All-oxide giant-magnetoresistive devices

xv, 133 leaves : ill. (some col.) ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577M AP 2009 ChanYGiant magnetoresistance effect (GMR) has been broadly employed in hard disk read heads and non-volatile memory devices since its discovery by Albert Fert and Peter Grunberg. The importance of GMR was further confirmed by their sharing of the Nobel Prize in 2007. Commercial GMR devices are dominated by spin valve (SV) structures, in which a thin non-magnetic layer is sandwiched between two ferromagnetic layers. Currently, commercial SV are dominated by metal-based structures, because of their simplicity in theoretical treatment and fabrications. Recently, all-oxide SVs and oxide-based SVs have attracted research interests, using oxide materials with high spin polarization such as CrO₂ and rare-earth doped manganites like (La,Sr)MnO₃. The aim of this project is to demonstrate GMR in all-oxide SVs with such oxides. In this work, I fabricated pseudo SV (PSV) devices, which do not rely on antiferromagnetic layers for introducing coercivity contrast between magnetic layers. Well-defined magnetic states were achieved by intrinsic differences between coercivities of the two ferromagnetic layers. The materials selected in this work were Lai₁₋ₓAₓMnO₃ (where A = Ca and Sr) for ferromagnetic electrodes and LaNiO₃ (LNO) for non-magnetic layer. All samples were fabricated by pulsed laser deposition (PLD). PLD is an advanced technique to deposit thin films which can retain the stoichiometry of target materials. This work was primarily divided into two parts. The first part was the optimization of deposition conditions for each layer in the PSV. The second part was concerned with measurements of the PSV devices.The surface morphology and crystallinity of thin films were investigated by atomic force microscopy and x-ray diffractometry. GMR responses are generally worsened by rough surfaces and poor crystallinity of thin films. Precise optimization of deposition parameters, such as substrate-to-target distances, laser fluence, repetition rate and oxygen pressure, are therefore necessary to obtain thin films with extremely flat surfaces and highly crystalline structure. Two PLD systems were utilized to deposit thin films in this project, one was a typical PLD system and the other one a laser molecular beam epitaxy (LMBE) system. Compared with PLD, LMBE can produce epitaxial films with atomic flatness. Experimental results showed that the roughness (~1 nm) and crystallinity (FWHM ~ 0.2° obtained from ω-scan) of the films fabricated by LMBE were superior to those produced by PLD. Magnetic properties, such as magnetization and coercive field of the separate layer were studied by vibrating sample magnetometer. For 100 nm thick La₀.₇Sr₀.₃MnO₃ (LSMO) thin films on LaAlO₃ (LAO) (001) substrates, Curie temperatures were about 330 K, as estimated by both magnetization-temperature and resistance-temperature measurements. After the optimization of deposition parameters, PSV devices were fabricated. All devices were prepared by standard processes of UV-lithography, dry etching and lift-off. There are two measurement configurations for demonstrating GMR effect, namely current-in-plane (CIP) and current-perpendicular-to-plane (CPP) geometries. CIP devices were fabricated with PSV structure La₀.₆₇Sr₀.₃₃Mn₀.₉₅Ru₀.₀₅O₃ (LSMRO) (100 nm)/ LNO(15 nm)/LSMO(50 nm). Ru doping in LSMO has been shown to induce coercivity enhancement 2.5 times that of LSMO (~100 Oe at 80K). Double coercivity was clearly observed in the devices. Magnetoresistance measurements were performed on the device at 10 K. LSMO(50 nm)/ LNO(15 nm)/La₀.₇Cao.₃MnO₃ (25 nm) PSV, prepared to be fabricated into CPP device, was deposited on LAO with LNO as the bottom electrode. Microstructural analysis showed the epitaxial nature of the films, and hysteresis loop measurements illustrated double coercivity characteristic of the heterostructure. Successful preparation of PSV structures described here would serve as a protocol for further investigations of spin transport in other oxide systems.Department of Applied PhysicsM.Phil., Dept. of Applied Physics, The Hong Kong Polytechnic University, 200