Study On Magnetic Properties Of Novel Nd₂Fe₁₄B/?''-Fe₁₆N₂ Nanocomposite Material

The rare earth permanent magnets have taken an important role in our lives,were widely used in automotive,medical,electronics,energy,transportation and other fields.Along with the improvement of the standard of living and the development of science and technology,people are in great demand of the properties of materials.Nanocomposite is a new type of rare earth permanent magnetic material.These materials with a hard phase to provide strong magnetic anisotropy and a soft phase to provide high saturation magnetization.The hard and soft phases are exchange-coupled in nanoscale,and widely regarded as the best available permanent magnetic materials.The nanocomposite magnets offer various technical advantages over traditional materials,such as high energy product,high remanence ratio,low rare-earth content,and low fabrication cost,which are of great value in practical application.Nanocomposite permanent magnets are widely regarded as the next generation permanent magnetic materials due to their excellent magnetic performance.At present,the types of nanocomposite permanent magnets are rich and varied,and different kinds of materials show different magnetic properties.??-Fe16N2 has a high saturation magnetization and has become a competitive candidate for soft material.Nd2Fe14 B is a high-performance hard magnetic material that possesses strong magnetocrystalline anisotropy.Therefore,a nanocomposite magnet with a Nd₂Fe₁₄B/?''-Fe₁₆N₂ bilayer structure may be one of the most promising exchange spring systems to achieve maximum energy product.Simultaneously,it will be able to meet the needs of the industry and greatly reduce the amount of rare earth.In this thesis,hysteresis loops,the magnetic reversal process,and energy products have been calculated systematically for a Nd₂Fe₁₄B/?''-Fe₁₆N₂ bilayer system with a deviation of easy axis ? taken into account,where ? is an angle between the easy axis and the applied field.Based on the micromagnetic theory,we discuss the magnetic behavior of the bilayer and multilayer systems with different thicknesses of soft and hard magnetic layer,system and system layer thickness changes.Meanwhile,angular distributions of magnetization at various applied magnetic fields are studied to understand the magnetic reversal process of the exchange spring bilayer structure.Main results for this work are as follows:?1?Results show that Nd₂Fe₁₄B/?''-Fe₁₆N₂ bilayer system with a deviation angle ? between the easy axis and the applied field plays an important role in the magnetization reversal process.When ?=0°,the nuclear field and coercivity of the bilayers drops monotonically,with the increase in thickness of the soft magnetic layer?Ls?.When ??0°,there will be no obvious nucleation phenomenon.Due to the existence of ? between easy axis and field,the magnetic moment is deviated from the field direction under the effect of the magnetic anisotropy energy in the magnetization reversal process.With decrease of the field,the magnetic moments gradually deviate from the initial direction.The soft magnetic phase thickness was fixed,remanence and coercivity decreases as ? increases,and the squareness ratio of the hysteresis loop also decreases,resulting in a sharp decrease of the energy product with the increase of the ?.?2?With the hard and soft magnetic layer thickness as the main parameters,the hysteresis loops and the changes of the magnetic moments with the applied field of Nd₂Fe₁₄B/?''-Fe₁₆N₂ bilayers have been investigated.The results show that the thicknesses of the hard and soft magnetic phases have great influence on the magnetic properties of the bilayer.With the increase of the thickness of hard layer Lh,the remanence and the maximum energy product is more and more smaller,the change of coercivity is not obvious.The largest energy product is obtained at Lh=2.5 nm.When Lh is fixed,the remanence increases,whereas the coercivity of the bilayers drops rapidly and then to be gentle,with the increase in thickness of the soft magnetic layer?Ls?.The maximum energy product increases rapidly,and then decreased gradually with the increase of Ls.The highest energy product 110.79 MGOe is obtained at the optimal thickness of the soft magnetic layer is 4 nm.Therefore,only in the appropriate ratio of the thickness of the hard and soft magnetic can the best double-layer film of the magnetic properties be obtained.?3?We discuss the magnetic behavior of the multilayer systems,with different thickness of the system and the number of layers.In the case of the same layer,with the increase of magnetic layer thickness,the remanence,coercivity and maximum energy product all decrease.When the total thickness of the system are fixed,in the magnetic layer thickness range,with the increase of the number of layers?each layer thickness decreases?,remanence and maximum energy product increase gradually.When the number of layers increases?magnetic layer thickness decreases?to a certain value,further increasing layers,remanence and maximum magnetic energy product tends to a constant value.Therefore,the thickness of the magnetic system and the system layers make a great difference to the magnetic properties of the system.The soft and hard layer in different thickness range plays different roles,which have different effects on the coercivity.According to the above research results,Nd2Fe14B/a"-Fe16N2 exchange coupling film has excellent magnetic properties.The result shows that magnetic properties change with film structures changing from bilayers to multilayer;maximum energy product increase greatly.The largest maximum energy product?121.96 MGOe?occurs at the optimal thickness of the total magnetic layer is 40 nm and Layers is 32.Our theoretical results provide basis for performing relevant experimental preparation of Nd₂Fe₁₄B/?''-Fe₁₆N₂ multilayer to achieve the maximum energy product.