Elelctrical and Magnetic Properties of Several Perovskite Oxide Films and Superlattices

Author:Zhang Zuo

Supervisor:zhang shan tao chen yan feng


Degree Year:2019





The oxides with perovskite structure are a family exihibiting not only numerous physical properties(such as ferroelectricity,ferromagnetism,pyroelectricity,superconductivity and catalysis),but also potential applications in spin electronic devices,information storage etc.al.Therefore,perovskite oxides have been the hot research field in material science and condensed matter physics for a long time.With great advances in the technology of plused laser deposition(PLD)and molecular beam epitaxy(MBE)during the past thirty years,many emergent physical phenomena,such as two-dimensional electron gas and high-temperature superconductivity in the interfaces,have been found in perovskite oxide films due to the competition and coupling among superlattice,orbit,charge and spin,which further attracted researchers’great interests.Based on above background,we investigated the crystal structure,electrical transport and magnetic properties of films with different types of structures,like Sr2CrWO6 double perovskite oxide films,SrRuO3/SrRuO3 supperlattices and LaMnO3:NiO nanocomposite films in this paper.The results are summarized as follows:1.The Sr2CrWO6(SCWO)double perovskite oxide thin films with different thickness were deposited by pulsed laser deposition.The SCWO is a perfect half-metal with obvious spin polarization and total net moment of 2.0 μB/f.u.by first-principles calculation.The magnetic hysteresis loop(M-H)confirmed that the SCWO films exhibited ferrimagntism.However,the value of the largest moment was lower than the theoretical value,which was attributed to negative effect of the oxygen vacancies and anti-site defects(ASD).The temperature dependent resistivity(ρ-T)confirmed the perfect metal behavior of SCWO films,which could be comparable with the alloys.The resistivity of the SCWO films with 12 nm thickness was 4.9×10-11 Ω·m and 9.5×10-7Ω·m at 2 K and 300 K.respectively.Meanwhile,the density and mobility of carries of SCWO films were very high,for example,that of the SCWO films with 12 nm thickness were 2.25×1028 m-3 and 5.66×104 cm2·V-1·s-,respectively.Besides,the SCWO films show unusually giant positive magnetoresistance(MR)under the low temperature.The value of positive MR for SCWO films with 12 nm,24 nm and 48 nm was 17200%,2060%and 1185%,respectively,which is the largest among the reports for perovskite oxides.After excluding some possible reasons(such as oxygen vacancies),the physical mechanism of the giant positive MR in SCWO films could be explained as follow:On the one hand,the high density and mobility of carries may have contributions to the giant positive MR.On the other hand,the external magnetic field may suppress the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations with disorder spins,which enhance electronic scattering and lead to the giant positive MR.2.The SrRuO3/CaRuO3(SRO/CRO)superlattices with 6,10,15 periods were deposited by pulsed laser deposition.The supperlattices were(00l)epitaxial growth on STO substrates confirmed by XRD and reciprocal space mapping(RSM).Besides,the superlattices exhibited layer-by-layer structures and sharp interfaces confirmed by TEM.The out-plane temperature dependent magnetization(M-T)under zero field cooling(ZFC)and field cooling(FC)conditions suggested that the superlattices showed nearly spin glass behavior under the low tempertuare.Meanwhile,the slightly pinched out-plane M-H loops suggested the competition of ferromagnetic and antiferromagnetic phase,which arised from the distortion of RuO6 in CRO layers.The p-T curves depicted the metal-semiconductor transition in the superlattices due to the two-dimensional weak localization effect at low temperature.3.The 0-3 type(1-x)LaMnO3:xNiO nanocomposite films were parpered by pulsed laser deposition,which was confirmed by XRD and TEM.The M-H and ZFC-FC cureves indicated that the composite films exhibited ferromagnetic behavior,and the p-T curves suggested excellent insulating behavior.Therefore,the LMO:NiO nanocomsite films were macroscopic ferromagnetic insulators.Besides,the nanocomposite films exhibited large exchange bias due to the ferromagnetic and antiferromagnetic interactions between the interfaces of LMO and NiO.The phenmonen of charge transfer from Mn to Ni ions was evidenced by X-ray absorption spectroscopy(XAS)and the first-priciple calculation.