The Fabrication And Characterization Of Low-dimensional Magnetic Materials

In this thesis,magnetic multilayer films and quasi-one-dimensional magnetic materials were prepared by physical and chemical methods.The crystallographic structures,magnetic properties,and magnetoelectric and magnetocaloric properties were further studied.This thesis is mainly divided into three aspects.The first is the preparation of the giant magnetoresistance?GMR?spin-valve structure by magnetron sputtering to characterize its magnetoresistance and magnetic Seebeck effect;the second is the preparation and characterization of Fe Sn nanowires and YIG nanotubes prepared by chemical methods;the third is the preparation and Lorentz transmission electron microscopy?LTEM?characterization of CoTb thin films and double anti-ferromagnetic/ferromagnetic pinned structure by magnetron sputtering.We have gotten the following achievements:1. We observed the GMR effect and magnetic Seebeck effect in Co/Cu/Co and NiFe/Cu/Co spin valves.The GMR effect leads to a negative magnetic Seebeck coefficient value,which reached-9% in our experiment,and-11% in our simple theoretical model prediction.The value of the magnetic Seebeck effect is much higher than the anisotropic Seebeck effect and other possible derived signals.The magnetic Seebeck effect comes from the non-zero spin polarization of the Seebeck coefficient of the ferromagnetic layer and the modulation of the spin polarization currents of different spin configurations.Our research is helpful for future highly sensitive temperature,magnetic field sensors,and temperature gradient-driven magnetic field sensors.The magnetic Seebeck effect also provides a method to estimate the spin polarization of the Seebeck coefficient in a specific ferromagnetic layer.In the current situation,the spin polarization of the Seebeck coefficient in the ferromagnetic layer is difficult to measure.2. We synthesized Fe₃Sn₂ nanowires with a diameter of 80-100nm and a length of about 10 microns by electrochemical deposition.The anisotropy is characterized along the axial direction,and the anisotropy of the annealed sample is stronger than the unannealed sample.The reason is that annealing makes the grain structure more uniform and the quality is thus better.From the change of the coercive force with the angle,we can judge that the inversion mode of the nanowire is a uniform rotation.The change of coercive force with temperature conforms to the thermal activation formula.The change of saturation magnetic moment with temperature satisfies three-half Bloch's law.The saturation magnetic moment is inversely proportional to the cubic power of the temperature.3. We have successfully synthesized Y₃Fe₅O₁₂?yttrium iron garnet,YIG?nanotubes ?NTs?with different diameters using the conventional sol-gel method in the anodized aluminum oxide?AAO?template.Annealing process at 800? can obtain pure phase.The measurement of magnetization at room temperature indicates that the diameter of the nanotube increases,and the coercive force decreases significantly.The results of the temperature-dependent field cooling?FC?and zero field cooling?ZFC?measurements indicate that the YIG nanotubes change from a ferromagnetic?FM-like?state to a superparamagnetic?SPM?state?310 K–400 K?as the temperature decreases.As the size of YIG NT decreases,the blocking temperature drops sharply.In the ferromagnetic state,the coercivity in three different samples satisfies the thermal activation formula.The temperature dependence of saturation magnetization satisfies Bloch's 3/2 law.The characterization of ferromagnetic resonance?FMR?shows that the damping parameter of Gilbert YIG NTs is about 7-9×10^-3.4. In ferrimagnetic CoTb alloy films,we have studied various magnetic domain states using LTEM.By varying the temperature and the external field,the evolution of the maze domain and the coexistence of magnetic skyrmions and magnetic bubbles can be observed.In the coexistence phase,when the external electric field was further increased,all the magnetic bubbles were eliminated only the skyrmions left.The results indicate that the skyrmions are more stable than the magnetic bubbles.5. We studied the magnetization reversal process of Pt?2?/IrMn?10?/Co?n?/IrMn?10?/SiN exchange bias structure with different magnetization layer thicknesses under external magnetic field using LTEM.When the applied magnetic field changes in a direction parallel to the exchange bias,the magnetization reversal of the sample undergoes a change from a single domain?saturated state?to a ripple domain to a 180-degree domain to a single domain?saturated?.The ripple and 180-degree domains appear and exist at the same time.When the thickness of the magnetization layer is low,there will always be closed domains that are difficult to saturate and disappear under higher fields.When the applied magnetic field changes along the direction perpendicular to the exchange bias,only 90-degree rotation and disappearance of the ripple domain appear in the sample,and then the rotation and appearance process.