Structural and electronic transformation pathways in morphotropic BiFeO3

Abstract

Phase boundaries in multiferroics, in which (anti-)ferromagnetic, ferroelectric and ferroelastic order parameters coexist, enable manipulation of magnetism and electronic properties by external electric fields through switching of the polarization in the material. It has been shown that the strain-driven morphotropic phase boundaries in a single-phase multiferroic such as BiFeO3 (BFO) can exhibit distinct electronic conductivity. However, the control of ferroelectric and phase switching and its correlation with phase boundary conductivity in this material has been a significant challenge. Supported by a thermodynamic approach, here we report a concept to precisely control different switching pathways and the associated control of electronic conductivity in mixed phase BFO. This work demonstrates a critical step to control and use non-volatile strain-conductivity coupling at the nanoscale. Beyond this observation, it provides a framework for exploring a route to control multiple order parameters coupled to ferroelastic and ferroelectric order in multiferroic materials.

Electromechanical-mnemonic effects in BiFeO3 for electric field history-dependent crystallographic phase patterning

“…The Vdc waveform as depicted in Figure 4 starts and ends at 0 V. The vertical displacement was simultaneously measured by the AFM z-sensor and indicates structural phase transitions by steep slopes, whereas less drastic changes can be attributed to field and polarization dependent piezoelectric response [20]. The resulting crystallographic and domain patterns (SI Figure SI4) are similar to those observed using the FORC waveform that starts and ends at -7 V, corroborating comparability between experiments.…”

“…Sequentially increasing the negative Vdc appears to prevent formation of long patterns of RT stripes in the field of view that were previously observed at domain walls (Figure 1) This contradiction suggests that the whole electric field history has an impact on the resulting RT patterns in mixed phase BFO films, rather than only the final voltage step. Moreover, literature suggests that for strained BFO, polarization switching in T phase is mediated by the R phase and, therefore, ferroelectric switching occurs in the sequence Tdown → Rdown → Rup → Tup from free energy density calculations [20]. However, an Rup phase is not observed in Vdc scanning experiments of Figure 3, in fact, the mixed RT phase appears always in a negative polarization orientation, whereas the polarization of T depends on poling voltage.…”

“…Despite such pronounced structural distortions, these films appear to be free from grains [17], grain boundaries [18] and dislocations [19], which are common in highly strained heteroepitaxial systems. Exposure to electric fields can induce crystallographic phase transitions from mixed RT phases to pure T phase and vice versa [2][3][4][20][21][22][23]. Crystallographic phase coexistence and transitions govern a variety of material properties, which unlock opportunities to tailor functional responses.…”

“…For example, the strong electromechanical coupling of mixed phase BFO [22,24] can enable lead-free micro-sensors and -actuators for biological applications, while its giant elastic tunability can be exploited in frequency-agile electroacoustic devices [2,21,25]. Enhanced conductivity at domain walls and phase boundaries [1,7,8,20,26,27] are envisioned to play a role in future nanoelectronic systems such as domain wall based memristors. Electrochromism and optical absorption both depend on crystallographic phases, yielding novel opportunities in photonics and acoustooptic 3 applications [3].…”

Structural, magnetic, and ferroelectric properties of T-like cobalt-doped BiFeO3 thin films

“…10 Usually, increasing the film thickness above 20-30 nm triggers strain relaxation, and, instead of the usual formation of misfit dislocations, a mixture of both the T phase and a more stable bulkderived rhombohedral-like (R ) phase 9 is formed. The presence of this "strain-induced morphotropic phase boundary" (MPB), 11 along with the added versatility that the T and R phase fractions can be modulated with an electric field, 11,12 has generated increased interest in BFO as a potential leadfree piezoelectric material. Furthermore, the significant electronic, magnetic, 13 elastic, 14 and optical modifications [15][16][17][18][19] related to the change in coordination, oxygen bond angles, and strain gradients offer interesting perspectives for multifunctional devices.…”