Materials Transactions, Vol. 47, No. 3 (2006) pp. 625 to 630
Special Issue on Shape Memory Alloys and Their Applications
#2006 The Japan Institute of Metals
Magnetic-Field Induced Two-Way Shape Memory Effect
of Ferromagnetic Ni2 MnGa Sputtered Films
Makoto Ohtsuka1 , Yuya Konno1; * , Minoru Matsumoto2 , Toshiyuki Takagi2 and Kimio Itagaki1
1
2
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan
The shape memory effect (SME) induced by the magnetic field is interesting and important for physics and application. The ferromagnetic
Ni2 MnGa films with various compositions were deposited on a poly-vinyl alcohol (PVA) substrate with a radio-frequency (RF) magnetron
sputtering apparatus using four kinds of Ni–Mn–Ga targets. After separating from the substrate, the films were heat-treated at 1073 K for 3.6 ks
for homogenization and aged at 673 K for 3.6 ks in a constraint condition. The reversible two-way SME by the thermal change was confirmed for
the constraint-aged films with various compositions. The gradient of the strain–temperature curve, the amount of strain accompanied by the twoway SME and the width of thermal hysteresis were dependent on the composition of the films. The strain–temperature curve shifted to a high
temperature region and the martensitic phase was stabilized by the applied magnetic field. Furthermore, the two-way SME by the magnetic field
was observed around the martensitic transformation temperature on cooling for the constraint-aged film, which showed the large gradient and
small thermal hysteresis in the strain–temperature curve.
(Received September 21, 2005; Accepted December 15, 2005; Published March 15, 2006)
Keywords: nickel–manganese–gallium alloy, sputtered film, ferromagnetic shape memory alloy, martensitic transformation, two-way shape
memory effect, magnetic field, constraint-aging
1.
Introduction
For the typical shape memory alloy (SMA) such as Ni–Ti,
the martensitic transformation occurs by temperature change
and stress field, and the re-orientation of twin variant is
induced by the stress field in the martensitic phase. On the
other hand, for the ferromagnetic shape memory alloy
(FSMA), the martensitic transformation and the re-orientation of twin variant are induced not only by temperature and
stress but also by magnetic field (see, for a recent review1)).
The ternary intermetallic compound Ni2 MnGa is an
intelligent material, which has both a shape memory effect
(SME) and a ferromagnetic property. It has the Heusler type
structure at high temperature and some martensitic structures
at low temperature.2–5) The martensitic transformation occurs
in the ferromagnetic region below the Curie temperature
(TC ). It can be controlled not only by temperature and stress
but also by magnetic field.6) Recent researches on the
Ni2 MnGa showed that large magnetic-field-induced strains
can be generated either by re-orientation of martensitic
variants7,8) or by shifting the martensitic transformation
temperature.9,10) If the SME appears under magnetic field, its
response will be faster than that of thermal one.
However, the Ni2 MnGa bulk alloy is too brittle to be
formed in a required shape. To solve this problem, use of the
sputtered films has been proposed by the authors.11,12) They
may be applied as actuators of micro-machines. Recently,
free-standing Ni2 MnGa sputtered films have been implemented in micro-devices such as a micro-scanner13) and a
micro-valve.14) The use of shape memory alloy films for an
actuator of micro-machines is very attractive because of its
large recovery force and quick response to the magnetic field.
It has been found by the authors that the Ni2 MnGa films,
prepared by the sputtering method and aged at various
*Graduate
Student, Tohoku University. Present address: Hiroshima
Research & Development Center, Mitsubishi Heavy Industries, Ltd.,
Hiroshima 733-0036, Japan
temperatures in a constrained condition after the heat
treatment at 1073 K, have the two-way SME by thermal
change.15,16) Furthermore, the magnetic-field induced SME
was found for the constraint-aged films by shifting the
martensitic transformation temperature.17,18)
However, the effect of alloy composition on the two-way
SME by temperature change for the ferromagnetic Ni2 MnGa
constraint-aged films has not been clarified. In the present
study, the effect of alloy composition on the two-way SME
by temperature change was investigated with respect to the
Ni2 MnGa constraint-aged films with different compositions.
Furthermore, the two-way SME induced by the magnetic
field was investigated.
2.
Experiment
2.1 Specimens preparation
The Ni2 MnGa films were deposited on a poly-vinyl
alcohol (PVA) substrate (thickness: 14 mm) with a radiofrequency (RF) magnetron sputtering apparatus (Shibaura,
CFS-4ES). Four kinds of Ni–Mn–Ga targets whose nominal
compositions were Ni49:5 Mn28 Ga22:5 , Ni52:5 Mn22 Ga25:5 ,
Ni54 Mn20 Ga26 and Ni55 Mn18 Ga27 were used. The sputtering
power (WS ) was 200 W and the substrate temperature was
kept at 323 K by cooling water. The thickness of the
deposited films attained to about 5 mm by controlling the
sputtering time.
After separating from the substrate, the films were fastened
between straight alumina plates and heat-treated at 1073 K
for 3.6 ks. The composition of the heat-treated films was
determined by an inductively coupled plasma (ICP) spectrometry (PerkinElmer, Optima 3300). The composition and
valence electron concentration (e=a) of these films are shown
in Table 1. The valence electrons (e) were assumed to be
3d8 4s2 (e ¼ 10) for Ni, 3d5 4s2 (e ¼ 7) for Mn and 4s2 4p1
(e ¼ 3) for Ga. Hereafter, these films are called as N51(HT),
N54(HT), N55(HT) and N57(HT) films using nickel content,
respectively. HT means the heat treatment.
7.70
Table 2 Martensitic transformation temperatures and Curie temperature
for the constraint-aged films with various compositions.
Sample
Ms
Mf
As
Af
TC
N51(HT-AG)
329 K
321 K
329 K
336 K
368 K
N54(HT-AG)
N55(HT-AG)
306 K
348 K
290 K
338 K
298 K
345 K
315 K
345 K
342 K
—
N57(HT-AG)
368 K
356 K
358 K
358 K
—
The heat-treated films with a straight shape were cut into
5 mm 12 mm bands and bended to a cylindrical shape. The
deformed films were fixed inside a silica tube whose inner
diameter was 4 mm. The constraint films were aged at 673 K
for 3.6 ks in a flow of argon gas, and then rapidly cooled to
room temperature (R.T.).
The martensitic transformation temperatures for the constraint-aged films were investigated using a differential
scanning calorimeter (DSC, TA instruments, DSC 2910).
The Curie temperature (TC ) was determined by a vibrating
sample magnetometer (VSM). The martensitic transformation temperatures and the TC of the constraint-aged films are
shown in Table 2. The martensitic transformation temperatures of these constraint-aged films were higher than R.T.
and increased with increasing e=a ratio. The TC was also
higher than R.T. Therefore, these films were in martensitic
phase and ferromagnetic condition at R.T. Hereafter, the
constraint-aged films are called as N51(HT-AG), N54(HTAG), N55(HT-AG) and N57(HT-AG), respectively.
2.2 Measurement of SME
The SME by thermal change for the constraint-aged films
was observed using a digital video camera (Sony, DCRPC120). The film was set in a thermostatic bath. One end of
the film was fixed on a sample holder in the thermostatic bath
and another end was free. The measurement was performed
from 295 to 360 K. Heating and cooling rates were 3:3 102 K/s.
Furthermore, the shape change under a magnetic field for
the constraint-aged films was also observed at a constant
temperature using a camera with a charge-coupled imaging
device (CCD) and recorded on a video tape. The magnetic
field was applied parallel to the film surface up to 5 T using a
super-conductor magnet (Sumitomo heavy industries, HF%–
100VT–50HT).
3.
Results and Discussion
3.1 Structural characterization
Figures 1(a) and (b) show the X-ray diffraction (XRD)
profiles at R.T. in scattering angle of 2 from 35 to 55 for
N55(HT-AG)
N57(HT-AG)
201014M
7.69
Ni56:6 Mn18:5 Ga24:9
12914M
Ni55:2 Mn20:6 Ga24:2
Ni55 Mn18 Ga27
12714M
Ni54 Mn20 Ga26
N57(HT)
12914M
N55(HT)
N57(HT-AG)
N55(HT-AG)
N54(HT-AG)
N51(HT-AG)
35
40
N54(HT-AG)
45
2θ / °
12510M
7.66
200bct, 20214M
7.73
Ni54:4 Mn21:3 Ga24:3
bct
7 layer modulated
structure (14M)
5 layer modulated
structure (10M)
001414M
Ni51:4 Mn28:3 Ga20:3
Ni52:5 Mn22 Ga25:5
12514M
Ni49:5 Mn28 Ga22:5
N54(HT)
12310M
001010M
200bct
N51(HT)
12510M
Valence electron
concentration (e=a)
12514M
Composition of film
(b)
112bct
20210M
Composition of
target
Intensity, I (arb. unit)
Sample
(a)
12714M
Table 1 Composition and valence electron concentration (e=a) of the heattreated films.
200bct, 20214M
112bct, 12714M
M. Ohtsuka, Y. Konno, M. Matsumoto, T. Takagi and K. Itagaki
12310M
626
N51(HT-AG)
50
35
40
45
2θ / °
50
55
Fig. 1 X-ray diffraction patterns for the constraint-aged films with various
compositions. (b) enlarged intensity of the diffraction patterns in (a).
the constraint-aged films with various compositions, respectively. The diffraction patterns in Fig. 1(b) are shown with an
enlarged scale of the ordinate in the Fig. 1(a). All constraintaged films show a martensitic structure at room temperature.
The XRD pattern obtained for the martensitic phase is
indexed as body centered tetragonal (bct), 5 layers modulated
structure (10M) and 7 layers modulated structure (14M),
which are reported by Wedel et al.3) and Pons et al.4) The
XRD peaks for the N51(HT-AG) film present the 5 layers
modulated martensitic structure and those of the other
constraint-aged films present the 7 layers modulated one.
The ð1 1 2Þbct peak shifts to higher angle with increasing
nickel content, whereas ð2 0 0Þbct peak does not shift. It is
found that the lattice constant of c-axis of bct structure
decreases with increasing nickel content.
3.2 Two-way SME by thermal change
The photos in Fig. 2 show the spontaneous shape change
of the N55(HT-AG) film by the thermal change during
heating (a)–(d) and cooling (d)–(f). They were taken by a
camera which was slightly tilted from the parallel direction of
the film surface because of the difficulty checking the shape
change of the film. In the heating step, the radius of curvature
for the film increased. However, the shape does not recover to
the original straight one. In the next cooling step, the film
automatically starts bending. It is clear that the constraintaged film shows a two-way SME by thermal change. It is
commonly accepted that the occurrence of the two-way SME
has been ascribed to the relaxation of inhomogeneous stress
field in the parent phase by the formation of the preferentially
oriented twin variants in martensitic phase. It was reported
that both the stress-induced martensitic phase and Ni3 (Mn,
Ga) precipitates existed as the inhomogeneous stress field in
the parent phase for the constraint-aged films.16)
The strain–temperature curves for the constraint-aged
films with various compositions are shown in Fig. 3. The
strain " (¼ ðdS =2Þ=r, dS : thickness of film, r: radius of
curvature) was estimated from the shape change recorded on
Magnetic-Field Induced Two-Way Shape Memory Effect of Ferromagnetic Ni2 MnGa Sputtered Films
(a) 310.2 K
(d) 348.3 K
(b) 324.7 K
(e) 326.7 K
(c) 332.1 K
(f) 319.8 K
627
5 mm
Two-way shape memory effect of N55(HT-AG) film by heating [(a)–(d)] and cooling [(d)–(f)].
the video tape. The symbols of (a)–(f) for N55(NT-AG) film
in Fig. 3 correspond to the photographs of (a)–(f) in Fig. 2.
The reversible two-way SME can be confirmed for all the
constraint-aged films.
The schematic strain–temperature curve of the constraintaged film is shown in Fig. 4. is the gradient of the strain–
temperature curve. It was defined as a ratio of the strain to the
difference in transformation temperatures. The gradient of
heating and cooling curves in the strain–temperature curve
was nearly the same for each film. The gradient of N51(HTAG) and N54(HT-AG) films are about 5 times of the others.
The amount of strain accompanied by the two-way SME
("TWME ), which was determined by the difference in the strain
between at low and high temperatures, was different from
each film. The value of "TWME was large for N51(HT-AG),
N54(HT-AG) and N57(HT-AG) compared to N55(HT-AG).
The width of thermal hysteresis for the N54(HT-AG) film is
about half of the others. The gradient, two-way SME strain
0.10
0.08
Strain, ε (%)
Fig. 2
N51(HT-AG)
0.06
0.04
N57(HT-AG)
(a)
0.02
N54(HT-AG)
0
(f)
(b)
(c)
(e)
(d)
N55(HT-AG)
300
320
340
Temperature, T / K
360
Fig. 3 Strain–temperature curves for the constraint-aged films with various
compositions.
628
M. Ohtsuka, Y. Konno, M. Matsumoto, T. Takagi and K. Itagaki
tion temperature changed by the magnetic field for a
Ni2 MnGa single crystal19) and their strain change by the
applied magnetic field up to 5 T corresponded to that
observed by cooling down to 4–5 K.20) This result shows
the good agreement with the present one. Therefore, it is
considered that the strain accompanied by the shape change
under a magnetic field will be large between the start and
finish temperatures (Ms and Mf ) of shape change on cooling.
If the width of thermal hysteresis becomes small, the twoway SME will be appeared by a small magnetic field change.
On the other hand, if the gradient in the strain–temperature
curve increases, the total strain change by the magnetic field
will be large.
Strain, ε
heating
cooling
∆T
α
ε TWME
Temperature, T
Fig. 4 Definition of gradient (), effective recovery strain accompanied by
two-way shape memory effect ("TWSE ) and thermal hysteresis (T) in
strain–temperature curve.
Table 3 Gradient of the strain–temperature curve (), strain accompanied
by two-way SME ("TWME ) and width of thermal hysteresis (T) for the
constraint-aged films with various compositions.
Sample
/K1
"TWME (%)
T/K
N51(HT-AG)
8:8 105
4:6 102
7
N54(HT-AG)
6:8 105
4:2 102
4
N55(HT-AG)
1:2 105
2:9 102
8
N57(HT-AG)
1:4 105
4:4 102
8
and thermal hysteresis for the constraint-aged films with
various compositions are listed in Table 3.
The strain–temperature curves in the thermal cycling
under the magnetic field of 0 and 5 T for the N51(HT-AG)
and N54(HT-AG) films are shown in Figs. 5(a) and (b),
respectively. The transformation temperatures increased by
the applied magnetic field of 5 T. It is considered that the
strain–temperature curve shifts to a high temperature region
and the martensitic phase is stabilized by the applied
magnetic field. Vasil’ev et al. reported that the transforma-
0.10
Conclusions
The results obtained in the present study are summarized
as follows:
(1) The XRD peaks of the N51(HT-AG) film presented 5
layers modulated martensitic structure and those of the
other constraint-aged films present 7 layers modulated
one at R.T.
0.06
0T 5T
0.07
0.06
0.05
N51(HT-AG)
0.03
310 320 330 340 350 360
Temperature, T / K
(b)
N54(HT-AG)
0.05
heating
cooling
0.08
0.04
4.
Strain, ε (%)
Strain, ε (%)
0.09
(a)
3.3 Two-way SME by magnetic field change
N54(HT-AG) film may be expected to have a magneticfield induced SME due to the large gradient and small
thermal hysteresis in the strain–temperature curve. The SME
of the constraint-aged films induced by the magnetic field
was investigated at constant temperature. The shape memory
behavior by a magnetic field for the constraint-aged films was
observed around the martensitic transformation temperature
on cooling. The shape change under the magnetic field from 0
to 5 T for the N54(HT-AG) film is shown in Fig. 6. The
radius of curvature of the bended film was changed by the
magnetic field. It is clear that the constraint-aged film shows
the two-way SME induced by the magnetic-field.
The strain of the N54(HT-AG) film against magnetic field
is shown in Fig. 7. The strain increased with increasing
magnetic field and decreased with decreasing magnetic field.
This behavior will be expected from the martensitic transformation induced by the applied magnetic field.
0.04
0.03
0.02
0.01
0
0T 5T
heating
cooling
-0.01
290 300 310 320 330 340
Temperature, T / K
Fig. 5 Effect of magnetic field (0 and 5 T) in strain–temperature curves of (a) N51(HT-AG) and (b) N54(HT-AG) films.
Magnetic-Field Induced Two-Way Shape Memory Effect of Ferromagnetic Ni2 MnGa Sputtered Films
(a) 0 T
(c) 5 T
(b) 3 T
(d) 0 T
629
5 mm
Fig. 6 Shape memory effect by the magnetic field change of N54(HT-AG) film at the temperature between Ms and Mf on cooling. The
magnetic field was applied perpendicular to the photos.
Strain, ε (%)
0.04
N54(HT-AG)
(c)
0.03
0.02
Acknowledgement
(d)
(b)
The authors grateful to Mr. K. Koike, Faculty of Engineering, Yamagata University, for the measurement by the
vibrating sample magnetometer (VSM). This research was
partly supported by the Grants-in-Aid for Scientific Research
by Japan Society for the Promotion of Science (JSPS).
0.01
0 (a)
0
the applied magnetic field.
(5) The two-way SME by the magnetic field was observed
for the N54(HT-AG) film around the martensitic
transformation temperature on cooling.
*
*
M f < T = 315 K <M s
1
2
3
4
Magnetic Field, B / T
5
Fig. 7 Strain-magnetic field curve of N54(HT-AG) film at the temperature
between Ms and Mf on cooling.
(2) The reversible two-way SME for the constraint-aged
films with various compositions was confirmed by the
temperature change.
(3) The N54(HT-AG) film showed a good shape memory
properties in which the gradient was large and the width
of thermal hysteresis was small in the strain–temperature curve.
(4) The strain–temperature curve shifted to a high temperature region and the martensitic phase is stabilized by
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