Preparation And Investigation Of Magnetic Property Of Self-Assembled Two-Dimensional CoFe₂O₄ Mesocrystals

Conventional hard disk drives face severe limitation of superparamagnetism in increasing storage density in big data world today.The increasing demand for hard drives with higher storage density has motivated a technology shift from continuous magnetic media to bit patterned media?BPM?with isolated storage unit.However,it's complicated to fabricate BPM composing of ordered arrays of isolated magnetic nanostructures as a lot of complex micromachining technologies are needed.Mesocrystal is an self-assembled ordered superstructure of isolated nanocrystals with specific crystallographic orientation.Hence magnetic mesocrystals possess great application potential in BPM.Through oxide self-assembly process,magnetic mesocrystals composed of discrete nanoscopic magnets embedded in a immiscible nonmagnetic matrix have been achieved.However,the embedded nanomagnets inevitably suffer from mismatch strain and defects at the interfaces of two mixed phases.This may produce uncontrollable magnetic property for each nanomagnet,such as wide variation of switching field that affects the addressability of individual predefined bits and may cause overwriting of adjacent bits in BPM.Thus,two-dimensional non-embedded magnetic mesocrystals composed of monodisperse well-organized nanocrystals have greater potential in novel high-density storage devices.Focusing on preparation and investigaion of magnetic properties of two-dimensional non-embedded magnetic mesocrystals and modulation of magneto-transport property of manganite by magnetic mesocrystals,we performed some works as follows:1.Utilizing the difference of surface/interface energy of perovskites and spinels,two-dimensional non-embedded CoFe₂O₄ mesocrystals were fabricated through a combination of nanoseeds layer growth and oxides self-assembly method using pulsed laser deposition.The structure of the CoFe₂O₄ mesocrystals was characterized by atomic force microscopy?AFM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?.The monodisperse CoFe₂O₄ nanopyramids shaped by four{111}crystal faces exhibit specific crystallographic orientation and are fully relaxed.2.The macroscopic and microscopic magnetic properties of CoFe₂O₄mesocrystals were investigated by superconducting quantum interference device?SQUID?magnetometry,magnetic force microscopy?MFM?and dynamic cantilever magnetometry?DCM?.This two-dimensional non-embedded CoFe₂O₄ mesocrystals exhibit tilted magnetic anisotropy,which may be induced by shape anisotropy of the pyramid-like structure;CoFe₂O₄ nanopyramids possess switchable single domain state,which has good thermal stability;Considering the structural symmetry of CoFe₂O₄ nanopyramids,four-fold magnetic anisotropy axis can be expected,which means eight different magnetization states can be achieved.At present,the switch between various magnetization states has been realized in this thesis.The experimental results show that CoFe₂O₄ mesocrystals can be expected to realize multi-state storage and further improve the storage capacity of hard disk.3.Columnar La_0.7Sr_0.3MnO₃ thin film was obtained by depositing La_0.7Sr_0.3MnO₃on the CoFe₂O₄ mesocrystal,which works as a growth template.Magneto-transport property was investigated by the standard four-terminal method.The CMR temperature range of this columnar La_0.7Sr_0.3MnO₃ thin film is much broader than that of epitaxial La_0.7Sr_0.3MnO₃ thin film.CMR with a wide temperature range from 2 K to 350 K was acquired due to the torpid paramagnetic-ferromagnetic transition,which may be caused by the local spin pinning effects or spin coupling on the columnar grain boundaries,as CMR effect is coupled with paramagnetic-ferromagnetic transition according to double-exchange theory.