Magnetic Domain Change Induced by In‐Plane Electric Polarization Switching in Bi(Fe, Co)O3 Thin Film

Perovskite BiFe0.9Co0.1O3 with electric‐field‐induced magnetization reversal is a promising material for low‐power non‐volatile memory devices because spontaneous magnetization perpendicular to the electric polarization can be reversed by out‐of‐plane polarization reversal. In this study, magnetic and ferroelectric domain changes in BiFe0.9Co0.1O3 after applying an in‐plane electric field with counter electrodes fabricated by lithography is investigated. Electric poling can be performed up to 100 times via 71° polarization switching without disrupting the ferroelectric domains. It is determined that the correlation between the ferroelectric and magnetic domains is preserved, while in‐plane 71° polarization switching without reconstructing the ferroelectric domains does not change its out‐of‐plane magnetization. Lastly, possible differences in the antiferromagnetic spin vector and magnetization direction for in‐plane and out‐of‐plane 71° switching is discussed.

DOI: 10.1002/apxr.202200099 of electric power is required to maintain the stored data.Hence, the explosive increase in the amount of data due to advances in information technology will lead to an increase in energy consumption on the same scale over the next few decades.[3] Hard disk drives are widely used for data storage, but electric power is consumed to generate the magnetic field needed for writing magnetic information.If the polarization reversal induced by an electric field accompanies magnetization reversal in multiferroic materials, such a phenomenon can be utilized for an ultra-low-consumption voltage-write magnetic-read-out memory device.
[6] However, BiFeO 3 lacks net magnetization due to the presence of a longrange cycloidal modulation, [7] which hampers straightforward magnetic actuation.10][11][12] In addition, a magnetic easy plane perpendicular to electric polarization has been verified in singlecrystalline bulk BFCO [13] and BFCO thin film on SrTiO 3 (111). [14]e previously investigated out-of-plane magnetization reversal without the reconstruction of the striped domain structure after 71°electric polarization switching in BFCO(001) pc on GdScO 3 (110) o (here "pc" and "o" denote pseudocubic and orthorhombic notations, respectively).The electric and magnetic domains were observed by piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM), and the polarization switching was carried out by scanning a biased cantilever. [15]e also demonstrated that the trailing field, which is the effective electric field induced by the motion of a biased cantilever, can be used to manipulate the domain structure. [16,17]However, the leaky nature of BFCO renders it difficult to repeatedly perform polarization switching with preserving the domain structure.To apply this phenomenon to magnetic memory, an electric field should be applied through the electrode rather than by scanning the PFM cantilever.However, the magnetic domain cannot be observed if the film is covered by an electrode.Thus, we fabricated a pair of counter electrodes and observed the magnetic domain between them.This would be also advantageous in avoiding possible damage to the BFCO caused by the electric field concentrated at the cantilever tip during the poling. [18]n contrast to our previous reports, this paper focuses on the magnetization reversal induced by the application of an in-plane electric field.BFCO/GdScO 3 (110) o has electric polarization in eight <111> pc directions, allowing either in-plane or out-of-plane switching.[21][22] In BFCO, the combination of inplane counter electrodes and MFM measurements makes it possible to observe the magnetization behavior in response to the repeated application of electric fields.In addition, in-plane and out-of-plane polarization switchings likely lead to different magnetization directions after the reversal, given that the inversion of the Dzyaloshinskii−Moriya (DM) vector of BFO depends on the path of polarization switching, [23] but this has yet to be clarified.To address these questions, we repeatedly applied in-plane electric fields to BFCO/GdScO 3 (110) o through electrodes fabricated on the thin film and observed ferroelectric and magnetic domains using PFM and MFM techniques.

Methods
BFCO epitaxial thin films were grown on GdScO 3 (110) o substrate by using pulsed laser deposition with a KrF excimer laser ( = 248 nm).A stoichiometric BFCO target was ablated at a substrate temperate of 600 °C in an oxygen partial pressure of 15 Pa.The repetition rate and fluence for the deposition were fixed at 5 Hz and 1.5 J cm −2 , respectively.Subsequently, the oxygen partial pressure was increased to 5000 Pa during cooling after deposition to compensate for oxygen vacancies in the film which can prevent the polarization switching. [24]The thickness of the BFCO thin film was ≈100 nm.The crystal structure of the thin films was investigated by X-ray diffractometry (XRD) with Cu K radiation (Rigaku SmartLab).For the in-plane poling, patterned Pt electrodes were prepared with the dc magnetron sputter via the standard lift-off process.The electric field was applied using a ferroelectric test system (TOYO FCE).The PFM and MFM observations were performed using a scanning probe microscope (Asylum Research Cypher).To determine the polarization direction in ferroelectric domains, two in-plane PFM scans with inplane scanning angles of 0°and 90°relative to the [100] pc directions and one out-of-plane PFM were performed, and 3D-PFM images were generated by using a commercial photoediting software.Detail of the measurement conditions can be found in our previous reports. [15,25]

Results and Discussion
XRD 2/ patterns for the as-grown BFCO/GdScO 3 (110) o thin films (Figure 1a) show 00h pc diffraction peaks in the perovskite structure without any detectable impurities, indicating highly crystalline BFCO. Figure 1b shows the reciprocal space maps around the 003 pc (GdScO 3 330 o ), 023 pc (GdScO 3 332 o ), 02_3 pc (GdScO 3 510 o ), and 113 pc (GdScO 3 422 o ) reflections.The BFCO 003 pc symmetric diffraction appears at the larger q z side of the 330 o peak of GdScO 3 .Peak splitting of BFCO 203 pc and 113 pc asymmetric diffractions indicates a monoclinic distortion of BFCO.In particular, one of the 113 pc peaks has the same q z value as the 003 pc peak, indicating that BFCO has a rhombohedrallike monoclinic structure (i.e., M A phase [26] ).A comparison of the reciprocal space map (RSM) images of 203 pc and 023 pc gives insight into the in-plane polarization variants of the M A phase BFCO.When the four possible in-plane polarization variants P 1 -P 4 are defined as illustrated in Figure 1c, the peaks in the 203 pc and 023 pc RSM images can be assigned as labeled in Figure 1b, and suppression of the P 3 and P 4 variants can be observed.This is attributed to the monoclinic distortion of the GdScO 3 substrate surface. [27]The lattice constants of BFCO/GdScO Figure 2 summarizes the results of the scanning probe measurements for as-grown BFCO/GdScO 3 (110) o .As seen in the topographic image (Figure 2a), the root mean square of the roughness is 0.48 nm, which is sufficiently small for conducting the PFM and MFM measurements.The in-plane PFM image with detecting [100] pc component (Figure 2b) shows a clear striped pattern in the polarization domains, while the one with detecting [010] pc component (Figure 2c) has minimal contrast.These results indicate that two of the four in-plane polarization variants (Figure 2d) are dominant because of the monoclinic distortion on the GdScO 3 substrate surface, which is consistent with the asymmetric peaks in RSM.In the MFM image (Figure 2e), a striped pattern corresponding to a phase of magnetic interaction between a surface and cantilever can be seen.The phase contrast originates from out-of-plane components of weakly ferromagnetic spontaneous magnetization arising from the canted colinear spin structure.The contrast pattern in the MFM image coincides with the boundary in the in-plane PFM image.The MFM image exhibits rather a strong signal inside the domain and a weak signal near the edge of the domain boundary as shown in Figure S1a,b (Supporting Information), unlike the general phenomenon in MFM measurements. [28]This should be attributed to the thick effective domain boundary region originating from the 71°ferroelectric domain wall inclining by 45°f rom the film normal as schematically illustrated in Figure S1 (Supporting Information) and small magnetic signal owing to the weak ferromagnetism.These results demonstrate that there is a correlation between electric polarization and magnetization in BFCO/GdScO 3 (110) o . [15]o investigate the magnetization change induced by the inplane polarization switching, the counter Pt electrodes (thickness: ≈200 nm) were patterned on top of the BFCO layer (Figure 3a).We aligned the edge of the Pt electrode parallel to the ferroelectric stripe so that the in-plane 71°polarization switching was accessible.The gap length l determines the magnitude of the applied voltage required for the poling.Figure 3b shows the topographic AFM image of BFCO between the Pt electrodes.A step-terrace structure is observed in the BFCO area, which may be attributed to the dissolution of surface precipitates caused by solutions used during the lithographic process.The distance between the Pt electrode area (l) was determined to be 1.4 μm.The width of electrodes w (≈100 μm) was designed to be substantially large compared to l so that the effect of roundness of an electric line of force can be neglected at the PFM/MFM measurement area.According to in previous study, [19] there is a quantitative relationship of (coercive field)∝(channel width) −k based on the finite element modeling, and the power law k was determined to be 0.62±0.03from the fitting of experimental data.By adopting the coercive field 400 kV cm −1 of 200-nm-thick in BFCO films, [8,14] the coercive field in the present planar electrode was estimated to be ≈120 kV cm −1 (corresponding to ≈17 V).Indeed, we experimentally found that 22.5 V was large enough to induce the inplane polarization switching.Figure 3c  Next, we describe how in-plane electric poling changes the ferroelectric and magnetic domain structures.Figure 4 shows the 3D-PFM and MFM images obtained in the regions that underwent polarization switching by an in-plane electric field of 22.5 V.In the PFM images, two of eight possible polarization variants can be observed which form a "head-to-tail" ferroelec-tric domain with the 71°domain boundary.However, the overall domain shape is completely reconfigured compared to the as-grown state, with increased domain size by more than three times after the poling process.Note that the contrast remained the same in the out-of-plane PFM images, indicating that there is no change in out-of-plane polarization and only in-plane polarization switching occurred.The MFM image obtained in the sample area is also shown in Figure 4.The magnetic domain is also fully reconstructed from the as-grown state upon the electric poling.Nevertheless, the magnetic and ferroelectric domain boundaries are closely matched, indicating that the ferroelectric and magnetic domains are correlated with each other.
We investigated how the shape of the electric polarization and magnetization changes with multiple in-plane poling by applying positive and negative voltage alternately and obtained PFM and MFM images each time.At the second and third polings, the typical 71°switchings where greenish stripes turn to bluish stripes (or vice versa)with preserving the head-to-tail ferroelectric domains were observed.After 100 cycles of poling, the striped domains are observed to remain in most of the measurement  area, but the merged stripes (called "blocked domains" by Zou et al. [21] ) indicated with dashed lines also appear concomitantly with the domain boundary perpendicular to the electrode/film interface (charged 71°tail-to-tail domain boundary).The generation of such domains originates from charge injection from the electrode and will lead to polarization fatigue upon further poling. [21]The charged domains are related to the higher leakage current of BFCO than BiFeO 3 and can cause polarization fatigue in BFCO after much fewer cycles than in BiFeO 3 (1.2 × 10 9 cycles [21] ).Nevertheless, in-plane Pt electrodes are preferable for investigating the magnetization change induced by an electric field multiple times via the 71°polarization switching without disrupting the ferroelectric domains.
The MFM images obtained in the same region are displayed next to the corresponding 3D-PFM images in Figure 4.The correlation between the ferroelectric and the magnetic domains was observed even after 100 times of polarization switching.Except for the "blocked domain" region, the light green and deep blue domains in the 3D-PFM images correspond to the red domain in the MFM images, and the deep green and light blue domains correspond to the blue domain as well.The light green and deep blue domains (deep green and light blue domains) are in an inplane 71°polarization switching relationship during the poling process.These results indicate that in-plane 71°polarization reversal without fully reconstructing striped ferroelectric domains does not change its out-of-plane magnetization.
BFCO has a magnetic easy plane perpendicular to the electric polarization axis. [13]Mössbauer spectroscopy on bulk, [29] (111) pcoriented thin film, [14] and (001) pc -oriented thin film [15] has revealed that the antiferromagnetic spin vector L in BFCO aligns perpendicular to the electric field gradient which is along the polarization axis P (drawn as a blue plane in Figure 5a).Note that the spontaneous magnetization M governed by the DM interaction is also expected to be in the plane perpendicular to P. Mössbauer spectroscopy on (001) pc -oriented thin film also demonstrated that the angle of L from the substrate normal direction was ≈63°.This angular condition is visualized as the two yellow cones with an apex angle of 2 × 63°in Figure 5a.From the above conditions, the possible directions of L in BFCO are limited to the four indicated by L 1 -L 4 , where the blue surface and the yellow cone intersect.Now we consider the change in spin direction in the 71°polarization reversal.We fix the initial polarization direction to P 1-11 as in Figure 5b.The experimental results in this study make it possible to compare the magnetization changes between out-ofplane 71°polarization switching (here P 1-11 → P 111 ) and in-plane 71°polarization switching (P 11-1 → P 1-1-1 ).There should be four possible directions of antiferromagnetic spin vectors L 1 '-L 4 ' perpendicular to P 111 or P 1-1-1 even after the polarization switching.In both cases, the out-of-plane magnetization is expected to reverse when the spin direction changes to L 2 ' or L 3 '.This is the case for our previous observation of the magnetization reversal induced by an out-of-plane electric field.However, no change was observed in the out-of-plane component of the magnetization in the present in-plane polarization reversal, indicating that the inplane polarization switching resulted in the spin direction L 1 ' or L 4 '.Thus, this experiment has demonstrated that the occurrence or absence of magnetization reversal by an electric field is deterministic and depends on the direction of the applied electric field (out-of-plane or in-plane).The above discussion is based on the  experimental results obtained from the (001) pc -oriented BFCO thin film, where the 71°polarization switching is most preferable.We expect if the similar planer electrodes are fabricated on (110) pc -oriented BFCO thin film and an electric field is applied along [1-10] pc or [1-11] pc direction, we can access the 109°and 180°polarization directions, and the out-of-plane magnetization reversal may be detected.

Conclusion
In conclusion, we explored the reversal phenomena induced by in-plane polarization switching in (001) pcoriented BFCO thin film on a GdScO 3 substrate.By using lithographically-fabricated Pt counter electrodes, we could observe the magnetic domain using MFM after in-plane polarization switching up to 100 times.The in-plane 71°polarization switching of striped ferroelectric domains does not change its out-of-plane magnetization, which is in contrast to the out-ofplane 71°polarization switching investigated in our previous work.The findings in this study should facilitate the design of memory devices utilizing BFCOs and other multiferroic materials.
shows the in-plane PFM image scanned along the [100] pc direction.Compared with Figure 2b, no degradation caused by the lithographic process was found in the ferroelectric domains.

Figure 3 .
Figure 3. a) Schematic top view of Pt electrodes deposited on BFCO for in-plane polarization switching.The parameters l and w indicate the width and gap length of the Pt electrodes, respectively.b) AFM topographic image and c) in-plane PFM phase images.

Figure 4 .
Figure 4. 3D-PFM and MFM images of BFCO/GdScO 3 (110) o after electrical switching via Pt electrode.The 3D-PFM images are colored in accordance with the schematic image.

Figure 5 .
Figure 5. a) Possible spin direction L i (i = 1-4) in BFCO/GdScO 3 (110) o thin film.b) Changes in spin direction L and magnetization M after magnetization reversal induced by an electric field.Out-of-plane and in-plane polarization switching are compared.