Direct Observation of Magnetization Reversal by Electric Field at

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Direct observation of magnetization reversal by electric field at room temperature in Co-substituted bismuth ferrite thin film Keisuke Shimizu, Ryo Kawabe, Hajime Hojo, Haruki Shimizu, Hajime Yamamoto, Marin Katsumata, Kei Shigematsu, Ko Mibu, Yu Kumagai, Fumiyasu Oba, and Masaki Azuma Nano Lett., Just Accepted Manuscript • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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Direct observation of magnetization reversal by electric field at room temperature in Co-substituted bismuth ferrite thin film Keisuke Shimizu,*,† Ryo Kawabe,† Hajime Hojo,*,†,‡ Haruki Shimizu,† Hajime Yamamoto,†,§ Marin Katsumata, Kei Shigematsu,† Ko Mibu,|| Yu Kumagai,# Fumiyasu Oba,† and Masaki Azuma† †

Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama 226-8503,

Japan ||

Graduate School of Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan

#

Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama

226-8503, Japan KEYWORDS. multiferroic material, magnetoelectric coupling, weak ferromagnetism, bismuth ferrite

ABSTRACT. Using the electric field to manipulate the magnetization of materials is a potential way of making low-power-consumption non-volatile magnetic memory devices. Despite concentrated effort in the last 15 years on magnetic multilayers and magnetoelectric multiferroic thin films, there has been no report on the reversal of out-of-plane magnetization by an electric field at room temperature without the aid of an electric current. Here, we report direct

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observation of out-of-plane magnetization reversal at room temperature by magnetic force microscopy after electric polarization switching of Cobalt-substituted bismuth ferrite thin film grown on (110)o-oriented GdScO3 substrate. A striped pattern of ferroelectric and weakly ferromagnetic domains was preserved after reversal of the out-of-plane electric polarization.

Bismuth ferrite, BiFeO3 (BFO), is the most promising magnetoelectric multiferroic material because of its high antiferromagnetic Néel temperature (TN) of about 600 K, high ferroelectric Curie temperature (TC) of about 1100 K, and strong coupling between these two orders.1-4 The crystal structure of BFO is a rhombohedral perovskite type with an R3c space group, where the electric polarization, mainly coming from Bi3+ ion displacement, is along the [111]pc or [001]h direction (pc and h denote pseudo-cubic and hexagonal notation, respectively), and the FeO6 octahedra have an anti-phase rotation around the polar axis5 (a-a-a- in Glazer’s notation).6 Spontaneous magnetization arising from spin canting due to the spin-orbit interaction and breaking inversion symmetry induced by the FeO6 octahedral rotation, known as the Dzyaloshinskii-Moriya (DM) interaction, has been predicted,7 but the presence of a long-range cycloidal modulation propagating along the h direction with a period of 62 nm superimposed on a G-type antiferromagnetic ordering prohibits the appearance of a net magnetization.8 This cycloidal spin modulation disappears under a strong magnetic field of over 18 T, and a weak ferromagnetic moment of about 0.03 µB/f.u. has been observed.2 It has also been reported that the spin structure of BFO is quite sensitive to epitaxial strain in thin film samples. In particular, BFO thin films grown on substrates with a large lattice mismatch, such as SrTiO3 (about −1.7%) and NdScO3 (about 0.9%), had a collinear spin structure.9 The free energy of the DM interaction is written as


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EDM = − D·(L×M)

(1)

where D is the DM vector, L is the antiferromagnetic spin vector, and M is the weak ferromagnetism vector.7 The DM vector in BFO is governed by the manner of rotation of the FeO6 octahedra and aligned parallel or antiparallel along the electric polarization vector P. Therefore, when the antiferromagnetic spin vector lies in the plane perpendicular to the polarization, a weak ferromagnetic moment is generated in the same plane, but so far there have been no reliable reports on such spontaneous magnetization in BFO film. Recently, J. T. Heron et al. predicted that 71° and 109° polarization switching can cause a change in the octahedral rotations and, hence, a change in the DM vector.4 They have also revealed that 180° switching takes place via sequential 71° and 109° switching in a step-wise process, leading to a change in the DM vector while it is preserved through direct 180° switching. Therefore, the weak magnetic moment should be able to be manipulated by polarization switching induced by application of an electric field. They demonstrated an in-plane magnetization reversal of a Co0.9Fe0.1 upper layer by two-step 180° polarization switching in BFO thin films by X-ray magnetic circular dichroism coupled to photoemission electron microscopy (XMCD-PEEM) imaging. However, the presence of a Co0.9Fe0.1 upper layer complicates the interpretation of the experimental results including the way of coupling between the magnetic moment of Co0.9Fe0.1 alloy and that of BFO.10 Direct observation of magnetization reversal on a single-phase material with ferroelectric and ferromagnetic orderings is crucial to the study of the intrinsic coupling between the two orders. Moreover, out-of-plane magnetization reversal is desirable from the viewpoint of integration.


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Cobalt-substitution is one of the powerful ways to modify the electric and magnetic properties of BFO in both bulk and thin-film forms.11-13 We have found that partial substitution of Co for Fe stabilizes the collinear phase with weak ferromagnetism preserving the R3c crystal structure up to x = 0.20 for bulk BiFe1-xCoxO3.14 Neutron powder diffraction and Mössbauer spectroscopy measurements on x = 0.2 and 0.1 bulk samples respectively revealed a change in the spin structure from a cycloidal one at low temperature to a collinear one at room temperature, as schematically illustrated in Figs. 1a and b.15 In the collinear phase, the ordered spin moment lies in the (001)h plane, perpendicular to the electric polarization vector, indicating that the abovediscussed magnetic easy plane is not affected by Co substitution.14, 15 This result is consistent with our DFT calculations, and thin film samples grown on (111)-oriented SrTiO3 substrates had a spontaneous magnetic moment of about 0.03 µB/f.u. in the (001)h plane.16 The DFT calculations also indicated that Co substitution increases the spin canting angle and enhances the spontaneous magnetization. Reorientation of the magnetic easy plane after ferroelectric poling was demonstrated through remnant magnetization measurements on single crystals.17 However, the in-plane nature of the magnetization of these thin film samples prohibited us from observing the magnetic domains by using a local probe, i.e., magnetic force microscopy (MFM). The (001)pc-oriented thin film is therefore favorable for the investigation of the magnetization reversal by electric polarization switching because the out-of-plane magnetization is expected to appear: the magnetic easy plane perpendicular to the polarization direction (pc) tilts from the film plane, as illustrated in Fig. 1c. Since the magnetic structure of BFO is quite sensitive to lattice strain, the choice of single crystal substrate for thin film growth is of great importance.9, 18 Here, (110)o-oriented (o denotes orthorhombic notation) GdScO3 (GSO) (apc about 3.969 Å) substrate was selected because of the


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small lattice mismatch of about 0.5% with BiFe0.9Co0.1O3 (BFCO). Indeed, BFO thin films grown on the GSO substrate have been reported to have a cycloidal spin structure, the same as that of bulk samples.9 Our DFT calculation showed that the presence of substituted Co and a 0.5% lattice mismatch do not affect the magnetic easy plane, as shown in Fig. S1. Figure 1d shows the θ-2θ XRD pattern of BFCO thin film grown on SrRuO3 (SRO)/(110)ooriented GSO substrate. It shows highly crystalline BFCO thin film that is (001)pc-oriented and without detectable impurity phases. Since the out-of-plane lattice constant of SRO layer is close to that of BFCO, their diffraction peaks overlap. The Laue fringes come from 10 nm-thick SRO layer. Figure 1e shows reciprocal space mappings (RSM) around the 510o(203pc) and 442o(113pc) reflections of GSO. The 203pc and 113pc reflections of BFCO split into two and three spots, respectively, indicating that the crystal structure is a rhombohedral-like monoclinic one, the same as that of BFO thin films on STO substrates.19 The lattice parameters, am, bm, cm, and , are 5.637 (√2 × 3.986) Å, 5.594 (√2 × 3.955) Å, 3.941 Å, and 89.33°, respectively. Note that the monoclinic unit cell is defined as √2 apc × √2 bpc × cpc,11, 19, 20 as shown in the inset of Fig. 1d. The out-of-plane lattice parameter cm is slightly smaller than the average of the in-plane lattice parameters ((am/√2 + bm/√2)/2, about 3.971 Å), indicating that moderate tensile strain (about 0.47%) is imposed on the BFCO thin film. Therefore, the BFCO thin film is expected to have a collinear spin structure like that of bulk samples. Conversion electron Mössbauer spectroscopy (CEMS) on a nearly 100%

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Fe-enriched thin

film sample confirmed that the BFCO thin film on GSO substrate has a collinear spin structure. The bottom of Figure 2a shows the CEMS spectrum of the BFCO thin film at room temperature. The spectrum is symmetric in the height and the width, whereas the spectrum of the BFO thin film on the same substrate with a cycloidal spin structure (the top of Fig. 2a) is asymmetric.9 The


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data is well fitted with one magnetic component, which has a Zeeman sextet with a symmetric height and an isomer shift (IS) of 0.385 mm/s, quadrupole shift (QS) of −0.217 mm/s, and hyperfine field (Bhf) of 47.7 T, indicating that the magnetic structure is a collinear one. The value of IS is characteristic of Fe3+ ions in octahedral coordination. The value of QS is close to that of bulk BFCO (-0.20 mm/s) and that of (111)pc-oriented BFCO thin films (-0.217 mm/s),15,

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strongly suggesting that the spins are aligned perpendicular to the electric field gradient (Vzz > 0),15 which is along the polar axis. This indicates that the BFCO thin film on GSO substrate has a magnetic easy plane perpendicular to the electric polarization axis, L ⊥ P (Fig. 2b), as in the case of bulk15 and (111)pc-oriented thin film samples.16 Here we define the polarization axis as [111� ]pc

since out-of-plane polarization is downward as discussed in the next paragraph. The area

intensity ratio of six lines is 3:2.62:1:1:2.62:3, indicating that the angle between the ordered spin moment (antiferromagnetic spin vector) and the incident direction of the γ-rays, the substrate normal direction in this case, is about 63° (Fig. 2c). Taking into account these conditions, the possible directions of L are limited to four, as schematically illustrated in Fig. 2d. These are quite ��]pc and [2�11�]pc. Since L are in four of six directions in the close to [1�21]pc, [21�1]pc, [121 easy plane perpendicular to P along the [111� ] direction (the blue directions in Fig. 2e), it is reasonable to assume that BFCO has six favorable magnetic spin directions and two of these

were not chosen most probably because of the epitaxial strain. The presence of such a magnetic easy axis has been predicted for highly strained BFO film.21 Our attempt to theoretically reproduce the experimentally observed magnetic easy axis on Bi(Fe,Co)O3 thin films with moderate tensile strain was not successful, probably because our model assumes order in the atoms, whereas the samples in the experiments show disorder, which can induce local strain in the lattice. Each ferroelectric domain has four possible magnetization directions, all of which


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have an out-of-plane ferromagnetic component. The spontaneous magnetization M is in four directions, perpendicular to both L and P. Magnetization reversal due to 71° ferroelectric switching is expected to occur if the octahedral rotation is accompanied by a change in the L direction (Fig. 2f). The presence of spontaneous magnetization was confirmed by making remnant magnetization measurements using a superconducting quantum interference device (SQUID) magnetometer, as shown in Fig. S2. By reducing the thickness of the GSO substrate, which has a large paramagnetic contribution from Gd3+, to about 40 m, we succeeded in observing a net out-of-plane remanent magnetization of the BFCO thin films at room temperature, which disappears at low temperature because of the change to the cycloidal spin structure. The correlation between the ferroelectric and ferromagnetic domains was investigated by performing piezoelectric force microscopy (PFM) and MFM on the as-grown BFCO thin film. Since the polarization directions in the monoclinic phase are close to pc, there are eight possible ferroelectric domain variants with four polarizations upward and four polarizations downward relative to the film normal, as shown in Fig. 3a. PFM and MFM measurements were performed before and after an electric poling, as shown in Fig. 3b. Figure 3c shows the total PFM image of a 6 µm×6 µm area on the as-prepared film. A striped domain configuration with four polarization down domains can be seen. All of the adjacent domains have 71° domain walls. Such striped domain structures commonly appear in BFO and other ferroelectric thin films in order to minimize the electrostatic and elastic energies.22-25 Figure 3d shows the MFM phase image of the same area. The ferromagnetic domains form a striped structure similar to that of the ferroelectric domain, indicating that both domains are strongly coupled with each other. Here, reversal of the magnetization of the cantilever causes a


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reversal of the MFM image contrast, confirming that the contrast comes from the magnetic interaction, not the electrostatic one (see, Fig. S3). These results are consistent with the X-ray magnetic linear dichroism coupled to photoemission electron microscopy (XMLD-PEEM) observations made by T. Zhao et al. on a BFO thin film, where the antiferromagnetic domains are coupled with the ferroelectric domains.3 We also conducted MFM observations on BiFe0.95Co0.05O3 thin film in which spin structure changes from low temperature cycloidal one to high temperature collinear one with spontaneous magnetization at 250-400 K15 to further confirm the origin of the MFM contrast. The MFM phase image at RT where cycloidal phase without spontaneous magnetization is dominant (Fig. S4)) shows no clear contrast while the image at 423 K has clear domain structure unambiguously confirming that the contrast is magnetic in origin. Next, magnetization reversal by polarization switching was demonstrated. A poling scan with a [11� 0]pc slow scan direction and a -7 V tip bias was applied to a square region measuring 4 m×4 m, as shown in Fig. 3b. The total PFM image after a poling scan is shown in Fig. 3e.

The out-of-plane polarization component is successfully switched from downward to upward in the entire poled region. There are only three domains, colored red, pink, and yellow, and with the exception of the boundary regions there is no orange domain with an [1 1� 0]pc in-plane polarization. This is because of an effective in-plane electric field created by a cantilever motion during the poling scan, the so-called trailing field.24-26 In the case of a box poling scan (like Fig. 3b), the in-plane field is only activated along the slow scan axis, because the trace and retrace scans cancel out the field during each fast scan.24 In the present poling scan, the induced trailing field points in the [11� 0]pc direction, so the orange domain with a [1�10]pc in-plane polarization is

electrostatically unfavorable and does not remain.


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Figure 3f shows an MFM phase image after electric poling. It is clear that the out-of-plane magnetization is changed drastically by the poling and the magnetic domain structure is coupled with the ferroelectric domain structure after the poling but not with the one before the poling. It seems that the phase contrast on the left and right side near the poling region gets a phase shift (i.e. becomes red in this image). This is presumably due to the influence of the electrostatic force in the poled region.27 Figures 4a-d, and 4e-h are magnified views of areas (i) and (ii) in Fig. 3c. An almost identical striped domain structure is observed after the poling in area (i) with the in-plane polarization direction unchanged, as shown in Figs. 4a and c, indicating that pure out-of-plane 71° domain switching occurred in this area. On the other hand, the striped domain structure is heavily modified in area (ii). The in-plane polarization direction is also different, as shown in Figs. 4e and g. Such differing behaviors between two regions can be attributed to the different in-plane polarization directions in the initial domain structures. The net in-plane polarization (Pip_net), i.e., the vector sum of the in-plane electric polarizations of each domain, in area (i) points toward the lower right. It therefore conforms to the [11�0]pc trailing field. On the other hand, Pip_net in area

(ii) points to the upper right and the trailing field changes it to pointing to the lower right. The magnetic domain structure in area (i) is also preserved, as shown by the MFM phase contrast in

Figs. 4b and d, but the color is reversed, indicating that the out-of-plane magnetization switched from downward to upward and vice versa in this area. This is the first direct observation of magnetization reversal by the electric field at room temperature. On the other hand, although the MFM phase contrast is certainly changed by the poling in area (ii), the contrast is correlated with the ferroelectric domain structure after the poling, as shown in Figs. 4f and g. These results indicate that an out-of-plane magnetization reversal can be achieved by 71° polarization


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switching without reconstruction of the ferroelectric domain. It should be noted that the positions of the ferroelectric and ferromagnetic domains in Fig. 4 do not completely match. We believe that this is due to the difference in surface sensitivity between the MFM and PFM methods and the inclination of the 71° ferroelectric domain wall relative to the film normal (shown in Figs. S5 and S6). In summary, we confirmed that a single-phase ferromagnetic ferroelectric BFCO thin film grown on (110)o-oriented GSO substrate has a canted collinear spin structure with an out-ofplane magnetization by CEMS. Moreover, we observed an out-of-plane magnetization reversal by 71° polarization switching without ferroelectric domain reconstruction by using MFM and PFM. The current demonstration of magnetic reversal by an electric field paves the way to lowpower-consumption magnetic memories such as magnetoresistive random access memory. Experimental and Calculation Details BFCO (x= 0.10) thin films were prepared on (110)o-oriented GdScO3 substrates by using pulsed laser deposition with a KrF excimer laser (λ= 248 nm). A pulsed laser was focused on stoichiometric targets with an energy density of 1.5 J/cm2. During the deposition, the substrate was kept at 690°C in an oxygen partial pressure of 15 Pa. The thicknesses of all thin films were 100 nm. A SrRuO3 layer with a thickness of 10 nm, estimated from the period of Laue fringes, was deposited as the bottom electrode. The crystal structure of the thin films was investigated by making X-ray diffraction measurements with Cu Kα radiation (XRD; Rigaku SmartLab). 57

Fe Mössbauer spectra were measured at room temperature on nearly 100%

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Fe-enriched

BFCO thin films by using conversion electron Mössbauer spectroscopy. 57Co in a Rh matrix was used as the γ-ray source, and the Doppler velocity of the source was calibrated by using the


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standard spectrum of α-Fe foil. The spectra were analyzed with standard software (Normos, made by R. A. Brand and commercially available from WissEl GmbH). Temperature dependence of remanent magnetization of BFCO thin film was measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS). After a magnetic poling of 5 T at room temperature, remnant magnetization (Mr) was measured at 0 T on cooling from 400 K to 5 K. To reduce the paramagnetic contribution of the GSO substrate, the thickness of GSO was reduced from ~500 µm to ~40 µm by polishing the substrate (like TEM sample preparation). Note that a non-magnetic sapphire substrate was glued to the sample by epoxy adhesive for reinforcement. The PFM and MFM observations were performed using a scanning probe microscope (Asylum Research Cypher). The vertical and lateral PFM images were obtained using dual ac resonance tracking PFM (DART-PFM). An ac voltage of 3 V at a frequency of about 250 kHz (vertical) or 900 kHz (lateral) was applied to a conductive cantilever (Asylum Research ASYELEC-01) to obtain the PFM contrast. All PFM images plot the amplitude × cos (phase) signal. An in-plane PFM scan performed along the pc direction can discriminate only two of four variants, corresponding to in-plane polarization direction being to the left or right of the cantilever. Hence, two in-plane PFM scans with in-plane scanning angles of 0° and 90° relative to the [110]pc direction and one out-of-plane PFM scan were performed to obtain the total PFM image. The total PFM image was created by merging the three obtained images, using commercial photo-editing software (see Fig. S7). MFM images were obtained with a hard magnetic cobalt alloy coated cantilever (NanoWorld MFMR) with a spring constant of 2.8 N/m and a resonance frequency of 75 kHz and a quality factor of ~ 300 at room temperature by using tapping/lift modes. The lift height in the scan was set at 20 nm. To distinguish the magnetic force


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from the electrostatic force, etc., we performed two MFM scans in which the magnetization direction on tip poled upward in one scan and downward in the other, as shown in Fig. S3. Precipitates or pits in the topographic images concomitantly obtained in PFM and MFM scans were utilized to align the MFM and PFM images. Firstly, a scan in a large area of 50 µm×50 µm was performed in tapping-mode to find the target area. Then, MFM or PFM scan in the area of 6 µm×6 µm was performed. High-temperature MFM observations were performed by using Asylum Research MPF3D (see Fig. S4). For the calculations of the magnetic energy landscapes of BiFeO3 and Bi(Fe,Co)O3 thin films on (110)pc-oriented GdScO3 substrates, we used the local spin density approximation (LSDA) and the projector augmented wave (PAW) method28 as implemented in VASP code.29 PAW radii of 1.6, 1.2, 1.2, and 0.8 Å and valence electrons of 6s2 6p3, 3d7 4s1, 3d8 4s1, and 2s2 2p4 were used for Bi, Fe, Co, and O, respectively. The plane-wave cutoff energy was set to 520 eV. The convergence criterion of the self-consistent electronic structure iterations was 10-8 eV. We used a 2 × 2 × 2 pseudo-cubic supercell containing 40 atoms, as illustrated in Fig. S1a. As the LSDA calculation underestimated the lattice constant of bulk BiFeO3 by 1.9%, we constructed the supercells by reducing the experimental lattice parameters of our thin films by the same ratio. A 4 × 4 × 4 Monkhorst-Pack mesh was used for the reciprocal space integration. The Co substituted model was constructed by replacing one-eighth of the Fe with Co, as illustrated in Fig. S1b. The concentration of Co in our model (12.5%) was very close to the experimental one (10%), even though the atomic disorder effect was not taken into account.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: *******. Calculated magnetic anisotropy, remanent magnetization measurement, and the details of PFM and MFM experiments. (PDF)

AUTHOR INFORMATION Corresponding Author *(K.Shimizu.) E-mail: [email protected] *(H.H.) E-mail: [email protected] *(M.A.) E-mail: [email protected] Present Addresses Department of Energy and Material Science, Kyushu University, Kasuga 816-8580, Japan

‡

§

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-

8577, Japan Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported by Grants-in-Aid for Scientific Research 15K14119, 16H00883, 16H02393, 17H04952 and 17K14105 from the Japan Society for the Promotion of Science, by


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the Kanagawa Institute of Industrial Science and Technology (KISTEC). The authors thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for PFM and MFM analysis. The Mössbauer spectroscopic study was supported by the Nanotechnology Platform Program of MEXT, Japan.

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Figure 1. a, The cycloidal phase with a modulation propagating along the h direction without a net magnetization, which is stable below 200 K for x=0.1 BFCO. b, The canted collinear phase with a weak ferromagnetic moment M (green arrow), which is stable at room temperature for x=0.1 BFCO. The weak ferromagnetic moment lies in a plane perpendicular to the polarization vector, P (pink arrow). Orange spheres represent magnetic Fe/Co ions, and small blue arrows represent the spins at Fe/Co sites. c, Schematic illustration of the out-of-plane magnetization Mz in (001)pc-oriented BFCO thin film. d, The XRD θ-2θ profile of the BFCO thin film. The inset in Fig. 1d is a monoclinic unit cell of BFCO defined as √2 apc ×√2 bpc × cpc. e, Reciprocal space maps of the BFCO thin film around 510o and 422o reflections of GSO substrate, indicating the rhombohedral-like monoclinic crystal structure with a moderate tensile strain (about 0.47%).


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BFO (ref. 9)

Figure 2. a, The CEMS spectrum for BFO thin film on GSO substrate (top) reported by D. Sando et al. (Adapted with permission9. Copyright 2013, Nature Publishing Group) and the BFCO thin film on GSO substrate (bottom). The insets are schematic illustrations of the cycloidal phase and the canted collinear phase. b, The spin direction L is determined from the QS value. The spin is on the magnetic easy plane (blue disk) perpendicular to the polarization vector P. c, The spin tilt angle of about 63° from the film normal is determined by the area intensity ratio of the CEMS spectra. The spin is on two yellow cones, the apex angle of which is about 2 × 63°. d, The possible spin structure of the BFCO thin film satisfying both conditions of 2b and 2c. Assuming that the polarization points in the [111� ] direction, four magnetic states are allowed. e, The L vectors in the easy plane as viewed from the P direction. These are aligned along four of six directions. f, One of the possible magnetization switching processes accompanying out-of-plane 71° polarization switching, where the out-of-plane magnetization is reversed.


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Figure 3. a, Possible polarization directions of the BFCO thin film. The framework indicates a pseudo-cubic cell. Each arrow indicates a polarization vector along the eight directions. b, The configuration of the poling scan. The trailing field along the slow scan direction [11� 0]pc is generated by cantilever motion. c-d, Total PFM and MFM phase images for as-grown BFCO thin film. The color of each domain in the PFM image corresponds to the polarization direction. The relationship is shown by the indicator at the bottom left. The colors in the MFM image correspond to out-of-plane magnetization directions. e-f, Total PFM and MFM phase images after poling for BFCO thin film. The yellow arrow indicates the trailing field generated by the poling scan. The white scale bars in Figs. 3e-f correspond to 1 µm.


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Figure 4. a-d, Total PFM (a,c) and MFM phase (b,d) images for as-grown (a,b) and after poling (c,d) BFCO thin film in area (i) in Fig. 3c. e-h, Total PFM (e,f) and MFM phase (g,h) images for as-grown and after poling in area (ii) in Fig. 3c. Out-of-plane magnetization switching can be achieved by using 71° polarization switching without reconstruction of the ferroelectric domain. The area in these images is 1µm×1 µm.


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ToC FIGURE


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Direct Observation of Magnetization Reversal by Electric Field at

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