Structure And Magnetic Properties Of Nanocrystalline Magnetic Materials Prepared By Using High-Pressure Torsion Method

The High-pressure torsion（HPT）was first proposed by Percy W.Bridgman in 1935.HPT has received much attention since its invention as a strong plastic deformation technique that is mainly used to enhance the mechanical properties of non-magnetic materials by nanocrystallization.The magnetic properties of most magnetic materials are usually affected by the grain size.The applications of high-pressure torsion to magnetic materials have been increased in recent years.In this thesis,nanocrystalline Mn₃O₄,Mn₄N and Sm-Fe-N were prepared by high-pressure torsion method.The effects of high-pressure torsion on the structure and magnetic properties of the materials were investigated systematically.The main contents are as follows:（1）Manganese nitrides with Mn₄N as the main phase were prepared by high-temperature gas-solid reaction.Nanocrystalline Mn₄N was prepared by using a high-pressure torsion process.The coercivity of the HPT-ed Mn₄N reached up to 0.75 T（5 K）,which is much higher than that of the original sample.The presence of antiferromagneticα-Mn impurities led to an exchange bias phenomena in the Mn₄N after high-pressure torsion.The coercivity and the exchange bias field of the sample decrease with the increase of temperature.（2）Nanocrystalline Mn3O4 samples were prepared by high-pressure torsion method.The Mn3O4 grain size was reduced from several tens of microns to about thirty nanometers after severe deformation.The coercivity of Mn3O4 increased from 0.538 T to 1.007 T after high-pressure torsion,while the saturation magnetization of the sample decreased after HPT.An exchange bias field of 148 m T was observed in the strongly deformed Mn3O4 sample.（3）The Sm-Fe-N nanocrystalline bulk disc alloys were prepared by high-pressure torsion.The HPT-ed Sm-Fe-N powder was compacted into a high density bulk disc with a coercivity of 1.204 T.HPT has been proved to be effective for preparing bulk Sm-Fe-N magnet.The Sm₂Fe₁₇N₃ decomposed and precipitatedα-Fe at a torsion number of 10 turns.