Structure, Magnetic Properties and Hyperfine Parameters of Nd-Substituted Ni-Fe-Ga Heusler Alloys
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Structure, Magnetic Properties and Hyperfine Parameters of Nd-Substituted Ni-Fe-Ga Heusler Alloys
Monica Sorescu1, Felicia Tolea2, Mihaela Valeanu2 and Mihaela Sofronie2 1
Duquesne University, Department of Physics, Fisher Hall, Pittsburgh, PA 15282, USA
2
National Institute of Materials Physics, P.O. Box MG/7, 077125 Bucharest-Magurele, Romania
ABSTRACT Samples of Ni57-xNdxFe18Ga25 with x=2 and 4 were prepared in ribbon form by rapid quenching via melt spinning route. The samples were analyzed by X-ray diffraction (XRD), magnetic measurements and Mössbauer spectroscopy, both in the as-quenched form and after thermal annealing at 900 oC for 2 min and 400 oC for 2 hours. For x=2 the Nd atoms are completely dissolved in the Ni-Fe-Ga matrix, while for x=4 the additional occurrence of the secondary 2:17 phase could be resolved. These findings were supported by the analysis of hyperfine magnetic field distributions obtained from the non-linear least-squares fitting of the Mössbauer spectra. INTRODUCTION The shape memory effect (SME) is present in a certain class of metallic alloys, including several Heusler alloys, which can recover their shape by heating after deformation. This thermal shape memory effect is governed by a diffusionless phase transition, called martensitic transformation (MT). The MT undergoes from a high-temperature, higher-symmetry austenite phase to a low-temperature, lower-symmetry martensite phase by shifting atoms over a small distance compared to the inter-atomic distance. Several Heusler alloy compositions have been synthesized and analyzed to determine the effect of substitutions on the structural and magnetic properties of these systems. The thermal characteristics of Ni-Fe-Ga-Mn and Ni-Fe-Ga-Co ferromagnetic shape memory alloys were discussed in [1] and the martensitic transformation features have been studied as function of compositions. Ni-Fe-Ga-Co was identified as a promising system for magnetic shape memory applications, due to its better ductility, mobile twin boundaries and the possibility to fine tune the transformation temperatures by slightly changing the stoichiometry [2]. In order to improve the magnetic and mechanical properties of Heusler type SMA different substitutions have been tested. Among these, the addition of Rare Earth (RE) elements [3, 4, 6, 7] was investigated, in order to improve the magnetic anisotropy and also the ductility of Ni-Mn-Ga ingots. It was concluded that due to the low solubility of RE elements in the Heusler type matrix (possibly less than 0.1 at %), there are no major changes in the MT characteristics; however, through segregation, new phases precipitate out at the grains boundaries that may favor the cohesion between grains. In particular, it was demonstrated in [5] that the addition of rare earth (Nd, Sm, Tb) to the Ni-Mn-Ga shape memory alloy significantly improves the compressive ductility of the alloy. To our knowledge there are no reported results
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