Magnetization reversal of [Co/Pd] perpendicular magnetic thin film dot on (Bi,La)(Fe,Co)O₃ multiferroic thin film by applying electric field

The magnetoelectric effect can be potentially exploited in the design of high-performance magnetic devices with low power consumption. For this purpose, the electric-field control of magnetization has been widely studied by several techniques. Among the structures reported are piezoelectric and magnetostrictive laminate composites [1], multilayer structures [2], magnetic tunnel junctions with a very-thin-CoFeB metallic magnetic layer [3, 4], a α-Cr₂O₃ dielectric and antiferromagnetic layer and a very-thin Co metallic magnetic layer [5, 6], a BiFeO₃ ferroelectric and antiferromagnetic layer and a very-thin Co metallic magnetic layer [7–9], a (Dy,Tb)FeO₃ ferroelectric and ferromagnetic material [10], and Ta/CoFeB/MgO devices with a perpendicular anisotropy [11]. Electric-field control of the perpendicular magnetic anisotropy has been also studied in multiferroic heterostructures of (Co/Pt)₃/PbMg_1/3Nb_2/3O₃-PbTiO₃ [12] and BiFeO₃/Al₂O₃/Pt/Co/Pt [13]. However, to realize magnetization reversal under an electric field, the systems or materials require a high operation speed of magnetization and an operation temperature considerably higher than room temperature. They should also not require a specific electric-field frequency, a DC magnetic field, a specific magnetic material or composition, or a specific film thickness (in most of the above examples, the films were very thin). As none of the systems and materials satisfy all of these requirements, they cannot be universally applied to magnetic devices driven by electric fields.

BiFeO₃ (BFO), which is both antiferromagnetic and ferroelectric at room temperature, is well-known as a typical multiferroic material [14]. Ferromagnetism and ferroelectricity have also been obtained in (Bi_1–x Ba_x )FeO₃ (BBFO), in which the Bi of BFO is partially substituted by Ba [15]. Moreover, partial substitution of the Bi (A-site) or Fe (B-site) with La or Co in thin films has introduced ferromagnetism in BFO [16–20]. However, the saturation magnetization (M_s) was smaller in these thin films than in BBFO compounds [15].

Previously, we fabricated high-quality BFO-based ferromagnetic/ferroelectric thin films using the pulsed-DC reactive sputtering method [21], which effectively deposits thin films of oxide and nitride by suppressing the arc discharge during sputtering. This method yielded a BBFO thin film with an M_s of approximately 90 emu cm⁻³ [22], 1.5 times higher than that of BBFO thin films prepared by the radio-frequency (RF) (13.56 MHz) sputtering method [23]. This improvement can be attributed to accelerated crystallization of the BFO phase caused by the enhanced surface diffusion of sputtered atoms on the substrate surface. We also fabricated high-quality (Bi_1–x La_x )(Fe,Co)O₃ (BLFCO) thin films with ferromagnetism/ferroelectricity using the pulsed-DC reactive sputtering method and detailed their magnetic properties. The maximum M_s, perpendicular coercivity (H _{c ⊥} ), and Curie temperature were 70 emu cm⁻³, 4.0 kOe, and 420 K, respectively. We then demonstrated magnetization reversal in BLFCO thin films under a local electric field with a width of several hundred nanometers [24, 25].

However, the magnetic properties of the above BLFCO thin films were insufficient for universal magnetic-device applications driven by electric fields. In fact, developing new multiferroic thin films with highly functional magnetic properties (high M_s, high perpendicular magnetic anisotropy, high spin polarization, and large magnetic optical Kerr effect) is a difficult task, although such magnetic properties would ensure high performance of magnetic device applications. If the magnetization direction of a highly functional magnetic thin film fabricated on a multiferroic thin film can be controlled by the multiferroic film under an electric field, high-performance magnetic devices can be easily realized. In this study, we fabricated multilayer structures of high-quality BLFCO multiferroic thin film/[Co/Pd] perpendicular magnetic thin film dots and demonstrated the transfer of magnetization reversal in the BLFCO thin film to magnetization reversal in the [Co/Pd] dots.

Ta (5 nm)/Pt (100 nm)/(Bi_0.41La_0.59)(Fe_0.72Co_0.28)O₃ (BLFCO) (200 nm) film was deposited on a thermally oxidized Si wafer in an ultrahigh-vacuum sputtering system. The Ta seedlayer, Pt underlayer, and BLFCO layer were deposited at room temperature, 400 °C, and 695 °C, respectively. The film thickness and deposition temperature of the Ta seedlayer and Pt underlayer were optimized to strengthen the (111) orientation of the Pt underlayer [26]. During the RF sputtering deposition of the Pt underlayer and the pulsed-DC reactive sputtering deposition of the BLFCO layer, plasma was irradiated at very high frequency (40.68 MHz) [27] and an electric power of 5 W, obtaining crystal grain growth of the face-centered cubic-(111)-oriented-Pt and BFO phases. The frequency of pulsed DC sputtering was fixed at 200 kHz. The duty ratios of ON and OFF sputtering were 3 and 2, respectively; for example, sputtering was turned ON for 3 μs and OFF for 2 μs at 200 kHz. The sputtering power of pulsed DC was fixed at 150 W. A [Co(0.7 nm)/Pd(2.0 nm)]₄ thin film (total thickness ∼10 nm) was deposited by normal DC sputtering onto the BLFCO thin film at room temperature. The [Co/Pd] dots with 3 μm diameter on the BLFCO layer were fabricated by photo lithography and liftoff process. Co_80.0Zr_5.3Nb_14.7 (10 nm) and/or MgO (5 nm) thin films were also deposited by normal DC and/or RF sputtering on a BLFCO thin film at room temperature.

The compositions of the fabricated thin films were analyzed by energy dispersive x-ray spectroscopy. The composition of thin films described in this section (experimental methods) were analyzed by this system. The crystallographic orientation, crystalline structure, and topographic image of the BLFCO thin film were analyzed by x-ray diffraction (XRD) and scanning probe microscopy (SPM). The magnetization curves of the BLFCO thin film, [Co/Pd] thin film, BLFCO/[Co/Pd] multilayer structure, Co-Zr-Nb thin film, BLFCO/Co-Zr-Nb multilayer structure, and BLFCO/MgO/Co-Zr-Nb multilayer structure were measured using a vibrating sample magnetometer while an in-plane/out-of-plane magnetic field was applied to the film surface, and/or using a magneto optical Kerr effect measurement system while an out-of-plane magnetic field was applied to the film surface. The ferroelectric hysteresis loops of the BLFCO thin film was measured using a ferroelectric tester. The local electric field was applied to the BLFCO/[Co/Pd] multilayer structure using SPM in contact mode with a conductive magnetic Co-Cr-Pt tip. The magnetic domain structures of the [Co/Pd] dots on the BLFCO thin film were analyzed by magnetic force microscopy (MFM) with a conductive magnetic Co-Cr-Pt tip.

Figure 1 shows the XRD profile of the BLFCO thin film on Ta/Pt layer. The topographic image of this thin film is also shown in this figure. The Pt underlayer was found to have a strong (111) orientation. The BLFCO film was found to have only (111) peak, this indicates that this film has a (111) orientation via hetero-epitaxial growth. Here, just before deposition of BLFCO thin film, the surface of Pt underlayer is exposed by Ar+O₂ atmosphere with 695 °C for several tens of seconds unintentionally. During this time, the surface of Pt underlayer should be partially oxidized unintentionally. Therefore, the peak intensity of BLFCO(111) was not so large. The grain size and surface roughness (R_a) were around 100 nm and 1.2 nm, respectively from topographic image, this indicates that this film has clear crystal grain growth without large roughness.

Zoom In Zoom Out Reset image size

Figure 2 shows the in-plane and out-of-plane magnetization (M–H) and ferroelectric (P–E) curves of the BLFCO thin film fabricated by pulsed-DC reactive sputtering. A clear hysteresis loop appeared in both curves. In the in-plane magnetization curves, the H_c was around 1.9 kOe and the squareness S (=remanent magnetization M_r/M_s) was around 0.34. In the out-of-plane direction, the H_c and S were 2.7 kOe and 0.41, respectively. These indicate that this BLFCO thin film has perpendicular magnetic anisotropy. Although the origin of this property is not clear yet, we will discuss about this by fabrication of many BiFeO₃-based thin films with various substituting elements. Previously, we showed that 300 nm thick BLFCO thin films fabricated by the pulsed-DC reactive sputtering technique possessed a single-phase crystalline structure and a Curie temperature of 420 K [25]. These results indicate a multiferroic property of the thin film with both ferromagnetism and ferroelectricity at room temperature. Here we measure only the minor loop of the electric property; the relationship between the magnetic and ferroelectric properties will be discussed in a future study.

Zoom In Zoom Out Reset image size

Figure 3 shows the out-of-plane magneto optical Kerr loops of the [Co/Pd], BLFCO thin film, the multilayered structure of BLFCO/[Co/Pd], and BLFCO/[Co/Pd] magnetized downward before measurement. The [Co/Pd] thin film showed a clear magnetic anisotropy perpendicular to the film plane (figure 3(a)). The H_c and S were 200 Oe and 0.70, respectively. The Kerr loop of the BLFCO thin film (figure 3(b)) resembled the M–H curve of figure 2(a). The coercivity (∼3 kOe) was larger than that of the [Co/Pd] thin film. The Kerr loop of the BLFCO/[Co/Pd] multilayer structure (figure 3(c)) roughly resembled that of the [Co/Pd] thin film shown in figure 3(a). As the applied magnetic field (1 kOe) was smaller than the coercivity of BLFCO film, the film maintained its demagnetized state during the measurement and the loop mainly reflected the Kerr loop of [Co/Pd] film. In figure 3(d), a very large downward magnetic field was applied before measurement, indicating a downward magnetization direction of BLFCO during the measurement under 1 kOe. Hence, the Kerr loop was shifted in the direction of downward magnetization. From these results, we inferred that the magnetization of the BLFCO layer magnetically interacted with the magnetization of the [Co/Pd] layer in this multilayered structure.

Zoom In Zoom Out Reset image size

Figure 4 shows cross-section and plane view schematics of the BLFCO/[Co/Pd] multilayer structure along with MFM images of the structure before and after applying the local electric field. The local electric field was applied through the conductive and magnetic Co-Cr-Pt tip of the SPM. Previously we showed that a local electric field induces magnetic and electric domains in BBFO and BLFCO thin films and that magnetization reversal occurs on the microscale [22, 24, 25]. In this figure, a −10 V local electric field is applied to the left [Co/Pd] dot. Before the electric field application, magnetic domains with random upward and downward magnetization directions were observed in both [Co/Pd] dots (MFM image (A) in figure 4), indicating a demagnetized domain structure of the [Co/Pd] dots. After applying the local electric field (MFM image (B)), the magnetic contrast of the left dot was considerably reduced while the contrast of the right dot remained unchanged.

Zoom In Zoom Out Reset image size

To analyze the different magnetization states of the two dots in the absence and presence of the local electric field, we compared the line profiles of their MFM images. Panels (a) and (b) of figure 5 show the line profiles along lines (a) and (b), respectively, of MFM image (B) in figure 4. Along line profile (b), the numbers of upward and downward magnetized domains were the same, indicating a demagnetized domain structure in the absence of the field. In contrast, along line profile (a), 86% of the magnetized domain was upward, indicating a nearly complete upward magnetization under the local electric field. Therefore, a downward magnetic moment of 75% was reversed to the upward moment under the local electric field. Magnetization transfer, occurring by magnetization reversal of the [Co/Pd] dots through magnetization reversal of the BLFCO layer, was deemed to be achieved by applying a local electric field to the multilayer structure. Here, magnetization reversal with a width of several hundred nanometers in BLFCO thin films under a local electric field was demonstrated as mentioned before. This indicates that the magnetization transfer with a width of several hundred nanometers will be performed in this system. We can find the magnetization reversal by applying electric field in multilayer structure such as piezoelectric and magnetostrictive multilayer structures, magnetic tunnel junctions with very thin CoFeB layer, α-Cr₂O₃ layer and very-thin Co layer, and BiFeO₃ layer and very-thin Co layer as mentioned in Introduction section. However, in these systems, magnetic material and its film thickness are restrictive. On the other hand, these restrictions are not serious in our system with the magnetization transfer, this indicates that our system has versatility against the other systems.

Zoom In Zoom Out Reset image size

To discuss the origin of magnetic interaction between the multiferroic and metallic magnetic layers, we compared the M–H curves of the BLFCO/Co-Zr-Nb multilayer structure and BLFCO/MgO/Co-Zr-Nb multilayer structures after applying an out-of-plane magnetic field to the film surfaces. The M–H curves of the BLFCO and Co-Zr-Nb films were also measured for reference. In this discussion, the [Co/Pd] thin film was omitted because its M–H curve can be influenced by the strain and roughness of growth on different underlayers [28]. Therefore, the investigated Co-Zr-Nb film had an amorphous structure [29] and low magnetostriction [30] and its magnetic properties were independent from those of the underlayer. In the M–H curve of the Co-Zr-Nb film (green curve in figure 6), the magnetization drastically decreased when the applied field was less than 10 kOe (the saturation field) because of the demagnetizing field. Below 10 kOe, the magnetization was a little larger in the M–H curve of BLFCO/Co-Zr-Nb (red curve in figure 6) than in the M–H curve of BLFCO/MgO/Co-Zr-Nb multilayer structure (black curve in figure 6). This indicates that the magnetic interaction between the BLFCO layer and Co-Zr-Nb layer includes the magneto-exchange interaction. Here, if the magnetic interaction between the multiferroic layer and metallic magnetic layer is only strong magneto-exchange interaction, the M–H curve of BLFCO/Co-Zr-Nb multilayer structure (red) is expected to be almost similar to the M–H curve of BLFCO film (blue). Because the magnetic domain size is large (several μm) and the domain wall width is wide (50 nm or more) in soft magnetic thin films [31], the direction of all magnetic moment in the thickness direction of the Co-Zr-Nb layer becomes uniform. And then, the direction of magnetic moment and magnetization of Co-Zr-Nb layer become same as the direction of magnetic moment and magnetization of the BLFCO layer by strong magneto-exchange interaction. However, the M–H curve of BLFCO/Co-Zr-Nb multilayer structure (red) and the M–H curve of BLFCO film (blue) are different. Therefore, it can be said that magnetic interaction includes not only magneto-exchange interaction but also magneto-static interaction. If we can fabricate and use the BFO-based multiferroic thin film with large M_s and perpendicular magnetic anisotropy, the effect of magneto-static interaction between BFO-based multiferroic layer and [Co/Pd] metallic magnetic layer will increase, and then, the probability of magnetization reversal of [Co/Pd] layer by magnetization transfer may be increased. To achieve this, we are investigating new BFO-based multiferroic thin films with various A-site substitution elements [32, 33] and B-site substitution elements [34, 35]. In order to discuss the magnitude ratio of magneto-exchange interaction and magneto-static interaction between the multiferroic layer and metallic magnetic layers in this system, multilayer structures with different ratio of film thickness of BLFCO, Co-Zr-Nb, and MgO should be fabricated, and M–H curves of the multilayer structures should be discussed.

Zoom In Zoom Out Reset image size

In summary, we fabricated a high-quality multilayered BLFCO multiferroic thin film/[Co/Pd] perpendicular magnetic thin film with a dotted structure. The out-of-plane M–H curve and P–E curve of the BLFCO thin film fabricated by pulsed-DC reactive sputtering showed clear hysteresis. The H_c and S in the out-of-plane direction of the BLFCO thin film were 2.7 kOe and 0.41, respectively, versus 200 Oe and 0.70, respectively, for the [Co/Pd] thin film. Magnetic interactions occurred at the interface between the BLFCO and [Co/Pd] layers in the BLFCO/[Co/Pd] multilayer structure. After applying a local electric field to the multilayer BLFCO/[Co/Pd] dot structure, the demagnetized structure of the [Co/Pd] dots transitioned to an 86% magnetized structure, demonstrating that magnetization transfer, i.e. the magnetization reversal of [Co/Pd] dots through the magnetization reversal of the BLFCO layer, was successfully achieved when a local electric field was applied to the multilayer structure. If we can apply this magnetization transfer to the magnetization reversal of metallic magnetic thin film with high perpendicular magnetic anisotropy or large optical Kerr effect, magnetic recording device or magneto optical device with very low power consumption will be realized.

This work was partially supported by JST/PRESTO (No. JPMJPR152C, ID: 15655293), and JSPS/KAKENHI (Grant No. 17K06784 and 20H02195).

All data that support the findings of this study are included within the article (and any supplementary files).

SY, NO, GE, and MK conceived the experiments. SY and MK performed the films fabrication and measurement of their properties. NO and GE performed the observation of domain structures and their analysis. All co-authors contributed to the written text with main contributions by SY and MK.