Phase Separation of Bismuth Ferrite into Magnetite under Voltage Stressing

Micro-Raman studies show that under ~700 kV/cm of d.c. voltage stressing for a few seconds, thin-film bismuth ferrite BiFeO3 phase separates into magnetite Fe3O4. No evidence is found spectroscopically of hemite alpha-Fe2O3, maghemite gamma-Fe2O3, or of Bi2O3. This relates to the controversy regarding the magnitude of magnetization in BiFeO3.

in the thinner films [11]. This is in accord with experiments [11], which show an order of magnitude enhancement in M in the thinnest films.
In the present work we have used micro-Raman techniques described previously [12] to examine the phase separation of bismuth ferrite under d.c. voltage stressing at room temperature, in order to see if magnetic phases are separated in the process. As described below, we have found that magnetite is formed under these conditions. Since the Raman spectra of the BiFeO 3 starting material agrees very well with those recently published on virgin thin-film samples [13][14][15][16][17], these studies show that the magnetite is formed entirely by the applied d.c. voltage and does not exist in significant concentrations in the unstressed specimens. We note that although Fe 3 O 4 and Fe 2 O 3 differ by only 10% oxygen concentration, their bondings differ and this gives rise to distinguishable Raman spectra.
The bismuth ferrite film with thickness of 185 nm was made by using sol-gel spin coating techniques on a Pt/TiO 2 /SiO 2 /Si commercialized substrate. The top electrodes (Pt dots of 100 m x 100 m) were then deposited by sputtering on the films via TEM grids. Micro-Raman spectra were recorded in backscattering geometry at ambient conditions by CCD collection. Two types of Raman instruments were used in the present work: RM1000 Renishaw systems which uses a 514.5 nm laser and LabRam 300 equipped with 632.8 nm laser. The electrical measurements were conducted using a Radiant RT 6000S ferroelectric system. The crystallization property of our film has been confirmed by XRD measurements.
The film was stressed by applying a d.c. voltage with increasing magnitudes. At low voltages, the film shows high resistivity. The d.c. breakdown voltage is about 13 V (i.e. E B breakdown field ~700 kV/cm,) for 2 second endurance and is relatively repeatable. The a.c. breakdown voltage is relatively higher as expected and is about 15 V (i.e. ~800 kV/cm) evaluated by hysteresis measurements on a number of electrodes at 1 kHz frequency. Prebreakdown could be achieved after applying a d.c. voltage slightly lower than the breakdown strength. This manifests itself by an abrupt increase of leakage current and the appearance of a few degraded spots on the electrode (Fig 1, the dark spots as indicated by the arrow are used for Raman measurements shown later).
Then we carried out Raman studies on this film as described in detail previously [12].
Remarkably, the Raman spectra collected from the degraded areas are identical and are comparable with those for Fe 3 O 4 phase (called magnetite) [19][20][21][22][23], evidenced by the strong peak at ~642 cm -1 and several other characteristic peaks at ~314 cm -1 , 395 cm -1 , and 517 cm -1 , although these peaks are slightly shifted to lower wavenumbers compared with bulk Fe 3 O 4 . This may be due to the effect of strain on these Fe 3 O 4 nano-particles.
Note that -Fe 2 O 3 has an inverse spinel structure with a tetragonal sublattice distortion. regarding the magnitude of magnetization in BiFeO 3 thin films. This point becomes a particular concern, considering the high coercive field E c observed for BiFeO 3 thin films: it has been shown that the coercive field in bismuth ferrite thin films is about 300 kV/cm, twice or three times larger than that in common ferroelectric thin films (<100 kV/cm) [13]; hysteresis measurements sometimes require an electric field as large as 1~2 MV/cm [13,[36][37][38], compatible or even larger with the field we applied in this work. Under such a high field, interfacial phase separation of BiFeO 3 into magnetite is reasonably expected due to intensive charge injection from the electrode.
Note that BiFeO 3 single crystal or ceramics show space-commensurate ferroelectricity below the Curie point (1103 K) and space-incommensurate antiferromagnetism below the Neel temperature (643 K), with a superimposed cycloidal modulation with a wavelength of ~62 nm [39]. Consequently, its remanent magnetization and potential magnetoelectric effect both vanish macroscopically, leading to a weak quadratic magnetoelectric behavior rather than a strong linear magnetoelectric behavior [4,5]. It has been found that the cycloidal order can be destroyed and the non-zero remanent magnetization (and potential magnetoelectric effect) can be released by applying a sufficiently large magnetic field of 20 T in bulk BiFeO 3 [40] or by chemical substitutions in thin films [41]. In this work, we show that phase separation of BiFeO 3 into nano-regions of magnetite under a d.c. field, comparable to the applied field for switching measurement, may also lead to a possibility that electrical testing of samples might itself produce extrinsic magnetization.  presents a magnified view of the spectra from ~250 cm -1 to 800 cm -1 .