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A large magnetic-field-induced strain in Ni–Fe–Mn–Ga–Co ferromagnetic shape memory alloy
Journal of Alloys and Compounds 577S (2013) S372–S375
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
A large magnetic-field-induced strain in Ni–Fe–Mn–Ga–Co ferromagnetic shape memory alloy Haruhiko Morito a,∗ , Katsunari Oikawa b , Asaya Fujita b , Kazuaki Fukamichi a , Ryosuke Kainuma b , Kiyohito Ishida b a b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-02, Sendai 980-8579, Japan
a r t i c l e
i n f o
Article history: Available online 2 May 2012
a b s t r a c t To obtain a large strain of Ni–Fe–Ga–Co ferromagnetic shape memory alloys, the effects of Mn on the magnetocrystalline anisotropy constant Ku , and the magnetic-field-induced strain (MFIS) under static stresses were investigated. The value of Ku was increased by the addition of Mn, and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy was accompanied by a relatively large value of −1.5 × 105 J/m3 at room temperature. From the stress–strain curves of this alloy, the twinning stress is estimated to be 8–10 MPa. Consequently, the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy provides a large MFIS of about 11.3% at room temperature under a static compressive stress of about 8 MPa. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ferromagnetic shape memory alloys (FSMAs) having a large magnetic-field-induced strain (MFIS) have attracted much attention, because they are promising as magnetic-controlled actuators and sensors. Large MFISs have been observed in martensite phases of Ni2 MnGa alloys [1–6] and other FSMAs [7–18]. The MFIS is explained by the martensite variant rearrangements caused by an external magnetic field [1,2]. When the magnetocrystalline anisotropy energy is larger than the energy driving variant rearrangement, the angle between magnetization and the applied magnetic field directions is lowered by not only the independent rotation of magnetization but also the variant rearrangement in order that the magnetic easy axis is aligned parallel to the magnetic field direction [1,2]. The MFIS has quantitatively also been discussed by introducing the magnetic stress mag [5,6]. For the variant rearrangement by applying magnetic field, the value of mag should be larger than the twinning stress tw . The value of mag is given by ε−1 Ku , where Ku and ε0 = (1−c/a) are the uniaxial mag0 netic anisotropy constant and the twinning strain calculated from the lattice parameters of the tetragonal phase, respectively. Recently, Ni–Fe–Ga Heusler-type alloys have drawn much attention as practical FSMAs [19–25]. The increase of the Curie temperature TC has been searched by adding Co [26–31]. The magnetocrystalline anisotropy constant Ku is effectively increased,
∗ Corresponding author. Address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 217 5814; fax: +81 22 217 5813. E-mail address: [email protected] (H. Morito). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.056
while the crystal structure of the martensite phase becomes the non-modulated structure in Ni–Fe–Ga–Co system, and hence the high twinning stress hinders the appearance of the large MFIS. In order to achieve a large MFIS, a static stress aiding the magnetic field was used, that is, the magnetic field was applied under a compressive stress. As a result, a large MFIS of 8.5% was observed under the static stress of 8 MPa in Ni49 Fe18 Ga27 Co6 martensite phase [30]. By using an assisted stress, a large MFIS can be obtained even in the non-modulated structure. However, the magnitude of MFIS is limited by the Ku value. Accordingly, a larger Ku value is required to obtain a larger MFIS over 10% in the vicinity of room temperature. In a previous work, we reported the TC can be controlled by changing composition in Ni–Fe–Ga and Ni–Fe–Ga–Co systems [26–31]. However, the increase of the TC by controlling the compositions of Ni, Fe and Co leads to a decrease of the martensitic transformation temperature below room temperature. Although the martensitic transformation temperature is increased by decrease of the Ga content, the sample progresses to two (ˇ + ) phases when the Ga content is less than 27at.% [20]. To overcome such practical problems, an addition of Mn is expected to be useful. It has been reported that the ˇ phase becomes stable by substituting Mn [32]. Consequently, we can decrease the Ga content and increase the TC with keeping the ˇ single-phase. In the present study, to obtain a larger MFIS over 10% in the vicinity of room temperature, we have tried to increase the value of Ku by the addition of Mn in relationship to the control of the TC . Furthermore, the MFIS under static compressive stresses in Ni–Fe–Mn–Ga–Co alloy has been investigated. In this paper, to make clear the effect of Mn, our previous results for Ni–Fe–Ga–Co alloy [30] are incorporated in the present discussion.
H. Morito et al. / Journal of Alloys and Compounds 577S (2013) S372–S375 2. Experimental Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single crystals were grown by an optical floating-zone method in a helium atmosphere. The single crystal specimens were annealed at 1433–1473 K for 48 h to homogenize and followed by quenching in ice water. After the homogenization, they were additionally heattreated at 673 K for 24 h to achieve a high degree of atomic order in the Heusler structure and followed by quenching in ice water. The crystallographic orientations were defined from the electron backscattering diffraction patterns. The cubic specimens in the parent phase were trimmed so that the <100>P (P: Parent) directions are parallel to the faces. To discuss the MFIS quantitatively, we treated the crystal structure of the martensite phase as the body-centered tetragonal (bct) structure. The <100>P (P: Parent) in the parent cubic phase with the lattice constant a0 corresponds to the a- or c-axis of the martensitic tetragonal phase in the bct unit cell. In order to obtain the single-variant specimen, uniaxial compressive stresses were applied to the [100]P and [010]P directions in the martensite phase of the above-mentioned single crystals. The crystal structure and lattice parameters of the samples were determined by powder X-ray diffraction using CuK˛. From the X-ray diffraction data analysis, it was confirmed that the modulated structure coexists with the nonmodulated structure in the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. By applying stresses, however, the non-modulated structure becomes more stable [24]. In this study, the measurements have been carried out by using single-variant samples with the nonmodulated structure. After applying compressive stress, the c-axis of the bct unit cell is oriented to the [001]P direction. The magnetization was measured up to 1.6 MA/m with a superconducting quantum interference device (SQUID) magnetometer. The MFIS was measured with an extensometer under a compressive stress in magnetic fields.
3. Results and discussion The thermomagnetization curves of the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-crystals are respectively shown by the dashed and solid lines in Fig. 1. In all the figures, each sample is expressed as NiFeGaCo and NiFeMnGaCo. Both the martensitic transformation starting temperature Ms and the Curie temperature TC were determined from the figures in the cooling process. The Ms and TC of the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 are 320 and 420 K, respectively, while those of the Ni49 Fe18 Ga27 Co6 , 325 and 398 K, respectively. The Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy has a high TC with keeping a martensitic transformation over room temperature. In the previous work, it was confirmed that the value of TC increases, whereas that of Ms decreases with increasing Mn content in the Ni54 Fe19-x Mnx Ga27 (x = 0–3) [25]. The increase of (Fe + Mn) content leads to the decrease of Ms , but the decrease of Ga content keeps the Ms above room temperature. By adjusting Mn composition, the value of TC is effectively increased.
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Fig. 2 shows the stress–strain curves at room temperature (R.T.) for the single-variant Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 . The dashed line stands for the result for the Ni49 Fe18 Ga27 Co6 alloy. The compressive stress was applied along the c-axis direction in the single variant state. In the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy, the variant rearrangement was initiated above the applied twinning stress tw of 8–10 MPa, corresponding to the plateau, and a twinning strain ε of about 15% is obtained. The values of tw and strain are equivalent to those of Ni49 Fe18 Ga27 Co6 alloy with the non-modulated structure. No observable effect of the Mn addition on the structure was confirmed. The magnetocrystalline anisotropy at R.T. for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy in the single-variant martensite phase was determined from the magnetization curves (M–H) along the c-axis ([001]P ) and c-plane ([010]P ). The measured M–H curves at R.T. for Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy are shown in Fig. 3, together with those of Ni49 Fe18 Ga27 Co6 alloy. The both M–H curves for the c-plane are easily saturated, while those for the c-axis are hardly saturated below 1.2 MA/m, revealing that the c-axis of the tetragonal unit cell in the martensite phase is the hard-axis. The curve of the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy at R.T. shows an easy saturation magnetization Msat of 50 Am2 /kg, much larger than 43 Am2 /kg for the Ni49 Fe18 Ga27 Co6 alloy. After correcting the demagnetizing field, the uniaxial magnetocrystalline anisotropy constant Ku is evaluated from the M–H curves as the area cross section between the easiest curve along c-plane and the hardest curve along c-axis directions. The R.T. value of Ku for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy in the martensite phase is estimated to be about −1.5 × 105 J/m3 . In comparison with the value Ku =−1.2 × 105 J/m3 of the Ni49 Fe18 Ga27 Co6 alloy, the magnitude of the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 martensite phase is relatively large. That is the |Ku | is increased due to the increase of TC by the addition of Mn. The variant rearrangement driven by applying magnetic field has been discussed quantitatively by introducing magnetic stress mag [5,6]. In order to satisfy the condition for the variant rearrangement by applying magnetic field, mag should be larger than the twinning stress tw . In other words, the criterion mag > tw should be satisfied. The value of mag is expressed by ε−1 Ku with ε0 = (1−c/a). The value of ε0 is calculated from the 0 lattice parameters ratio c/a = 1.19 estimated from X-ray diffrac-
25
Ms
Ni Fe Mn Ga Co Ni Fe Ga Co
20
(MPa)
M (arb. units.)
Ni Fe Mn Ga Co
TC
15
c-axis
10 Ni Fe Ga Co
5
H = 40 kA/m
R. T.
0 300
350
400
0
450
4
8
12
16
(%)
Temperature (K) Fig. 1. Thermomagnetization Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloys.
curves
of
the
Ni49 Fe18 Ga27 Co6
and
Fig. 2. Compressive stress–strain curves along the c-axis direction at room temperature (R.T.) in the martensite phase for the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-crystals.
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H. Morito et al. / Journal of Alloys and Compounds 577S (2013) S372–S375
0
60
c-plane
R. T.
Ni Fe Mn Ga Co
50
-2 -4
Ni Fe Ga Co (%)
M (Am2/kg)
40 30 20
c-axis
-10
R. T.
0 0.2
0.4
0.6
Ni Fe Ga Co
-8
10
0.0
-6
0.8
1.0
1.2
1.4
1.6
Ni Fe Mn Ga Co
-12 0.0
0.2
H (MA/m) Fig. 3. Magnetization curves along the c-axis and c-plane directions at R.T. for the single-variant martensite phase in the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloys.
(a) before applying
assist
dies sample
tion pattern. Meanwhile, the room temperature value of Ku is −1.5 × 105 J/m3 . Therefore, the value of mag is evaluated to be 0.79 MPa. On the other hand, tw is estimated to be 8–10 MPa from the applied twinning stress tw in Fig. 2, that is, the variant rearrangement occurs within the region of 2 MPa. This region is defined as tw evaluated from the strain–stress curve given in Fig. 2. As it turned out, the magnitude of stress becomes mag < tw and is not satisfied with the appearance condition of MFIS. Consequently, no MFIS is obtainable at R.T. under the condition mentioned above. Recently, in order to achieve a large MFIS, a static stress aiding the magnetic field has been demonstrated, that is, a large stressassisted magnetic-field-induced strain (SAMFIS) of 8.5% under the assisted stress assist of 8 MPa in Ni49 Fe18 Ga27 Co6 martensite phase is obtained [30]. By using such assisted stress, a large MFIS can be obtained in FMSAs [17,18,30]. Therefore, to obtain a large MFIS, a static stress has been applied as the assist of magnetic field for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. Shown in Fig. 4 are the SAMFIS curves at R.T. for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-crystals. To make clear the effect of Mn, our previous results for Ni49 Fe18 Ga27 Co6 [30] are incorporated in the present result. Before applying magnetic field, the assisted stress assist of 8 MPa was applied along the c-axis hard direction in the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. As a result, a shrinkage of 2% was observed (Fig. 4b). Next, the magnetic stress mag of about 0.79 MPa was added by applying the magnetic field parallel to the same direction. There is a plateau in the stress–strain curve as seen from Fig. 2, corresponding to tw = 8–10 MPa. By applying both assisted and magnetic stresses, therefore, the strain corresponding to the total applied stress assist + mag is obtained from the stress–strain curve. During applying magnetic field, a steep shrinkage is caused by the variant rearrangement, which the hard axis (c-axis) in the variant was aligned perpendicular to the magnetic field direction (Fig. 4c), was observed around H = 0.3 MA/m. In the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy, the maximal strain comes to 11.3% in the field of 0.8 MA/m, which value is larger than that of the Ni49 Fe18 Ga27 Co6 alloy. In the previous study, we clearly claimed the conditions of SAMFIS by studying some FSMAs [18]. The value of the SAMFIS of the Co41 Ni32 Al27 is smaller than that of Co47.5 Ni22.5 Ga30.0 and Ni49 Fe18 Ga27 Co6 , while both the values of Ku and mag of the former are larger than those of the latter. That is, the value
c-axis
0.4 0.6 H (MA/m) (b) after applying
assist
assist
c-axis
0.8
1.0
(c) after applying assist
c-axis
H
Fig. 4. Magnetic-field-induced strain ε measured parallel to the direction of magnetic field H applied along the c-axis at R.T. for the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-variant specimens under the stress level of about 8 MPa. The bottom sample figures are the schematic images of sequences for applying (a) assisted stress and (b) magnetic field.
of tw is also significant for the appearance of SAMFIS. The SAMFIS of the Co41 Ni32 Al27 alloy is small because of the large value of tw , though the value of mag is relatively large. As a result, to obtain a large SAMFIS, the alloy should exhibit a narrow region, that is, a low slope in the stress–strain curve in Fig. 2, for the slope of twinning stress tw , together with a large mag . In the present study, the tw of Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy is comparable in magnitude to that of Ni49 Fe18 Ga27 Co6 alloy. By applying assisted stress of 8 MPa, a shrinkage of 5% was observed in Ni49 Fe18 Ga27 Co6 alloy[30], while that of 2% was observed in Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. Even if the more small assisted stress which is corresponding to the strain of 2% was applied to Ni49 Fe18 Ga27 Co6 alloy, the MFIS was not obtained because of the low level of mag . By addition of Mn, however, the mag is increased with keeping the small tw , leading to the large SAMFIS.
4. Conclusions In conclusion, the increase of the Curie temperature TC by the Mn addition effectively increases the uniaxial magnetocrystalline anisotropy constant Ku of Ni–Fe–Ga–Co alloy. The room temperature value of Ku at for the present Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy is evaluated to be −1.5 × 105 J/m3 , nearly equivalent to that of Ni2 MnGa alloy [1,5,6]. In the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy, the variant rearrangement associated with the magnetization rotation was observed, and the maximal strain of 11.3 % was confirmed under a compressive stress of about 8 MPa. From these results, it has been revealed that the present Ni–Fe–Mn–Ga–Co ferromagnetic shape memory alloys have great potential as large MFISs assisted by static compressive stresses.
H. Morito et al. / Journal of Alloys and Compounds 577S (2013) S372–S375
Acknowledgements The authors wish to thank Dr. Y. Sutou, Mr. T. Ota and Mr. T. Takagi for their experimental supports. A part of the present study was supported by the Grant-in-Aids for Scientific Research (S) from the Japan Society for the Promotion of Science. The support from the Global COE Project is also acknowledged. References [1] K. Ullakko, J.K. Huang, C. Kantner, R.C. O’Handley, V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966. [2] R.C. O’Handley, J. Appl. Phys. 83 (1998) 3263. [3] O. Heczko, A. Sozinov, K. Ullakko, IEEE T. Magn. 36 (2000) 3266. [4] S.J. Murray, M. Marioni, S.M. Allen, R.C. O’Handley, T.A. Lograsso, Appl. Phys. Lett. 77 (2000) 886. [5] A. Sozinov, A.A. Likhachev, N. Lanska, K. Ullakko, Appl. Phys. Lett. 80 (2002) 1746. [6] A. Sozinov, A.A. Likhachev, K. Ullakko, IEEE T. Magn. 38 (2002) 2814. [7] R.D. James, M. Wuttig, Phil. Mag. A77 (1998) 1273. [8] T. Kakeshita, T. Takeuchi, T. Fukuda, M. Tsujiguchi, T. Saburi, R. Oshima, S. Muto, Appl. Phys. Lett. 77 (2000) 1502. [9] A. Fujita, K. Fukamichi, F. Gejima, R. Kainuma, K. Ishida, Appl. Phys. Lett. 77 (2000) 3054. [10] K. Oikawa, L. Wulff, T. Iijima, F. Gejima, T. Ohmori, A. Fujita, K. Fukamichi, R. Kainuma, K. Ishida, Appl. Phys. Lett. 79 (2001) 3290. [11] K. Oikawa, T. Ota, F. Gejima, T. Ohmori, R. Kainuma, K. Ishida, Mater. Trans. 42 (2001) 2472. [12] H. Morito, A. Fujita, K. Fukamichi, R. Kainuma, K. Ishida, K. Oikawa, Appl. Phys. Lett. 81 (2002) 1657. [13] M. Wuttig, J. Li, C. Craciunescu, Scripta Mater. 44 (2001) 2393. [14] R. Kainuma, Y. Imano, W. Ito, Y. Sutou, H. Morito, S. Okamoto, O. Kitakami, K. Oikawa, A. Fujita, T. Kanomata, K. Ishida, Nature 439 (2006) 957.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
A large magnetic-field-induced strain in Ni–Fe–Mn–Ga–Co ferromagnetic shape memory alloy Haruhiko Morito a,∗ , Katsunari Oikawa b , Asaya Fujita b , Kazuaki Fukamichi a , Ryosuke Kainuma b , Kiyohito Ishida b a b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-02, Sendai 980-8579, Japan
a r t i c l e
i n f o
Article history: Available online 2 May 2012
a b s t r a c t To obtain a large strain of Ni–Fe–Ga–Co ferromagnetic shape memory alloys, the effects of Mn on the magnetocrystalline anisotropy constant Ku , and the magnetic-field-induced strain (MFIS) under static stresses were investigated. The value of Ku was increased by the addition of Mn, and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy was accompanied by a relatively large value of −1.5 × 105 J/m3 at room temperature. From the stress–strain curves of this alloy, the twinning stress is estimated to be 8–10 MPa. Consequently, the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy provides a large MFIS of about 11.3% at room temperature under a static compressive stress of about 8 MPa. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ferromagnetic shape memory alloys (FSMAs) having a large magnetic-field-induced strain (MFIS) have attracted much attention, because they are promising as magnetic-controlled actuators and sensors. Large MFISs have been observed in martensite phases of Ni2 MnGa alloys [1–6] and other FSMAs [7–18]. The MFIS is explained by the martensite variant rearrangements caused by an external magnetic field [1,2]. When the magnetocrystalline anisotropy energy is larger than the energy driving variant rearrangement, the angle between magnetization and the applied magnetic field directions is lowered by not only the independent rotation of magnetization but also the variant rearrangement in order that the magnetic easy axis is aligned parallel to the magnetic field direction [1,2]. The MFIS has quantitatively also been discussed by introducing the magnetic stress mag [5,6]. For the variant rearrangement by applying magnetic field, the value of mag should be larger than the twinning stress tw . The value of mag is given by ε−1 Ku , where Ku and ε0 = (1−c/a) are the uniaxial mag0 netic anisotropy constant and the twinning strain calculated from the lattice parameters of the tetragonal phase, respectively. Recently, Ni–Fe–Ga Heusler-type alloys have drawn much attention as practical FSMAs [19–25]. The increase of the Curie temperature TC has been searched by adding Co [26–31]. The magnetocrystalline anisotropy constant Ku is effectively increased,
∗ Corresponding author. Address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 217 5814; fax: +81 22 217 5813. E-mail address: [email protected] (H. Morito). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.056
while the crystal structure of the martensite phase becomes the non-modulated structure in Ni–Fe–Ga–Co system, and hence the high twinning stress hinders the appearance of the large MFIS. In order to achieve a large MFIS, a static stress aiding the magnetic field was used, that is, the magnetic field was applied under a compressive stress. As a result, a large MFIS of 8.5% was observed under the static stress of 8 MPa in Ni49 Fe18 Ga27 Co6 martensite phase [30]. By using an assisted stress, a large MFIS can be obtained even in the non-modulated structure. However, the magnitude of MFIS is limited by the Ku value. Accordingly, a larger Ku value is required to obtain a larger MFIS over 10% in the vicinity of room temperature. In a previous work, we reported the TC can be controlled by changing composition in Ni–Fe–Ga and Ni–Fe–Ga–Co systems [26–31]. However, the increase of the TC by controlling the compositions of Ni, Fe and Co leads to a decrease of the martensitic transformation temperature below room temperature. Although the martensitic transformation temperature is increased by decrease of the Ga content, the sample progresses to two (ˇ + ) phases when the Ga content is less than 27at.% [20]. To overcome such practical problems, an addition of Mn is expected to be useful. It has been reported that the ˇ phase becomes stable by substituting Mn [32]. Consequently, we can decrease the Ga content and increase the TC with keeping the ˇ single-phase. In the present study, to obtain a larger MFIS over 10% in the vicinity of room temperature, we have tried to increase the value of Ku by the addition of Mn in relationship to the control of the TC . Furthermore, the MFIS under static compressive stresses in Ni–Fe–Mn–Ga–Co alloy has been investigated. In this paper, to make clear the effect of Mn, our previous results for Ni–Fe–Ga–Co alloy [30] are incorporated in the present discussion.
H. Morito et al. / Journal of Alloys and Compounds 577S (2013) S372–S375 2. Experimental Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single crystals were grown by an optical floating-zone method in a helium atmosphere. The single crystal specimens were annealed at 1433–1473 K for 48 h to homogenize and followed by quenching in ice water. After the homogenization, they were additionally heattreated at 673 K for 24 h to achieve a high degree of atomic order in the Heusler structure and followed by quenching in ice water. The crystallographic orientations were defined from the electron backscattering diffraction patterns. The cubic specimens in the parent phase were trimmed so that the <100>P (P: Parent) directions are parallel to the faces. To discuss the MFIS quantitatively, we treated the crystal structure of the martensite phase as the body-centered tetragonal (bct) structure. The <100>P (P: Parent) in the parent cubic phase with the lattice constant a0 corresponds to the a- or c-axis of the martensitic tetragonal phase in the bct unit cell. In order to obtain the single-variant specimen, uniaxial compressive stresses were applied to the [100]P and [010]P directions in the martensite phase of the above-mentioned single crystals. The crystal structure and lattice parameters of the samples were determined by powder X-ray diffraction using CuK˛. From the X-ray diffraction data analysis, it was confirmed that the modulated structure coexists with the nonmodulated structure in the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. By applying stresses, however, the non-modulated structure becomes more stable [24]. In this study, the measurements have been carried out by using single-variant samples with the nonmodulated structure. After applying compressive stress, the c-axis of the bct unit cell is oriented to the [001]P direction. The magnetization was measured up to 1.6 MA/m with a superconducting quantum interference device (SQUID) magnetometer. The MFIS was measured with an extensometer under a compressive stress in magnetic fields.
3. Results and discussion The thermomagnetization curves of the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-crystals are respectively shown by the dashed and solid lines in Fig. 1. In all the figures, each sample is expressed as NiFeGaCo and NiFeMnGaCo. Both the martensitic transformation starting temperature Ms and the Curie temperature TC were determined from the figures in the cooling process. The Ms and TC of the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 are 320 and 420 K, respectively, while those of the Ni49 Fe18 Ga27 Co6 , 325 and 398 K, respectively. The Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy has a high TC with keeping a martensitic transformation over room temperature. In the previous work, it was confirmed that the value of TC increases, whereas that of Ms decreases with increasing Mn content in the Ni54 Fe19-x Mnx Ga27 (x = 0–3) [25]. The increase of (Fe + Mn) content leads to the decrease of Ms , but the decrease of Ga content keeps the Ms above room temperature. By adjusting Mn composition, the value of TC is effectively increased.
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Fig. 2 shows the stress–strain curves at room temperature (R.T.) for the single-variant Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 . The dashed line stands for the result for the Ni49 Fe18 Ga27 Co6 alloy. The compressive stress was applied along the c-axis direction in the single variant state. In the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy, the variant rearrangement was initiated above the applied twinning stress tw of 8–10 MPa, corresponding to the plateau, and a twinning strain ε of about 15% is obtained. The values of tw and strain are equivalent to those of Ni49 Fe18 Ga27 Co6 alloy with the non-modulated structure. No observable effect of the Mn addition on the structure was confirmed. The magnetocrystalline anisotropy at R.T. for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy in the single-variant martensite phase was determined from the magnetization curves (M–H) along the c-axis ([001]P ) and c-plane ([010]P ). The measured M–H curves at R.T. for Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy are shown in Fig. 3, together with those of Ni49 Fe18 Ga27 Co6 alloy. The both M–H curves for the c-plane are easily saturated, while those for the c-axis are hardly saturated below 1.2 MA/m, revealing that the c-axis of the tetragonal unit cell in the martensite phase is the hard-axis. The curve of the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy at R.T. shows an easy saturation magnetization Msat of 50 Am2 /kg, much larger than 43 Am2 /kg for the Ni49 Fe18 Ga27 Co6 alloy. After correcting the demagnetizing field, the uniaxial magnetocrystalline anisotropy constant Ku is evaluated from the M–H curves as the area cross section between the easiest curve along c-plane and the hardest curve along c-axis directions. The R.T. value of Ku for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy in the martensite phase is estimated to be about −1.5 × 105 J/m3 . In comparison with the value Ku =−1.2 × 105 J/m3 of the Ni49 Fe18 Ga27 Co6 alloy, the magnitude of the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 martensite phase is relatively large. That is the |Ku | is increased due to the increase of TC by the addition of Mn. The variant rearrangement driven by applying magnetic field has been discussed quantitatively by introducing magnetic stress mag [5,6]. In order to satisfy the condition for the variant rearrangement by applying magnetic field, mag should be larger than the twinning stress tw . In other words, the criterion mag > tw should be satisfied. The value of mag is expressed by ε−1 Ku with ε0 = (1−c/a). The value of ε0 is calculated from the 0 lattice parameters ratio c/a = 1.19 estimated from X-ray diffrac-
25
Ms
Ni Fe Mn Ga Co Ni Fe Ga Co
20
(MPa)
M (arb. units.)
Ni Fe Mn Ga Co
TC
15
c-axis
10 Ni Fe Ga Co
5
H = 40 kA/m
R. T.
0 300
350
400
0
450
4
8
12
16
(%)
Temperature (K) Fig. 1. Thermomagnetization Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloys.
curves
of
the
Ni49 Fe18 Ga27 Co6
and
Fig. 2. Compressive stress–strain curves along the c-axis direction at room temperature (R.T.) in the martensite phase for the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-crystals.
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0
60
c-plane
R. T.
Ni Fe Mn Ga Co
50
-2 -4
Ni Fe Ga Co (%)
M (Am2/kg)
40 30 20
c-axis
-10
R. T.
0 0.2
0.4
0.6
Ni Fe Ga Co
-8
10
0.0
-6
0.8
1.0
1.2
1.4
1.6
Ni Fe Mn Ga Co
-12 0.0
0.2
H (MA/m) Fig. 3. Magnetization curves along the c-axis and c-plane directions at R.T. for the single-variant martensite phase in the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloys.
(a) before applying
assist
dies sample
tion pattern. Meanwhile, the room temperature value of Ku is −1.5 × 105 J/m3 . Therefore, the value of mag is evaluated to be 0.79 MPa. On the other hand, tw is estimated to be 8–10 MPa from the applied twinning stress tw in Fig. 2, that is, the variant rearrangement occurs within the region of 2 MPa. This region is defined as tw evaluated from the strain–stress curve given in Fig. 2. As it turned out, the magnitude of stress becomes mag < tw and is not satisfied with the appearance condition of MFIS. Consequently, no MFIS is obtainable at R.T. under the condition mentioned above. Recently, in order to achieve a large MFIS, a static stress aiding the magnetic field has been demonstrated, that is, a large stressassisted magnetic-field-induced strain (SAMFIS) of 8.5% under the assisted stress assist of 8 MPa in Ni49 Fe18 Ga27 Co6 martensite phase is obtained [30]. By using such assisted stress, a large MFIS can be obtained in FMSAs [17,18,30]. Therefore, to obtain a large MFIS, a static stress has been applied as the assist of magnetic field for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. Shown in Fig. 4 are the SAMFIS curves at R.T. for the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-crystals. To make clear the effect of Mn, our previous results for Ni49 Fe18 Ga27 Co6 [30] are incorporated in the present result. Before applying magnetic field, the assisted stress assist of 8 MPa was applied along the c-axis hard direction in the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. As a result, a shrinkage of 2% was observed (Fig. 4b). Next, the magnetic stress mag of about 0.79 MPa was added by applying the magnetic field parallel to the same direction. There is a plateau in the stress–strain curve as seen from Fig. 2, corresponding to tw = 8–10 MPa. By applying both assisted and magnetic stresses, therefore, the strain corresponding to the total applied stress assist + mag is obtained from the stress–strain curve. During applying magnetic field, a steep shrinkage is caused by the variant rearrangement, which the hard axis (c-axis) in the variant was aligned perpendicular to the magnetic field direction (Fig. 4c), was observed around H = 0.3 MA/m. In the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy, the maximal strain comes to 11.3% in the field of 0.8 MA/m, which value is larger than that of the Ni49 Fe18 Ga27 Co6 alloy. In the previous study, we clearly claimed the conditions of SAMFIS by studying some FSMAs [18]. The value of the SAMFIS of the Co41 Ni32 Al27 is smaller than that of Co47.5 Ni22.5 Ga30.0 and Ni49 Fe18 Ga27 Co6 , while both the values of Ku and mag of the former are larger than those of the latter. That is, the value
c-axis
0.4 0.6 H (MA/m) (b) after applying
assist
assist
c-axis
0.8
1.0
(c) after applying assist
c-axis
H
Fig. 4. Magnetic-field-induced strain ε measured parallel to the direction of magnetic field H applied along the c-axis at R.T. for the Ni49 Fe18 Ga27 Co6 and Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 single-variant specimens under the stress level of about 8 MPa. The bottom sample figures are the schematic images of sequences for applying (a) assisted stress and (b) magnetic field.
of tw is also significant for the appearance of SAMFIS. The SAMFIS of the Co41 Ni32 Al27 alloy is small because of the large value of tw , though the value of mag is relatively large. As a result, to obtain a large SAMFIS, the alloy should exhibit a narrow region, that is, a low slope in the stress–strain curve in Fig. 2, for the slope of twinning stress tw , together with a large mag . In the present study, the tw of Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy is comparable in magnitude to that of Ni49 Fe18 Ga27 Co6 alloy. By applying assisted stress of 8 MPa, a shrinkage of 5% was observed in Ni49 Fe18 Ga27 Co6 alloy[30], while that of 2% was observed in Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy. Even if the more small assisted stress which is corresponding to the strain of 2% was applied to Ni49 Fe18 Ga27 Co6 alloy, the MFIS was not obtained because of the low level of mag . By addition of Mn, however, the mag is increased with keeping the small tw , leading to the large SAMFIS.
4. Conclusions In conclusion, the increase of the Curie temperature TC by the Mn addition effectively increases the uniaxial magnetocrystalline anisotropy constant Ku of Ni–Fe–Ga–Co alloy. The room temperature value of Ku at for the present Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy is evaluated to be −1.5 × 105 J/m3 , nearly equivalent to that of Ni2 MnGa alloy [1,5,6]. In the Ni49.5 Fe14.5 Mn4.0 Ga26.0 Co6.0 alloy, the variant rearrangement associated with the magnetization rotation was observed, and the maximal strain of 11.3 % was confirmed under a compressive stress of about 8 MPa. From these results, it has been revealed that the present Ni–Fe–Mn–Ga–Co ferromagnetic shape memory alloys have great potential as large MFISs assisted by static compressive stresses.
H. Morito et al. / Journal of Alloys and Compounds 577S (2013) S372–S375
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