Engeering Sciences Reports, Kyushu University
Vol. 28, No. 2, pp. 237-239, OCT. 2006

九州大学大学院総合理工学報告第28巻 第2号 237-239頁 平成18年10月

Structural Transition and Magnetic Field Induced Strainin Ni50Mn26.5Ga23.5 Alloy
Ngo Van NONG*1 and Michitaka OHTAKI*1,*2†E-mail of corresponding author: nvnong@mm.kyushu-u.ac.jp(Received May 22, 2006)
Polycrystalline samples of Ni50Mn26.5Ga23.5 were prepared by arc-melting method. The structuraltransformation from tetragonal to cubic was observed from X-ray diffraction analysis as a function oftemperature in the temperature range from 100 K to 370 K. Thermal magnetization curves measured at a low magnetic field of 300 Oe showed a sharp structural transformation at TM = 260 K and amagnetic transition at Curie temperature TC = 375 K. Thermal expansions were investigated in thevicinity of TM after different cooling procedures: zero-field-cooling (ZFC) and field-cooling (FC). A
magnetic-field-induced strain (MFIS) of ~ 0.2 % obtained from the difference between the ZFC andFC thermal expansions curves is remarkable.
Key words: Shape memory alloy, Martensite, Austenite, Twin variants

1.

Introduction

Ferromagnetic materials constitute anemerging class of active materials because of
their possibility to generate strains up to a few
percent by the application of a magnetic fieldalong specific crystallographic directions, andto control the martensitic transformation withmagnetic field, temperature and stress. The
mechanism is based on the magneticfield-induced rearrangement of the crystallographic domains (twin variants). A great dealof interest has been attracted by NiMnGa alloys. Ullakko et al. first reported a magneticfield-induced extensional strain (0.2%) in an
un-stressed stoichiometric single crystal of
Ni2MnGa at 0.8 kOe [1]. Later, a 4.3% extensional strain was observed for stress-biased
samples [2]. A shear strain of 6% at an appliedfield of 4 kOe was reported by Murray et al.
in 2001 [3]. More recently, Sozinov et al. havesucceeded in fabricating a single crystal
Ni48.8Mn29.7Ga21.5 with a giant magneticfield-induced strain of about 9.5% at an applied magnetic field of 1 T [4]. The strain
provedtobecausedbymagneticSciences, Department of Molecular and Material*1 Interdisciplinary Graduate School of Engineering
Sciences*2 CREST, Japan Science and Technology Agency,
Kawaguchi, Saitama, 332-0012

field-controlled twin boundary motion. Largemagnetic-field-induced strains in single crystals have also been observed for the applicationof a magnetic filed during cooling processacross the transition. However, the extensionof these results to polycrystalline samples
would markedly increase the application capabilities of this class of materials. A detailed
study on magnetic-field-induced strain ofpolycrystalline materials is still lacking. In thepresent work, the structural transformation inpolycrystalline sample Ni50Mn26.5Ga23.5 wasinvestigated by X-ray diffraction on changing
temperature.
Themagnetic-field-induced
strain was evaluated by investigating
self-strain without applied magnetic field(ZFC) and strain when applied field of 4.2 kOe(FC) during the cooling process in the temperature range of 200 K to 300 K. Other interesting magnetic properties of the samples
will also be discussed.

2.

Experiment

Polycrystalline samples at a nominal composition of Ni50Mn26.5Ga23.5 were prepared byarc-melting starting from pure elements of Ni(4N), Mn (4N) and Ga (4N) in a water-cooled
crucible under a pure Ar atmosphere. The resulting ingot was turned over and remelted several times to ensure the sample homogeneity.

Structural Transition and Magnetic Field Induced Strain in Ni50Mn26.5Ga23.5 Alloy

The ingot has been high vacuum-sealed inquartz tube and annealed at 1100 oC for ten
days. Then it was quenched in ice water. A
powder specimen for X-ray analysis was prepared from the same ingot, which was studied
using a Phillips X’Pert X-ray diffractometerwith Cu Kα radiation. Thermal magnetization
curves M(T ) were measured at a low magnetic
field of 300 Oe on a VSM upon heating after
different cooling procedures: zero field cooling(ZFC) and field cooling (FC, H = 300 Oe).
Magnetization
measurements
inthetemperature range of 10 K to 400 K weremeasured on a SQUID with the field up to 5T.
Thermal expansion measurement was carriedout by using the strain-gauge technique in thesteady magnetic field of up to 4.2 kOe. Themagnetic field was applied parallel to themeasuring direction.

from 260 K to 280 K. It is clear in X-ray diffraction patterns that the structure transforms
from cubic to tetragonal with decreasing temperature.
87Magnetization (emu/g)

238

6ZFC
FC
H = 300 Oe

5432
10100 150 200 250 300 350 400 450 500
Temperature (K)Fig. 2 The magnetization as a function oftemperature measured upon heating after different cooling ZFC and FC procedures.

Magnetization (emu/g)

10010 K
240 K
260 K

80290 K

60360 K

40375 K
400 K

2000102030405060

Magnetic Field (kOe)2θ (deg.)
F i g . 1 X-ray diffraction patterns at different
temperatures from 100 K to 370 K.

3.

Results and discussion

Crystal structure of the sample at different
temperature from 100 K to 370 K was analyzed
by X-ray diffraction. It is clearly shown in Fig.
1 that at temperatures above 280 K, two peaks
appear near 2θ = 44° and 81° which reveal thesingle phase of cubic austenite structure. At
temperatures below 260 K, four peaks near 2θ
= 43°,45°,80° and 83° were observed, indicating that a pure tetragonal martensite phasecrystallizes. The cubic and the tetragonalphase co-exist in the temperature range

Fig. 3 The magnetization isotherms measured
at different temperatures in the vicinity oftransition points.

Thermal magnetization curves measured
upon heating after different cooling procedures(ZFC and FC, H = 300 Oe) are shown in Fig. 2.
Two sharp transitions from martensitic
tetragonal to austenitic cubic was observed at
TM = 260 K and from ferromagnetic to paramagnetic at Curie temperature TC = 375 K.
The value of TM is in good agreement with theX-ray diffraction analyses. The very clear
separation between ZFC and FC magnetization curves below 260 K revealed that themartensitic phase has a high anisotropy.
Shown in Fig. 3 is a series of magnetization
isotherms at different temperatures in the九 州 大 学 大 学 院 総 合 理 工 学 報 告range from 10 K to 400 K. These curves manifest the whole transformation process. It isclear that at low temperature below the transition point, for example at T = 240 K, themagnetization is hard to saturate, which is acharacter of the martensitic phase, while athigh temperature above the transition point,for example at T = 290 K, the magnetization iseasy to saturate, indicating the austenitic
phase. The magnetization curve at T = 400 K
clearly shows the character of paramagneticstate.

Thermal expansion (%)

0.0

TM = 260 K

第28巻

239

第2号

orientation, which is parallel to the appliedmagnetic field, is the c axis (shorter than thecubic axis).

0.00

H = 4.2 kOe-0.05
Strain, ε (%)

平成18年

-0.10
-0.15
-0.20

-0.1
180

200

-0.3

180

240

260

280300Fig. 5 Magnetic-field-induced strain parallel tothe applied magnetic field of Ni50Mn26.5Ga23.5
alloy obtained upon heating after FC in H =4.2 kOe.

ZFC

-0.4
-0.5

220

Temperature (K)

-0.2

FC200

220

240

260280300

Temperature (K)Fig. 4 Linear thermal expansion measured
upon heating after different cooling procedures: ZFC (H = 0) and FC (H = 4.2 kOe).
The linear thermal expansion of the sample
measured upon heating from 195 K to 300 K
after different cooling procedures is shown in
Fig. 4. The ZFC (H = 0) curve represents thespontaneous linear thermal expansion afterthe sample was zero-field cooled. The changein ∆l/l is associated with the martensitic
transformation. The application of a magnetic
field when cooling the sample through TM gives
a dramatic effect on the thermal expansion, as
observed in the FC curve (FC at 4.2 kOe, field
applied parallel to the measuring direction). In
this case, the change in ∆l/l is two times larger.
These results suggest that the applied magnetic field facilitates the growth of specificorientation variants along the field direction as
the sample is cooled down through the martensitic transition. This preferential orientation
takes place to minimize the Zeeman energy,
favoring the growth of variants with the easymagnetization direction parallel to the appliedmagnetic field. The value of ∆l/l is negative.
The sample contracted corresponds to the applied field direction, indicating that in thetetragonal martensitic phase the preferential

The magnetic-field-induced strain (MFIS)
was obtained from the difference between thelinear thermal expansion at an applied magnetic field of 4.2 kOe and zero fields. The resultis shown in Fig. 5. The MFIS of about 0.2 % isremarkable for polycrystalline samples. The
temperature behavior observed in the MFIS
indicates that the major effect of the magnetic
field occurred upon cooling through the martensitic transformation.
In conclusion, the very clear picture ofstructural martensitic transformation process
was observed by the X-ray diffraction at different temperatures through the transition
point. Field cooling of 4.2 kOe for the sample
across the transformation temperature
brought about a MFIS of ~ 0.2 %, which value
is significantly large for polycrystalline samples.

References[1]. K. Ullakko, J. K. Huang, et al., Appl. Phys.
Lett., vol 69, p. 1966, 1996.
[2]. R. Tickle and R. D. James, J. Magn. Magn.
Mater., vol 195, p. 627, 1999.
[3]. S. J. Murray, M. A. Marioni et al., J. Appl.
Phys., vol 87, p. 5774, 2000.
[4]. A. Sozinov, A. A. Likhachev, N. Lanska andK. Ullakko, Appl. Phys. Lett., vol 80, p.
1746, 2002.