Microstructure and low-temperature phase transition in Ni 2 FeGa Heusler alloy

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1 Journal of Alloys and Compounds 425 (2006) Microstructure and low-temperature phase transition in Ni 2 FeGa Heusler alloy Libao Liu a,b,c,, Shiyou Fu c, Zhuhong Liu b, Guangheng Wu b, Xiudong Sun a, Jianqi Li b a Department of Physics, Harbin Institute of Technology, Harbin , China b Institute of Physics, Chinese Academy of Sciences, Beijing , China c Harbin Institute of Technology at Weihai, Weihai , China Received 7 November 2005; received in revised form 13 January 2006; accepted 17 January 2006 Available online 28 February 2006 Abstract The microstructure features and structural phase transition in the Ni 2 FeGa alloy has been systematically investigated by means of transmission electron microscopy (TEM). A number of ordered states have been observed at room temperature; certain short-range orders are found to be in metastable states which are temperature sensitive and become invisible when annealed. In situ cooling TEM observations revealed evident structural changes along with the martensitic transition with T c 145 K. Low-temperature microstructure domains, superstructures and variations of monoclinic distortion have been analyzed in detail Elsevier B.V. All rights reserved. Keywords: Ni 2 FeGa alloy; Magnetostriction; TEM; Low-temperature phase transition 1. Introduction Ferromagnetic shape memory alloys (FSMA), showing a reversible structural phase transition between high and low symmetric phases, combine the characteristics of giant strains in shape memory alloys with rapid response, as conventional magnetostrictive materials. The Ni 2 MnGa Heusler-type alloys and several notable systems, such as Ni Fe Ga, Co Ni Ga, Ni Mn Fe Ga, Co Ni Al, Ni 2 MnAl, Fe Pd, etc., have been extensively studied in recent investigations [1 10]. It is generally accepted that Ni Fe Ga alloy has a chemically ordered L2 1 structure and exhibits a thermoelastic martensitic transformation from a cubic to an orthorhombic structure at 142 K [3]. Previously, numerous investigations focusing on martensitic transformation (MT), magnetization, and microstructure features have been performed on materials prepared under different conditions. A series of investigations on, for example, microstructural change near the martensitic transformation, the highly undercooled Ni Fe Ga samples, crystalline anisotropy Corresponding author. Tel.: ; fax: address: lbliu@blem.ac.cn (L. Liu). and magnetic-field-induced strain, etc., have been extensively discussed [11 17]. It is also noted that numerous types of superstructure modulations appear along with martensitic transition at low temperatures [18,19]. For Ni 2 FeGa, it is found that, with increasing Ni concentration in ribbons, the martensitic transformation temperature increases progressively, on the other hand, the Curie temperature decreases from 440 to 312 K [3]. In the present study, the structural properties of the Ni 2 FeGa Heusler phase have been investigated by means of TEM. The ordered states appearing at room temperature and structural changes with the MT, have been analyzed in detail. 2. Experimental High quality, pure-phase Ni 2 FeGa (single-rolled) ribbons were synthesized in an induction furnace; the experimental details on the sample preparation have been reported in Ref. [20]. X-ray diffraction results indicate that all melt-spun samples were well crystallized in a Heusler phase. The specimens for TEM observations were prepared by mechanical polishing, dimpling, and then ion milling from both sides, thus the structural features inside the ribbons can be observed. Structural investigations were performed on an H- 9000NA TEM and a Tecnai F20 TEM both equipped with cooling sample stages /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 L. Liu et al. / Journal of Alloys and Compounds 425 (2006) Results Fig. 1. XRD pattern of Ni 2 FeGa alloy. Fig. 1 shows a typical powder XRD pattern from the Ni 2 FeGa ribbon. All main reflection peaks can be well indexed to a cubic structure with lattice parameters of a = Å [3]. The lowtemperature XRD measurements revealed a clear martensitic transformation with decreasing temperature as discussed in the NiMnGa Heusler alloy [6]. Microstructure investigation indicated that the Ni 2 FeGa alloy contains a rich variety of domain structures in addition to the complex grain boundaries. These domain structures, with size ranging from a few nanometers to several microns, can be clearly characterized in the bright- and dark-field TEM images. Fig. 2(a) and (b) shows respectively the bright-field (BF) images taken from two different areas, illustrating the granular structure in the Ni 2 FeGa ribbon. It is noted that the pentagonal grain in Fig. 2(a) contains many domain patterns with evident contrast alternation. Regular arrays of Ni 2 FeGa grains are also frequently observed as shown in Fig. 2(b). Symmetrical electron-diffraction observations suggested that the Ni 2 FeGa alloy has an average cubic structure with space group Fm3m; on the other hand, superstructure and short-range orders commonly appear in these kinds of materials. Fig. 2 shows the electron-diffraction patterns demonstrating the presence of complex superstructure spots (streaks) in the Ni 2 FeGa alloy at room temperature. These patterns are obtained with the electron beam along the [1 0 0] zone-axis. The main diffraction spots in all patterns can be well indexed to a cubic cell with lattice parameter of a = 5.74 Å in agreement with the results of X-ray diffraction. It suggested a well-defined L2 1 chemical order (Heusler phase) in current system by appearance of (1 1 1) diffraction spot in selected area diffraction (SAD) patterns along [0 1 1] axis [20 22]. Fig. 2(c) (f) shows a number of SAD patterns illustrating the presence of complex superstructure spots or streaks following the main diffraction spots. Considering the complexity of these superstructures, we first analyzed the superstructure with clear weak spots as shown in Fig. 2(d) and (e). The spots in these patterns appear regularly at positions ((±7/18) (±11/18) 0) and ((±11/18) (±7/18) 0). In most case, they appear as diffused pairwise reflections positioned symmetrically along the direction. Though the superstructure spots in the diffraction patterns (Fig. 2(c) (e)) looked notably different, the most notable superstructure spots appear repeatedly at the systematical positions q =((±7/18) (±11/18) 0) as indicated by the white arrows. The presence of multi-modulated structures in the present system often gives rise to complex diffraction patterns with remarkable additional satellite reflections. In Fig. 2(c), the superstructure reflections appear as two parallel lines roughly along 110 direction. This pattern, as discussed in following context, actually comes from a multi-modulated area where modulations exist with different wave vectors and periodicities. Fig. 2(f) shows the electron-diffraction patterns showing the fundamental structural features of the Ni 2 FeGa alloy at room temperature. Our investigations on the Ni 2 FeGa ribbons suggested that most superstructures, as shown for example in Fig. 2(c) (e), corresponds to certain metastable states which change rapidly with the sample annealed at the high temperatures. Fig. 2(f) shows a typical diffraction pattern from the Ni 2 FeGa samples annealed in a vacuum for about 0.5 h at around 800 K. It is noted that the other types of superstructures become almost invisible. The most striking features in this diffraction pattern are the presence of additional reflection spots and streaks following the main diffractions spots. These diffuse reflections emanate from each of the Bragg diffraction spots along the 110 directions. In the previous studies of Ni Mn Ga and Ni x Al 100 x alloys, the diffuse diffractions along 110 direction are discussed in connection with premartensitic transition and phonon anomalies in the [ζζ0] TA 2 branch of a Heusler phase. Fig. 3(a) shows a high-resolution TEM image taken from an area with multi-modulations. The fast Fourier transform (FFT) pattern is displayed in Fig. 3(b), showing the satellite spots and diffusion near the main spots, in good agreement with the diffraction patterns in Fig. 2. The diffuse superstructure spots directly suggest that the structural modulations in the present case have a short coherent length as apparent in the high-resolution image; each well-defined modulation (q 1, q 2 and q 3 ) appears respectively in the small area of a few nanometers in size. In Fig. 3(a), we can address the complex structures roughly as three fundamental modulations, indicated by q 1, q 2 and q 3. Arrows q 1 and q 3 appear respectively along the 1 10 and 110 directions with periodicities of about 5d 110 and 2d 110. q 2 =((±7/18) (±11/18) 0) yields the notable superstructure spots as discuss in the above context. The modulations in general result in a combined effect appearing as a complex contrast. The intensity of each modulation changes markedly from one area to another. As well, we can also find different sets of structural modulations with properties similar to q 1, q 2 and q 3. The common feature noted in our investigation is that all structural modulations are evidently temperature dependent. Specifically, clear changes for either average structure or the complex modulated structures are demonstrated in the lowtemperature observations.

3 178 L. Liu et al. / Journal of Alloys and Compounds 425 (2006) Fig. 2. BF TEM images and SAD patterns of Ni 2 FeGa alloy taken at room temperature: (a) many domains seen along [0 0 1]; (b) grains of different shape; (c), (d), (e) and (f) complex diffraction patterns taken along [0 0 1] axis ((f) heat-treated sample). In order to understand the structural changes along with the MT at a temperature T m = 145 K, we have performed extensive in situ cooling TEM observations on the Ni 2 FeGa alloys prepared under slight different conditions. It is noted that both the average structure and modulations show remarkable changes with decreasing temperature. Above the martensitic critical point (T m ), the diffuse streaks (shown in Fig. 2(f)) change progressively toward superstructure spots in the low-temperature range [18], and below T m (145 K), satellite reflections turn out to be sharp spots corresponding to several typical superstructures. Fig. 4(b) (f) shows a series of typical diffraction patterns taken at 100 K. It is noted that the superstructure can be fundamentally explained as a 5M transition, 6M and 7M superstructures were also found in some areas as reported in ref [18]. The striking feature revealed in these diffraction patterns indicates the presence of a clear structural transition from a cubic to a monoclinic phase as indicated for the main diffraction spots. Systematic analysis indicates that this monoclinic type of structural distortion can change from one area to another with the distortion angles ranging from 1 to 4. In order to understand the structural features of the monoclinic phase, we have performed our TEM observations on numerous samples, it has been demonstrated that the monoclinic transition always appears below the martensitic transition.

4 L. Liu et al. / Journal of Alloys and Compounds 425 (2006) Fig. 3. (a) HRTEM image of Ni 2 FeGa alloy taken along [0 0 1] zone-axis at room temperature; (b) FFT of (a). Fig. 4. (a) SAD patterns taken at room temperature; (b), (c), (d), (e) and (f) show monoclinic structural distortions at low temperature. Fig. 5 gives the statistical data for this structural distortion taken at a temperature of around 100 K, which shows two notable maximums at distortion angles of around 1 and 3. It should be pointed out that these results are notably dependent on the experimental temperatures. This kind of monoclinic distortion could be fundamentally explained as planar shear appearing during the martensitic transformation with the habit plane parallel to the [1 1 0] direction. Based on our experimental results, the displacement arising from the shear strain is estimated to be (1.7 7%)d 220 (1.7 7%) 5.74 Å Å, this result is in agreement with the reported data [3]. In the present system, the structural distortion can often give rise to several clear superstructure modulations below the MT as reported in Ref. [18,19]. Analysis indicates that the monoclinic distortion and structural modulation in general appears simultaneously and results in integrated effects, and as a result the directions of the modulations change clearly into a monoclinic structure, as seen from Fig. 4(c). Fig. 5. Distribution of the distortion angles for the low-temperature monoclinic lattice.

5 180 L. Liu et al. / Journal of Alloys and Compounds 425 (2006) Fig. 6. BF images and SAD patterns taken at a temperature of 100 K: (a) low-temperature domain structures in different grains with 90 rotation; (b) SAD pattern taken from the area shown in (a). Following the MT, bright field TEM observations reveal clear microstructure changes with decreasing temperature. Regular domain structures are clearly visible at a temperature of around 100 K. Fig. 6(a) shows a TEM image of an area with the notable 90 domain structure in the Ni 2 FeGa martensite. Fig. 6(b) shows the corresponding electron diffraction pattern with a clear 5M type structural modulation and recognizable monoclinic distortion. In this pattern, two perpendicular sets of satellite reflections are clearly visible around each basic Bragg spot. These are considered to originate from twin domains where the modulation wave vectors are rotated by 90 with respect to one another. This structural feature can also be seen in Fig. 6(a). The low-temperature domain lamella in general appear along the 010 or 100 direction with an average periodicity of about nm. The structural distortion across the domain boundary changes in an alternating fashion as indicated by the dashed-line. 4. Conclusions TEM observations revealed the presence of a rich variety of microstructure phenomena in the Ni 2 FeGa alloy. Certain metastable ordered states have been observed and analyzed at room temperature. In particular, our in situ cooling TEM observations revealed evident structural changes with the martensitic transition at T c 145 K. Low-temperature microstructure domains, superstructures and variations of monoclinic distortion have been extensively analyzed in connection with the MT phase transition. Acknowledgments The authors would like to thank Prof. Y.Q. Zhou for his help with the experimental works, we also thank N. Pemberton-Pigott for his help during manuscript preparation. The work reported here was supported by the National Natural Science Foundation of China and by the Ministry of Science and Technology of China (973 project No. 2006CB601001). References [1] K. Ullakko, J.K. Huang, C. Kantner, R.C. O Handley, Appl. Phys. Lett. 69 (1996) [2] M. Wuttig, J. Li, C. Craciunescu, Scripta Mater. 44 (10) (2001) [3] Z.H. Liu, H. Liu, X.X. Zhang, M. Zhang, X.F. Dai, H.N. Hu, J.L. Chen, G.H. Wu, Phys. Lett. A329 (3) (2004) 214. [4] A.A. Cherechukin, I.E. Dikshtein, D.I. Ermakov, A.V. Glebov, V.V. Koledov, D.A. Kosolapov, V.G. Shavrov, A.A. Tulaikova, E.P. Krasnoperov, T. Takagi, Phys. Lett. A 291 (2/3) (2001) 175. [5] K. Oikawa, L. Wulff, T. Iijima, Appl. Phys. Lett. 79 (2001) [6] Y. Ma, S. Awaji, K. Watanabe, M. Matsumoto, N. Kobayashi, Solid State Commun. 113 (2000) 671. [7] H.X. Zheng, M.X. Xia, J. Liu, J.G. Li, J. Alloys Compd. 385 (1/2) (2004) 144. [8] F. Gejima, Y. Sutou, R. Kainuma, K. Ishida, Metall. Mater. Trans. A 30A (10) (1999) [9] G.H. Wu, C.H. Yu, L.Q. Meng, J.L. Chen, F.M. Yang, S.R. Qi, W.S. Zhan, Z. Wang, Y.F. Zheng, L.C. Zhao, Appl. Phys. Lett. 75 (1999) [10] R.D. James, M. Wuttig, Philos. Mag. A 77 (5) (1998) [11] Y. Li, C. Jiang, T. Liang, Y. Ma, H. Xu, Scripta Mater. 48 (2003) [12] Y. Murakami, D. Shindo, K. Oikawa, R. Kainuma, K. Ishida, Appl. Phys. Lett. 85 (2004) [13] T. Omori, N. Kamiya, Y. Sutou, K. Oikawa, R. Kainuma, K. Ishida, Mater. Sci. Eng. A 378 (1/2) (2004) 403. [14] Y. Sutou, N. Kamiya, T. Omori, R. Kainuma, K. Ishida, Appl. Phys. Lett. 84 (2004) [15] V.A. Chernenko, J. Pons, E. Cesari, I.K. Zasimchuk, Scripta Mater. 50 (2004) 225. [16] H.X. Zheng, M.X. Xia, J. Liu, J.G. Li, J. Alloys Compd. 388 (2) (2005) 172. [17] H. Morito, A. Fujita, K. Fukamichi, R. Kainuma, K. Ishida, Appl. Phys. Lett. 83 (2003) [18] J.Q. Li, Z.H. Liu, H.C. Yu, M. Zhang, Y.Q. Zhou, G.H. Wu, Solid State Commun. 126 (2003) 323. [19] K. Oikawa, T. Ota, T. Ohmori, Y. Tanaka, H. Morito, A. Fujita, R. Kainuma, K. Fukamichi, K. Ishida, Appl. Phys. Lett. 81 (2002) [20] Z.H. Liu, M. Zhang, Y.T. Cui, Y.Q. Zhou, W.H. Wang, G.H. Wu, Appl. Phys. Lett. 82 (2003) 424. [21] Y. Sutou, I. Ohnuma, R. Kainuma, K. Ishida, Metall. Mater. Trans. 29A (1998) [22] V.V. Khovailo, T. Takagi, A.N. Vasilev, H. Miki, M. Matsumoto, R. Kainuma, Phys. Status Solidi (A) 183 (2001) R1.