| Shape memory alloys are new type of multi-functional intelligent material that have received much attention in recent years.Ni-Mn-Ga magnetic shape memory alloy materials have thermoelastic shape memory effect controlled by temperature field,and magnetic shape controlled by external magnetic field.It has broad application prospects in the field of intelligent drive and sensing.The macroscopic properties of ferromagnetic shape memory alloy materials depend on its magnetical and mechanical properties,but most of the current research focus on macroscopic performance,whilethe existing characterization methods of local scale properties have the disadvantages of certain destructiveness to the sample,low resolution,and slow imaging.Atomic force microscopy(AFM)has advantages of non-destructive,high-resolution,fast imaging of materials,and is adopted to study the local magnetical and mechanical properties of Ni-Mn-Ga ferromagnetic shape memory alloy films,which also is confirmed by the macroscopic characterization method.This thesis mainly includes the following aspects:(1)Based on the AFM morphology characterization technique,the evolution of surface local microstructure during the phase transformation of Ni-Mn-Ga alloy thin films was studied.It is found that the surface morphology of the martensite structure at room temperature exhibits a clear band structure,and as the martensite and austenite transformation occurs,the strip structures on the surface in the alloy films disappear and reappear.The martensitic strip domains reappear with the same orientation or a 90° switch.The corresponding martensitic-austenite transformation temperatures are obtained by temperature-driven morphology changes,which agrees well with the phase transition temperatures measured by macroscopic method,suggesting that the characterization of AFM surface topography can be used to study the phase transitions in shape memory alloy materials.(2)Using the magnetic force microscope(MFM)and variable field module(VFM)characterization methods in atomic force microscopy,the local magnetical properties of Ni-Mn-Ga alloy films were studied,and the changes of magnetic domain structures driven by temperature field and external magnetic field were investigated.It is found that in the phase transition of the temperature-driven alloy films,the magnetic domain wall moves due to the phase transition,the magnetic domain structure changes,and the magnetic domain gradually disappears with the increase intemperature.Otherwise,when the temperature decreases,the local magnetic domain of the alloy film is gradually recovered,and the magnetic strength is restored.Due to the reorientation of the magnetic moment,the final magnetic domain structure is no longer the same as the initial one.The external magnetic field control magnetic domain structures results show that when the external magnetic field strength is higher than its coercive field,the magnetic moment in order to keep in line with the direction of the external magnetic field,resulting in the movement of magnetic domain walland the magnetic domain structures changed.When the film reaches saturation magnetization,the local domain structure remains stable.The anti-parallel strong magnetic field can be repeated,the magnetic domain structure changes cyclically,the orientation of the magnetic moment has reversible repeatability,and the alloy films show time magnetic memory storage effect.(3)The local mechanical properties of Ni-Mn-Ga alloy films under temperature were studied by single point force curve and amplitude modulated-frequency modulated(AM-FM)characterization in atomic force microscopy.The results show that the elastic modulus of the martensite strips boundary is significantly higher than the inner-domain.After the phase transformation of alloy films,the austenite modulus is lower than that of martensite phase. |
