Modulation of spin dynamics in Ni/Pb(Mg1/3Nb2/3)O3-PbTiO3 multiferroic heterostructure

Motivated by the fast-developing spin dynamics in ferromagnetic/piezoelectric structures, this study attempts to manipulate magnons (spin-wave excitations) by the converse magnetoelectric (ME) coupling. Herein, electric field (E-field) tuning magnetism, especially the surface spin wave, is accomplished in Ni/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) multiferroic heterostructures. The Kerr signal (directly proportional to magnetization) changes of Ni film are observed when direct current (DC) or alternative current (AC) voltage is applied to PMN-PT substrate, where the signal can be modulated breezily even without extra magnetic field (H-field) in AC-mode measurement. Deserved to be mentioned, a surface spin wave switch of “1” (i.e., “on”) and “0” (i.e., “off”) has been created at room temperature upon applying an E-field. In addition, the magnetic anisotropy of heterostructures has been investigated by E-field-induced ferromagnetic resonance (FMR) shift, and a large 490 Oe shift of FMR is determined at the angle of 45° between H-field and heterostructure plane.

As we all know, the ME coupling effects include strain/stress, interfacial charge, and exchange bias in FM/FE multiferroic heterostructures [27,28]. Clearing the competition and coexistence of those mechanisms also presents an arresting diversity in metal and metallic oxide. As a typical magnetoresistive material, Ni is usually chosen to be used as the FM phase since it is a prototype of 3D itinerant ferromagnet with a high Curie temperature (T C = 631 K) [29]. Moreover, Ni is also an ideal material for extensive applications in sensors due to its high-temperature stability, anti-corrosion, and abrasive resistance [30][31][32][33][34]. 0.7Pb(Mg 1/3 Nb 2/3 )O 3 -0.3PbTiO 3 (PMN-PT) single crystal is a well-known piezoelectric material, which is desired for FM/FE heterostructures because of its giant anisotropic piezoelectric coefficients and low loss tangents [35][36][37][38][39].
Different from the correlated transition metal oxides such as La 2/3 Sr 1/3 MnO 3 and Fe 3 O 4 , whose electronphonon interaction is strong enough to obtain a substantial strain-modulable electronic/magnetic structure, pure 3D metal Ni with a higher carrier density is conventionally treated as a kind of stubborn materials leaking efficient approaches for tunability [27,32,40]. Nonetheless, to achieve the information of possible spin dynamic response, which may benefit the further spintronics devices and new energy applications, we focus on the multiferroic heterostructure composed of Ni and PMN-PT to seek a comprehensive understanding of the characteristic of 3D metal as a function of strain. In 2017, Zhu et al. [41] explored the excitation of surface spin wave in La 0.7 Sr 0.3 MnO 3 (LSMO)/PMN-PT based on strain/stress at 193 K. Enlightened by the accomplishment of the above, we are devoted to studying E-field which dominates the surface spin wave at room temperature in Ni/PMN-PT. In this study, it was observed that the application of voltage not only causes an FMR shift but also excites the surface spin wave at room temperature. The tunable multiferroic heterostructures represent great opportunities for new electronic devices.

Experimental
In this experiment, the Ni/PMN-PT multiferroic heterostructures were fabricated by direct sedimentation of Ni films onto the commercial and polished (011)-oriented PMN-PT using a magnetron sputtering (MS) method. The pure Ni target with a diameter of 50.8 mm was used, and the argon was used as the sputtering gas. At first, the vacuum of the main chamber reached 7×10 −7 Torr before growing Ni film. Then the temperature of PMN-PT was maintained at 200 , ℃ and the sputtering power was 80 W with the constant argon pressure of 4×10 −2 Torr during deposition. Finally, the Ni/PMN-PT samples were annealed to room temperature.
The surface and cross-sectional morphologies of the Ni/PMN-PT heterostructures were characterized by atomic force microscope (AFM; Asylum Research, MFP-3D, UK) and focused ion beam scanning electron microscope (FIB-SEM; Thermo Scientific, Scios 2, USA), respectively. The Kerr hysteresis loops were determined by magneto-optical Kerr effect (MOKE; Durham Magneto Optics Ltd., Nano MOKE TM 3, UK) magnetometer, and the FMR was performed with an electron spin resonance (ESR; JEOL, JES-FA200, Japan) system. The direct current (DC) voltage was applied by a Keithley 2410 (USA), and the alternative current (AC) voltage was applied by a Keysight 6804A (USA).

Results and discussion
In this study, a reversible E-field tuning FMR shift in Ni/PMN-PT heterostructures was achieved by using the ESR device. The back of PMN-PT (5 mm × 4 mm × 0.3 mm) was coated by a thin layer of platinum as the base electrode, the Ni layer as the top electrode. Figure 1(a) shows the schematic diagram of E-field control ESR measurement in Ni/PMN-PT heterostructures at out-of-plane orientation, i.e., H-field perpendicular to the sample plane (90°), where in-plane orientation is H-field parallel to the sample plane (0°). The PMN-PT single crystal produced strain when a vertical applied E-field was across it, and the P-E loop and ε 33 -E curve of PMN-PT are displayed in Fig. 1(b), which presents the PMN-PT with a coercive field of 2.6 kV/cm, and a remnant and saturation polarization of 52 and 62 μC/cm 2 under 10 kV/cm, respectively.
As shown in Fig. 2(a), the Ni film exhibits a smooth  and dense surface at the range of 2 μm × 2 μm. The cross-sectional morphology image presents that the thickness of Ni film is about 140 nm in Fig. 2(b). Figure 3(a) shows the schematic diagram of MOKE measurement under varying applied voltages. A laser beam was focused on the surface of Ni film, and then DC or AC voltage would be applied to the PMN-PT substrate, where no external H-field was used in AC-mode measurement. Figures 3(b) and 3(c) display the in-plane Kerr hysteresis loops of Ni/PMN-PT under different E-fields when DC voltages were applied perpendicularly to the substrate. When the E-field increases to 6.7 and −6.7 kV/cm, it can be calculated that the coercive field will decrease by 10 and 20 Oe, respectively. Figure 3(d) presents a butterfly-shaped Kerr-E curve, which is similar to the ε 33 -E loop of PMN-PT ( Fig. 1(b)), providing specific evidence for stain-mediated converse ME coupling in Ni/PMN-PT.
Under an E-field, the lattice constant of the PMN-PT substrate can be significantly changed due to its great piezoelectric property and E-field-induced strain in PMN-PT, and then delivered to Ni layer, leading to a deformation of the Ni film and attributing to the magnetoelastic effect. Because of the different piezoresponses of the PMN-PT under AC or DC voltage, the piezoresponse induced by AC is superior to the scenario of DC, which could be the intrinsic effect of our observation. In addition, although the electrodes were treated very carefully, the unperfect interface between electrode and PMN-PT may introduce space charges. Thus, under AC and DC voltages, these space charges may influence the effective E-field on PMN-PT as the defect effect.  Under a cyclic E-field of ±6.7 kV/cm, FMR spectra were obtained at the angles of 0° (i.e., in-plane direction), 45°, and 90° (i.e., out-of-plane direction) between H-field and heterostructure plane, which are shown in Fig. 4. The maximum shifts of FMR field (ΔH r ) are 410, 490, and 210 Oe at 0°, 45°, and 90° as shown in Figs. 4(a)-4(c), corresponding to mean ME coupling coefficients α = ΔH r /ΔE of 102.5, 122.5, and 52.5 Oe·cm/kV, respectively. The complete spectra of FMR varying with E-field at the three angles are presented in Figs. S1-S3 in the Electronic Supplementary Material (ESM). It is worth mentioning that the observed value of 410 Oe is approximate to the calculated value of 469 Oe (Eq. (4)). In the test of E-field regulated FMR shift, we note that the in-plane ME coupling coefficient α = 102.5 Oe·cm/kV is higher than the previously reported maximum (94 Oe·cm/kV) at room temperature [27]. The shift of FMR caused by the E-field in the Ni film is due to the piezoelectric strain effect in the ferroelectric PMN-PT substrate.
Moreover, the relationship between FMR and E-field is also plotted in Figs. 4(d)-4(f). By comparing the three results, it can be found that the shift of FMR at 45° is significantly larger than those at the other two angles. That is because the E-field produces strain on PMN-PT, that is, it causes a large lattice strain at 45°, leading to a large magnetic anisotropy field at 45°, and further results in a large FMR shift in Ni film at 45°. Interestingly, the tunable FMR by E-field exhibits a characteristic butterfly shape, further revealing the converse ME coupling caused by strain/stress mechanism in Ni/PMN-PT multiferroic heterostructure. Meanwhile, the three butterfly-shaped curves appear two different variation trends, indicating that magnetic anisotropy exists in the Ni thin film, and these different variation trends maybe result from strain responses in different crystalline directions of the (011) cut PMN-PT substrate. At the above experimental stages, not only a large FMR shift of 490 Oe can be clearly observed, but also  a reversible E-field tunability is shown completely in Ni/PMN-PT multiferroic heterostructure.
It is worth mentioning that both FMR and surface spin wave modes are observed under E-field (as shown in Fig. 5(a)) at the angle of 80° between H-field and heterostructure plane in Ni/PMN-PT heterostructure. Clearly, the strongest surface spin wave is excited by a small E-field of 3.3 kV/cm at room temperature, which has never been reported. This is probably owing to the unequal upper and lower interface conditions of the Ni layer, that is, one free surface and one faying surface with the PMN-PT. The relatively large lattice mismatch between Ni and PMN-PT causes larger lattice strain, which makes the exchange coupling interaction among spin in Ni/PMN-PT stronger than that in LSMO/PMN-PT multiferroic heterostructure, so that the surface spin wave can be excited by a smaller E-field of 3.3 kV/cm at room temperature. At this angle, E-field can regulate the surface spin waves "1" and "0" as shown in Fig. 5(b). Therefore, switching control can be realized in spin dynamics devices.

Conclusions
In summary, E-field-tuning non-volatile magnetic anisotropy and surface spin wave switch have been successfully certified in Ni/PMN-PT multiferroic heterostructures, wherein a conspicuous E-field regulated FMR shift of ca. 490 Oe arisen from strain/stress effect was observed, and the ME coupling coefficient α came to 122.5 Oe·cm/kV. Furthermore, at the angle of 80° between H-field and heterostructure plane, the surface spin wave mode can be switched to "1" or "0" via the applied E-field, particularly at room temperature, which might be enabled for E-field controllable logic device.
The achievements of this study make the Ni/PMN-PT multiferroic heterostructures promising candidates for developing novel E-field tunable magnonics or spintronics devices.