Synthesis And Characteristics Of Gete-based Phase-Change Magnetic Material

Phase change magnetic material (PCMM) has the characteristic of both phase change material (PCM) and diluted magnetic semiconductor (DMS). It is expected that not only the optical and electrical properties vary with phase change between the amorphous and crystalline states, but also the magnetic property varies during phase change when the spin is effectively injected into the non-magnetic phase change materials, which offer an effective way to contol of ferromagnetism. This new material may lead to new applications in data storage, sensor and logical devices, and multifunctional spintronic devices. As a newly studied topic, the application prospect of phase change magnetic magerial is attractive. But spin is introduced as another external freedom into phase change material, some fundamental material science, like the magnetic ground state as well as the mechanism of phase change control of ferromagnetism, are still unknown. There is no systemic research on phase change magnetic material. This thesis has systematically studied on the preparation, structure, magnetic and phase change properties of phase change magnetic material. The origin of ferromagnetism has been discussed and the mechanism of phase change control of ferromagnetism has been proposed. It could be considered as a fundamental material research on phase change magnetic material.A binary phase change material GeTe has been chosen as the basic material. Spin has been introduced into nonmagnetic phase change material GeTe through the injection of magnetic element. This study begins with the structure characteristics of GeTe. It is known that the crystalline rocksalt GeTe contains huge structural defects, one of which is Ge vacancy. But the vacancy ratio is still unknown. We have proposed an effective method to identify the vacancy ratio in rockralst GeTe, named substitution. Magnetic element cobalt, of which the electronic radius is closed with Ge, was used to incorporate into the interstitial site induced by the Ge vacancy. Co-sputtering was used to prepare homogeneous Ge1-xCoxTe thin film. X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) measurements indicated that Co had been incorporated into the lattice of GeTe. This method confirms the ratio of Ge vacancy in rocksalt GeTe is 8%. In addition, the spin-polarization calculation supports the reasonable 8% vacancy ratio compared with the Superconducting QUantum Interference Device (SQUID) results. The 8% Ge vacancy offers the opportunity that the magnetic atom could be incorporated. Pulsed Laser Deposition (PLD) was usesd to prepare Ge1-xFexTe thin film. The prepared condition has been optimized and the deposited film has high quality.The magnetization of prepared amorphous and crystalline Ge1-xFexTe thin films has been measured by SQUID. The typical magnetization loop and large coercivity indicate that the film is ferromagnetic. The saturated magnetization decreases as the magnetic ion increases, indicating that there is anti-ferromagnetic interaction in the film. The crystalline Ge1-xFexTe film has high hole concentration measured by the hall effect. When the magnetic ion content is 0.02, the hole concentration reaches up to 1021cm-3. And this high hole concentration hardly changes with temperature. It is a typical RKKY interaction that the high hole concentration modulates the long range ferromagnetic interaction. The field-cooling (FC) and zero-field-cooling (ZFC) magnetization has been performed. The fittings of 3-D spin wave model and modified ZFC model indicate that the ferromagnetism in Ge1-xFexTe film is intrinsic related with the particular growth. Magnetic measurements combined with mean field theory and modified ZFC model, reveal that two distinct ferromagnetic phase transitions occur. We suggest that the second-order ferromagnetic phase transition in high Fe content thin film is from a ferromagnetic phase with long range exchange interaction to a superparamagnetic phase with dipole interaction between Fe clusters, which can be viewed as a signature of spinodal decomposition in Ge1-xFexTe material.Based on the fact that the ferromagnetism is intrinsic in low magnetic-ion content Ge1-xFexTe film, we measured the temperature dependent magnetization of both amorphous and crystalline Ge0.98Fe0.02Te films to investigate the change of ferromagnetic interaction. It shows a long range ferromagnetic interaction in ordered crystalline phase and a short range ferromagnetic interaction in frustrated amorphous phase, which is consistent with phase change.Finally, we report the lattice strain induced phase selection and epitaxial relaxation in crystalline Ge1-xFexTe thin films by pulsed laser deposition. The single-crystal substrates of MgO and BaF2 are designed to match the lattice of low-temperatureα-GeTe phase and high-temperatureβ-GeTe phase, respectively. The structures of deposited Ge1-xFexTe films show lattice-match dependence rather than temperature dependence. Raman analysis indicates that the a-GeTe to (3-GeTe ferroelectric phase transition accompanies an increase of local six-coordinated Ge atoms, which is analogous to the phase transition from amorphous to crystalline for memory application.