| Spintronics is a new scientific field that focuses on generation, injection, transport and detection of spin polarized carrier. As known to all, electron has two intrinsic properties, charge and spin. The traditional microelectronics, which mainly studies and utilizes the transport properties of electron as a charge carrier, has become the cornerstone of modern information technology, while the spins, occupy a central place in information storage. As development of microelectronics technology, the dimension of microelectronic device becomes smaller and smaller and had reached several tens nanometers limit. In the nanoscal limit, the microelectronics had met problems such as heat damage and thermal stability in devices. Furthermore, the quantum confinement effect is the insuperable obstacle that will result in electronic band theory failure finally. One of most important challenge to microelectronics is look for novel devices to overcome this obstacle. It will be less energy consumption and faster speed if we can control spin to do logic operation than that of charge. As discover of GMR device and many proposal of prototype spin-base devices, Spintronics has becomes one of hottest research field recently. To realize spin-base device, the foremost mission is to fabricate a novel material which has room temper ferromagnetic and high spin polarized carrier.Magnetic semiconductor is one of most promising material for spintronics application. According to concentration of transition metal in semiconductor, there are two types of magnetic semiconductors:diluted magnetic semiconductor and concentrated magnetic semiconductor. The preparation method of a typical diluted ferromagnetic semiconductor was to dope transition element into the current used semiconductor system. It is expected that the transition elements may enter the crystal lattice by substituting some cation’s positions, and through ferromagnetic coupling between localized transition metal ions and itinerant carriers, ferromagnetism and spin-polarized carrier may appear in semiconductors. The Curie temperature is low for transition metal doped tranditional semiconductor as GaAs, Si, Ge et al. Mn doped GaAs prepared by low temperature molecular beam epitaxy method is one of the most matured diluted ferromagnetic semiconductor syste such ms. The Curie temperature of (Ga,Mn)As is lower than200K, which does not meet the requirements of room temperature application. In order to obtain material that has room temperature ferromagnetism, researchers turn their focus into oxide semiconductor such as ZnO、 TiO2、SnO2、In2O3and so on. On the one hand, most researcher pay their attention to origin of ferromagnetic of magnetic semiconductor while there are very few report s on the most important property of spintronics materials, spin polarization. On the other hand, the low solubility of transition metal elements in semiconductor lattice has limited the further increases of the institutional doping level that results in low Curie temperature and saturated magnetic moment of diluted magnetic semiconductor.In order to overcome mentioned problem, our group turn research into concentrated magnetic semiconductor. We proposal that grown of amorphous concentrated magnetic semiconductors by using the creative thin film growth method that alternatively deposit an atomic-thin layer of semiconductor and transition metal elements under thermal nonequilibrium condition. In our previous work, we had fabricated concentrated magnetic semiconductor with room temperature ferromagnetic and high saturated magnetic moment. However, there are still several basic problems in this material. The first one is that we need to fabricate uniform thin film to offer evidence for the ferromagnetic origin in this material. The second one is we hadn’t yet observed any spin polarized carrier signal indirectly or directly in our previous research. These two problems are necessary mission to magnetic semiconductor devices application.In this dissertation, the author had grown amorphous oxide concentrated magnetic semiconductor (In0.27Co0.73)2O3-v and Zn0.32Co0.68O1-v.We proved that ferromagnetic origin from amorphous phase of thin film. In transport measurements, we found the spin polarized carrier signal-anomalous Hall effect (AHE). More important, we reported spin polarization of67%in Zn0.32Co0.68O1-v, which is the highest spin polarization in oxide magnetic semiconductor as far as we know. In the final part of this dissertation, we found two successive spin-flip transitions in La0.25Pr0.75Co2P2。This result indicates that equal electron doping disturb antiferromagnetic coupling as well as electron and hole doping in iron-base superconductor.There are three parts for this dissertation.For the first part, we investigated fabrication, oxygen partial pressure tuned magnetism,transport and anomalous Hall Effect in concontrated magnetic semiconductor (Ino.27Coo.73)203-v.(1) We prepared concentrated magnetic semiconductor (Ino.27Coo.73)203-v by alternating sputtering under nonequilibrium thermal substrate, which has high Cobalt concentration. The magnetism can be control through tuning of oxygen partial pressure in sputtering process. Room temperature ferromagnetic was observed in this series magnetic semiconductor thin film. The saturated moment, carrier density and conductivity increases as oxygen partial pressure in sputtering process decreasing. The graze incident X-ray diffraction results, soft X-ray absorption spectra and HRTEM results show that ferromagnetism originated from amorphous thin film while crystallization and weaker ferromagnetic occur as oxygen partial pressure increasing in sputtering process.(2) The concentrated magnetic semiconductor (Ino.23Coo.77)203-v with higher Co concentration were grown under different oxgen pressure. All samples belong to variable range hopping regime(VRH) at low temperature. However, those samples show positive or negative magnetoresistance. The spin dependent VRH fitting indicates that the sign of magnetoresistance depends on exchange interaction of electron-hole during VRH process. We proposed a new method to measure exchange interaction item through transport measurement.(3) The conducting properties of can be tuned by oxygen partial pressure in sputtering process. The transport regime transit from metallic to insulating as oxygen partial pressure increasing. The transport regime is Variable Range Hopping (VRH) for insulating samples at low temperature range. We reported firstly the experimental observation of sign change of anomalous Hall resistivity in variable range hopping. We also observed that the scaling relationship between the anomalous Hall resistivity and the longitudinal resistivity disagrees with the existed theoretical prediction. According to our experimental results, we proposed that the spin-orbital coupling of electrons in the VRH region changes as temperature varies, in which has been considered as a constant in previous theory. As a result, our experiment is of significant importance for a better understanding of AHE in the VRH regime.The second part is about spin polarization of concentrated magnetic semiconductor Zn0.32Co0.68O1-v and reflectionless tunneling in disordered magnetic semiconductor/superconductor hybrid junction. (1) We fabricated Zn0.32Co0.68O1-v/Pb superconducting hybrid junction. From Andreev Reflection spectroscopy, we got the spin polarization of Zn0.32Co0.68O1-v is as high as67%, which is the highest spin polarization in oxide magnetic semiconductor as far as we know. This indicates that amorphous phase and high Cobalt concentration can enhanced spin polarization effectively. On the other hand, spin polarization is reduced due to interface contamination of interface such as reduction of superconducting gap and inelastic scattering.(2) We introduced ZnO barrier into the interface of Zn0.32Co0.68O1-v/Pb superconducting hybrid junction. The enhanced zero bias conductance peak (ZBCP) was observed at the background of tunneling. As temperature decreasing, the finite bias conductance peak (FBCP) occurs in differential conductance spectroscopy. The ZBCP and FBCP can be understood in the regime of reflectionless tunneling. Furthermore, we observed above-gap conductance peak which originates from localized states below Fermi level of impurity state in disordered Zn0.32Co0.68O1-v magnetic semiconductor.At the third part, we studied magnetic and magnetore si stance properties of one possible parent material of iron-based superconductor, La0.25Pr0.75Co2P2. For magnetic field H//c, two antiferromagnetic (AFM) transitions were observed at Tn1=240K, Tn2=11K that originated from Co and Pr magnetic moments, respectively. Compared to PrCo2P2, magnetization, electrical transport measurements show two spin-flip transitions in La0.25Pr0.75Co2P2below the AFM transition temperature Tn2.These two spin-flip transitions originate from antiferromagnetically coupled Pr magnetic moments along the c axis. Furthermore, one small spin flop transition originated from antiferromagnetically coupled Co moments was observed in La0.25Pr0.75Co2P2above TN2. The two spin-flip transitions were explained in different magnetic environments based on magnetic structure of La0.25Pr0.75Co2P2. Our results indicate that equi-valence electron doping also disturbs the antiferromagnetical coupling of magnetic layers as well as electron or hole doping in iron-based superconductor. This is may shed light on mechanism of iron-based superconductor research. |
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