Phase equilibria in the Mn-rich portion of Mn–Ga binary system Kazuhiro Minakuchi a , Rie Y. Umetsu b,⇑ , Kiyohito Ishida a , Ryosuke Kainuma a a Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-02 Aoba-yama, Sendai 980-8579, Japan b Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan article info Article history: Received 10 March 2012 Received in revised form 13 April 2012 Accepted 18 April 2012 Available online 26 April 2012 Keywords: Phase equilibria Mn–Ga binary system Magnetization Mn 3 Ga abstract The phase equilibria in the Mn-rich portion of Mn–Ga binary system were experimentally determined. The topological feature of the determined diagram was found to be similar to that reported by Wachtel and Nier [17] while the g phase with the D0 19 structure exists in a wider temperature and composition range and the s phase has a wider solubility range. The complicated composition dependence on magne- tization of the Mn-rich alloys reported by Masumoto et al. can successfully be explained with the deter- mined phase diagram. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Recently, Mn–Ga-based ferromagnetic ternary alloys, such as Ni 2 MnGa [1], Mn 2 NiGa [2] and Fe 2 MnGa [3,4], showing thermo- elastic martensitic transformation, have been receiving much attention as promising candidates for magnetic refrigerant (MR) material using the magnetocaloric effect (MCE) [5] and for magnetic actuator material using magnetic field-induced strain [3,6,7]. In the Mn–Ga binary system, there are many intermetallic compounds and especially, two compounds, MnGa-L1 0 and Mn 3 Ga-D0 22 , with ordered fct structures are known to exhibit a ferromagnetic or ferrimagnetic properties with high coercivity [8]. The L1 0 phase was first reported by Tsuboya and Sugihara to appear in the Mn composition ranging from 66 to 68.5 at.% [9]. On the other hand, the fct phase, actu- ally corresponding to the D0 22 phase, was first mentioned by Zwicker to appear in 77.5 at.% Mn alloy annealed at 400 °C after quenching from 850 °C [10]. Krén and Kádár have confirmed with neutron diffraction that the 72.25 at.% Mn alloy obtained by annealing at about 480 °C after solution treatment possesses the D0 22 ordered structure [11]. It is important to note that in the Mn 3 Ga alloy there is another intermetallic compound with the D0 19 hexagonal structure [12–14]. Masumoto et al. have clarified that the D0 22 phase is a metastable phase precip- itating in the disordered fcc cMn phase before the stable D0 19 phase appears by further annealing, and that the D0 19 phase possesses relatively wide solubility range at temperatures below about 700 °C [14]. Very recently, Balke et al. have proposed that the D0 22 phase is applicable as a magnetic material for spin torque transfer applica- tions in magnetoelectronic devices [15]. In spite of the importance of this binary alloy system, as mentioned below, the Mn–Ga binary phase diagram has not yet been fixed. In the present work, the phase equilibria in the Mn-rich portion of Mn–Ga binary system were experimentally determined and the stabilities and formation processes of the L1 0 and the D0 22 phases were examined. 2. Literature data To date, some versions of Mn–Ga binary phase diagram have been reported by Lu et al. [16], Wachtel and Nier [17], and Meiss- ner and Schubert [18]. Fig. 1 shows these three versions of phase diagrams in the Mn-rich portion of Mn–Ga binary system. 2.1. Lu version (see Fig. 1(a)) The dMn phase is relatively stable and no direct equilibrium be- tween liquid and cMn phases appears. Metastable D0 22 is pre- sented as a stable phase and the phase stabilities of the D0 22 and L1 0 phases are abnormal, for instance showing an impossible com- position dependence in which the order–disorder transformation temperatures among A1, D0 22 and L1 0 structures in the Mn-rich re- gion of over 75 at.% monotonically increase with increasing Mn composition. This version was selected in ‘‘Binary alloy phase dia- grams (second edition)’’ edited by Okamoto and Massalski in 1990 [19]. 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.065 ⇑ Corresponding author at: Tohoku Univ, Inst Mat Res, 2-1-1 Aoba ku Katahira Sendai Miyagi 980-8577, Japan. Tel.: +81 22 215 2492; fax: +81 22 215 2381. E-mail address: rieume@imr.tohoku.ac.jp (R.Y. Umetsu). Journal of Alloys and Compounds 537 (2012) 332–337 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom