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Science and Technology of Advanced Materials 6 (2005) 772–777
www.elsevier.com/locate/stam
Investigation on ferromagnetic shape memory alloys
G.D. Liu, Z.H. Liu, X.F. Dai, S.Y. Yu, J.L. Chen, G.H. Wu*
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,
P. O. Box 603-18, Beijing 100080, People’s Republic of China
Received 14 March 2005; revised 10 June 2005; accepted 10 June 2005
Available online 13 September 2005
Abstract
Ferromagnetic shape memory alloys, exhibiting large recoverable strain and rapid frequency response, appear to be promising shape
memory actuator material. These materials exhibit large shape memory effect associating with martensitic transformation, and magneticfield-induced strain in the martensite state. The recent development in researches on NiMnGa, NiFeGa, and CoNiGa in our group is briefly
reviewed. The perspectives of the ferromagnetic shape memory alloy are also described.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Ferromagnetic shape memory alloy; Heusler alloy; Martensitic transformation
1. Introduction
2. Field controlled-shape memory and magnetostrain
In conventional shape memory alloys, which are
paramagnetic, the martensitic transformation underlying
the shape memory effect is induced by changes in
temperature or stress or both. The same transformation in
ferromagnetic shape memory alloys (FSMAs) can be
triggered not only by changes in temperature and stress,
but also by changes in the applied magnetic field. Therefore,
it has attracted much attention as high performance solidstate actuator materials. Many FSMAs systems have been
developed from Heusler alloy system, such as Ni2MnGa [1–
3], Ni2MnAl [4], Co–Ni–Ga (Al) [5,6], and Ni–Fe–Ga [7,8].
There have been many reports on their structure, magnetic
properties, martensitic transformation, magnetically controlled shape memory effect, superelasticity and magneticfield-induced strains (MFIS). In this paper, we review some
specific contributions carried out by Institute of Physics,
CAS to this field since 1999.
The typical FSMA is Heusler alloy Ni2MnGa with cubic
L21 structure as the parent phase. It is ferromagnetic and
exhibits a thermoelastic martensitic transformation at lower
temperature, which indicates that the shape memory
behavior is possible to be affected by an external field [1].
However, it lasts too long time for people to concern the
shape memory behavior in an applied magnetic field. The
earliest related observation was reported by Ullakko et al. at
MTI in 1996 [2]. After their work, we modified the
composition to adjust the operating temperature of the
shape memory close to the room temperature and used cold
crucible technique to grow single crystals with high
performance. Based on these studies, two functional
behaviors, the external field-controllable shape memory
and the large magnetostrain in NiMnGa FSMA were clearly
revealed by our research work in 1999–2001, respectively
[3,9,10], as shown in Fig. 1.
3. Dynamics of martensitic transformation
* Corresponding author. Tel.: C86 10 8264 9247; fax: 86 10 6256 9068.
E-mail address: [email protected] (G.H. Wu).
1468-6996/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.stam.2005.06.009
Generally, it is believed that the free samples only exhibit
a little strain upon martensitic transformation due to the
intrinsic self-accommodation, and the large strain and the
magnetostrain are attributed to the preferential orientation
of the martensitic variants. But why was a large strain
G.D. Liu et al. / Science and Technology of Advanced Materials 6 (2005) 772–777
20000
1.4
f
10000
1.2
0
1.0
-10000
a
-20000
b
-30000
c
-40000
d
e
275
MFIS (%)
Strain(ppm)
773
Strain in [001]
Field in [001]
0T
0.2T
0.6T
0.8T
1.2T
strain in [001]
Field in [010]
1.2T
280
285
290
295
300
T=285 K
T=280 K
T=275 K
0.8
0.6
T=270 K
0.4
T=250 K
0.2
0.0
305
-20 -15 -10
T(K)
-5
0
5
10
15
20
Magnetic field (kOe)
Fig. 1. Single crystal with modified composition of Ni52Mn24Ga24 exhibited a field controlled-shape memory behavior (left): the magnitude and the sign of the
shape memory sample can be controlled by an external magnetic field. The single crystals also showed a large magnetostrain up to 1.2% with 100% free
recoverability (right).
observed in our free single crystal? Our work revealed that
the residual stress caused by the directional solidification
during the growth worked as an extrinsic factor imposed on
the sample and reduced the intrinsic self-accommodation
and establishing a preferential orientation of the variants [9].
The systematical research further indicated that the
martensitic transformation is very sensitive to the internal
stress. Based on our calculations and modeling, an average
internal stress of 13.8 MPa purposely induced in the
distorted lattice will force the parent phase to take a totally
different martensitic transformation path: which results in
the entire suppression of seven-layer modulation (7M) and
lead the transition going to the five-layer modulation (5M)
directly [11]. The related observations were shown in Fig. 2.
0.5
(a)
Tc
TM
(220)
As-ground
TM
(b)
300k
300k
(400)
(202)
(220)
(022)
(422)
240
280
320
360
30 40 50 60 70 80 90
2 Theta (deg)
40
50
60
70
(214)
(422)
(004)
(400)
(311)
30
(222)
(224)
(422)
(214)
(004)
(400)
(222)
(311)
200
(214)
(422)
(242)
(224)
(400)
(311)
(022)
(222)
(224)
(422)
150K
150K
0.0
160
(040)
(004)
(022)
(220)
<50 µm
0.4
270K
(214)
50-100 µm
TR
270K
(400)
360
(004)
320
(311)
280
(222)
240
(022)
200
(220)
100-200 µm
TI
Intensity (arb.units)
χ (arb. units)
0.1
160
(220)
TR
0.2
0.8
(400)
200-400 µm
0.3
(422)
0.4
80
(224)
1.2
After annealing
for D<50µm
(220)
0.6
For the thermodynamics of NiMnGa alloy, we
calculated the energy consumed for phase boundary
motion in a Ni52Mn23Ga25 single-crystalline sample during
martensitic transformation using a boundary friction
phenomenological theory. It was found that the energy
consumed for phase boundary motion is 13.14 J/mol, only
a small part of the latent heat of martensitic transformation. Furthermore, the results of transformation loops
measured by ac magnetic susceptibility proved that the
thermal hysteresis of martensitic transformation is in direct
proportion to the volume fraction of martensite. It was also
indicated that the thermal hysteresis of martensitic
transformation originates from the friction of phase
boundary motion [12].
90
2 Theta (deg)
T (K)
Fig. 2. Temperature dependence of ac magnetic susceptibility (left). The intermartensitic transformation disappeared when the internal stress was induced in
ground powder sample with varying particle size D. Inset: the 7M martensite recovered in the powder sample with D!50 mm after annealing at 500 8C for
eliminating the stress. Right: the different martensitic transforming path examined by XRD for stressed powder (a) and annealed powder (b).
774
G.D. Liu et al. / Science and Technology of Advanced Materials 6 (2005) 772–777
4. New FSMAs
Although large MFIS and transformation strain has been
obtained in Ni–Mn–Ga single crystal, the alloys are very
brittle, which might restrict their application. In order to
improve the magnetic and mechanical properties for the
practical application, single crystals of Ni52Mn16Fe8Ga24
have been grown. The substitution of Fe for Mn strengthens
the magnetic exchange interactions, increasing the Curie
temperature to 381 K. The shape memory strain in this
pseudoquaternary Heusler alloy increased to 2.4% in zero
field and can be enhanced to 4.2% by a field of 1.2 T. A
field-induced strain of 1.15% was obtained which only
decreased to 0.73% as the temperature was lowered to
170 K. This temperature dependence of the magnetostrain is
much better than that in undoped samples where a dramatic
decrease of MFIS from 1.2 to 0.36% just occurred only as
the temperature was lowered from 280 to 250 K. This is a
quite successful attempt to improve the material properties
by dopant for FSMAs [13]. Further, systematical investigation on quaternary Heusler alloy of Ni50.5Mn25KxFexGa24
and Ni 50.4Mn28KxFe xGa 21 focused on the structure,
martensitic transformation, and magnetic properties. It
was found that, substituting Fe for Mn up to about 70%,
the pure L21 phase and the thermoelastic martensitic
transformation still can be observed in these quaternary
systems. Fe dopant dropped the martensitic transformation
temperature from 220 to 140 K, increased the Curie
temperature from 351 to 429 K, and broadened the thermal
hysteresis from about 7 to 18 K. Magnetic analysis revealed
that Fe atoms contribute to the net magnetization of the
material with a moment lower than that of Mn. The
temperature dependence of magnetic-field-induced strains
has been improved by this doping method [14].
In 2003, we reported a class of new promising FSMAs of
Ni–Fe–Ga synthesized by using the melt-spinning technique
[8]. It was verified that the new alloys have a high chemical
ordering L21 structure for parent phase at high-temperature
and exhibit a thermoelastic martensitic transformation from
cubic to orthorhombic structure at low temperature. The
stoichiometric Ni2FeGa alloy has a martensitic transformation temperature of 142 K, a relatively high Curie
temperature of 430 K, a magnetization of 73 A m2/kg, and
a low saturated field of 0.6 T. The textured samples with
preferentially oriented grains show a completely recoverable two-way shape memory effect with a strain of 0.3%
upon the thermoelastic martensitic transformation. It should
be noted the stoichiometric Heusler alloy Ni2FeGa can only
be synthesized by melt-spun method, otherwise, the second
g phase would be formed if a normal solidifying way was
used. Comparably, the B2 phase of Ni–Fe–Ga can only be
obtained when the composition seriously deviates the
Heusler compound, as reported by Oikawa et al. [7].
This work indicates that the fast-solidifying is an
effective way to synthesize some chemical formula of
X2YZ in Heusler alloy and avoid forming the fcc disordered
structure (g phase) in the alloys. Some new ferromagnetic
Heusler alloys, such as Cu2FeAl and NiFeSb have been
successfully synthesized by this way and the characterization was confirmed [15,16].
For the purpose to deeply understand the magnetism and
martensitic transformation, we calculated the electronic
structures of the Heusler alloy Ni2FeGa for both the cubic
and the orthorhombic structures by self-consistent fullpotential linearized-augmented plane-wave method. The
localized moment of Fe atom is interpreted based on
the electronic structure and the popular explanation of the
localized moment of Mn in Heusler alloy X2MnY.
Comparing the density of states of cubic and orthorhombic
structures, we observed that a Ni peak near the density of
states of d band for the cubic structure splits for the
orthorhombic structure, indicating a band Jahn-Teller
mechanism responsible for the structural transition.
Accompanied by this transformation, an increase of Ni
moment and magnetization redistribution occurred. Temperature-dependence anisotropy field shows an evidence of
martensitic transformation between 125 and 190 K. The
magnetic behavior seems to contain a transition from
Heisenberg-like at temperature below 70 K to itinerant
magnetism at temperature higher than 160 K upon
martensitic transformation. Temperature dependence of
saturation magnetization reveals the spontaneous magnetization at martensite and parent phase are 3.170 and
3.035 mB, respectively. The calculated magnetic moment
at martensite is 3.171 mB, which is quite consistent with the
experimental value. The magnetic moment of Fe and Ni
atom in Heusler alloy Ni2FeGa is analyzed based on the
computational results and the experimental magnetization
curves. It is found that the magnetic moment of Fe atoms is
about 10–43% larger than that of a-Fe [17].
For other FSMAs, we studied the field-controlled shape
memory [18] in single crystals of CoNiGa. We obtained that
two-way shape memory with 22.3% strain has been
obtained in free samples. By applying a bias field of up to
2 T, the shape memory strain can be continuously controlled
from negative 2.3% to positive 2.2% giving it a total strain
of 4.5%. This work proved that CoNiGa is a good shape
memory material working at relatively high-temperature of
up to 450 K and has a lower magnetic anisotropy than
NiMnGa.
Other workers have reported that CoNiGa alloys have a
good superelastic property. In order to develop it for the
practical application, we doped Fe in the single crystal of
Co50Ni22Ga28: Fex (xZ0, 1.5, 2, 2.5) single crystals. It
resulted in that the samples showed strong anisotropic
superelasticity behaviors, including the varied strains,
superelastic parameters and even transformation path
related to the different crystalline orientations under
compression. A large superelastic strain up to 11%
has been obtained in tension test. The iron-doped materials
also displayed perfect superelasticity under bending and
torsion [19].
G.D. Liu et al. / Science and Technology of Advanced Materials 6 (2005) 772–777
5. Micromagnetism of Ni–Mn–Ga Heusler alloys
6. Half metallic materials
In Heusler alloy X2MnZ, the separation of the Mn
ions (O4 Å) is too large for direct d–d coupling. Thus,
the magnetic interaction is thought to arise from an
indirect interaction that takes place by means of the
polarization of the conduction electrons. Electronic
band-structure calculations have predicted the existence
of half metals, with a perfectly spin-polarization (wi.e.
PZ100%). Notable among the half metallic candidates
are a number of the Heusler alloys [21]. We
investigated on structural, magnetic, transport, and
spin-polarization measurements of the Heusler alloys
Co2MnSi and NiMnSb. Room temperature transport
measurements showed a negative magnetoresistance in
NiMnSb. Point-contact Andreev reflection measurements
of the spin polarization yield polarization values for
Co2MnSi and NiMnSb of 56 and 45%, respectively.
Temperature dependence of resistivity for Co2MnSi
reveals a relatively large residual resistivity ratio
(r293 K/r5 K) typical of single-crystal Heusler alloys.
In NiMnSb, resistivity and magnetization as a function
of temperature show evidence of a magnetic phase
transition near 90 K [22].
magnetostriction(10-6)
(a)
-300
-600
-900
-1200
600
(b)
magnetostriction(10-6)
In view of micromagnetism, we studied the first in situ
observation of temperature-dependent micromagnetic and
twin structure in oriented single crystals of Ni–Mn–Ga
Heusler alloys. Micromagnetic measurements were made
over a temperature interval of 50 to K35 8C covering both
forward and reverse martensitic transformation. Magnetic
domains in the martensite phase were found to be uniformly
spaced (25–30 mm); the direction of the domain walls
conforms to the changing direction of the magnetic easy
axis as they traverse from one twin to another. The
martensite twins could be reoriented in applied fields as
low as 1300 Oe [20].
0
775
400
200
0
-20000
-10000
0
10000
20000
Applied Magnetic Field (Oe)
Fig. 3. Magnetostriction measured from the stacked sample along the
ribbon length direction (a) and thickness direction (b).
reflects the magnetoelastic competition occurring in
those strong textured and thin ribbon samples. For
FeGa alloys, large magnetostrictions up to K1300 and
C1100 ppm related in the different directions were
obtained in our stacked Fe85Ga15 ribbon samples, as
shown in Fig. 3 [24]. In the case of non-1808 domain
magnetization in the high anisotropic samples, the
magnetostriction was mainly attributed to the existence
of Ga clusters which preferentially orient with the ribbon
normal due to the ribbon grain texturing. Forming the
modified DO3 structure, the Ga–Ga atom pairs distribute
in the matrix and cause the X-ray diffraction peak split
in melt-spun ribbons. As a special micromorphology, Ga
clusters highly condensed in some nanoscale dots have
also been experimentally observed for the first time, as
shown in Fig. 4 [24].
7. Magnetostrictive materials
As mentioned above, we found the high chemical
ordering could be achieved by a fast-cooling solidifying
method. This method was also used for other magnetostrictive materials. Magnetostriction of Fe–Al alloys has
been largely improved by using melt-spun method. The
large magnetostriction up to K700 ppm obtained in
Fe81Al19 sample is about five times as large as that in
conventional bulk samples of Fe–Al composition [23]. It
has been ascribed to the high concentration of Al–Al
atom pairs created by melting–spinning method and their
strongly preferential orientation in [100] textured ribbon
plane. The remarkable anisotropic magnetostriction
8. Large magnetoresistance in quaternary Heusler alloy
Ni–Mn–Fe–Ga
Quaternary Heusler alloy Ni50Mn8Fe17Ga25 ribbons
have been prepared by melt-spun method. The ribbons
exhibit large negative magnetoresistance up to 9% over a
wide temperature region, particularly in the region during
the martensitic phase transformation. MR deceases
significantly after annealing. The large MR is isotropic
and is mainly attributed to the local magnetic disorders,
magnetic clusters and heterogeneity. The maximum MR
at martensitic transformation may be due to the
redistribution of electrons and the increase of phase
776
G.D. Liu et al. / Science and Technology of Advanced Materials 6 (2005) 772–777
Fig. 4. TEM examinations: bright-field image on textured grains (a) and in a thinner area (b), electron pattern diffracted from the bright area along the [100]
zone axis (c) and from a dark speckles along the [011] axis (d).
boundary scattering. This feature adds a new useful
functionality to Heusler alloys [25].
9. Conclusions
Investigation on ferromagnetic shape memory alloys
has been carried out by our research group since 1999.
Some interesting results on the magnetostrain, fieldcontrolled shape memory, superelasticity, magnetostriction and magnetoresistance have been obtained. It was
found the fast-solidifying is an effective way to
synthesize some Heusler alloy. By this way, some
new Heusler alloys with ferromagnetic shape memory
effect have been successfully synthesized. These studies
indicate that FSMAs is a very useful functional
materials and a potential source for farther research in
the fields of physics and material science. We will keep
hunting for new candidates for actuator materials in this
field and promise to report new FSMA in very close
future.
Acknowledgements
The work described in this paper, was supported by Key
Project of National Natural Science Foundation of China
Grant No. 50371101 and a grant from the Research Grants
Council of the Hong Kong Special Administration Region,
China (Project No. HKUST6059/02E).
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