Electric-field tuning of magnetic anisotropy in the artificial multiferroic Fe3O4/PMN–PT heterostructure

ABSTRACT Modulation of the magnetic anisotropy by the electric field in epitaxial Fe3O4/PMN–PT multiferroic heterostructure is studied, which shows that the coercive field of Fe3O4 thin films can be decreased from 368 to 113 Oe by applying an electric field in PMN–PT [011] direction. Meanwhile, most shift of ferromagnetic resonance (FMR) field Hr () can reach up to −483 Oe by applying an external electric field. The giant shift of Hr from 251 to 9681 Oe is obtained as the sample rotated from θ = 90° (H // to θ = 0° (H // [100]) when the electric field was applied in Fe3O4/PMN–PT heterostructure. GRAPHICAL ABSTRACT IMPACT STATEMENT A giant shift of ferromagnetic resonance field was obtained by simultaneously applying an external electric field and a rotating sample along axes both parallel and perpendicular to the magnetic field.

Multiferroic materials allow the realization of the magnetization change by an external electric field and vice versa. The magnetic structure variation driven by an electric input (electric field or current) has been proposed in broad and bright aspects regarding the potential applications in spintronics [1][2][3]. Compared with conventional electrical current, electric-field manipulation of magnetization has some significant advantages, including miniaturization, high efficiency, low energy consumption and multi-functionalization [4][5][6]. In addition, multiferroic materials have a promising prospect in next-generation devices, such as magnetic field sensors, microwave/radio frequency systems, radar and information storage devices [7][8][9][10]. However, the scarcity of single-phase multiferroic materials above room temperature (RT) has driven an intense research of artificial multiferroic heterostructures, which consist of ferromagnetic and piezoelectric phases [11]. So far, the magnetoelectric coupling has been observed in metal-and transition-metal-oxidebased multiferroic heterostructures. As we know that the magnetoelectric coupling involves the effect of charge, strain and exchange bias; so obviously the competition and coexistence of these factors show a dramatic difference in metal and oxide from the carrier density, dielectric constant and lattice-strain interaction point of view. Thus, a detailed picture of the magnetic structure change of the ferromagnetic layer by the electric field may supply the significant information to reveal the fundamental physics of magnetoelectric coupling in the artificial heterostructure as well as the magnetite [12][13][14][15][16].
Fe 3 O 4 has been investigated to be a promising ingredient for several decades as it possesses a high Curie temperature (850 K) and extremely high (80%) spin polarization [17,18]. Magnetite offers exciting opportunities in fundamental study and technological application, such as in electromagnetic wave absorption, data storage and photovoltaic devices among many others [19][20][21]. The magnetic anisotropy of Fe 3 O 4 is highly expected as the in-plane anisotropy magnetoresistance (AMR), which can be reached at 10.7% at room temperature [22,23]. Generally, the AMR realization of an Fe 3 O 4 single crystal thin film is based on the rigid strain effect of a non-ferroic substrate, which lacks tunability [24][25][26]. In order to in situ modulate the magnetic anisotropy of  [27][28][29]. In this multiferroic structure, the tunable Verway transition, resistance switching and even non-volatile state are exhibited by the electric field, besides a high AMR can be achieved below Verwey transition temperature [30][31][32]. Here we report a room temperature anisotropy of 30% in remanence ratio, strongly suggesting that the strain-engineered Fe 3 O 4 could be a candidate for low energy consumption data storage. Meanwhile, the structural transition of Fe 3 O 4 thin films can lead to a distinct change of the magnetization by the electric field. We have comprehensively studied the magnetic anisotropy of Fe 3 O 4 variation by an electric field in the respective inand out-of-plane orientation of magnetic field.
The Fe 3 O 4 /PMN-PT multiferroic heterostructures were prepared by pulsed laser deposition (PLD). The Fe 3 O 4 thin films were deposited on a single crystal PMN-PT (011) substrate (5 × 4 × 0.3 mm 3 ) using a KrF excimer laser with a frequency of 10 Hz and a pulsed laser energy of 2 J/cm 2 . The substrate was kept at 350°C in an oxygen pressure of 2 × 10 −5 Torr, for 20 minutes in the course of deposition. The samples were cooled down naturally to room temperature under the same pressure.
The surface morphology of the magnetic film was performed by atomic force microscope (AFM, Asylum Research, MFP-3D) in contact mode. The cross-sectional morphology image of Fe 3 O 4 /PMN-PT heterostructure was characterized using a field emission scanning electron microscope (FESEM, JOEL, JSM-7800F). The structure of the Fe 3 O 4 /PMN-PT heterostructure was verified by standard θ-2θ scan using X-ray diffraction (XRD, Rigaku, D/max-2500/PC) with Cu K α radiation in the angular range of 20-80°. The ferroelectric hysteresis loop and strain-electric-field curve of the single-crystal PMN-PT were measured by a ferroelectric tester (Radiant Technology, Precision Premier II). Magnetic hysteresis loops were obtained via magnetic property measurement system (MPMS, Quantum Design, SQUID-VSM) and magneto-optical Kerr effect magnetometer (MOKE, Durham Magneto Optics Ltd, Nano MOKE TM 3). Ferromagnetic resonance spectra were measured by electron paramagnetic resonance (EPR, JEOL, JES-FA200) system with a TE 102 cavity in the X band.   On the other hand, the coercive field drops to 350 Oe and the remanence ratio was only 22% along the hard axis (PMN-PT [100]). Figure 2(b) displays the obtained P-E hysteresis loop of the PMN-PT substrate at room temperature. It was observed that the saturation polarization was around 42 μC/cm 2 , the coercive field was 4.2 kV/cm and the remnant polarization was 37 μC/cm 2 . In addition, the out-of-plane strain-electric-field curve was also measured, as shown in Figure 2(b). The maximum strain was around 0.27%. Figure 3 shows the Kerr hysteresis loops of Fe 3 O 4 / PMN-PT heterostructure at different electric fields. As shown in Figure 3(a), strain-induced magnetic anisotropy leads to a relatively small change, the remnant magnetization ratio decreases from 72% at 0 kV/cm to 65% at 6.7 kV/cm and the coercive field decreases from 466 Oe at 0 kV/cm to 447 Oe at 6.7 kV/cm when magnetic field along [011] direction of the PMN-PT substrate.
However, the Kerr hysteresis loops have stronger alteration when magnetic field is applied along PMN-PT [100], the remnant magnetization ratio decreases from 37% at 0 kV/cm to 7% at 6.7 kV/cm and the coercive field decreases from 368 Oe at 0 kV/cm to 113 Oe at 6.7 kV/cm (Figure 3(b)). Clearly, there are relatively significant differences in the changes of the remnant magnetization ratio and coercive field in Fe 3  shows a non-external electric-field sensitive behavior due to the small d 32 , although a tensile strain exists.
Furthermore, electric-field tuning of magnetic anisotropy can be demonstrated by the electric-field-induced Kerr signal (remanent magnetization) change using the AC-mode MOKE technique without external magnetic field (as shown in Figure 3(c) and (d)). The result indicated that the butterfly-shaped Kerr loop corresponds to the piezostrain loop of PMN-PT single crystal (Figure 2(b)), demonstrating a converse ME effect by strain transformation across the Fe 3 O 4 /PMN-PT multiferroic composite interface. Meanwhile, the contrary tendency along the in-plane orthogonal [011] and [100] directions was also observed, manifesting that the magnetic anisotropy existed in the Fe 3 O 4 thin film. The contrary tendency may be the result of opposite inplane piezoelectric strain response of the PMN-PT substrate [34].
In addition, electric-field tuning magnetic anisotropy of the Fe 3 O 4 /PMN-PT heterostructure was quantitatively   (1)). On the other hand, as shown in Figure 4  In conclusion, we successfully observed the electricfield-induced switching of magnetic anisotropy in the Fe 3 O 4 /PMN-PT multiferroic heterostructure. The lattice parameter of the Fe 3 O 4 film was changed due to the piezo strain/stress effect. We acquired a larger change in remanence ratio (30%) and coercive field (255 Oe) along [100] directions under the applying of the electric field. The shifts of H r reached to −483 and 414 Oe along [011] and [100] directions of PMN-PT substrate respectively when the electric field of 6.7 kV/cm was applied. The giant shift of H r from 251 to 9681 Oe by rotating the sample from θ = 90°(H // [011]) to θ = 0°(H // [100]) was obtained under the electric field of 6.7 kV/cm. The tunable magnetic properties via external electric field provide more options for multifunctional multiferroic device applications.

Disclosure statement
No potential conflict of interest was reported by the authors.