Engineering room-temperature multiferroicity in Bi and Fe codoped BaTiO3

Abstract

Fe doping into BaTiO3 stabilizes the paraelectric hexagonal phase in place of the ferroelectric tetragonal one. We show that simultaneous doping of Bi along with Fe into BaTiO3 effectively enhances the magnetoelectric (ME) multiferroic response (both ferromagnetism and ferroelectricity) at room temperature, through careful tuning of Fe valency along with the controlled recovery of the ferroelectric-tetragonal phase. We also report a systematic increase in large dielectric constant values as well as reduction in loss tangent values with relatively moderate temperature variation of the dielectric constant around room temperature with increasing Bi doping content in Ba1−xBixTi0.90Fe0.10O3 (0 ≤ x ≤ 0.10), which makes the higher Bi–Fe codoped sample (x = 0.08) promising for use as a room-temperature high-κ dielectric material. Interestingly, the x = 0.08 (Bi–Fe codoped) sample is not only found to be ferroelectrically (∼20 times) and ferromagnetically (∼6 times) stronger than x = 0 (only Fe-doped) at room temperature, but also observed to be better insulating (larger bandgap) with indirect signatures of larger ME coupling as indicated from anomalous reduction of the magnetic coercive field with decreasing temperature. Thus, room-temperature ME multiferroicity has been engineered in Bi and Fe codoped BTO (BaTiO3) compounds.

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“…Interestingly, such a nonmonotonic variation of polarization with x (i.e., Bi doping) is found to be identical with that of the corresponding sample tetragonality, as shown in the inset of Figure d. Also, note Figure S1d of the Supporting Information for the variation of the ferroelectric polarization (dPr), normalized to the tetragonal phase fraction (%) with x (Bi doping amount) in the BBTFO series, which, however, exhibits a direct correlation with the c/a ratio of the corresponding unit-cell (which is caused by Ti off-centric distortion , ), as shown in Figure d. However, such nonmonotonic variation of tetragonality ( c/a ) (or similarly the ferroelectric polarization) with x (Bi doping content) can be understood by considering the discussions in the following paragraph.…”

“…% + tetra. % = 100%] present in these samples are determined by Rietveld refinement − using the FULLPROF suite (for technical details of the Rietveld refinement, see references , ). For BB10TFO (i.e., x = 0.10), where Bi doping is equal to Fe amount, the system becomes fully tetragonal, though very close to the cubic limit, as seen in the inset of Figure d.…”

“…As lower Bi-doped compounds (i.e., lower x values) are a bit more conductive (compared to pristine BTO; to be discussed in the later sections), they exhibit distorted PE loops (not shown here). Hence, to extract out the intrinsic ferroelectric properties, we employ the PUND technique, which is more effective for such kinds of compounds and, thus, are widely employed (further details regarding PUND techniques are described in our earlier works , ). In Figure c, we see nonmonotonic variation of the ferroelectric remanent polarization (dPr) in BBTFO series (which has also been found to switch to negative values upon the reversal of applied electric field pulse in PUND measurements, a signature of the intrinsic ferroelectric character; for details, note ref ).…”

“…Inset to part d shows the variation of sample tetragonality ( c/a ) (as obtained from Rietveld refinements) with x (Bi doping amount). Part c and the inset to part d are reprinted with permission from ref . Copyright 2020 AIP Publishing.…”

“…Although Ti-site specific substitution in BaTiO 3 by other non-d 0 elements (e.g., Fe, Mn, Co, etc.) lowers the band gap substantially, − it also strongly reduces the ferroelectricity, since they lead to the stabilization of paraelectric hexagonal BTO phase. , Similarly, a naive attempt to increase the conductivity of BTO through the incorporation of oxygen vacancies (each of which donates two electrons) also leads to the stabilization of the paraelectric BTO hexagonal phase in place of the ferroelectric tetragonal one at room temperature. , Interestingly, while only Fe-doped BTO (BaTi 0.9 Fe 0.1 O 3−δ ) stabilizes in an almost paraelectric hexagonal phase, the ferroelectric tetragonal phase can systematically be recovered back in Bi–Fe codoped BTO (Ba 1– x Bi x Ti 0.9 Fe 0.1 O 3−δ ) compounds. , …”