Microstructure And Magnetic Properties Of Ni/Co Based Ferromagnetic Heusler Alloy

Ferromagnetic Heusler alloy has rich physical properties,such as ferromagnetism,thermoelectric effect,magnetoresistance,half-metallic properties,the thermal elastic martensitic transformation,shape memory effect,superconductivity,magnetic optical effect,large magnetic field induced strain effect,magnetic refrigeration effect and Magnetocaloric effect etc.These peculiar properties contain profound physical connotation and great potential application prospects.In recent twenty years,with the notion proposed of new materials,Heusler alloy has become a hot topic in the field of materials science.In this work,the physical properties such as microstructure,martensitic transformation,magnetocaloric effect,magnetoresistance effect and dynamic magnetic properties of several Ni/Co based ferromagnetic Heusler alloy systems have been studied.For Ni-Co-Mn-Ge alloy system,we studied the crystal structure,microstructure,magnetic behavior and dynamic magnetic properties of(Ni1-xCox)2MnGe(x=0?1.0)and(Ni0.2Co0.8)2Mn1+yGe1-y(y=-0.8?0.8)Heusler alloy systematically.The evolution process of crystal structure and magnetic structure with composition is clarified,as well as the preferential occupation of Ni atoms and the reason of the second phase appearing in the Ni-rich(Ni1-xCox)2MnGe system especially.The Ni/Co of the X location forms a ferromagnetic coupling with the Mn of the Y location,and the Mn of the Y location forms an antiferromagnetic coupling with the Mn in the Z location.The dynamic magnetic properties of the alloy show that the Landau g factor of the Mn atom in the L21 structure is 2.15,and the damping coefficient of the alloy is 0.22.The large damping coefficient is related to the Mn atom with antiferromagnetic coupling in the alloy.For the Ni-Co-Fe-Ge alloy system,the crystal structure evolution,magnetic behavior,and the dynamic magnetic properties of the different structure of the alloy system were studied.In Ni50-xCoxFe25Ge25 system,when the Co content is lower than 18%,the alloy shows B2 structure whose symmetrical degree is low.With the increase of Co content,the alloy structure gradually transformed into L21 structure with high symmetry,and the saturation magnetization of alloy increase.In the Ni25Co25Fe50-yGey system,the crystal structure of the alloy is very sensitive to the content of Fe.The Ni25Co25Fe50_yGey alloys have high saturation magnetization,of which the saturation magnetization of Gel 0 is up to 130Am2/kg.Ferromagnetic resonance study shows that the Gilbert damping coefficient of pure L21 phase alloy Ni25Co25Fe25Ge25 is 0.1704.For the Cr doped Ni43Co7Mn39Sn11 alloy system,the crystal structure,martensitic transformation,magnetocaloric effect,magnetoresistance effect and dynamic magnetic properties of Ni43Co7Mn39-xCrxSn11 alloy were studied.The effect of Cr doping on the magnetic entropy change is clarified,and the physical mechanism of the asymmetric magnetoresistance curve and the calculation of the equivalent temperature of the magnetic field inducing phase transition are given.It is shown that a small amount of Cr doping change the structure and significantly reduce the transformation temperature of the alloy.With the martensitic phase transition,the alloy exhibits significant magnetocaloric effect and magnetoresistance effect.The magnetic entropy change at the phase transition temperature of 254K reaches-11.3 J/Kg/K for the Ni43Co7Mn38Cr1Sn11 alloy.Due to the magnetic field induced phase transition,the R-H curve of the alloy appears asymmetrical in the vicinity of the phase transition temperature.In order to characterize the driving force,a new method of calculating the value of ?=dT/dH is proposed in this paper.It is calculated that the y of Ni43Co7Mn38Cr-Sn11 alloy is 5.3 K/T,which is coincident with that calculated by the Clausius-Clapeyron equation(-dT/dH).The y of the alloy increases with the increase of Cr content,indicating that the doping of Cr can help to promote the martensitic transformation of NiCoMnSn alloy.For nanocrystalline Ni-Fe-Ga alloy,SBA-15 template method and high temperature hydrogen annealing were used to prepare Ni-Fe-Ga nano particles.The preparation,structure control,microstructure and dynamic magnetic properties of the samples were studied.By adjusting the components and template concentration,the y phase is effectively suppressed in the nanocrystalline,and the pure L21 phase Ni-Fe-Ga single crystal nano particles are obtained.The content of Fe plays a crucial role in the formation of ? phase.The magnetic measurements show that all samples exhibit ferromagnetism at room temperature.There is a mutation on the thermal magnetic curve of Ni52Fe19Ga29,indicating the magnetic phase transition.The ferromagnetic resonance damping coefficient?=0.0188 at 300K,and ?=0.0213 at 150K were obtained for the Ni52Fe19Ga29,respectively.For Co-Fe-Ga nanocrystalline,a large number of L21 phase Co-Fe-Ga alloy nanowires were obtained by template.The nanowire is about 125-200nm in length,4-7nm in diameter,and the saturation magnetization is 87.5Am2/kg at room temperature.Magnetic analysis shows that the effective anisotropy coefficient of Co55Fe31Ga14 is 2.99 × 105 J/m3,Co54Fe29Ga17 is 5.65 ×105/m3,and Co55Fe28Ga17 is 1.02×106J/m3,respectively.The effective anisotropy coefficients of Co55Fe31Ga14 and Co54Fe29Ga17 are lower than that of bulk Co2FeGa,while the Keff of the Co55Fe28Ga17 is higher than the bulk for the large shape anisotropy of the nanocrystal.The dynamic magnetic test shows that the Gilbert damping factor of Co55Fe28Ga17 is 0.1944.The high damping factor indicates the material potential applications in the GHz band.