Metamagnetic shape memory effect in NiMnbased Heusler-type alloys

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Metamagnetic shape memory effect in NiMn-based Heusler-type alloys

ARTICLE in JOURNAL OF MATERIALS CHEMISTRY JANUARY 2008

Impact Factor: 7.44 DOI: 10.1039/b713947k

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Katsunari Oikawa

Tohoku University

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Wataru Ito

Sendai National College of Technology

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Metamagnetic shape memory effect in NiMn-based Heusler-type alloys

Ryosuke Kainuma,*a Katsunari Oikawa,b Wataru Ito,b Yuji Sutou,b Takeshi Kanomatac and Kiyohito Ishidab

Received 11th September 2007, Accepted 7th January 2008

First published as an Advance Article on the web 21st January 2008

DOI: 10.1039/b713947k

Recent findings on Heusler-type alloys in NiMnIn- and NiMnSn-based systems show a specific

martensitic (i.e., diffusionless) transformation from a ferromagnetic parent (P) phase to an

antiferromagnetic-like martensite (M) phase. In this paper, the magnetic and martensitic properties

and the details of the magnetic field-induced shape memory effect (SME) are introduced in

NiMnIn-based alloys. The martensitic transformation temperatures of these alloys significantly

decrease with the application of a magnetic field, and a metamagnetic phase transformation occurs

from the M phase to the P phase. By using this transition, a magnetic field-induced strain of

approximately 3%, namely, a metamagnetic SME, is confirmed.

Introduction

Based on diffusionless phase transformation, which is termed

martensitic transformation, the shape memory effect (SME) in

alloys is known to be unique behavior by which an alloy

deformed in the low-temperature phase recovers its original

shape by reverse transformation upon heating to the reverse

transformation temperature. This effect was first observed in

AuCd alloys in 1951 and became well known with its discovery

in TiNi alloys in 1963.1 TiNi alloys are the most familiar shape

memory alloys (SMA) with applications in various fields such as

medical guidewires, cellar-phone antennae, and smart actuators.

Because the strain and stress generated by the SME are

extremely large as compared to those generated in piezoelectric

and magnetostrictive materials, SMAs are potential candidates

for actuators such as motors and supersonic oscillators.

However, since the output actuation in SMAs occurs through

temperature change for an input signal, it is not easy to obtain

a rapid response to the input signal at frequencies greater than

5 Hz because the thermal conductivity of the alloys is a ratedetermining factor of the response.2 This fatal drawback restricts

the application of SMAs as actuators. Magnetic shape memory

alloys (MSMAs) in which a rapid output strain is achieved

through the application of a magnetic field have been developed

to overcome this obstacle.

Since Ullakko et al.3 first reported the existence of magnetic

field-induced strain (MFIS) in ferromagnetic Ni2MnGa single

crystals in 1996, research in this field has drastically progressed

and current studies have reported large MFIS values greater

than 9%.4 The MFIS obtained in the ferromagnetic Ni2MnGa

single crystal is explained by the rearrangement of martensite

variants due to an external field. When the crystalline magnetic

anisotropy energy is greater than the energy driving the variantboundaries, the angle between the 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. Thus, the variant rearrangement yields

the MFIS, which is comparable to the strain induced by the

monoaxial stress applied to the martensite (M) phase. Details

on the MFIS in Ni2MnGa alloys have recently been reviewed

Ryosuke Kainuma

Ryosuke Kainuma was born in

Hokkaido, Japan in 1961. He

received diploma and Dr Eng

degrees in materials science

from Tohoku University in

1983 and 1988, respectively.

He is a full professor at

IMRAM, Tohoku University.

His research field is materials

science, covering phase

diagrams, phase transforma-

tions and microstructure control

of alloys and intermetallic

compounds. Katsunari Oikawa

Katsunari Oikawa was born in

Aomori, Japan in 1968. He

studied materials science at

Tohoku University and received

his PhD at the same University

in 1996. He is an associate

professor of the solidification

process at Department of

Metallurgy, Graduate School

of Engineering, Tohoku

University. His current research

field is phase diagrams, thermo-

dynamic and microstructure

control.

aInstitute of Multidisciplinary Research for Advanced Materials(IMRAM), Tohoku University, Sendai, 980-8577, Japan. E-mail:[email protected] of Materials Science, Graduate School of Engineering,Tohoku University, Sendai, 980-8579, JapancFaculty of Engineering, Tohoku Gakuin University, Tagajo, 980-8579,Japan

This journal is The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 18371842 | 1837

APPLICATION www.rsc.org/materials | Journal of Materials Chemistry

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by Marioni et al.5 Although large output strain and rapid

response can be confirmed in Ni2MnGa alloys, the output stress

is principally lower than 5 MPa.6 Furthermore, the significant

brittleness of Ni2MnGa single crystals is a serious problem

that prevents the application of this material. On the other

hand, it has been reported that some trials use phase transforma-

tion to obtain an MFIS.79 However, in Ni2MnGa alloys, a huge

magnetic field is required to obtain magnetic field-induced trans-

formation because both the parent (P) and M phases exhibitferromagnetism and the saturated magnetization of the M phase

is comparable to that of the P phase.10

Since 2000, the present authors have developed many Ni-based

MSMAs such as NiMnAl,11,12 NiCoAl,13,14 NiCoGa,14,15 and

NiFeGa,16,17 and clarified the characteristic features of their

magnetic and martensitic transformations. Very recently, we

observed an unusual transformation from the ferromagnetic P

phase to the antiferromagnetic-like M phase in NiMnIn- and

NiMnSn-based Heusler alloys,18 which exhibit behaviors

completely different from those of the previous MSMAs. In this

paper, the magnetic and mechanical properties, mainly those of

the NiMnIn-based alloys, are reviewed and the SME induced

by a magnetic field is introduced.

Metamagnetic phase transition in NiMnIn-based

alloys

Although stoichiometric Ni2MnIn and Ni2MnSn Heusler alloys

with bcc-based (L21) crystal structures, as shown inFig. 1(a), are

known to exhibit ferromagnetism,19,20 martensitic transforma-

tion in these alloys has not yet been reported. The authors

observed that martensitic transformation from the Heusler-type

P phase to the monoclinic or orthorhombic M phase occurs in

the NiMnNi2MnX sections.18 Fig. 1(b)shows the thermomag-

netization curve of the Ni50Mn34In16alloy (at%) for a magnetic

field of 0.05 T.21 The Curie temperature Tc of the P phase is

detected at around 280 K, and the martensitic transformation

starting temperatureTMsappears at around 230 K. It is observedthat the magnetization drastically decreases with temperature

from the TMs to the martensitic transformation finishing

temperatureTMfand that the thermal hysteresis is approximately

20 K. Such thermomagnetization behavior in a weak magnetic

field is sometimes observed in the M phase of conventional

FSMAs with large crystalline magnetic anisotropy energy.10

However, in the present case, the magnetization of the M phase

is very small, even in the presence of strong magnetic fields.

Fig. 222 shows the thermomagnetization curves for magnetic

fields of 0.05 T and 7 T and the magnetization curves at various

temperatures for a Ni46Mn41In13 alloy whose composition is

slightly different from that of the previous alloy. InFig. 2(a), it

is observed that the difference in the saturated magnetizationbetween the P and M phases is approximately 100 emu g1 and

that the TMs and TMf temperatures and the temperatures TAsand TAf at which the reverse transformation starts and ends,

respectively, decrease by 4050 K due to the increase in the

magnetic field from 0.05 T to 7 T. This result suggests that at

temperatures between 180 and 220 K, the M phase transforms

to the P phase due to application of the magnetic field of

7 T. This behavior, i.e., the magnetic field-induced reverse

Fig. 1 Crystal structure of Ni2MnZ Heusler phase (a) and thermomag-

netization curve21 of Ni50Mn34In16 in a magnetic field ofH 0.05 (b).

Magnetic moments of Mn atoms in each state are schematically indicated

with arrows in insets of (b), where M(AF?), P(FM) and P(PM) are the

martensite phase with antiferromagnetic-like magnetism and the parent

phases with ferromagnetism and paramagnetism, respectively.

Fig. 2 Thermomagnetization curves in magnetic fields of H 0.05

and 7 T (a) and magnetization curves at various temperatures (b) in

the Ni46Mn41In13alloy.22

1838 | J. Mater. Chem., 2008, 18, 18371842 This journal is The Royal Society of Chemistry 2008

View Article Online
http://dx.doi.org/10.1039/b713947k


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transformation (MFIRT), was confirmed in the magnetization

curves at 180 and 200 K, as shown in Fig. 2(b). InFig. 2(b), it

is observed that the curves at 273 K and 100 K that correspond

to the P and M phases exhibit simple ferromagnetic and weak-

magnetic behavior, respectively; however, those at 180 and 200

K show drastic variations in magnetization due to the MFIRT

at approximately 5 and 7.5 T, respectively, at which the weak

magnetism of the M phase appears to be antiferromagnetic

because the L10 martensite phase in the binary NiMn alloyexhibits antiferromagnetism.23 Such a magnetic field-induced

transformation, the so-called metamagnetic phase transition,

has also been reported in alloys such as MnAs and FeSiLa24

and has recently attracted considerable interest due to the

possibility for the application of magnetocaloric materials in

magneticrefrigeration systems. In the NiMnIn alloy, the magnetic

entropy change induced by a magnetic field is evaluated from the

magnetization curves inFig. 2(b)to be approximately 13 J kg1

K1 at 190 K,22 and this alloy is a promising magnetocaloric

material. Thus, the decrease in the martensitic transformation

temperatures can be attributed to the magnetically induced

stabilization of the P phase, which is mainly caused by the differ-

ence in the saturated magnetization between the P and M phases.Because theTcof the NiMnIn ternary alloys lies in the temper-

ature range between 280 and 300 K, it is difficult to obtain

a metamagnetic phase transition at room temperature. In order

to increase the Tc of the ternary alloys, we attempted the

substitution of Co for Ni. Fig. 3 shows the phase diagram for

the martensitic and magnetic transformations determined by

differential scanning calorimetric (DSC) measurements in the

Ni(50x)CoxMn(50y)Iny (x: 0, 5, and 7.5; y: 1016) alloys,21

where the solid and broken lines indicate TMs and Tc, respec-

tively. It is observed that the value ofTc increases with the Co

composition and is almost independent of the In composition,

whereas the value ofTMsdecreases with increase in the Co and

In compositions. Furthermore, abnormal behavior of TMs isdetected, as shown in Fig. 3, i.e., although the variation in the

temperature is almost linear to the In composition in the para-

magnetic region of the P phase, the value ofTMsdeviates from

the linear relation and decreases in the ferromagnetic region.

Figs. 4(a) and (b)show the thermomagnetization curves in the

magnetic fields of 0.05 and 7 T and the magnetization curves at

200, 270, 290, and 320 K in the Ni45Co5Mn36.7In13.3alloy, respec-

tively.25 The characteristic features of the quaternary alloy are

basically similar to those of the NiMnIn ternary alloy;

however, the decrease in the value ofTMs, which is only approx-

imately 25 K in a magnetic field of 7 T, is smaller than that in the

Ni46Mn41In13alloy. This issue is discussed in the next section. It

is also observed that the addition of Co results in a decrease in thesaturation magnetization in the M phase region. The origin of

this phenomenon is not yet clear, but it appears that the doped

Co has a strong magnetic interaction with Mn, which should

be dominant in the magnetic properties in this alloy. It should

be emphasized here that an apparent metamagnetic phase transi-

tion is detected at 290 K,as shown in Fig. 4(b). This indicates that

this quaternary alloy possibly has a magnetic field-induced SME

due to this metamagnetic phase transition at room temperature.

Transformation entropy change21

As shown inFigs. 2and4, while the TMsand TMftemperatures

decrease with increase in the magnetic field in all the alloys, thedegree of the decrease differs in the various alloys. For instance,

the decrease in the value ofTMs for a magnetic field of 7 T is

approximately 50 K in Ni46Mn41In13 but approximately 25 K

in Ni45Co5Mn36.7In13.3. The transformation temperature change

(DT) induced by the magnetic field change (DH) is approximately

given by the ClausiusClapeyron relation in the magnetic phase

diagram:

dH

dT

DS

DI

DTz

DI

DS

DH

(1)

Fig. 3 Composition dependence of indium on TMs and Tc for

Ni50MnIn, Ni45Co5MnIn and Ni42.5Co7.5MnIn alloys.21

Fig. 4 Thermomagnetization curves in magnetic fields of H 0.05

and 7 T (a) and magnetization curves at various temperatures (b) in

the Ni45Co5Mn36.7In13.3alloy.25


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where T denotes the absolute temperature; H, the applied

magnetic field; and DIand DS, the differences in magnetization

and entropy between the P and M phases, respectively.25 Accord-

ing to eqn (1), the decrease in the value ofTMsinduced by the

magnetic field is proportional to the value of DHwhen DI/DS

is constant. In the NiMnIn and NiCoMnIn alloys shown in

Fig. 2 and 4, while the values of DI are approximately 100

emu/g in both the cases, the value ofDT for the same magnetic

field DHis different in each case. This suggests that the differencein the values ofDT is due to the difference in the value ofDS

between these alloys. In fact, the values of DSexperimentally

determined by DSC measurements in the Ni46Mn41In13 and

Ni45Co5Mn36.7In13.3alloys are 17.1 J K1 kg1 21 and 27.0 J K1

kg1,25 and the change in the value ofTMs, namely, DT, estimated

by Eq. 1 on the basis of the experimental values ofDIand DHare

41 and 26 K, respectively, which are in agreement with the

experimental values. This result implies that a large value of

DI and a small value of DSmust accompany the martensitic

transformation if a large variation in TMsis desired.

Fig. 5(a)shows the value ofDSdetermined by DSC measure-

ments for Ni(50x)CoxMn(50y)Inyalloys.21 In ordinary paramag-

netic SMAs such as TiNi and Cu-based alloys, the value ofDSinthe martensitic transformation, which is mainly caused by the

difference in the vibration entropy between the P and M

phases,26 is not drastically altered with variation in the alloy

composition when the crystal structures of both the P and M

phases are fixed.27 In the transformation from the paramagnetic

P to the antiferromagnetic-like M phase, which is indicated by

the open symbols inFig. 5(a), the values ofDSslightly decrease

with increase in the In composition at approximately 50 J K1

kg1 in the ternary system and at 6070 J K1 kg1 in the quater-

nary system, which indicates normal behavior. On the other

hand, the values ofDS drastically decrease with increase in the

In composition during the transformation from the ferromag-

netic P to the antiferromagnetic-like M phase, as indicated by

the closed symbols in Fig. 5(a), thereby exhibiting a singularpoint at compositions in which the P phase changes from the

paramagnetic to the ferromagnetic condition. Since the crystal

structure of the M phase obtained from the paramagnetic P

phase is not basically different from that obtained from the

ferromagnetic P phase,21 this behavior cannot be attributed to

the change in the crystal structure. Such experimental results

on theTMstemperature and the value ofDScan be qualitatively

explained by a thermodynamic consideration that takes into

account the magnetic contribution to the Gibbs energy of the

P phase.21 Fig. 5(b) shows the relationship between the values

ofDSagainst the temperatures Tc TMs. A strong correlation

expected between the temperatures Tc TMs and DS was

confirmed, while the curve for the 0Co alloys does not coincidewith that for the quaternary alloys. This relationship implies

that a small value ofDScan be obtained from a specimen with

a large value ofTc TMs.

Metamagnetic shape memory effect25

Figs. 6(a) and (b) show the stressstrain curves of the

Ni45Co5Mn36.7In13.3 and Ni45Co5Mn36.5In13.5 alloys, respec-

tively, to which were applied a compressive strain of approxi-

mately 7% at 298 K.25 The stressstrain characteristics at the

testing temperature of Tt 298 K are dependent on the TAsand TAf temperatures of the specimens, that is, when TAf< Tt,

typical pseudoelastic (PE) behavior with a closed loop appeared,as shown inFig. 6(b), and when TAf> Tt, the deformed strain

remained even after the stress was removed, as shown in

Fig. 6(a). An almost perfect SM effect in the as-deformed

specimens was confirmed by dilatometric examination whilst

heating to 373 K. These results confirm that the NiCoMnIn

alloys are generally SM and PE materials.

The shape recovery induced by the magnetic field was

examined by a three-terminal capacitance method with the

Ni45Co5Mn36.7In13.3specimen.25 Fig. 7shows the recovery strain

induced by the magnetic field at 298 K at which a compressive

pre-strain of approximately 3% was applied along the direction

plotted with a filled circle in the stereographic triangle shown

in Fig. 7, and the magnetic field was applied vertically to thecompressive axis of the specimen. The recovery strain started

to increase at approximately 2 T and rose sharply at approxi-

mately 3.6 T and then gradually increased to 8 T with the

magnetic field. A recovery strain of approximately 2.9%, almost

equal to the pre-strain of 3%, was obtained with a magnetic field

of 8 T. However, only a slight change in length was observed

from approximately 3 T when the magnetic field was removed.

This behavior induced by the magnetic field is comparable to

the general SM effect due to the reverse transformation induced

by heating, as shown in Fig. 6(a), where the application of

magnetic field corresponds to heating. We propose that the

Fig. 5 Entropy change DSdue to martensitic transformation vs. (a)

indium composition and (b) Tc TMs. While the transformation from

the paramagnetic parent phase shows a large DSas plotted with open

symbols in (a), the DSdrastically decreases in the transformation from

ferromagnetic parent phase as indicated with solid symbols.21


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SM effect due to MFIRT, which is a metamagnetic transition, be

called the metamagnetic shape memory effect (MMSME) and

that such a SMA be termed a metamagnetic shape memory

alloy (MMSMA).25

In contrast with the previous ferromagnetic SMAs such as

Ni2MnGa and FePd, this MMSMA system has many advan-

tages for practical applications. The most important advantage

may be that the present MFIS can yield a high output stress

due to the magnetic field. The ClausiusClapeyron relation on

the critical stress for the stress-induced martensitic transforma-

tion is given by

dsc

dT

DS

3,Vm (2)

where 3denotes the difference in the lattice strain between the P

and M phases in a corresponding direction and Vmdenotes the

molar volume of the alloy. The magnetic field-induced change

in the critical stress corresponds to the output stress obtained

by the combination with eqn (1), as follows:

DsczDS

3,Vm,DTz

DI

3,Vm,DH (3)

This equation shows that the output stress yielded by the

MMSMA is proportional to the magnetic field. In eqn (3),

the fact that Dscis inversely proportional to 3may be one of themost important points. The parameter 3 significantly depends on

the deformation mode and the direction of monoaxial strain

applied in a single crystal. For instance, in the tensile mode for

a NiFeGa single crystal,28 which has a crystal structure in the

M phase similar to that of the NiCoMnIn alloy, the value of 3

is approximately 6% along the p direction but only

approximately 0.6% along the p direction. This suggests

that the value ofDsccan be varied by the selection of the defor-

mation direction in a single crystal. By using some appropriate

data for the NiCoMnIn alloy and the values of3 for the NiFeGa

single crystal, the value ofDsc in the tensile deformation mode

induced by a magnetic field of 1 T is evaluated to be

approximately 13 and 130 MPa along the p and pdirections, respectively. Since the value of 3continuously varies

with change in the direction from pto p, the output

stress can be selected by the selection of the deformation direction

in a single crystal. However, it is impossible to obtain a combina-

tion of large values ofDscand 3over the magnetic input energy

corresponding to DI DH. It is worth noting that eqn (3) only

expresses the conversion between the magnetic and mechanical

energies. Thus, in NiCoMnIn alloys, the output stress is expected

to be greater than that in NiMnGa alloys, whereas the output

strain is relatively smaller. Very recently, MFIRT under a static

compressive stress of 50 MPa was experimentally confirmed in

NiCoMnIn alloys by using X-ray diffraction examination by

Wang et al.29 It is known that monoaxial strain generallystabilizes the M phase, as given by eqn (2). However, the

magnetic field always stabilizes the P phase in the MMSMA.

This relationship suggests that precise control of the strain is

possible by a combination of the strain and magnetic field.

Recently, the MMSME was also confirmed in polycrystalline

NiCoMnSn alloys.30 The MMSME appears not only in single

crystals but also in polycrystals and reversible strains as the

two-way memory effect is obtained by deformation of the

polycrystalline specimen.30 The NiCoMnSn alloy system, which

does not include expensive elements, may be the most promising

MMSMA candidate for industrial applications in the future.

Fig. 7 Recovery strain at 298 K induced by a magnetic field for the

Ni45Co5Mn36.7In13.3 single crystal in which a compressive pre-strain of

about 3% was applied, where the magnetic field was applied vertically

to the compressive axis of the specimen and the length change parallel

to the compressive axis was measured. Shape recovery is due to magnetic

field-induced reverse transformation which is termed the metamagnetic

shape memory effect.25

Fig. 6 Compressive stressstrain curves for (a) Ni45Co5Mn36.7In13.3and (b) Ni45Co5Mn36.5In13.5 single crystals at Tt 298 K. The

Ni45Co5Mn36.5In13.5 alloy in (b) shows almost perfect pseudoelasticity.

Almost perfect shape recovery due to heating to 373 K after deformation

was also confirmed by examination using a dilatometer for the

Ni45Co5Mn36.7In13.3alloy as demonstrated in (a).25 Here, shape memory

effect results from twin boundary deformation of the M phase and

reverse transformation induced by heating as demonstrated in (a). On

the other hand, pseudoelasticity is due to stress-induced martensitic

transformation during application of stress and reverse transformation

during release of stress as shown in (b).


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In the MMSME that does not require any change in

temperature, a rapid response to an input signal is expected.

Very recently, Sakonet al.31 reported that a Ni45Co5Mn36.7In13.3single crystal exhibited an almost perfect MMSME for a single

magnetic field pulse of 200 Hz, whose characteristic features

are similar to that in a static magnetic field.

Conclusion

The phase transformation from the ferromagnetic to the anti-

ferromagnetic phase has already been reported for several alloy

compounds and ceramics. In the NiMn-based alloys discussed

in this study, the magnetic transition occurs with the thermoelas-

tic martensite transformation accompanying the shape memory

effect and pseudoelasticity, which is different from that in the

previous materials. This combination of magnetic and structural

transitions has high potential for the development of many types

of multiferroic devices such as sensors, actuators, and thermo-

magnetic engines and can by controlled by three factors, namely,

temperature, stress, and magnetic field.

Since the reports on MMSMAs, many investigations have been

performed and some unique physical properties such as giantmagnetoresistance,32,33 giant magnetothermal conductivity,34

and inverse magnetocaloric effect35,36,37 have been reported. These

properties are also very interesting.

Acknowledgements

The authors are grateful to Mr Y. Imano, Dr H. Morito and

Prof. S. Okamoto, O. Kitakami and A. Fujita, Tohoku

University, for their assistance with the experiments. This study

was supported by a Grant-in-Aid from CREST, Japan Science

and Technology Agency (JST), Grants-in-Aid for Scientific

Research from the Japan Society for the Promotion of Science

(JSPS), Japan, and Global COE Project.

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Metamagnetic shape memory effect in NiMnbased Heusler-type alloys

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