With the rapid increasing requirements for information storage, developing compact, innovative devices that offer fast, energy-efficient nonvolatile random access memory is becoming a significant and challenging task. To meet this challenge, a new way to control magnetism via electric fields, using the converse magnetoelectric (ME) effect, rather than electric currents or magnetic fields, is attracting tremendous attention. Initial research suggests that the top candidates for realizing electric-field control of magnetism are single-phase multiferroic materials with simultaneous magnetic and ferroelectric orders; however, more recent experiments show that these materials have small converse ME effects and are unsuited to practical application. As an alternative, artificial multiphase systems that consist of both ferromagnetic (FM) and ferroelectric (FE) materials are receiving more attention in recent years. Coupling of the two ferroic phases suggests that an electric field may be able to control magnetic properties. Previous experimental work has demonstrated that an electric field can control magnetic anisotropy and remnant magnetization by using strain-mediated ME coupling in heterostructures with FM films grown on FE substrates such as epitaxial La0.67Sr0.33MnO3/BiTiO34, La2/3Sr1/3MnO3/PMN-PT5, polycrystalline Ni/PMN-PT, Fe3O4/PZNPT, CoFe2O4/PMN-PT, and CoFe/BiFeO3. For the most part, these studies used the linear-converse piezoelectric response to induce changes in magnetic anisotropy and remnant magnetization, which typically return to their initial state once the driving electric field is removed. However, a nonvolatile tuning of the magnetic state by the electric field, namely the memory effect, is required for information storage.
 
Perovskite manganites contain rich physical phenomena: the Jahn-Teller (JT) distortion, double-exchange coupling, metal-insulator transition, and phase separation due to the strong interplay among lattice, charge, spin, and orbital degrees of freedom. Meanwhile, the lattice strain can modulate most of these properties by modifying MnO6-octahedron distortion and thus the strength of the double-exchange interaction and the JT coupling. Especially, due to the strong coupling between lattice and spin, an electric field is able to control the magnetic properties of manganite films through the converse piezoelectric effect or the polarizing effect of the FE substrate2. It is well accepted that heterostructures composed of piezoelectric oxide and manganite film are a promising platform for studying strain-mediated ME coupling, either for revealing fundamental principles or for exploring new functional devices. Recently, Chen et. al observed a nonvolatile memory effect of ME coupling/strain in narrow-band, phase-separated Pr0.6Ca0.4MnO3(PCMO)/PMN-PT heterostructures, and demonstrated that the electric-field control of magnetization is dominated by the change of phase separation in PCMO, which is nonvolatile and manifests a memory effect of ME coupling. It was assumed that such a nonvolatile magnetic memory effect was due to the modulated energy balance between coexisting FM and charge-ordered antiferromagnetic (COAFM) phases in the phase-separated manganite system.
 
Here, we report a memory effect of ME coupling in the heterostructure composed of a wide-band Pr0.7Sr0.3MnO3 (PSMO) manganite film grown on (011)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) substrate. Heterostructures were fabricated by using the pulsed-laser deposition (PLD) technique. The PSMO thin film was used as a model system because of its relatively large eg bandwidth with trivial phase separation and optimized metal-insulator/ferromagnetism-paramagnetism transition temperature. In the absence of Sr doping, the Pr0.7Ca0.3MnO3 system has a narrow, one-electron eg bandwidth and exhibits strong COAFM. When Ca is successively substituted by Sr, the average ion radius is altered because Sr ions are larger, leading to an increase in electron bandwidth (W) and hence the hopping amplitude for electrons in the eg band. Such an increase in W stabilizes the FM state by enhancing the double-exchange (DE) interaction, and hence favors the FM-metallic state over the COAFM state. As a result, the system behaves strong phase-separation when Ca is partially substituted by Sr. However, in the case of a full substitution (i.e. Pr0.7Sr0.3MnO3), the COAFM state and phase separation become trivial while the FM-metallic state due to DE interaction dominates the transport process. The (011)-oriented PMN-PT single crystal was chosen as the substrate because of its perovskite-cubic structure (aPMN-PT = 4.017 Å) and excellent anisotropic transverse piezoelectric effect. The (011)-cut PMN-PT single-crystal slab behaves opposite piezoelectric behavior in the in-plane [100] and  directions when an electric field is applied along the out-plane [011] crystalline direction, which could generate in-plane compressive stress along [100] and tensile stress along .This strong in-plane anisotropic piezoelectric effect provides an exceptional opportunity for generating a large in-plane anisotropic strain in the epitaxially grown PSMO film on the PMN-PT substrate. Thus, a different electric-field-tuning magnetic memory effect due to the large in-plane anisotropic strains may be expected in the two in-plane directions.
 
In this study, by introducing an in-plane anisotropic strain field using the converse piezoelectric effect of the substrate, we observed an in-plane anisotropic, nonvolatile change of magnetization in the low-temperature FM states. More interestingly, these anisotropic modulations of magnetization persisted after the removal of the electric field (anisotropic strain-field) at low temperatures, indicating a nonvolatile magnetic memory effect in the wide-band PSMO film. Our analysis reveals that the preferential seeding and growth of FM domains, driven by the anisotropic strain field during the formation of FM ordering, lead to an induced-magnetic anisotropy in the FM film. After the electric filed is removed, this induced-anisotropy field results in a metastable magnetic state, which accounts for the nonvolatile memory effect in the wide-band PSMO film. Furthermore, we found that this anisotropic memory effect of ME coupling gradually disappears when the temperature approaches the metal-insulator transition, which can be ascribed to the collapse of the metastable magnetic state caused by increasing thermal energy. Our results clearly evidence that an electric-field-induced anisotropic strain can lead to a nonvolatile magnetic memory effect by forming a metastable magnetic state in a wide-band manganite, where phase separation is trivial. The competition between the barrier energy of the metastable magnetic state and the thermal energy is responsible for the observed temperature-dependent memory effect of ME coupling.