ , resulting in a chemical expansion of the crystal lattice, relieving the strain.…”

“…Herein, moreover, high-polar and low-polar state domains can interact together with the presence of in-plane and out-of-plane polarization gradients. In order for this configuration to be stable, the polarization discontinuities may be screened by the presence of charged defects, such as the ubiquitous oxygen vacancies in such films localized at domain walls 28 , accommodating the locally different symmetry. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 Earlier works in ferroelectric HoMnO 3 epitaxial films pointed out the interplay between strain gradients and the presence of oxygen vacancies and the possibility to modulate the strain gradients, and thus flexoelectricity and polarization, as a function of the oxygen pressure during the film growth 19 . Here, the oxygen-pressure dependence on the perovskite phase stabilization is the direct evidence that the formation of oxygen vacancies during thin film growth plays a central role in its stability, since perovskite SMO is only stabilized under low oxygen pressure 28,36,37 . The formation of oxygen vacancies is charge compensated by a reduction of the formal oxidation state of some Mn 4+ to Mn 3+ , resulting in a chemical expansion of the crystal lattice, relieving the strain.…”

Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films

“…A recent theoretical study suggests that, in addition to directly affecting internal bond lengths and oxygen octahedral rotations, coherent epitaxial strain can facilitate spontaneous formation of oxygen vacancies and even influence defect-site preference leading to vacancy ordering in CaMnO3 and similar materials 15,16 . This prediction opens the door for exploring and controlling new properties and functionalities stemming from ionic activity.…”

Interplay between strain, defect charge state, and functionality in complex oxides

“…[http://dx.doi.org/10.1063/1.4958716] Defects in oxide thin films are increasingly being considered as active elements that can lead to added functionality. [1][2][3][4][5] One promising route to engineering new material behaviors is to exploit the simultaneous coupling between strain and structural or electronic properties, and strain and defect formation energies. This concept is well established in bulk ceramics, where defects are known to cause isotropic or anisotropic changes in volume.…”