Effect of adding cobalt on the shape memory properties of Ni-Mn-Ga sputtered films was investigated. It was found that the films containing cobalt less than 5.1 ...
Materials Transactions, Vol. 45, No. 2 (2004) pp. 350 to 352 #2004 The Japan Institute of Metals
Effect of Cobalt Addition on Shape Memory Properties of Ferromagnetic Ni-Mn-Ga Sputtered Films*1 Yoshiharu Katano1; *2 , Makoto Ohtsuka1 , Minoru Matsumoto1 , Kunihiro Koike2 and Kimio Itagaki1 1 2
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Department of Basic Technology, Faculty of Engineering, Yamagata University, Yonezawa 992-8510, Japan
Effect of adding cobalt on the shape memory properties of Ni-Mn-Ga sputtered films was investigated. It was found that the films containing cobalt less than 5.1 mol% has a single phase and those with more than 6.8 mol%Co has two phases. The Curie temperature increased and the saturation magnetization decreased with increasing cobalt content. These films exhibited one way shape memory effect. The shape recovery start temperature increased and the temperature range of shape change broadened with increasing cobalt content. (Received November 7, 2003; Accepted December 24, 2003) Keywords: nickel-manganese-gallium, ferromagnetic shape memory alloy, Heusler type crystal structure, sputtered film, shape memory effect
1.
Introduction
Ferromagnetic shape memory alloys of Ni-Mn-Ga, Fe-Pd and so on are new shape memory alloys that exhibit a shape memory effect by a magnetic field.1) This shape memory effect will enable the quick response and remote control. Ni2 MnGa has a Heusler type crystal structure and a ferromagnetic property.2) The bulk material of Ni2 MnGa is brittle and processing to wire or plate is very difficult. Our research group succeeded to make sputtered films which are amenable to bending.3) These films exhibited the shape memory effect by change of temperature4) and magnetic field.5) The addition of ferromagnetic 3d transition metal such as iron, cobalt and so on in the Ni-Mn-Ga alloy will be very effective for the improvement of shape memory properties induced by the magnetic field. The cobalt addition in the NiMn-Ga film will affect the magnetic properties, Curie temperature and martensitic transformation temperatures. The aim of this study is to investigate the effect of adding cobalt on the shape memory properties of ferromagnetic NiMn-Ga sputtered film. 2.
Experiment
2.1 Preparation of specimens The Ni-Mn-Ga films containing cobalt were deposited on a poly-vinyl alcohol (PVA) substrate (thickness: 14 mm) using a dual magnetron sputtering system (Shibaura, CFS-4ES). The sputtering apparatus has the radio-frequency (RF) and the direct current (DC) power sources. Two kinds of targets of a Ni-Mn-Ga alloy (Ni-24 mol%Mn-24 mol%Ga sintered from the mixture powders of Ni, Mn and Ni30 Mn70 ) and a cobalt metal (purity: 99.99%) were used. The RF sputtering power for the Ni52 Mn24 Ga24 alloy target was kept at 200 W and the DC sputtering power for the cobalt metal target was changed from 0 to 16 W, respectively. The cobalt content of *1This
Paper was Presented at the Autumn Meeting of Japan Institute of Metals, held in Sapporo, on October 13, 2003. *2Graduate Student, Tohoku University
films increased with increasing DC sputtering power for the cobalt metal target. The thickness of obtained film was about 5 mm. The films were heat-treated at 1073 K for 3.6 ks in vacuum of 2 104 Pa for the homogenization. 2.2 Measurement of properties The chemical composition of these heat-treated films was determined by the inductively coupled plasma (ICP) spectroscopy (Seiko, SPS-1200A). The crystal structures of these films were identified using a conventional X-ray diffractometer (XRD) equipment (Rigaku, RINT2000) with Cu-K radiation at room temperature. The thermo-magnetic curve of films was measured during heating by a vibrating sample magnetometer (VSM) (Tamagawa, model 3200) under an external magnetic field of 40 Am1 . The temperature was changed from 77 to 450 K. The magnetization curve of films at 77 K was also measured at magnetic field up to 1.2 kAm1 . Furthermore, the shape memory effect of films was observed in a thermostatic bath. The temperature distribution in the bath was controlled within 0:5 K. The shape change of films was observed by a digital video camera (Sony, DCRPC120). One end of the film was fixed on a sample holder in the bath and another end was made free. The flat film was bent to the right angle at room temperature and the change of angle with elevating temperature was observed. 3.
Results and Discussion
3.1 Composition As shown in Fig. 1, the composition of cobalt in the heattreated films changes linearly with increasing DC sputtering power for the cobalt metal target and the rate of cobalt content per 1 W of DC sputtering power is about 0.38 mol%. The valence electron concentration, e/a, of the films, which was calculated with the number of valence electron for Ni (e ¼ 10), Mn (e ¼ 7), Ga (e ¼ 3) and Co (e ¼ 9), as shown in Table 1, increases with increasing cobalt composition. 3.2 Crystal structure Figures 2(a) and (b) show the X-ray diffraction pattern of
Effect of Cobalt Addition on Shape Memory Properties of Ferromagnetic Ni-Mn-Ga Sputtered Films
Fig. 1
-1
m g
4
2
0
0 5 10 15 20 DC Sputtering Power, WDC / W
Table 1 Composition and valence electron concentration of Ni-Mn-Ga films containing cobalt. mol%Mn
mol%Ga
200
0
54.0
23.4
22.6
0
7.72
200
5
53.7
22.1
22.6
1.6
7.74
200 200
6 8
53.0 52.2
22.4 22.9
22.3 21.5
2.3 3.4
7.74 7.77
200
12
50.9
22.9
21.1
5.1
7.79
200
16
50.1
22.4
20.7
6.8
7.81
Intensity, I (arb. unit)
e/a
6.8 mol%Co 6.8 mol%Co 5.1 mol%Co
5.1 mol%Co
3.4 mol%Co
3.4 mol%Co
1257-Layer
2.3 mol%Co
35
2.3 mol%Co
1.6 mol%Co
1.6 mol%Co
0 mol%Co
40 45 50 2θ (Cu-Kα )
0 mol%Co
35
5.1 mol%Co
30
1.6 mol%Co
20
3.4 mol%Co
10
0 mol%Co (a) H = 40 A
0 0
m-1
100 200 300 400 Temperatuer, T / K
40 45 50 2θ (Cu-Kα )
(b)
400
380
360
340 7.70 7.72 7.74 7.76 7.78 7.80 Valance Electron Concentration , e / a Fig. 3 (a) Magnetization-temperature curves under external magnetic field of 40 Am1 . (b) Curie temperature vs. e/a.
Precipitate
(b)
mol%Co
1277-Layer
mol%Ni
1297-Layer
WDC /W
00147-Layer, 200BCT 1277-Layer, 112BCT
WRF /W
: 7 Layer : BCT
2.3 mol%Co
40
420
Cobalt content vs. DC sputtering power for the cobalt metal target.
(a)
50
Magnetization, M / nWb
6
Curie Temperature, TC / K
Co Content (mol%)
8
351
55
Fig. 2 (a) X-ray diffraction patterns of heat-treated films with 0 6:8 mol%Co composition at room temperature in scattering angles of 2 from 35 to 55 . (b) enlarged intensity of the diffraction in (a).
the heat-treated films in scattering angles of 2 from 35 to 55 . The diffraction patterns in Fig. 2(b) is shown with an enlarged scale of the ordinate in Fig. 2(a). All the films with 0 6:8 mol%Co composition show a martensite phase at room temperature. The diffraction peaks from the body centered tetragonal (bct) and 7-layers modulated structures were clearly observed in all the films. As shown in Fig. 2(b), weak diffraction peak from the secondary phase is also observed for the film with 6.8 mol%Co. Although the secondary phase seems to be a precipitate with the excess content of cobalt, its identification was not possible. It is
suggested from these results that a solid solution is formed in the films with 0 5:1 mol%Co. 3.3 Magnetic properties The thermo-magnetic curves of the heat-treated films during heating under the external magnetic field of 40 Am1 are shown in Fig. 3(a). Curie temperatures were determined by extrapolating the magnetization curve to the temperature axis. As shown in Fig. 3(b), the Curie temperature increases linearly with increasing e/a. Fig. 4(a) shows the magnetization curves of films at 77 K. The magnetization of all films approximately saturated above 0.8 kAm1 . The saturation magnetization is slightly decreased with increasing cobalt composition, as shown in Fig. 4(b). 3.4 Shape change The shape recovery behavior of the heat-treated films during heating was investigated. Figure 5(a) shows the change of angle for the films during heating. The angle () between two straight parts of the bent film is defined as the magnitude of bending deformation. ¼ 0 means that the film is straight. All films changed their shape above room temperatures. It is found that the reverse martensitic transformation start temperature, As , defined as the temperature at which the right angle started to decrease, and the temperature range of shape change increase with increasing cobalt content. The relation between As and valence electron concentration (e=a) is shown in Fig. 5(b). It is found that
Y. Katano, M. Ohtsuka, M. Matsumoto, K. Koike and K. Itagaki
100
(a)
140
T=77K
120
: : : : :
(a)
80
100 Angle , θ
60 40
0 mol%Co
80 60
1.6 mol%Co
2.3 mol%Co
3.4 mol%Co 5.1 mol%Co
20
0 0.5 1 0 0.5 1 0 0.5 1 0 0.5 1 0 0.5 1 -1 Magnetic Field, H / A m
Sat. Magn., Msat / nWb m
0 300
110
350 400 450 Temperatuer, T / K
500
420
100
(b)
400
90
As / K
0
0 mol%Co 1.6 mol%Co 2.3 mol%Co 3.4 mol%Co 5.1 mol%Co
40
20
g-1
Magnetization, M / nWb m
g
-1
352
80 (b) 70 7.70
7.72
T=77K 7.74
7.76
7.78
7.80
380
360
Valance Electron Concentration , e / a Fig. 4 (a) Magnetization curves at 77 K. (b) Saturation magnetization vs. e/a.
As increases with increasing e/a. 4.
Conclusion
The prepared sputtered Ni-Mn-Ga films containing cobalt have a martensite phase at room temperature. The body centered tetragonal and 7-layered modulated structures are observed. The secondary phase, which is considered to be a precipitate of cobalt compound, appeared in the cobalt-rich film. The Curie temperature increased and the saturation magnetization decreased with increasing cobalt composition. The shape recovery start temperature increased and the temperature range of shape change broadened with increasing cobalt composition. These films may be used as a high temperature shape memory alloy. Acknowledgments This research was partly supported by Industrial Technology Research Grant Program in ’03 from New Energy and
340 7.70 7.72 7.74 7.76 7.78 7.80 Valance Electron Concentration, e / a Fig. 5 (a) Change of angle determined from shape change-temperature curves. (b) As vs. e/a.
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