Microstructural Evolution on Annealing and its Correlation to Structural and Magnetic Properties of Melt-Spun Ni 50 Mn 2
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ORIGINAL PAPER
Microstructural Evolution on Annealing and its Correlation to Structural and Magnetic Properties of Melt-Spun Ni50Mn29Ga21 Ribbons A. Satish Kumar 1 & M. Ramudu 2
&
V. Seshubai 3
Received: 19 September 2020 / Accepted: 29 September 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Polycrystalline Ni50Mn29Ga21 thin ribbons were prepared by melt-spinning and the effect of annealing temperatures on the microstructural evolution, phase transformations, crystal structure, and the magnetic properties of the ribbons was systematically investigated. Different annealing temperatures from 473 to 1100 K were used to alter the degree of quenched-in disorder gradually, which led to evolution of microstructure from cellular to well-defined grain structure. The crystal structure was found to be 7 M monoclinic in ribbons annealed at 773 K or below, while a minority phase with 5 M tetragonal structure is found to coexist on annealing at 1100 K. The as-spun ribbon exhibit superparamagnetic behavior whereas the sample annealed at 1100 K did not exhibit the same. The variations in the saturation magnetization with annealing temperatures were systematically correlated. The saturation magnetization and the martensitic transformation temperature show a correlation to the increase in the degree of Mn ordering. Keywords Melt-spun ribbons . Ferromagnetic shape memory alloys . Martensitic transformations . Atomic order
1 Introduction Ni-Mn-Ga–based ferromagnetic shape memory alloys (FSMAs) have emerged as potential active materials due to their multifunctional properties like ferromagnetic shape memory effect [1], magnetoresistance [2], magneto-caloric effect [3], elastocaloric applications [4], and spin glass behavior [5]. It has been established that Ni50Mn50-xGax (15 ≤ x ≤ 35) alloys solidify from the melt to a B2 structure on cooling [6]. The austenitic L21 structure is reached through a B2–L21 ordering process, which takes place on cooling to ~ 950 K, depending on the chemical composition [6]. In stoichiometric Ni2MnGa, it is well known that the L21 structure shows
* M. Ramudu [email protected] 1
Department of Physics, Rajiv Gandhi University of Knowledge Technologies, Nuzvid 521201, India
2
Department of Physics, School of Sciences, National Institute of Technology Andhra Pradesh, Tadepalligudem 534101, India
3
School of Physics, University of Hyderabad, Hyderabad 500046, India
structural ordering with Mn occupying the corners of the cube with ferromagnetic exchange and Ga preferentially occupying the edge centers. In B2 structure, the random occupation of Mn and Ga atoms at Mn/Ga sites leads to site disorder. Such a site disorder can cause antiferromagnetic exchange locally between the nearest Mn atoms that are located at neighboring Mn-Ga or Ni-Ga sites. In Mn rich Ni-Mn-Ga alloys, the excess Mn atoms that occupy the Ni or Ga sites lead to antiferromagnetism and modify the magneto-mechanical properties of the alloys [7]. The effect of the varying degree of long-range a
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