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

Abstract

We present a comprehensive study of the physical properties of epitaxial cobalt-doped BiFeO3 films ∼50 nm thick grown on (001) LaAlO3 substrates. X-ray diffraction and magnetic characterization demonstrate high quality purely tetragonal-like (T′) phase films with no parasitic impurities. Remarkably, the step-and-terrace film surface morphology can be fully recovered following a local electric-field-induced rhombohedral-like to T′ phase transformation. Local switching spectroscopy experiments confirm the ferroelectric switching to follow previously reported transition pathways. Critically, we show unequivocal evidence for conduction at domain walls between polarization variants in T′-like BFO, making this material system an attractive candidate for domain wall-based nanoelectronics.

Superior polarization retention through engineered domain wall pinning

“…The clear atomic steps in a representative topography image in Fig. 1a show a high-quality surface of the thin film and no R-like striped domains are found, consistent with previous studies on similar films 44,45 . Figure 1b presents the corresponding IP piezoresponse force microscopy (PFM) phase image.…”

“…Figure 1b presents the corresponding IP piezoresponse force microscopy (PFM) phase image. The striped IP domains oriented along <110> -type directions indicate a monoclinic structure (M c ) [46][47][48] , in agreement with previous X-ray diffraction (XRD) reciprocal space mapping (RSM) data 44 . In Fig.…”

“…This mixed-phase BFO system is a fertile ground for intriguing physical properties including large piezoelectric responses 34,35 and fieldinduced strains 36 , electrochromic effects 37 , non-zero magnetic moments 38 , electrical conductivity [39][40][41] , interesting mechanical properties 42,43 , and is a suitable system for reducing the polarization retention loss 30 . Herein, we demonstrate that by intentionally introducing designer defects, macroscale strain coherence is maintained throughout the thickness of the film, which leads to the preservation of the T' BFO phase and thus a complete suppression of the mixed-phase structures throughout the film 44,45 . These defects pervade the entire film uniformly and apply a local compressive strain.…”

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

“…The large elongation in the [001] direction of T‐like BFO strongly depresses the antiferromagnetic Néel temperature [ 68,171 ] and gives rise to a rich array of structural and/or ferroelectric phase transitions just above room temperature. Various studies have confirmed the presence of G‐type AFM order with a wFM moment, [ 66,265 ] with one study suggesting the coexistence of G‐type and C‐type AFM order, [ 254 ] possibly caused by a weakening of the exchange coupling in the [001] direction. With regard to the cycloid, despite various efforts by neutrons and CEMS, clear evidence of the existence of a long period modulation is still forthcoming.…”

“…This section would not be complete without reminding the reader that BFO exhibits very rich strain diagrams, most emphatically shown by the formation under strong compressive strain of a giant axial ratio ( c / a ≈ 1.25) tetragonal‐like polymorph, T‐like BFO. [ 65–68,171,265–270 ] In addition to its various interesting piezoelectric, [ 65 ] optical, [ 64 ] and magnetoelectric facets, [ 67 ] T‐like BFO has an intriguing antiferromagnetic structure. [ 11 ]…”