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Martensitic and intermartensitic transitions in Ni50Mn20Cu5Ga21Al4 Heusler alloy
Journal of Alloys and Compounds 586 (2014) 718–721
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Martensitic and intermartensitic transitions in Ni50Mn20Cu5Ga21Al4 Heusler alloy C. Salazar Mejía ⇑,1, A.M. Gomes Instituto de Física, Universidade Federal do Rio de Janeiro, C.P. 68528, Rio de Janeiro 21941-972, Brazil
a r t i c l e
i n f o
Article history: Received 26 April 2013 Received in revised form 27 September 2013 Accepted 30 September 2013 Available online 14 October 2013 Keywords: Heusler alloys Martensitic transition Intermartensitic transition Magnetization measurements
a b s t r a c t Intermartensitic and martensitic transitions on polycrystalline Ni50Mn20Cu5Ga21Al4 Heusler alloys were studied through magnetization, heat capacity and electrical resistivity measurements. The occurrence of an intermartensitic transition depends on whether the sample undergoes heat treatment, while the martensitic transition is independent of the annealing process. After an annealing treatment, martensitic and intermartensitic transitions are observed at 264 K and 181 K respectively, both showing large thermal hysteresis. Both transitions affects the magnetic response of the zero field cooled and field cooled magnetic measurements. Similar effects are also observed on the electrical resistivity and specific heat of the alloys. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The Heusler alloys are known to be ferromagnetic intermetallic materials with a face-centered cubic structure and X2YZ stoichiometry type. Some Heusler alloys, as the Ni–Mn–Ga and Ni–Mn–In, present a remarkable property known as magnetic shape memory effect, due to the martensitic transformation, in which the alloy is deformed in the low-temperature phase and it recovers its original shape by reverse transformation upon heating to the reverse transformation temperature [1,2]. The Ni2MnGa alloys present a ferromagnetic to paramagnetic transition around TC = 380 K and a martensitic transition at TM 220 K from a cubic L21-type structure to a tetragonal martensitic structure [3]. It was found that off-stoichiometry Ni–Mn–Ga also show large magnetocaloric effect[4], related to a magnetic and structural transitions occurring at the same temperature. The magnetic (TC) and martensitic (TM) transitions can be also tuned through atomic substitution of Cu on Mn site, leading to a giant magnetocaloric effect in Ni2Mn1 xCuxGa alloy at room temperature [5] for the ideal x = 0.25 composition. Recently, the partial substitution of Ga by Al has been studied as a way to improve the ductility and the fabrication cost of the Ni–Mn–Ga alloys [6–8]. The fact of the MCE been related to the structural transitions in these alloys, reinforces the impor-
⇑ Corresponding author. E-mail addresses: [email protected] (C. Salazar Mejía), [email protected] (A.M. Gomes). 1 Present address: Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany. 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.204
tance of the studies on the structural properties in order to get a better understanding of the phenomenon underlying the structural transitions. Some Ni–Mn–Ga samples present low-temperature intermartensitic transitions (IMT) in addition to the martensitic transformation (MT) or instead of the pre-martensitic transitions[9]. These IMT transformations could be also explained as the martensitic transition occurring in different steps. For example, first the sample acquires a modulated martensitic structure (M) and then the final, more stable, tetragonal (T) one [10] or following P(cubic) ? 5M ? 7M ? T [11] and these are exhibited by single and polycrystalline samples. IMT have been reported for other materials as Ni–Mn–In–Sb [12], Ni–Mn–Co–Ga [13] and Ni–Mn– Fe–Cu–Ga [10]. These transformations have been extensively studied by dynamical mechanical analysis, calorimetry, dilatometry, transmission electron microscopy [14], resistivity [15], magnetization [13] and X-ray diffraction [12]. In this work we present the study of the intermartensitic and martensitic transitions in a polycrystalline Ni50Mn20Cu5Ga21Al4 alloy by means of magnetization, electrical resistivity and specific heat measurements. The effects of quenching and heat treatments in the MT and IMT of the sample are also presented.
2. Materials and methods A polycrystalline Ni50Mn20Cu5Ga21Al4 sample were prepared using a standard arc-melting method with high purity metallic elements in argon atmosphere. The sample were re-melted 4 times to assure homogeneity and annealed at 1273 K for 3 days followed by quenching in room temperature water. An additional annealing at 673 K for one day was also performed. The annealing process was made in quartz ampoules with a partial argon atmosphere. Another sample with similar
C. Salazar Mejía, A.M. Gomes / Journal of Alloys and Compounds 586 (2014) 718–721
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composition was also melted and cut for different heat treatments. Identification of the crystal structure was achieved by analysis of X-ray powder diffraction (XPD) done in a Bruker D8 diffractometer, using Cu Ka radiation. Magnetization, resistance and heat capacity measurements were performed in a Physical Property Measurement System (PPMS) from Quantum Design, Inc.
3. Results and discussion From X-ray diffractograms obtained at room temperature (RT), presented in Fig. 1(a), we determined that the austenite phase of the Ni50Mn20Cu5Ga21Al4 sample is a cubic L21 structure with lattice parameters a = b = c = 5.796 Å. The peaks observed at 43.3° and 83.3°, indicated by arrows at the diffractogram, are related to the martensitic tetragonal structure still present at room temperature. From the diffractogram, we can determine that there are no spurious phases present in the sample. The inset of Fig. 1 shows the results for the as-cast sample. It can be seen that the peak at 65° is not well defined, and the signal-to-noise ratio is not comparable to the annealed sample. This can be attributed to the heat treatment that promotes a higher crystalline alloy. The martensite structure at 100 K, presented in Fig. 1(b), can be described with a monoclinic structure, parameters am = 4.205 Å, bm = 5.548 Å, cm = 21.060 Å and b = 88.527°. Magnetization curves presented in Fig. 2, were measured between 2 K and 320 K, following the Zero Field Cooling (ZFC), the Field Cooled Cooling (FCC) and the Field Cooled Warming (FCW) procedures, with an applied magnetic field of 200 Oe. From the results for the cooling run, it can be seen a para-ferromagnetic second-order transition at Curie temperature TC = 305 K, followed by a martensitic transformation at TM = 264 K and also an intermartensitic transformation is present at TI = 181 K. The start and finish martensitic transition temperatures, MS and MF respectively, and the reverse IMT temperature, TR, are also indicated in the figure. The MT and the IMT have a thermal hysteresis of 10 K and 49 K respectively. The same measurements, made in the as-cast sample, are presented in the inset of Fig. 2. It is important to note that the as-cast sample exhibits what appears to be a broad martensitic transition, with hysteresis between the cooling and warming curves, but not an abrupt change in magnetization as expected for first-order transitions, as the as-cast sample is not fully homogeneous, a broad distribution of transitions temperatures is expected. In this case, the annealing process is necessary to form the required phase. No sign of intermartensitic transition is present
e
Fig. 2. Magnetization as a function of temperature measurement for Ni50Mn20Cu5Ga21Al4, following ZFC, FCC and FCW procedures, in an applied magnetic field of 200 Oe. Inset shows the same measurements for the as-cast sample.
in the sample without heat treatment. These last features will be discussed later. The martensitic transitions were studied in detail by magnetization measurements. The ZFC, FCC and FCW curves were repeated 3 times and the results are showed in Fig. 3, where each run was measured with a sweep rate of 1 K/min. It can be seen that each FCC curve is different from the others around the direct IMT, while the reverse intermartensitic transformation is quite reproducible. This feature of the direct intermartensitic transformation can also be seen in Fig. 4 where FCC and FCW curves were measured for different values of applied magnetic field. The forward IMT is controlled by the internal stresses and elastic energy, which causes a notable variation of the forward transformation temperatures while the corresponding reverse transformations are much more reproducible [16], as it comes from a more stable phase, which is the final martensitic state; while the forward IMT (on cooling) comes from an intermediate state. Also in Fig. 3 is observed how the martensitic and the intermartensitic transitions are affected by the applied magnetic field. The martensitic transitions temperatures (TA, TM, MS. . .) and the reverse intermartensitic transition temperature (TR) are hardly affected by the magnitude of the field while the direct intermartensitic transition (at cooling) takes place in multiple and different steps each time, as mentioned before. It
(a)
(b)
Fig. 1. (a) X-ray diffraction at room temperature for the sample after the heat treatment and the sample as-cast (inset). The principal peaks of the cubic L21-type structure are indexed in the diffractogram. The small peaks indicated by the arrows are related to the tetragonal martensitic structure still present at room temperature. (b) X-ray diffraction at 100 K. Can be described with a monoclinic structure.
Fig. 3. Magnetization as a function of temperature for Ni50Mn20Cu5Ga21Al4, following FCC (top) and FCW (bottom) procedures, with an applied magnetic field of 200 Oe.
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Fig. 4. Magnetization as a function of temperature measurements for different applied magnetic field values for Ni50Mn20Cu5Ga21Al4, following FCC (solid symbols) and FCW (open symbols) procedures.
can be also note that the FCC curve recorded at 200 Oe is different of that presented in Fig. 2. From Fig. 4 is also possible to confirm that the martensite phase has a higher anisotropy than the austenite phase, yet higher magnetic saturation value [3,17]. For the present material, a magnetic field between 1 kOe and 3 kOe is sufficient to overcome the anisotropy energy. The effect of the intermartensitic transformation on the magnetization of the material is reduced as the magnetic field increases, being almost negligible for a field of 50 kOe, while the martensitic transformation still causes a step-like change in the magnetization for that field. It was reported for other compounds that the occurrence of an intermartensitic process is not necessary for a martensite to transform back to the parent phase, when a sufficiently high magnetic field is applied [12]. However, this is not the case for our sample, both transitions still occur even for higher applied magnetic field and the transition temperatures are almost field independent, as can be seen in the resistivity measurements. The resistivity of the sample as a function of temperature was measured for an applied magnetic field of 0 and 50 kOe, during warming and cooling. The results, presented in Fig. 5(a) (where a bias of 0.15 lX m was applied at the 50 kOe curve), show a
(a)
(b)
noticeable jump corresponding to the martensitic transition; a change in the slope corresponding to the ferro-paramagnetic transition and a small jump at the reverse intermartensitic transition. These transport effects, at the martensitic transitions, are related to variations on the electronic structure of the sample as consequence of the structural transformation while the change in the slope of the curve at TC is related to the decrease of electron-magnon scattering [15]. The direct IMT (on cooling) does have a small effect on the resistivity. The applied magnetic field also produces a small effect on the transport properties of the material, except for the change in the Curie temperature. Fig. 5(b) shows the specific heat of the sample obtained in absence of magnetic field from 2 K to 380 K. A small peak at the reverse intermartensitic transition is observed. Even though the IMT is reported to be of first-order [18], the endothermic effect (when warming) is low compared to the exothermic effect (when cooling) [19,16,20] and that is reflected in the peaks of the specific heat measurement. The peak observed at the martensitic transition is typical of first-order transitions, as expected. It was not possible to obtain the curve with applied magnetic field because the experimental setup for specific heat is not suitable for measurements under applied magnetic field at high temperature as the sample tends to move away from the sample holder when crossing the intermartensitic transition. To study the effect of the heat treatment on the martensitic transitions (MT and IMT) an additional sample was melted and cut for different heat treatments. The previous sample showed in this work will be called from now on sample A, and the second sample B, according to the designations resumed in Table 1, where we have a heat treatment at 1273 K for 72 h followed by quenching in room temperature water or followed by slow cooling in the furnace and an additional heat treatment at 673 K for 24 h followed by quenching either by slow cooling at ca. 13 K/min. In Fig. 6 the samples with the two heat treatments are compared. The main difference in Curie temperature between samples A and B is attributed to the small differences in Mn composition for both specimens being Ni49.3Mn19.4Cu5.4Ga20.1Al5.7 for sample A, and Ni48.4Mn17.3Cu5.2Ga20.6Al8.5 for sample B, as determined by Energy-dispersive X-ray spectroscopy measurements. The MT and the IMT are present in all samples, showing that quenching does not induce these kind of transitions and that this feature is intrinsic to the sample. To obtain the desired magnetic and structural properties of the material (for comparison see sample as-cast showed in the inset of Fig. 2), heat treatment is necessary to increase the degree of atomic order [21,22] and acquire a homogeneous sample. Comparing the results for the sample Bq2 and Bs2 it is also possible to observe that finishing the heat treatment with quenching or slow cooling procedures leads to some changes in the transition temperatures of the samples. The quenching procedure seems to induce some additional stress on the sample that increases the anisotropy of the material, as reflected in the magnetization measurements. In Fig. 7 the magnetization curves as a function of field and as a function of temperature (inset) for samples Bq1, with the first heat treatment followed by quenching, and the sample Bq2, with both heat treatments finished with quenching, are compared. The goal of the second heat treatment is to increase the degree of long-
Table 1 Sample description.
Fig. 5. (a) Resistivity as a function of temperature for applied magnetic field of 0 and 50 kOe. A bias of 0.15 lX m was applied at the 50 kOe curve for better visualization. The measures were performed during warming (solid black symbols) and cooling (open red symbols). (b) Specific heat obtained in absence of magnetic field from 2 K to 380 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Sample label
Heat treatment
A Bq1 Bq2 Bs1 Bs2
1273 K-quenching + 673 K-quenching 1273 K-quenching 1273 K-quenching + 673 K-quenching 1273 K-slow cooling 1273 K-slow cooling + 673 K-slow cooling
C. Salazar Mejía, A.M. Gomes / Journal of Alloys and Compounds 586 (2014) 718–721
Fig. 6. Magnetization as a function of temperature measurements for sample A, Bq2 and Bs2, following FCC (top) and FCW (bottom) procedures in an applied magnetic field of 200 Oe.
721
Heusler alloy, by means of magnetization, resistivity and heat capacity measurements. We found that after annealing treatment, the sample exhibits an intermartensitic transition, while this behavior is not observed on the as-cast sample. Both intermartensitic and martensitic transitions are independent of quenching procedure and they are intrinsic of the sample. While the martensitic transition temperatures and dynamics are independent of field, the intermartensitic transition, specially the direct one (when cooling) is almost different in each measurement. The martensitic and intermartensitic transitions affect the magnetic response, the electrical resistivity and the specific heat of the sample, being that the effect of the IMT in the specific heat and electrical resistivity is much lower than the effect of the MT. The presence of different phase transitions in this kind of materials and the effect that these transitions have on the different physical properties open the possibility to use this multifunctional materials as actuators. For that purpose, the understanding of the characteristics, properties and origin of those transitions is fundamental. Acknowledgements The authors would like to thank CNPq and FAPERJ for financial support. We are grateful to Professor Renato Guimaraes for assistance with X-ray measurements at LDRX-Universidade Federal Fluminense and to CBPF and CAPES/Pro-Equipamentos. References
Fig. 7. Magnetization as a function of applied magnetic field for sample Bq1 (triangles) and Bq2 (circles) at 10 K. The inset shows the magnetization as a function of temperature measurements for sample Bq1 and Bq2 following FCC (solid symbols) and FCW (open symbols) procedures in an applied magnetic field of 200 Oe.
range order of the L21 phase[7], but in this case, it is not observed. As can be seen more clearly in the inset of Fig. 7, the second heat treatment seems to increase the anisotropy of the sample as the magnetization values with low applied magnetic field are lower in the Bq2 sample than in the Bq1. A similar result is seen for the samples Bs1 and Bs2 with slow cooling instead of quenching (not show here). Besides the change in the magnetization value, the reverse IMT becomes sharper with the additional treatment and all the transition temperatures have a change, being more significant the effect in the direct IMT, TI, as reported for other samples [21,20]. 4. Conclusions We presented in this paper a study of the martensitic and intermartensitic transitions both exhibited by the Ni50Mn20Cu5Ga21Al4
[1] K. Ullakko, J.K. Huang, V.V. Kokorin, R.C. OHandley, Scripta Mater. 36 (1997) 1133. [2] R. Kainuma, K. Oikawa, W. Ito, Y. Sutou, T. Kanomata, K. Ishida, J. Mater. Chem. 18 (2008) 1837. [3] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Philos. Mag. B 49 (1984) 295– 310. [4] L. Pareti, M. Solzi, F. Albertini, A. Paoluzi, Eur. Phys. J. B 32 (2003) 303–307. [5] S. Stadler, M. Khan, J. Mitchell, N. Ali, A.M. Gomes, I. Dubenko, A.Y. Takeuchi, A.P. Guimaraes, Appl. Phys. Lett. 88 (2006) 192511. [6] A.C. Abhyankar, Y.T. Yu, Y.K. Kuo, G.W. Huang, C.S. Lue, Intermetallics 18 (2010) 2090. [7] H. Ishikawa, R.Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida, Acta Mater. 56 (2008) 4789–4797. [8] C. Salazar Mejía, A.M. Gomes, L.A.S. de Oliveira, J. Appl. Phys. 111 (2012) 07A923. [9] A. Gonzalez-Comas, E. Obrado, L. Manosa, A. Planes, V.A. Chernenko, B.J. Hattink, A. Labarta, Phys. Rev. B 60 (1999) 7085–7090. [10] M. Khan, B. Gautam, A. Pathak, I. Dubenko, S. Stadler, N. Ali, J. Phys.: Condens. Matter 20 (2008) 505206. [11] O. Soderberg, A. Sozinov, N. Lanska, Y. Ge, V.K. Lindroos, S.P. Hannula, Mater. Sci. Eng. A 438 (2006) 957–960. [12] S.Y. Yu, S.S. Yan, L. Zhao, L. Feng, J.L. Chen, G.H. Wu, J. Magn. Magn. Mater 322 (2010) 2541–2544. [13] M. Khan, S. Stadler, N. Ali, J. Appl. Phys. 99 (2006) 08M705. [14] V.A. Chernenko, C. Segui, E. Cesari, J. Pons, V.V. Kokorin, Phys. Rev. B 57 (1998) 2659–2662. [15] V.V. Khovailo, K. Oikawa, C. Wedel, T. Takagi, T. Abe, K. Sugiyama, J. Phys.: Condens. Matter 16 (2004) 1951–1961. [16] C. Segui, V.A. Chernenko, J. Pons, E. Cesari, V. Khovailo, T. Takagi, Acta Mater. 53 (2005) 111–120. [17] F.Q. Zhu, F.Y. Yang, C.L. Chien, L. Ritchie, G. Xiao, G.H. Wu, J. Magn. Magn. Mater. 288 (2005) 79–83. [18] W.H. Wang, G.H. Wu, J.L. Chen, S.X. Gao, W.S. Zhang, Appl. Phys. Lett. 79 (2001) 1148. [19] V.A. Chernenko, A. Amengual, E. Cesari, V.V. Kokorin, I.K. Zasimchuk, J. Phys. IV 5 (1995) 95–98. [20] X.G. Sun, J. Song, X.N. Zhang, C.Y. Xie, J. Alloys Comp. 509 (2011) 4868–4870. [21] C. Segui, J. Pons, E. Cesari, Acta Mater. 55 (2007) 1649–1655. [22] V. Sánchez-Alarcos, V. Recarte, J.I. Pérez-Landazábal, G.J. Cuello, Acta Mater. 55 (2007) 3883–3889.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Martensitic and intermartensitic transitions in Ni50Mn20Cu5Ga21Al4 Heusler alloy C. Salazar Mejía ⇑,1, A.M. Gomes Instituto de Física, Universidade Federal do Rio de Janeiro, C.P. 68528, Rio de Janeiro 21941-972, Brazil
a r t i c l e
i n f o
Article history: Received 26 April 2013 Received in revised form 27 September 2013 Accepted 30 September 2013 Available online 14 October 2013 Keywords: Heusler alloys Martensitic transition Intermartensitic transition Magnetization measurements
a b s t r a c t Intermartensitic and martensitic transitions on polycrystalline Ni50Mn20Cu5Ga21Al4 Heusler alloys were studied through magnetization, heat capacity and electrical resistivity measurements. The occurrence of an intermartensitic transition depends on whether the sample undergoes heat treatment, while the martensitic transition is independent of the annealing process. After an annealing treatment, martensitic and intermartensitic transitions are observed at 264 K and 181 K respectively, both showing large thermal hysteresis. Both transitions affects the magnetic response of the zero field cooled and field cooled magnetic measurements. Similar effects are also observed on the electrical resistivity and specific heat of the alloys. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The Heusler alloys are known to be ferromagnetic intermetallic materials with a face-centered cubic structure and X2YZ stoichiometry type. Some Heusler alloys, as the Ni–Mn–Ga and Ni–Mn–In, present a remarkable property known as magnetic shape memory effect, due to the martensitic transformation, in which the alloy is deformed in the low-temperature phase and it recovers its original shape by reverse transformation upon heating to the reverse transformation temperature [1,2]. The Ni2MnGa alloys present a ferromagnetic to paramagnetic transition around TC = 380 K and a martensitic transition at TM 220 K from a cubic L21-type structure to a tetragonal martensitic structure [3]. It was found that off-stoichiometry Ni–Mn–Ga also show large magnetocaloric effect[4], related to a magnetic and structural transitions occurring at the same temperature. The magnetic (TC) and martensitic (TM) transitions can be also tuned through atomic substitution of Cu on Mn site, leading to a giant magnetocaloric effect in Ni2Mn1 xCuxGa alloy at room temperature [5] for the ideal x = 0.25 composition. Recently, the partial substitution of Ga by Al has been studied as a way to improve the ductility and the fabrication cost of the Ni–Mn–Ga alloys [6–8]. The fact of the MCE been related to the structural transitions in these alloys, reinforces the impor-
⇑ Corresponding author. E-mail addresses: [email protected] (C. Salazar Mejía), [email protected] (A.M. Gomes). 1 Present address: Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany. 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.204
tance of the studies on the structural properties in order to get a better understanding of the phenomenon underlying the structural transitions. Some Ni–Mn–Ga samples present low-temperature intermartensitic transitions (IMT) in addition to the martensitic transformation (MT) or instead of the pre-martensitic transitions[9]. These IMT transformations could be also explained as the martensitic transition occurring in different steps. For example, first the sample acquires a modulated martensitic structure (M) and then the final, more stable, tetragonal (T) one [10] or following P(cubic) ? 5M ? 7M ? T [11] and these are exhibited by single and polycrystalline samples. IMT have been reported for other materials as Ni–Mn–In–Sb [12], Ni–Mn–Co–Ga [13] and Ni–Mn– Fe–Cu–Ga [10]. These transformations have been extensively studied by dynamical mechanical analysis, calorimetry, dilatometry, transmission electron microscopy [14], resistivity [15], magnetization [13] and X-ray diffraction [12]. In this work we present the study of the intermartensitic and martensitic transitions in a polycrystalline Ni50Mn20Cu5Ga21Al4 alloy by means of magnetization, electrical resistivity and specific heat measurements. The effects of quenching and heat treatments in the MT and IMT of the sample are also presented.
2. Materials and methods A polycrystalline Ni50Mn20Cu5Ga21Al4 sample were prepared using a standard arc-melting method with high purity metallic elements in argon atmosphere. The sample were re-melted 4 times to assure homogeneity and annealed at 1273 K for 3 days followed by quenching in room temperature water. An additional annealing at 673 K for one day was also performed. The annealing process was made in quartz ampoules with a partial argon atmosphere. Another sample with similar
C. Salazar Mejía, A.M. Gomes / Journal of Alloys and Compounds 586 (2014) 718–721
719
composition was also melted and cut for different heat treatments. Identification of the crystal structure was achieved by analysis of X-ray powder diffraction (XPD) done in a Bruker D8 diffractometer, using Cu Ka radiation. Magnetization, resistance and heat capacity measurements were performed in a Physical Property Measurement System (PPMS) from Quantum Design, Inc.
3. Results and discussion From X-ray diffractograms obtained at room temperature (RT), presented in Fig. 1(a), we determined that the austenite phase of the Ni50Mn20Cu5Ga21Al4 sample is a cubic L21 structure with lattice parameters a = b = c = 5.796 Å. The peaks observed at 43.3° and 83.3°, indicated by arrows at the diffractogram, are related to the martensitic tetragonal structure still present at room temperature. From the diffractogram, we can determine that there are no spurious phases present in the sample. The inset of Fig. 1 shows the results for the as-cast sample. It can be seen that the peak at 65° is not well defined, and the signal-to-noise ratio is not comparable to the annealed sample. This can be attributed to the heat treatment that promotes a higher crystalline alloy. The martensite structure at 100 K, presented in Fig. 1(b), can be described with a monoclinic structure, parameters am = 4.205 Å, bm = 5.548 Å, cm = 21.060 Å and b = 88.527°. Magnetization curves presented in Fig. 2, were measured between 2 K and 320 K, following the Zero Field Cooling (ZFC), the Field Cooled Cooling (FCC) and the Field Cooled Warming (FCW) procedures, with an applied magnetic field of 200 Oe. From the results for the cooling run, it can be seen a para-ferromagnetic second-order transition at Curie temperature TC = 305 K, followed by a martensitic transformation at TM = 264 K and also an intermartensitic transformation is present at TI = 181 K. The start and finish martensitic transition temperatures, MS and MF respectively, and the reverse IMT temperature, TR, are also indicated in the figure. The MT and the IMT have a thermal hysteresis of 10 K and 49 K respectively. The same measurements, made in the as-cast sample, are presented in the inset of Fig. 2. It is important to note that the as-cast sample exhibits what appears to be a broad martensitic transition, with hysteresis between the cooling and warming curves, but not an abrupt change in magnetization as expected for first-order transitions, as the as-cast sample is not fully homogeneous, a broad distribution of transitions temperatures is expected. In this case, the annealing process is necessary to form the required phase. No sign of intermartensitic transition is present
e
Fig. 2. Magnetization as a function of temperature measurement for Ni50Mn20Cu5Ga21Al4, following ZFC, FCC and FCW procedures, in an applied magnetic field of 200 Oe. Inset shows the same measurements for the as-cast sample.
in the sample without heat treatment. These last features will be discussed later. The martensitic transitions were studied in detail by magnetization measurements. The ZFC, FCC and FCW curves were repeated 3 times and the results are showed in Fig. 3, where each run was measured with a sweep rate of 1 K/min. It can be seen that each FCC curve is different from the others around the direct IMT, while the reverse intermartensitic transformation is quite reproducible. This feature of the direct intermartensitic transformation can also be seen in Fig. 4 where FCC and FCW curves were measured for different values of applied magnetic field. The forward IMT is controlled by the internal stresses and elastic energy, which causes a notable variation of the forward transformation temperatures while the corresponding reverse transformations are much more reproducible [16], as it comes from a more stable phase, which is the final martensitic state; while the forward IMT (on cooling) comes from an intermediate state. Also in Fig. 3 is observed how the martensitic and the intermartensitic transitions are affected by the applied magnetic field. The martensitic transitions temperatures (TA, TM, MS. . .) and the reverse intermartensitic transition temperature (TR) are hardly affected by the magnitude of the field while the direct intermartensitic transition (at cooling) takes place in multiple and different steps each time, as mentioned before. It
(a)
(b)
Fig. 1. (a) X-ray diffraction at room temperature for the sample after the heat treatment and the sample as-cast (inset). The principal peaks of the cubic L21-type structure are indexed in the diffractogram. The small peaks indicated by the arrows are related to the tetragonal martensitic structure still present at room temperature. (b) X-ray diffraction at 100 K. Can be described with a monoclinic structure.
Fig. 3. Magnetization as a function of temperature for Ni50Mn20Cu5Ga21Al4, following FCC (top) and FCW (bottom) procedures, with an applied magnetic field of 200 Oe.
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C. Salazar Mejía, A.M. Gomes / Journal of Alloys and Compounds 586 (2014) 718–721
Fig. 4. Magnetization as a function of temperature measurements for different applied magnetic field values for Ni50Mn20Cu5Ga21Al4, following FCC (solid symbols) and FCW (open symbols) procedures.
can be also note that the FCC curve recorded at 200 Oe is different of that presented in Fig. 2. From Fig. 4 is also possible to confirm that the martensite phase has a higher anisotropy than the austenite phase, yet higher magnetic saturation value [3,17]. For the present material, a magnetic field between 1 kOe and 3 kOe is sufficient to overcome the anisotropy energy. The effect of the intermartensitic transformation on the magnetization of the material is reduced as the magnetic field increases, being almost negligible for a field of 50 kOe, while the martensitic transformation still causes a step-like change in the magnetization for that field. It was reported for other compounds that the occurrence of an intermartensitic process is not necessary for a martensite to transform back to the parent phase, when a sufficiently high magnetic field is applied [12]. However, this is not the case for our sample, both transitions still occur even for higher applied magnetic field and the transition temperatures are almost field independent, as can be seen in the resistivity measurements. The resistivity of the sample as a function of temperature was measured for an applied magnetic field of 0 and 50 kOe, during warming and cooling. The results, presented in Fig. 5(a) (where a bias of 0.15 lX m was applied at the 50 kOe curve), show a
(a)
(b)
noticeable jump corresponding to the martensitic transition; a change in the slope corresponding to the ferro-paramagnetic transition and a small jump at the reverse intermartensitic transition. These transport effects, at the martensitic transitions, are related to variations on the electronic structure of the sample as consequence of the structural transformation while the change in the slope of the curve at TC is related to the decrease of electron-magnon scattering [15]. The direct IMT (on cooling) does have a small effect on the resistivity. The applied magnetic field also produces a small effect on the transport properties of the material, except for the change in the Curie temperature. Fig. 5(b) shows the specific heat of the sample obtained in absence of magnetic field from 2 K to 380 K. A small peak at the reverse intermartensitic transition is observed. Even though the IMT is reported to be of first-order [18], the endothermic effect (when warming) is low compared to the exothermic effect (when cooling) [19,16,20] and that is reflected in the peaks of the specific heat measurement. The peak observed at the martensitic transition is typical of first-order transitions, as expected. It was not possible to obtain the curve with applied magnetic field because the experimental setup for specific heat is not suitable for measurements under applied magnetic field at high temperature as the sample tends to move away from the sample holder when crossing the intermartensitic transition. To study the effect of the heat treatment on the martensitic transitions (MT and IMT) an additional sample was melted and cut for different heat treatments. The previous sample showed in this work will be called from now on sample A, and the second sample B, according to the designations resumed in Table 1, where we have a heat treatment at 1273 K for 72 h followed by quenching in room temperature water or followed by slow cooling in the furnace and an additional heat treatment at 673 K for 24 h followed by quenching either by slow cooling at ca. 13 K/min. In Fig. 6 the samples with the two heat treatments are compared. The main difference in Curie temperature between samples A and B is attributed to the small differences in Mn composition for both specimens being Ni49.3Mn19.4Cu5.4Ga20.1Al5.7 for sample A, and Ni48.4Mn17.3Cu5.2Ga20.6Al8.5 for sample B, as determined by Energy-dispersive X-ray spectroscopy measurements. The MT and the IMT are present in all samples, showing that quenching does not induce these kind of transitions and that this feature is intrinsic to the sample. To obtain the desired magnetic and structural properties of the material (for comparison see sample as-cast showed in the inset of Fig. 2), heat treatment is necessary to increase the degree of atomic order [21,22] and acquire a homogeneous sample. Comparing the results for the sample Bq2 and Bs2 it is also possible to observe that finishing the heat treatment with quenching or slow cooling procedures leads to some changes in the transition temperatures of the samples. The quenching procedure seems to induce some additional stress on the sample that increases the anisotropy of the material, as reflected in the magnetization measurements. In Fig. 7 the magnetization curves as a function of field and as a function of temperature (inset) for samples Bq1, with the first heat treatment followed by quenching, and the sample Bq2, with both heat treatments finished with quenching, are compared. The goal of the second heat treatment is to increase the degree of long-
Table 1 Sample description.
Fig. 5. (a) Resistivity as a function of temperature for applied magnetic field of 0 and 50 kOe. A bias of 0.15 lX m was applied at the 50 kOe curve for better visualization. The measures were performed during warming (solid black symbols) and cooling (open red symbols). (b) Specific heat obtained in absence of magnetic field from 2 K to 380 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Sample label
Heat treatment
A Bq1 Bq2 Bs1 Bs2
1273 K-quenching + 673 K-quenching 1273 K-quenching 1273 K-quenching + 673 K-quenching 1273 K-slow cooling 1273 K-slow cooling + 673 K-slow cooling
C. Salazar Mejía, A.M. Gomes / Journal of Alloys and Compounds 586 (2014) 718–721
Fig. 6. Magnetization as a function of temperature measurements for sample A, Bq2 and Bs2, following FCC (top) and FCW (bottom) procedures in an applied magnetic field of 200 Oe.
721
Heusler alloy, by means of magnetization, resistivity and heat capacity measurements. We found that after annealing treatment, the sample exhibits an intermartensitic transition, while this behavior is not observed on the as-cast sample. Both intermartensitic and martensitic transitions are independent of quenching procedure and they are intrinsic of the sample. While the martensitic transition temperatures and dynamics are independent of field, the intermartensitic transition, specially the direct one (when cooling) is almost different in each measurement. The martensitic and intermartensitic transitions affect the magnetic response, the electrical resistivity and the specific heat of the sample, being that the effect of the IMT in the specific heat and electrical resistivity is much lower than the effect of the MT. The presence of different phase transitions in this kind of materials and the effect that these transitions have on the different physical properties open the possibility to use this multifunctional materials as actuators. For that purpose, the understanding of the characteristics, properties and origin of those transitions is fundamental. Acknowledgements The authors would like to thank CNPq and FAPERJ for financial support. We are grateful to Professor Renato Guimaraes for assistance with X-ray measurements at LDRX-Universidade Federal Fluminense and to CBPF and CAPES/Pro-Equipamentos. References
Fig. 7. Magnetization as a function of applied magnetic field for sample Bq1 (triangles) and Bq2 (circles) at 10 K. The inset shows the magnetization as a function of temperature measurements for sample Bq1 and Bq2 following FCC (solid symbols) and FCW (open symbols) procedures in an applied magnetic field of 200 Oe.
range order of the L21 phase[7], but in this case, it is not observed. As can be seen more clearly in the inset of Fig. 7, the second heat treatment seems to increase the anisotropy of the sample as the magnetization values with low applied magnetic field are lower in the Bq2 sample than in the Bq1. A similar result is seen for the samples Bs1 and Bs2 with slow cooling instead of quenching (not show here). Besides the change in the magnetization value, the reverse IMT becomes sharper with the additional treatment and all the transition temperatures have a change, being more significant the effect in the direct IMT, TI, as reported for other samples [21,20]. 4. Conclusions We presented in this paper a study of the martensitic and intermartensitic transitions both exhibited by the Ni50Mn20Cu5Ga21Al4
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