Studies On The Strain States And Thickness Effects Of Epitaxial (La_1-xCa_x)MnO₃ Thin Films

Since the discovery of colossal magnetoresistance (CMR) in perovskite manganites films in 1990s, these materials have attracted much attention for their potential application in high density memory, magnetic random access memory and spin-polarized all-oxides devices due to their high performance on low field magnetoresistance and high spin polarization. Also owing to the coexistence and competition between several orders (crystal lattice, spin and orbits et al.), perovskite manganites show intriguing physical phenomena such as charge order, orbital order and phase separation, and becomes one of main topics in condensed matter physics. Hence, the investigation in CMR films has great significance for both spintronics fundamental physics research and all oxides or oxides-metal devices.In this thesis, the influence of films preparation conditions, strain and thickness of perovskite manganites films on both magnetic and transport properties was carefully investigated, and special effort was devoted to the detail discussion of relationship between film performance and crystal mismatches on lattice and angle.The whole thesis consists of six chapters.Chapter 1:The general introduction of physical properties, strain and thickness effect on perovskite manganites films is given. First, we briefly review the physical properties of manganites including crystal structure, splitting of crystal field, ordering phases and phase separation, and then discuss thickness effect induced by strain in manganites films, containing the influences of in plane lattice distortion induced by mismatch on crystal structure, transport and magnetic properties, and thickness effect on crystal parameters and physical properties. In the end of chapter 1 we simply introduce some models of thickness effect induced by strain.Chapter 2: The film preparation method and samples measurement are introduced. Especially, the pulsed laser deposition (PLD) method and X-ray diffraction reciprocal space mapping are described in details. Chapter 3: The relationship between in-situ deposition oxygen pressures and electro-magnetic properties of La_0.7Ca_0.3MnO₃(LCMO) thin films was discussed. The experimental results indicated that the transitions temperature on resistivity and magnetism in LCMO ultrathin films were mainly relied on in-situ oxygen pressures. While for thicker films, the in-situ deposition oxygen pressures and the post annealing are all important. Furthermore, it was found that when the in-situ oxygen pressures were lowered than 25 Pa either thin or thick films could not reach the bulk-like properties after post annealing. However, as the in-situ oxygen pressures higher than 25 Pa, the thick films could have the same electronic and magnetic performance as bulk after post annealing. Therefore, it should use high in-situ oxygen pressure for high performance CMR epitaxial films.Chapter 4: Effects of lattice mismatch in La_0.7Ca_0.3MnO₃(LCMO) epitaxial films on electro-magnetic properties were studied. The experimental data confirmed that the strain state should have direct impact on film transport properties. For free-strain film samples the properties were only decided by thickness, while for mismatched film samples, the relaxation appeared accompanied increments of transition temperature as film thickness increased. In particular range of thickness, the partial relaxation could be observed by microstructure and magnetic properties correspondingly. After fully relaxed, transition temperature and crystal structure of the films could reach the bulk LCMO. Hence, it should take lattice mismatch into consideration to prepare high quality CMR films.Chapter 5: The correlation between angular distortion in out plane of orthorhombic structure and electro-magnetic properties in La_0.67Ca_0.33MnO₃(LCMO) epitaxial films grown on NdGaO3(NGO)(110) was studied. Due to the orthorhombic symmetry of NGO (LCMO), the angle was 89.265°(89.864°) in pseudocubic lattice, which is in (001) plane in orthorhombic lattice. LCMO has small mismatch with NGO in length (<0.05%), while 0.64% mismatch in angle. The results show that when the thickness of LCMO grown on NGO(110) is thinner than 30 nm,T_P decreased rapidly due to the shear strain, while thicker than 30 nm, an angle relaxation in pseudocubic lattice changing occurs in LCMO films, and T_P is close to bulk LCMO value.So, despite the mismatch in length is neglected, mismatch in angle plays important role in film crystal structure and properties.Chapter 6: The influence of in-plane angle mismatch induced strain between La_0.67Ca_0.33MnO₃ (LCMO) and NdGaO₃[NGO(001)] on films structure and properties was studied. Because of choosing the NGO (001) as substrates, the LCMO angular mismatch is in plane. The results showed that LCMO films grown on NGO with the ferromagnetic metal ground state exhibited like charge ordering behavior in transport properties after post annealing, resulting in great low field magnetoresistance effect in extensive temperature range. This may originate from shear strain induced by in-plane angular mismatch, which forces the film lattice to array in periods after post annealing. It may be responsible for the charge ordering behavior in films mentioned above.