Magnetism And Electronic Transport Properties Of Phase-change Magnetic Material Ge_1-xFe_xTe

Chalcogenide have attracted a great deal of attention since they have been utilized in phase-change memory. They exhibit fast and reversible transitions between amorphous and crystalline phase. These two phases show remarkably different optical and electrical properties, making widely used in non-volatile memory technology. However, a concept of magnetic phase-change material has been proposed. Magnetic phase-change materials are synthesized by doping phase-change materials with magnetic elements to inject spins into matrix. The magnetic properties, as well as optical and electrical properties, could be conveniently controlled by the phase change feature. This material system exhibits potential for some interesting applicaitons, such as multilevel storage, sensor, integrated circuit, and multifunctional spintronic devices.Magnetic phase-change material belongs to phase-change material as well as dilulted magnetic semiconductor(DMS). The phase change feature and magnetic semiconductor properties make magnetic phase-change material attractive. Thus the investigation of magnetic phase-change material transport properties, magnetic behaviors and electronic structure are important. As a typical kind of magnetic phase-change materials, the properties of Ge1-xFexTe have not been studied in detail. In this thesis, we synthesized the magnetic phase-change materials Ge1-x Fex Te and carried out a systematic research on the magnetic behaviors, transport properties and electronic structure of Ge1-xFexTe.Firstly, I investigated the charge transport properties of the Ge1-x Fex Te polycrystalline films and observed the Fe composition-driven metal-insulator transition(MIT). In order to explain the MIT phenomenon, I studied the electronic structure of the Ge1-xFexTe polycrystalline films. The MIT behavior is attributed to disorder-induced localization. The Fermi energy is shifted downward with increasing Fe concentration. The transition from insulating to metallic state occurs where the Fermi energy exceeds mobility edge. This work shows that the MIT behavior of Ge1-xFexTe is tuned by Fe-doping.As a IV-VI diluted magnetic semiconductor, the investigation of magnetism in Ge1-xFexTe is essential. In order to investigate the magnetism, high-quality epitaxial Ge1-xFexTe thin films with various Fe concentrations were deposited on BaF2 substrate. The epitaxial Ge1-xFexTe thin films show a face-centered cubic structure. I found a large solid solubility of Fe(higher than 8 at. %) in GeTe. For the Ge1-xFexTe thin films with x<0.08, the lattice constant shows a monotonic and linear decrease with increasing Fe content.The magnetic properties of the alloy were studied using superconducting quantum interference device(SQUID) magnetometry. The magnetic interaction depends on the Fe composition. On one hand, the ferromagnetism in epitaxial Ge1-xFexTe thin films with x=0.02 is caused by Ruderman–Kittel–Kasuya–Yoshida(RKKY) interaction and the epitaxial Ge0.98Fe0.02 Te thin films are ferromagnetic with a Curie temperature Tc~160K. At low temperature, a strong magnetic anisotropy was observed in Ge0.98Fe0.02 Te thin films and the easy axis is within the film plane. On the other hand, the magnetic phase in the ones with x=0.08 is a mixture of spin glass and ferromagnetic phase.The Curie temperature of Ge0.92Fe0.08 Te is around 170 K. We have observed a series of interesting magnetic phenomena in Ge0.92Fe0.08 Te thin films at low temperature: the magnetization does not saturate up to 2T; a large bifurcation between the zero-field cooling(ZFC) and field cooling(FC) temperature-dependent magnetization was observed; the hysteresis loops after ZFC and FC show an exchange bias effect; a time-dependent thermoremanent magnetization follows power-law decay. All the above magnetic phenomena confirm the existence of spin glass.The transport behaviors in Ge1-x FexTe thin films are closely related to magnetism, which makes it possible to use the transport measurements to characterize the magnetism in Ge1-xFexTe. Electrical characterization of the Ge1-xFexTe thin films indicates that the films are p-type with a high carrier concentration ~3×1020 cm-3. Thus it is reasonable to attribute the ferromagnetism to RKKY interaction, which relies on the semiconductor layers possessing a high carrier density. In the Ge0.98Fe0.02 Te layer, anomalous Hall effect was observed clearly, which indicates that spin polarized charge carriers play an important role in the origin of ferromagnetism in the Ge0.98Fe0.02 Te layer. The anomalous Hall effect also supports the intrinsic ferromagnetic interaction in Ge0.98Fe0.02 Te film. Besides, the minimum carrier concentration is achieved at Tc~160K, suggesting a close relationship between transport behaviors and ferromagnetism. A shift from positive to negative magnetoresistance(MR) with increasing temperature was observed, which is attributed to a competition between spontaneous magnetization and thermal effects. For the Ge1-xFexTe layer with x=0.08, different transport properties were observed. The minimum carrier concentration is achieved at around 5K, suggesting spin glass freezing temperature Tg~5K. The ZFC and FC MR curves show much differernce, indicating the existence of frustration. The ZFC MR is asymmetric and has a large positive MR component at low magnetic field, which is due to spin-flip scattering.