Elsevier

Journal of Alloys and Compounds

Volume 644, 25 September 2015, Pages 883-887
Journal of Alloys and Compounds

Magnetic analysis of martensitic and austenitic phases in metamagnetic NiMn(In, Sn) alloys

https://doi.org/10.1016/j.jallcom.2015.04.232Get rights and content

Highlights

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    NiMnIn austenite and martensite have similar Ising-type critical exponents.

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    NiMnIn critical exponents rule out disordered states as spin-glass in martensite.

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    In NiMnIn alloys, magnetism arises mainly from moments localized at Mn atoms.

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    NiCoMnSn critical exponents are close to the ones from tricritical mean field model.

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    NiCoMnSn complex magnetic state results from three different magnetic atoms.

Abstract

Two different metamagnetic shape memory alloys of nominal composition Ni50Mn36In14 and Ni42Co8Mn39Sn11 have been studied by means of modified Arrott plots to give insight into the magnetic states of both the austenitic and martensitic phases. For Ni50Mn36In14 alloy, the same critical exponents (β = 0.32 and γ = 2.0) are obtained in austenite and martensite. They suggest that localized moments at Mn atoms are responsible for the magnetism of both phases according to the Ising model. The martensite, however, displays a rather complex behavior because β continuously changes with temperature. In Ni43Co6.5Mn39Sn11.5, critical exponents in the austenite are β = 0.27 and γ = 1.0. They are close to the tricritical mean field model, but no reliable fits were obtained in the martensite. The results are discussed in terms of microscopically different magnetic states in two alloys reflecting a complex interplay between the ferromagnetic and antiferromagnetic contributions.

Introduction

Metamagnetic Shape Memory Alloys (MMSMAs) are a special kind of Magnetic Shape Memory Alloys (MSMA) in which the low temperature phase (martensite) is weakly magnetic, while the high temperature one (austenite) is ferromagnetic [1], [2], [3], [4]. The martensitic transformation (MT) is accompanied with a large drop in magnetization on cooling and, because of this, the reverse transformation into austenite can be induced by a moderate magnetic field [1], [5], [6]. Thus, MT shows the typical metamagnetic behavior, i.e. a sharp step in the magnetization curve and related large inverse magnetocaloric, magnetoresistance and magnetostrain effects [7], [8], [9], which attract interest for their possible technical applications. These effects are much lower in normal MSMAs, such as Heusler-type Ni–Mn–Ga or Ni–Fe–Ga, where the magnetization changes between austenite and martensite are small [10]. Typical MMSMAs compositions are Heusler-type Mn-rich Ni–Mn–X (X = In, Sn, Sb) alloys with or without additions of Co (see, e.g., [5] and references therein).

While a Curie temperature, TCM, is frequently found well below the martensitic one, TM, hinting for a pure paramagnetic state between TCM and TM, no definitive proof has been obtained of such hypothesis. The ferrimagnetism of the austenitic phase is well documented in most alloys and both ferro-magnetic (FM) and antiferro-magnetic (AF) interactions are present between the magnetic atoms, mainly Mn ones. However, the magnetic character of the martensitic phase is controversial and several options have been proposed, such as spin glass [11], [12], antiferromagnetic [13] or simply paramagnetic [14].

The magnetism of the martensitic phase is important to know, both from a basic point of view and to tailor the compositions in view of new future applications of the metamagnetic shape memory alloys. Different approaches have been used up to now. For instance, Mössbauer measurements of Ni50Mn36.5Fe0.557Sn13 samples indicate a paramagnetic state below TM [14]. Magnetization results on Ni48Co6Mn26Al20 treated by Arrott plots analysis were interpreted as arising from a complex cluster structure with frozen disorder at low temperatures although fitting to traditional Arrott plots was poor and gave little information [15]. Analysis of “Modified Arrott Plots” used in Ref. [12] derives a change from short range to long range magnetic interactions at the Curie temperature of the martensite, in Ni49.5Mn32.5Cu4Sn14. This alloy, however, is far from having a pronounced metamagnetic character, because no change in the martensitic transformation appears in fields up to 4 T [16].

In this work we systematically study two different metamagnetic shape memory alloys, Ni–Mn–X, where X = In and Sn, respectively, to shed light on their magnetic character around the Curie temperature in the austenitic phase as well as in the martensitic one, by determining their critical exponents using “Modified Arrott Plots” and fitting to a magnetic equation of state for the alloys.

Section snippets

Experimental

Two different alloys of nominal composition Ni50Mn36In14 and Ni42Co8Mn39Sn11 were prepared from pure elements by multiple arc remelting and subjected to different heat treatments. The Ni–Mn–In alloy was annealed at 1170 K for 24 h and water quenched. Then, the alloy was heated at 773 K for 30 min and slowly cooled to room temperature. Ni(Co)–Mn–Sn was annealed at 1173 K for 14 days and then quenched in water. In a second treatment it was heated at 1073 K for 30 min in argon atmosphere and finally was

Results and discussion

Fig. 1(a) depicts the diffraction patterns of the two alloys. The diffraction peaks for Ni–Mn–In at T = 343 K are indexed in terms of a L21-ordered structure of cubic austenite, whereas Ni(Co)–Mn–Sn shows at room temperature the coexistence of the L21-ordered cubic austenite, with composition Ni43Co6.5Mn39Sn11.5, and a non-transforming gamma phase, with composition of Ni39Co17.5Mn42Sn1.5, plus some amount of the orthorhombic martensite, which is present because MT develops around room temperature.

Conclusions

In conclusion, we have determined the critical exponents for Ni50Mn36In14 and Ni43Co6.5Mn39Sn11.5 metamagnetic alloys in an attempt to elucidate the magnetic state across the second order magnetic transition in the martensite and compare it with the one of austenite, in a temperature range far from the metamagnetic transition. For Ni50Mn36In14 alloy similar critical exponents (β = 0.32 and γ = 2.0) are obtained for the paramagnetic and ferromagnetic phases of austenite as well as in martensite. The

Acknowledgments

The authors thank to V.A. L’vov, E. Cesari and J. Feuchtwanger for useful discussions. Technical and human support provided by SGIker is gratefully acknowledged. Funding of the Spanish Ministry of Economy and Competitiveness (MINECO) under Grants MAT2011-28217-C02-02 and MAT2014-56116-C4-3-4-R, the Department of Education, Basque Government, Project No. IT-711-13 and UPV/EHU, Project “Grupos Consolidados GIC12/10” are greatly acknowledged.

References (28)

  • V. Sánchez-Alarcos et al.

    Acta Mater.

    (2013)
  • G. Sharma et al.

    JMMM

    (2014)
  • M. Ge et al.

    Solid State Commun.

    (2013)
  • P. Entel et al.

    J. Magn. Magn. Mater.

    (2015)
  • R. Kainuma et al.

    Nature

    (2006)
  • T. Krenke et al.

    Phys. Rev. B

    (2006)
  • W. Ito et al.

    Appl. Phys. Lett.

    (2008)
  • P. Entel et al.

    Eur. Phys. J. B

    (2013)
  • A. Planes et al.

    J. Phys.: Cond. Mat.

    (2009)
  • V.K. Sharma et al.

    J. Phys. D: Appl. Phys.

    (2009)
  • J.M. Barandiarán et al.

    Appl. Phys. Lett.

    (2013)
  • P. Lázpita et al.

    Mater. Sci. Forum

    (2010)
  • V.A. Chernenko et al.

    Phys. Rev. B

    (2003)
  • D.Y. Cong et al.

    Appl. Phys. Lett.

    (2010)
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