Preparation And Magnetic Properties Of (110)_A Oriented Ni-Mn-based Ferromagnetic Shape Memory Alloy Thin Films

Ferromagnetic shape memory alloys are new kind of smart materials,which are capble of large magnetic-field-induced strains as well as fast dynamic response.They have potential application in many fields,such as sensing and actuating.Epitaxial growth of Ni-Mn-based shape memory alloy thin films can overcome the lack of bulk materials and can be directly applied to microelectron-mechanical system(MEMS)have become attracted considerable attention.However,due to the complicated microstructure and phase composition of epitaxial Ni-Mn-Ga thin films grown on MgO(100)substrate,have not obtained large magnetostriction at present.Ni-Mn-Sn thin film epitaxially grown on MgO(100)substrate due to the large mismatch between the thin film and the substrate,broadening the martensitic transformation temperature and having large hysteresis,which is not conducive to obtained a large magnetocaloric effect.The present study is focus on preparation of(110)A oriented epitaxial Ni-Mn-Ga and Ni-Mn-Sn films and the system analysis of the effect of chemical composition on film microstructure,crystal structure,martensite transformation and magnetic properties.The main results are following:The(110)A oriented epitaxial growth of Ni2MnGa thin films was prepared on Al2O3(1120)substrates by magnetron sputtering.Due to it is of autensite phase at room temperature,hysteresis loop measurements showed that there was no magnetic field induced reorientation of the martensite variant which severely affected its magnetostictive properties.Based on the investigation mentioned above,the(110)A oriented epitaxial growth Ni-Mn-Ga alloy thin films with a ferromagnetic martensite phase at room temperature was prepared by adjusting the composition and the thickness.Finally the magnetic-field-induced martensitic variant reorientation were observed in the Ni51.0Mn27.5Ga21.5 alloy films with a thickness of 200 nm.The results of transmission electron microscopy showed that the number of martensite variants increased significantly with the increase of film thickness,and the number of matensite variants in films with thickness of 200 nm was less than that of other martensite variants,which is the main reason that leads to the magnetic-field-induced martensite variant reorientation.The obvious martensitic transformation was obtained in the(110)A-oriented Ni49.8Mn36.4Sn13.8 thin films prepared on the Al2O3(1120)substrate by control the compositional.The larger magnetization difference ?M between austenite phase and martensite phase were obtained.Magnetization curve measurement shows the existence of magnetic field induced martensitic transformation.However,the martensitic transformation of Ni-Mn-Sn films with different compositions epitaxially grown on MgO(100)substrates are not obvious,and the difference in magnetization ?M between the austenite phase and martensite phase are small,which indicates that it is not conducive to obtain a large magnetocaloric effect.