The Preparation And Study Of In₂O₃Diluted Magnetic Semiconductors And Their Heterostructures With La_0.7Ca_0.3MnO₃

Oxide-based diluted magnetic semiconductors （DMSs） haverecently attracted heightened interest because of their promisingpotential in spintronics. In₂O₃is a transparent semiconductor with awide band gap. It is expected as a spintronics material integratingphotonic, electronic, and magnetic properties by transition metaldoping. However, the acquisition of the homogenous DMSs and theorigin of ferromagnetism （FM） are still controversial.La_0.7Ca_0.3MnO₃（LCMO）, a representative manganite with colossalmagnetoresistance （MR） and spin polarization of100%, has attractedmuch attention because of its potential application in magnetic read,magnetic sensors and magnetic random access memory. However,the Curie temperature （Tc） and metal-insulator transition （MIT）temperature （Tp） of LCMO film are below room temperature. Theseproperties severely hinder the application of LCMO in practical device.So the enhancement of Tcand Tpis especially important.In this work,(In_1-xNi_x)₂O₃（x=0.03,0.06） samples were prepared bysolid state reaction and a vacuum annealing process. The influencesof vacuum annealing on the structural and magnetic properties of (In_1-xNi_x)₂O₃powders were studied. The origin and mechanism of FM insamples were discussed. The single-layer LCMO and La_0.8Sr_0.2MnO₃（LSMO） films were prepared by pulsed laser deposition technique. Theinfluences of preparation condition and thickness on the structureand properties of LCMO and LSMO films were compared. LCMO, In₂O₃DMSs with room temperature FM, and pure In₂O₃were combined toform different heterostructures. The influences of these structureson magnetic and transport properties of LCMO film were investigated.The results are summarized as follows：（1）(In_1-xNi_x)₂O₃（x=0.03,0.06） samples were prepared by solid statereaction and a vacuum annealing process. The influences of Niconcentration and vacuum annealing on the structural and magneticproperties of (In_1-xNi_x)₂O₃powders were studied. The air-sinteredsamples initially showed paramagnetism, and then exhibited obviousroom temperature FM after vacuum annealing. The ferromagneticsignal almost disappeared after air annealing and reappeared aftervacuum annealing. Hence, the FM can be switched “on” and “off” byalternate air and vacuum annealing. This finding indicates that theoxygen vacancies in the samples play a critical role in inducing FM.X-ray diffraction, high-resolution transmission electron microscopy,and field-cooled/zero-field-cooled measurements were performed.These analyses confirmed that there was no detectable trace of Ni orNi oxide secondary phase in vacuum-annealed (In_1-xNi_x)₂O₃samples.No ferromagnetic signal emerged from pure In₂O₃powders regardlessof undergoing vacuum or air annealing. These results revealed thatoxygen vacancy from vacuum annealing and local spin from Ni dopingare two factors that indispensably affect the ferromagnetic properties of the Ni-doped In₂O₃system. The BMP mechanism suitablyexplains the FM in our vacuum-annealed (In_1-xNi_x)₂O₃samples. Toexplore further the possibility of the existence of d0FM, the vacuumannealing for pure ZnO, TiO₂and CuO powders was carried out. Theresult indicated that all vacuum-annealed oxides powders didn’texhibit room temperature FM.（2） The single-layer LCMO and LSMO films were prepared by pulsedlaser deposition technique. The influences of preparation conditionand thickness on the structure, magnetic and transport properties ofLCMO and LSMO films were studied. The high oxygen partial pressureis very important to prepare the epitaxial LCMO and LSMO films withstoichiometric ratio. The saturation magnetization, Tcand Tpallincrease with increased thickness in the both films. Tcand Tpof LSMOfilm are above300K when the thickness is more than100nm. Themaximum MR value of209nm-thick LSMO film is37%at5T. Tcand Tpof LCMO film with various thicknesses are far lower than300K. Forexample, the maximum MR value of45nm-thick LCMO film can reach100%at5T, however the Tpand Tcare only145K and190K,respectively.（3） The LCMO/(In_0.95Fe_0.05)₂O₃and LCMO/Fe （0.5nm）/In₂O₃heterostructures were deposited on single-crystal （001） orientedSrTiO3substrates using pulsed laser deposition. The total number ofFe atoms in both samples is almost the same. In order to comparewith them, the LCMO/Fe （0.5nm） and LCMO/In₂O₃heterostructures alsowere prepared. The influences of these structures on magnetic andtransport properties of LCMO film were investigated. Compared withthe LCMO single layer, remarkably enhanced magnetization and Tp were observed for the sandwich structure LCMO/Fe （0.5nm）/In₂O₃. Thesaturation magnetization increased by³5%, and the Tpincreasedfrom75K to145K at zero field. However, the saturationmagnetization all decreased for the LCMO/Fe （0.5nm）, LCMO/In₂O₃andLCMO/(In_0.95Fe_0.05)₂O₃heterostructures. The resistance of LCMO/Fe（0.5nm） is above107Ω exceeding the limit of our measurementcapacity, the samples LCMO/In₂O₃and LCMO/(In_0.95Fe_0.05)₂O₃shownsemiconductor behavior. An extensive investigation of x-rayphotoelectron spectroscopy and atomic force microscope revealedthat sputtering0.5nm-thick Fe was oxidized and formed nanodots.Due to the exchange coupling between the Fe ions’ spins in ironoxides nanodots and the Mn’s spins in the LCMO, the Tpof LCMO/Fe（0.5nm）/In₂O₃sample was increased significantly.In summary, we have prepared (In_1-xNi_x)₂O₃powders with roomtemperature FM, and discussed the origin and mechanism of FM. Theresult indicated that oxygen vacancy and local spin are twoindispensable factors effecting the induction of long-rangeferromagnetic coupling in Ni-doped In₂O₃. We have raised successfullythe Tpof LCMO by75K. Although the values of transition temperatureare still lower than room temperature, the enhancement is significant.It could act to push the strongly correlated LCMO material intopractical devices.