Crystallographic Features,Microstructure,Martensitic Transformation And Magnetic Properties Of<100>_A And<110>_A Oriented Epitaxial Ni-Mn-Ga Ferromagnetic Shape Memory Alloy Thin Films

Epitaxial Ni-Mn-Ga thin films have attracted considerable attention,since they are promising candidates for magnetic sensors and actuators in micro-electro-mechanical systems.Comprehensive information on the microstructural and crystallographic features of the Ni-Mn-Ga films and their relationship with the constraints of the substrate is essential for further property optimization.In the present work,epitaxial Ni-Mn-Ga thin films were produced by DC magnetron sputtering and then characterized by x-ray diffraction technique(XRD)and backscatter electron diffraction equipped in scanning electron microscope(SEM-EBSD).The<001>A oriented Ni-Mn-Ga thin films with different composition were fabricated on Mg(100)substrate.XRD measurements demonstrate that when the composition is Ni49Mn25Ga23,the Ni-Mn-Ga are of Austenite.When the composition is Ni49Mn29Ga22 and Ni50Mn28Ga22,the Ni-Mn-Ga thin films are composed of three phases:austenite,NM martensite and 7M martensite.When the composition is Ni49Mn35Ga16,the Ni-Mn-Ga thin films are composed of NM martensite and 7M martensite.With the optimized measurement geometries,maximum number of diffraction peaks of the concerning phases,especially of the low symmetrical 7M martensite,are acquired and analyzed.The lattice constants of all the three phases under the constraints of the substrate in the in<001>A oriented Ni50.33Mn29 92Ga19.75 thin films are fully determined.Austenite is of L2i crystal structure with lattice constant:aA=0.5773 nm.The 7M martensite is of monoclinic crystal structure with lattice constant:a7M=0.4262 nm,b7M=0.5442 nm,C7M=41.997 nm,β=93.7°.The NM martensite is of tetragonal crystal structure with lattice constant:aNM= bNM =0.0.3835 nm,CNM=0.6680 nm.Microstructure characterization shows that both the 7M martensite and NM martensite are of plate morphology and organized into two characteristic zones featured with low and high relative second electron image contrast.Local martensite plates with similar plate morphology orientation are organized into plate groups or groups or variant colonies.SEM-EBSD in film depth analyses further verified the co-existence situation of the three constituent phases:austenite,7M martensite and NM martensite.NM martensite is located near the free surface of the film,austenite above the substrate surface,and 7M martensite in the intermediate layers between austenite and NM martensite.Further EBSD characterization indicates that there are four distinct martensite plates in each variant groups for both NM and 7M martensite.Each NM martensite plate is composed of paired major and minor lamellar variants in terms of their thicknesses having a coherent interlamellar interface,whereas,each 7M martensite plate contains one orientation variant.Thus,there are four orientation 7M martensite variants and eight orientation NM martensite variants in one variant group.According to the crystallographic orientation of martensites and the crystallographic calculation,for NM martensite,the inter-plate interfaces are composed of compound twins in adjacent NM plates.The symmetrically distribution of compound twins results in the long and straight plate interfaces in the low relative contrast zone.The asymmetrically distribution leads to the change of inter-plate interface orientation in the high relative contrast zone.For 7M martensite,both Type-I and Type-II twin interfaces are nearly perpendicular to the substrate surface in the low relative contrast zones.The Type-I twin pairs appear with much higher frequency,as compared with that of the Type-Ⅱ twin pairs.However,there are two Type-II twin interface trace orientations and one Type-I twin interface trace orientation in the high relative contrast zones.The Type-Ⅱ twin pairs are more frequent than the Type-I twin pairs.The inconsistent occurrences of the different types of twins in different zones are originated from the substrate constrain.Further EBSD analysis revealed that thereare six variant groups for NM martensite and 7m martensite in<001>A oriented Ni50.33Mn29.92Ga19.75 thin film.And there are total 48 NM martensite variants and 24 7M martensite variants,which indicates that the martensitic transformation sequence in<001>A oriented Ni50.33Mn29.92Ga19.75 thin film is from Austenite to 7M martensite and then transform into NM martensite(A→7M→NM).Base on the investigation mentioned above,the present study proposed to fabricate<1 10>A orientated Ni-Mn-Ga thin films on MgO(111)and Al2O3(110)substrates.SEM and XRD analysis demonstrate that with optimization of chemical composition and film thickness,the continues<1 10>A orientated Ni-Mn-Ga thin films can be obtained.For the<1 10>A orientated Ni48.65Mn28.94Ga22.41 thin film grown on MgO(111)substrate there are three oriented Austenite,whereas there are only one oriented Austenite for the<11 0>A orientated Ni50.57Mn27.01Ga22.42 thin films grown on Al2O3(110)substrate.Finally the magnetic field induced martensite variant reorientation were observed in the<1 10>A orientated Ni48.65Mn28.94Ga22.41 thin films.In summary,the present study revealed that the 7M martensite is of monoclinic crystal structure other than the pseudo-Orthorhombic or adaptive phase and the correlation between microstructure and crystallographic features of 7M martensite and NM martensite.Moreover,the consequence and crystallographic orientation relationship of martensitic transformation in epitaxial Ni-Mn-Ga thin films were determined.Last but not least,the giant magnetic field induced martensite variant reorientation,which was observed in the<1 10>A oriented Ni-Mn-Ga thin films.The present study intends to offer deep insights into the crystallographic features and martensitic transformation of epitaxial Ni-Mn-Ga thin films.