Research On Preparation And Properties Of All Perovskite-Based Magnetic Tunneling Junctions

The aim of this study is to fabricate all perovskite-based magnetic tunneling junctions, under combining the advantages of conventional magnetic tunneling junctions (FM/I/FM, the abbreviations of FM and I mean ferromagnetic metallic electrodes and insulating barrier) with large tunnel magnetoresistance (TMR) in low magnetic fields and perovskite manganites, Re_1-xTXMnO₃ (Re is a trivalent rare-earth ion and T is a divalent dopant) in high spin polarization. The HRTEM image shows epitaxial growth of LSMO trilayers on STO substrate and electron holography of barrier layer reveals that the thickness of barrier is about 5nm. A colossal TMR value of 9050% is observed at 4.2K under the external magnetic field of 12T, which is 5 times greater than the highest value reported previously. As a result, a polarization value of 99% for LSMO film is first obtained from experiment, which is consistent with the value of 100% for calculation of energy band theory and measurement of spin-resolved photoemission spectra theoretical. The present results directly indicate the half-metallic ferromagnetism in La_0.7Sr_0.3MnO₃ from experiment and the potential application of all perovskite-based magnetic tunneling junctions. Firstly, a comparative study of crystalline structure, magnetic properties and electronic transport in epitaxial LSMO films have been conducted, which are deposited on (100), (110) and (111) LaAlO₃ substrates using RF magnetron sputtering. It has been found that the (111) oriented LSMO film has the lowest resistance value and the biggest saturation magnetization value compared with the values of (100) and (110) oriented LSMO films. This results can be attributed as the small lattice distort in the (111) LSMO films which can result in little distortion of magnetic domain parallel direction. From this research, the relationship between structure and properties of LSMO films can be further understood. To improve the properties of LSMO films, the effects of annealing procedures on the structural and magnetic properties of epitaxial LSMO films grown on LAO and STO substrates have been studied. Annealing procedures at the higher temperatures can enhance the ferromagnetic order and Curie temperature (Tc) and dramatically reduce the coercivity values due to the increase in the oxygen content. Especially, the magnetic properties of the LSMO films by the means of in-situ annealing procedures have been improved without increase the surface roughness. On the basis of all that has been studied above, a trilayers of LSMO/La0.96Sr0.04MnO3/LSMO by magnetron sputtering method on (100) STO substrate have been deposited, and then anti-ferromagnetic Ir22Mn78/Ni79Fe21 was incorporated as the pinning layer of the top electrode of the trilayers. The junctions area with size of 8×4μm2 were patterned by photolithography and ion beam etching. The effects of barrier thickness, interface and anti-ferromagnetic pinning on the TMR values of the MTJs have been studied. The high TMR value of 9050% can be attributed to the use of the La0.96Sr0.04MnO3 barrier which can minimizes interfacial disorder and strain.