| Nowadays with the development of modern technologies in electric devices, the urgent need for device miniaturization and multi-functionalization as well as low energy consumption is rapidly increased, which has caused the restriction for the development of related fields. Thus finding a new material with two or even more functional features for designing multi-functional devices has attracted much attentions. Since the multiferroic materials have ferroelectric, ferromagnetic and unique magnetoelectric coupling properties, it is possible to rotate magnetization and manipulate spin states via electric fields, finally leading to the breakthrough in spintronics with advantage in miniaturization and multi-functionalization. However, such a magnetoelectric coupling effect in single phase multiferroic materials is weak and only exists at low temperature and high magnetic field, which is not favorable for practical applications. Fortunately, in the multiferroic heterostructures, one can achieve significant magnetoelectric coupling effect, such as manipulating magnetization rotation by electric control at room temperature, via design the heterostrucutre configurations. Although great progresses have been obtained in multiferroic heterostructures, there are still some pending problems need to be addressed, such as how to achieve a reversible non-volatile magnetization rotation over a large area of heterostructure by a convenient method, how to realize anisotropic resistive switching by converse magnetoelectric coupling, and how to achieve significant magnetodielectric effect at room temperature and low magnetic fields.Aiming at these problems, this dissertation focus on the nonvolatile magnetization rotation and related anisotropic resistive switching as well as the strain mediated magnetic variation by electric fields to design a purely electric field driven nonvolatile memory device. In addition, the magnetic field effect on the dielectric and ferroelectric properties in multiferroic heterostructures are also studied. The detailed experimental results and discussion are shown as follows:In chapter1, we introduce the recent research progress on the strain mediated and charge mediated converse magnetoelectric coupling effect in multiferroic heterostructures. The review focuses on the nonvolatile magnetic variation induced by piezostrain from ferroelectric single crystal substrates. And several questions are proposed according to the current research progress.In chapter2, Co/0.7Pb(Mgi/3Nb2/3)O3-0.3PbTiO3(Co/PMN-PT) multiferroic heterostructures were fabricated, and a new and promising side polarization configuration is employed to effectively rule out the impact of the ferroelectric field effect, and provide a significant piezostrain for the magnetization rotation. Via electric polarization control, the magnetoelastic anisotropy induced by the strain transferred from polarized PMN-PT competes with the interface magnetic anisotropy Co/PMN-PT multiferroic heterostructuie, leading to a90°rotation of magnetic easy axis. Without applying external magnetic fields, besides the reversible and non-volatile90°rotation of magnetization, the electric driven180°nonvolatile magnetization rotations are achieved at room temperature over a large spatial area with perturbation magnetic field induced by ferroelectric switching current. And a reversible180°magnetization reversal can be realized as well with an about5Oe auxiliary magnetic field. The anisotropic resistive switching synchronously occurs with the magnetization rotations. Based on this effect, an electric controlled tri-state spin valve prototype device as well as a digital single-pole double-throw switch and a three-state output gate logical devices are designed.In chapter3, we report variations of the in-plane anisotropic resistivity corresponding to the piezostrain-mediated nonvolatile rotation of magnetic easy axis in Co/PMN-PT multiferroic heterostructures. The variations of anisotropic resistivity are closely related to the magnetic-easy-axis rotation. The resistivity of easy axis is always bigger than that of hard axis, and under the manipulation of electric field, the anisotropic resistivity can be switched between7.7%and-9.4%. According to the systematical anisotropic magnetoresistivity measurements in different magnetic fields, different magnetization dynamic properties are studied. A nearly90°phase shift of AMR between two opposite polarization states in magnetic fields lower than magnetic anisotropy field corresponds to the90°-rotation of magnetization. While in a magnetic field higher than anisotropy field, the asymmetric cos2(θ)-like behaviors of anisotropic magnetoresistivity reveal the asymmetric rotation of magnetization. The polarization-electric-field manipulation of anisotropic electronic transport behavior may provide the multifunctionality for device performance by adding multi-controllable parameters and achieving nondestructively electrical read-out.In chapter4, we report the nonvolatile rotation of magnetic easy axis in La2/3Sr1/3MnO3/0.7Pb(Mgi/3Nb2/3)O3-0.3PbTiO3(LSMO/PMN-PT) induced by piezostrain, and the magnetic related anisotropic resistivity. Based on the characterization of the in-plane strain, the surface topography and ferroelectric properties in the heterostructures, we investigate the origin of magnetic anisotropy in LSMO film. The90°rotation of magnetic easy axis corns from the competition between the magnetoelastic anisotropy induced by the strain transferred from polarized PMN-PT and the initial magnetic anisotropy. When the LSMO film is etched into electrode pattern for anisotropic resistivity measurement, the shape anisotropy would not apparently affect the intrinsic magnetic anisotropy of LSMO film, and different areas of LSMO film present almost uniform orientation of magnetic easy axis. Meanwhile, the LSMO film shows a in-plane anisotropic resistivity related to the magnetic anisotropy with higher resistivity along magnetic hard axis. And the anisotropic magnetoresistivities at different magnetic fields reveal different magnetization rotation processes.In chapter5, we fabricated the PbZro.52Tio.48O3/Lao.625Cao.375MnO3(PZT/LCMO) heterostnicture to realize a simpler magnetic interface and investigate the dielectric response and ferroelectric properties at different magnetic fields and temperatures in the frequency range from1kHz to100MHz. A significant magnetic field dependent dielectric behavior in PZT/LCMO thin films with a giant magnetodielectric effect up to52%near Tc of LCMO was observed. Through the equivalent RC-circuit fitting, impressive magnetoresistance and magnetodielectric coupling effect of interface, which is closely related to the inhomogeneity of interface, is believed to be responsible for the great magnetodielectric effect of the system. Meanwhile, the Ec and Pr decrease with increasing magnetic fields near Tc and show abnormal changes below Tc. These extrinsic changes of ferroelectric properties can be well explained in terms of voltage drop model and space-charge related polarization. |
PDF Full Text Request