| "Strain engineering" is a means to manipulating physical properties of films using strain. It is proven that strain is an effective method to tune the band structures of semicondutors, critical temperature of superconductors, phase separation of colossal manganites, polarized states of ferroelectric, resonant frequency of ferrites and magnetoelectric coupling of multiferroics, etc.. The mechanism of these phenomena lies in the strain-tunable competing of charge, orbit, spin and lattice freedoms and strong interactions among them. Consequently, strain engineering is now regarded as an emergent object in condensed matter physics all over the world.On the other hand, a large amount of promising devices based on the strain engineering have bee proposed, for example, strain-controlled light emitting diode, ferrite microwave devices, high sensitive magnetic field sensors and magnetoelectric memory with high density and low power cost. Therefore, this thesis studies the influences of strain on the electrical and magnetic properties, and crystal structures in metal and oxide films, and show the coupling between properties and strain, and then develops and demonstrates some prototype of novel and multifunctional devices (for example, strain-controlled magnetoelectric memory).We chose spinel (001)-Zno.4Fe2.604(ZFO) and pervoskite (011)-La2/3Sr1/3MnO3(LSMO) as the typical transition metal oxide films, and artificially nano-structure magnetic tunnel junctions as CoFeB/MgO/CoFeB/Ru/CoFe/IrMn/CoFe multilayers. The in situ strain induced by electric field in0.7Pb(Mg2/3Nbi/3)O3-0.3PbTiO3(PMN-0.3PT) substate is used to tune the strain states in the above films or structures and then we figure out the relationships between properties and strain. The strian in the films as a function of electric field applied to the PMN-0.3PT will be characterized by high resolution sychrotron radiation X-ray diffraction. Combined with the characaterization of the electrical and magnetic properties, we studied systematicially the strain-properties in the above films and provide an effective route to tune the magnetization and transport properties with strain. With the help of strain-property relations, we constructed a non-volatile and multi-state memory by strain through cotrolling polarization switching of the PMN-0.3PT substrates. Our reseach will give a better understandings of the coupling among the intrinsic freedoms, and also contributes to the technique of "straintronics".In chapter one, we will introduce some backgrounds of strain engineering and problems in this field. The routes of electric-field-controlled magnetization are summarized and we will emphasize the advantages of strain engineering for electric-field-controlled magnetic and electrical properties. At last, the research objects, contents, and key scientific problems will be listed.In chapter two, we introduce some fundamental physics on for electric-field-controlled magnetization and some physics concepts on magnetism. We will give an interpretation about magnetoelectric coupling in the frame of phenomenological thermodynamics.In chapter three, some important techniques for sample fabrications and processings, will be introduced, including synchrotron radiation high resolution X-ray diffraction, strain measurements, magnetization measurements under in situ strain and design of magnetoresistance measurement system.In chapter four, magnetic semiconductor oxide film ZFO was chosen as one typical system for static (on SrTiO3(STO) substrates) and dynamic strain engneering. We observed the anomalous strain states in the ZFO/STO films, originating from the V-W growth mode.(001)-ZFO/PMN-0.3PT epitaxial heterostructures have been investigated to demonstrate the electric-field-controlled resistance and magnetization switching at room temperature. The tunabilitiy of resistance of the ZFO film is about-0.1%under the in-plane strain-0.02%at296K, and the tunabilitiy of magnetization is about1.1%under the in-plane strain-0.11%at296K. A possible microscopic mechanism of the manipulation of resistance and magnetization is the enhancement of hopping amplitude of electrons between mixed-valent Fe2+and Fe3+ions under the electric-field-induced in-plane compressive strain.In chapter five, for a larger electric-field-controlled remnant magnetization, we choose the (011)-PMN-0.3PT substrate with anisotropic in-plane strain and grow half metallic LSMO expitaxial films on it. A large anisotropic remnant magnetization tunability was observed in multiferroic (011)-LSMO/PMN-0.3PT epitaxial heterostructures. The remnant magnetization along [100] direction was suppressed by an electric field applied to the substrate while the remnant magnetization along [011] was enhanced. The tunabilities of the remnant magnetization along the [100] and [011] directions are about-17.9%and+157%under electric field of+7.27kV/cm, respectively. We also found non-volatile and multi-state resistance in this heterostructures at room temperature. This large anisotropic remnant magnetization tunability and non-volatile resistance states may find potential applications in the electrically written and magnetically read memories.In chapter six, we studied transportation properties of MTJs/PMN-0.3PT heterostructures. The details of MTJs fabrication and processing procedures are presented. The180°switching of magnetization is observed under in situ strain. We believe that the electric-field-controlled magnetization switching originated from electric-field-induced change of magnetic anisotropy of CoFeB free layer. Our results show that a progressive step towards to electrically written and magnetically read memories. |
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