Studies On M(?)ssbauer Spectroscopy And Electromagnetic Properties Of Fe-based Nanocrystalline Magnetic Materials

Fe-based nanocrystalline alloy has good soft magnetic properties,but it has low cutoff frequency which is below GHz range and suffers from a narrow absorbing bandwidth.Therefore,it is very important to improve the high frequency performance of Fe-based nanocrystalline alloy.At the same time,Understanding the phase transition of Fe-based nanocrystalline alloy and the relationship between the nanostructures and the performance will be helpful.The M???ssbauer spectrum has extremely high energy resolution,which can help us understand the information about the internal structure at the levlel of atoms.In this thesis,we have employed the M???ssbauer spectrum to study the phase transitions and properties at different temperatures and annealing time by different heat treatment.The details are as follows:In chapter 4,the phase transitions,nanostructure change and electromagnetic properties of Finemet nanocrystalline alloy of different temperatures during the conventional annealing are studied.Finemet nanocrystalline alloy structure has a secondary phase transition.It consists of Fe₃Si phase,amorphous phase and Fe₂B phase when the annealing temperature is higher than the secondary crystallization temperature.Saturation magnetization increases with the increase of temperature,however an obvious turning point at 700?is observed,which may be caused by the appearance of Fe₂B phase.In chapter 5,the phase change process,structure change and electromagnetic properties of Finemet nanocrystalline alloy of different temperature and different annealing time during the rapid annealing are studied.The phase transitions of Finemet during rapid heat treatment are similar to that of conventional heat treatment,but the crystallization ratio of Finemet nanocrystalline alloy powder obtained by rapid heat treatment is smaller when annealing at the same temperature.In the process of rapid heat treatment,the grain has no time to grow and grain is refined,so the dielectric constant decrease,the electromagnetic parameter matching problem is improved effectively,and the high frequency performance of Fe-based nanocrystalline alloy is optimized.In chapter 6,Finemet flakes with multiphase nanostructures are obtained by conventional annealing the amorphous ribbon at 900?.Multiple magnetic loss peaks in a permeability spectrum is observed.Multiple resonance phenomenon can be helpful to broaden the absorbing bandwidth.The structure of Finemet nanocrystalline alloy with multiphase nanostructures is studied.And we explain the origin of the multiple magnetic loss peaks.