Microstructure And Magnetic Behavior Of Mn Doped GeTe Phase Change Materials

Phase change magnetic materials (PCMM) such as Mn doped GeTe thin films are considered as one of the most attractive chalcogenide materials that used in various applications. They show unique electronic, magnetic and electrical characteristics, which can be controlled by phase change between amorphous and crystalline structures. These characteristics give tremendous and exceptional potential for these materials to be implemented in the emerging field of multi-functional spintronic devices, novel solid state data storage technology and non-volatile logic devices. This thesis has systematically studied the preparation and properties of Mn doped GeTe phase change magnetic material. The microstructural, optical, magnetic and electrical properties of the PCMM Ge1-xMnxTe have been investigated and an effective method to modulate the magnetic properties of Mn doped GeTe chalcogenide-based phase change materials has been proposed.The phase-change magnetic Ge1-xMnxTe films were deposited by the sputtering system under different conditions, and the microstructure of the films were studied. The X-ray diffraction (XRD) results demonstrate that the as-deposited films are amorphous, while the annealed films are crystalline with a stable rhombohedral face centred cubic structure. Crystallographic structure analysis shows the existence of secondary magnetic phases, the intensities of more pronounce peaks become weaker with the decreased thickness as well. The crystallinity was enhanced when the film thickness increased. The lattice parameters of Mn-doped GeTe thin films as function of the thickness were calculated. The crystallite sizes were increased with the films thickness while the full width at half maximum, plane distance and the lattice strain were decreased. In addition, the HR-TEM was used to investigate the microstructure information of Geo.94Mno.o6Te thin films. The HR-TEM image shows two large crystallites which are good evidence for the polycrystalline structure of the films.The optical properties of the amorphous and crystalline Ge1-xMnxTe films with different thickness were measured. The obtained results demonstrated that the optical properties of the films are quite different between amorphous and crystalline phases and they are dependent on the films thickness.The magnetic properties of the amorphous and crystalline Ge1-xMnxTe thin films have been measured. Based on the fact that the ferromagnetism is intrinsic in low magnetic-ion content Ge1-xMnxTe thin films, the magnetization (M) as a function of the magnetic field (H) of amorphous and crystalline Ge0.96Mn0.04Te films was measured to investigate the change of ferromagnetic interaction. The M-H hysteresis behavior was observed for both amorphous and crystalline phase Ge0.96Mn0.04Te films and the saturation magnetization value was reduced by37.7%from crystalline to amorphous state. Therefore, the typical hysteresis loop and large coercivity indicate that the interaction between magnetic ions in Mn doped GeTe film is ferromagnetic, and there is drastic variation between amorphous and crystalline states. The temperature (T) dependences of magnetization (M) at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Geo.96Mno.04Te thin film under different applied magnetic fields were performed. The measured data at100and300Oe applied magnetic field show a large bifurcation of the ZFC and FC curves while, there is no deviation at5000Oe magnetic field. The bifurcation of the ZFC and FC curves at a lower magnetic field demonstrates superparamagnetic behavior, which may indicate the existence of ferromagnetic clusters inside the film. Furthermore, the (M-T) analysis of Ge1-xMnxTe films with different thickness show that the maximum magnetization values occur for the thicker film at the lowest temperature and a significant decrease was observed when the thickness decreased and temperature increased. This might due to the alignments of magnetic moments of Mn-doped GeTe thin films along the external magnetic field at low temperatures which exhibit ferromagnetic (FM) property, while at high temperatures the films show a paramagnetic (PM) behavior, which is consistent with phase change. Moreover, the data analysis results of Mn-doped GeTe films with thickness60,120and200nm measured at FC condition showed that all the films have the ferromagnetic ordering at Tc=48,97and110K, respectively. The ferromagnetic Curie temperature gradually increases with the thickness and approaches nearly110K for the film with200nm thickness.Consequently, the thickness dependence of electrical properties of Ge0.89Mn0.11Te films has been studied. It was found that the carrier concentrations of all films were larger than1×1021cm-3. The electrical resistivity decreases with the film thickness while the mobility increases. The Hall resistance as a function of the magnetic field of Ge0.89Mn0.11Te films was measured perpendicular to the film plane at various temperatures. The hysteresis loops show that the magnitude of the Anomalous Hall Effect AHE decreases with increased temperature indicating the existence of intrinsic carrier induced ferromagnetism.Therefore, these materials could be considered as a fundamental research material on the phase change magnetic materials, since they offer an opportunity to initiate a new generation of devices that combine both the standard microelectronics and magnetic properties of the materials.