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Journal of Materials ScienceFull Set - Includes `Journal of MaterialsScience Letters' ISSN 0022-2461Volume 47Number 8 J Mater Sci (2012) 47:3658-3662DOI 10.1007/s10853-011-6212-2

Fabrication conditions and transformationbehavior of epitaxial Ni–Mn–Ga thin films

I. R. Aseguinolaza, I. Orue, A. V. Svalov,V. A. Chernenko, S. Besseghini &J. M. Barandiarán

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Fabrication conditions and transformation behavior of epitaxialNi–Mn–Ga thin films

I. R. Aseguinolaza • I. Orue • A. V. Svalov •

V. A. Chernenko • S. Besseghini • J. M. Barandiaran

Received: 2 November 2011 / Accepted: 14 December 2011 / Published online: 28 December 2011

� Springer Science+Business Media, LLC 2011

Abstract Epitaxial Ni–Mn–Ga films have been grown

onto heated substrates by sputtering. Their chemical compo-

sition depends on the sputtering argon pressure. Represen-

tative epitaxial films of Ni52.3Mn26.8Ga20.9, 0.5 lm-thick,

transform martensitically at about 120 �C, accompanied by

sharp changes in the lattice parameter and resistivity, and

orders ferromagnetically below 98�. The observed high

transformation temperature, orthorhombic martensitic

structure, twinning mode and film morphology, indicate a

potential multifunctional behavior of the film, such as

high-temperature shape-memory effect and magnetic field

actuation.

Introduction

Ferromagnetic shape memory alloys (FSMAs), such as the

off-stoichiometric Ni–Mn–Ga Heusler compounds, are

subject of a continuing intense research, because of the

interesting underlying physics and potential applications.

The FSMAs exhibit a giant recoverable strain of about 10%

(that can be induced by temperature, mechanical stress, and

magnetic field) alongside remarkable magnetocaloric,

magnetoresistance and damping characteristics [1–3]. The

ferromagnetic ordering and martensitic transformation

(MT) are the fundamental ingredients governing these

outstanding properties.

In view of the large energy per mass rate, thin-film

technology has been already probed to obtain the best of

the properties of FSMAs on (nano-)microscale [4–14]. In

fact, both grain-textured and epitaxially grown single

crystalline Ni–Mn–Ga thin films, in a free-standing and/or

attached-to-the-substrate form, revealed expected func-

tionality [6–12]. Incidentally, magnetic field-induced strain

activation was possible owing to reduced internal geo-

metrical constraints in thin film samples where grain size is

comparable to plate thickness [4, 6, 11].

Thin-film technology of FSMAs is still under develop-

ment. In order to meet application conditions, one should

control crystal structure, crystallographic orientation, and

twinning substructure of the films alongside the transfor-

mation behaviour and magnetic properties. All these

characteristics depend on the constraints from substrate,

film thickness, and thermal treatment. For any technical

application, it is essential that the films display ferromag-

netism and martensitic structure at room temperature.

Magnetron sputtering has proved to be the most suitable

technique to fabricate Ni–Mn–Ga/substrate thin films. By

this technique, epitaxial growth has been achieved onto some

appropriate substrates, e.g., MgO(001) [5–14]. Alongside

the target composition, substrate type and temperature, the

sputtering power and argon pressure were reported to be the

significant factors in controlling properties of films deposited

onto cold substrates [15, 16]. In particular, the influences of

sputtering power on the transformation temperatures and

structure of martensitic phase in polycrystalline Ni–Mn–Ga

I. R. Aseguinolaza � A. V. Svalov � V. A. Chernenko (&) �J. M. Barandiaran

Departamento de Electricidad y Electronica, Universidad

del Paıs Vasco, P.O. Box 644, 48080 Bilbao, Spain

e-mail: volodymyr_chern@yahoo.com

I. Orue

SGiker, Vicerrectorado de Investigacion UPV/EHU,

Sarriena s/n, 48940 Leioa, Spain

V. A. Chernenko

Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain

S. Besseghini

CNR-IENI, 23900 Lecco, Italy

123

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DOI 10.1007/s10853-011-6212-2

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films are explained by the variation of composition and the

corresponding valence electron concentration in the multi-

component alloy [15, 17]. In the case of Ni–Mn–Ga films

epitaxially grown on heated substrates, further advancement

needs also a detailed study of the influence of sputtering

parameters on their properties.

In this study, we report the preparation of epitaxial

Ni–Mn–Ga films by magnetron sputtering to address how

the composition and transformation behavior of the films

depend on the preparation parameters. We show that the

film composition and the related transformation behavior

are mainly controlled by the Ar pressure. The results of

extensive measurements of physical properties of a repre-

sentative film are presented to demonstrate its potential for

magnetic and high-temperature actuations.

Experimental

Thin films have been deposited by DC magnetron sput-

tering, using a Pfeiffer Vacuum Classic 500 station. Tar-

gets of Ni49.9Mn27.8Ga22.3 (at.%), each having the electron

concentration e/a = 7.61, were fabricated by a plasma-

melting technique and by subsequent spark-cutting the

ingot into disks of 50-mm diameter and 3-mm thickness.

The targets displayed the MT at 25 �C and the Curie

temperature, (TC), at 88 �C with few degrees of variation

across the target. Deposition parameters, such as sputtering

power and Ar pressures varied from 50 to 200 W and from

2.8 9 10-3 to 2.6 9 10-2 mbar, respectively. The base

pressure was always 4.1 9 10-7 mbar and the target-sub-

strate distance was 9 cm. Single crystalline plates of

MgO(001) were used as substrates. They were held either

at room temperature (RT), 400 �C or 500 �C during the

deposition. The RT and 400 �C deposited films were sub-

jected to subsequent annealing to obtain more ordered

structures. The thickness of the films was fixed at 0.5 lm.

Compositions of the target and films were determined

with an accuracy better than 0.5 at.% by energy-dispersive

X-ray spectroscopy (EDX), using a SEM (scanning elec-

tron microscope) Jeol JSM-6400. SEM was also used for

the microstructure observations in the secondary electron

mode [18].

Crystal structure of the thin films was examined by

X-ray diffraction (Philips X0Pert PRO), using CuKa radi-

ation. Diffractograms were taken at 35 different tempera-

tures from 30 to 200 �C, with a heating step of 5 �C. The

uncertainty in the relative change of lattice spacing was

better than 5 9 10-5nm.

Magnetic measurements were carried out in a vibrating

sample magnetometer recording in-plane and out-of-plane

hysteresis loops, M(H), in fields up to 1.8 T at RT, as well

as the temperature dependence of magnetization, M(T). In

the latter case, the applied magnetic field was 0.05 T, and

the temperature was varied from –120 to ?120 �C.

Electric resistivity, R(T), was measured in the temper-

ature range from 0 to 200 �C, at a rate of 1 �C/min, in the

four-probe configuration by using an in-house made set-up

with a temperature sensor in direct contact with the sample.

Results and discussion

Influence of substrate temperature and post-deposition

annealing

Depositions made at temperatures up to 400 �C resulted in

films showing typical features of magnetically and struc-

turally disordered compounds. To promote atomic order-

ing, a 500 �C post-deposition annealing was done, whereby

films showed ferromagnetism together with a certain

paramagnetic contribution as demonstrated by the pres-

ence of large slopes in the hysteresis loops at high fields.

Two TC, of about -10 and 60 �C, were detected in the

M(T) curve (not shown), presumably related to two dif-

ferent phases. A 700 �C annealing did not lead to any

improved properties either, showing even stronger para-

magnetic contribution at RT and a low TC (&-30 �C).

On the other hand, the depositions at 500 �C produced

highly ordered single-phase films, so that no additional heat

treatment was needed. Thus, all subsequent depositions

were made at this temperature.

Influence of Ar pressure and deposition power

Several series of films with different deposition pressure

have been fabricated, keeping other parameters constant.

Among them, a series of the three samples grown onto

heated MgO wafers, using 200 W power and Ar pressures

of 2.8 9 10-3, 1.1 9 10-2, and 2.6 9 10-2 mbar (films 1,

2, and 3, respectively) was the most indicative.

Figure 1 shows that the composition of the epitaxial

films depends on Ar pressure, and suffers a considerable

and systematic shift from the target one, enriching in Ni

and losing Mn and Ga. The trend shown by the experi-

mental points on the right panel of Fig. 1 can be used as a

guide for a proper calibration and control of the film

composition by sputtering.

Table 1 shows that the evolution of the TC versus

electron concentration follows the well-known general

trend, increasing as e/a decreases, although the absolute

values of TC deviate from those of the bulk [17]. The sat-

uration magnetization value is above 50 emu/g for films 2

and 3, which is close to the previous results [5]. However,

it is much smaller for film 1, pointing to its less homoge-

neous state in this case.

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The effect of sputtering power on the film composition

in the case of epitaxial growth was marginal comparatively

to Ar pressure, therefore, 150 W power was selected in all

further experiments as a technically optimal value.

Crystal structure and morphology

Hereafter we will focus on the results obtained on the

representative 0.5 lm-thick film of Ni52.3Mn26.8Ga20.9,

epitaxially grown on MgO(001), heated at 500 �C under

2.6 9 10-2 mbar pressure and 150 W power. The electron

concentration of this film, e/a = 7.73, is almost identical to

film 3 and belongs to the TC * TM zone (Fig. 1), which is

interesting for applications [19].

X-ray diffraction pattern of the film shows only one

single peak at 2h = 63.5�, which indicates a preferential

lattice orientation with the h100i axis along the film nor-

mal. Temperature-induced changes in the position and

intensity of this peak are depicted in Fig. 2, and unequiv-

ocally indicate a transformation from the cubic to the

martensitic phase. The value of lattice parameter (5.83 A)

and its small change when the sample transforms into cubic

phase (about 0.3%, Fig. 2) suggests that it corresponds to

the lattice parameter b of the quasi-orthorhombic 14 M

structure, in agreement with Refs [10, 11, 20]. The thermal

expansion coefficient can be evaluated from the slope in

the martensitic and austenitic phases to be equal to

38.0 9 10-6 K-1 and 12.0 9 10-6 K-1, respectively

(cf. Ref. [8]). The role played by the differences between

the MgO substrate and the Ni–Mn–Ga films, both the in

thermal expansion and lattice constant, has been already

highlighted in the context of internal biaxial tensile stresses

measured in the film plane [10].

The local surface topography was studied by scanning

electron microscopy (SEM), secondary electrons signal, and

one of the selected areas image was shown in Ref. [18]. The

twinning pattern in the martensite reflects a well-resolved

surface relief, and the twin boundaries aligned along or

perpendicular to h110iMgO directions confirm a quasi-single

crystalline nature of the film (in austenitic phase) and

the epitaxial relationship Ni–Mn–Ga(001)[110]//MgO(001)

[100] between austenite and substrate (see also [10, 11, 20]).

Fig. 1 (Color online) Leftpanel films composition as a

function of Ar pressure. Films

were deposited with 200 W on

MgO(001) heated at 500 �C.

Right panel compositions of

three films and target are shown

together with e/a isolines

providing a guide for a proper

calibration and control of the

composition by sputtering

process. TM and TC are the

martensitic and ferromagnetic

transitions temperatures,

respectively

Table 1 TC and saturation

magnetization of the films

obtained at various sputtering

Ar pressure and 200 W power

on heated (500 �C) substrate

MgO(001)

Film Argon pressure,

mbar

Composition,

at.%

e/a TC, �C TC, �C

(expected [17])

Ms, emu/g

(5 K)

1 2.8 9 10-3 Ni56.1Mn25.8Ga18.1 7.96 29 49 41

2 1.1 9 10-2 Ni54.0Mn26.4Ga19.6 7.84 44 71 54

3 2.6 9 10-2 Ni52.2Mn26.8Ga21.0 7.73 98 88 53

Fig. 2 (Color online) Temperature evolution of the lattice parameter

b and (400)-peak intensity of Ni52.3Mn26.8Ga20.9/MgO(001) film

composite during step-wise heating. Inset evidences an MT through a

conversion of (004)-peak in the cubic phase into (040)-peak in the

orthorhombic phase

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Repeated analysis of the twin morphology has forced us

to reconsider the previous description of the film micro-

structure given in Ref. [18]. X-rays measurements are

averaged along the full sample, and indicate that the b axis

of the orthorhombic structure is out of the film plane. This

means that, in average, the martensite exhibits ac twinning

in the film plane. Such a twinning implies that twin

boundaries should be perpendicular to the film plane (see

also Ref [21].) and the resulting surface topology would be

hardly resolved. On the other hand, local SEM observations

[18] show well-developed surface relief produced by out-

of-plane ac twinning in some parts of the film, indicating

that twin boundaries are aligned at 45� with respect to the

film plane (see also Ref [11].). Although this reconsidera-

tion has not impact in any deduction of Ref [18]. (the

micromagnetic structure and all the parameters of model in

Ref. [18] do not depend on the spatial coordinates), it

removes the apparent contradiction between X-rays results

and local SEM observations, by assuming a mixed type of

ac-twinning in the film with a dominance of the in-plane

orientation. Moreover, the micrograph given in Ref [18]

was useful to clarify the underlying physics of the FMR

anomalies, serving as a kind of microstructure probe.

Magnetic and resistivity measurements

The transformation and magnetic behavior of the films have

been studied by magnetization and resistivity measurements.

Typical results are shown in Fig. 3. The TC was determined

to be 98 �C. No evidences of MT appears in the M(T) curve,

because MT takes place in the paramagnetic zone. The MT

already observed by X-rays measurements are confirmed by

the large abrupt anomaly (about 25%, cf. Ref [17]) on the

resistivity curve in Fig. 3. This curve yields TM of about

120 �C and a transformation hysteresis of 8 �C. The

enhanced MT temperature and narrow temperature hyster-

esis are the prerequisites of advanced functional properties of

the studied films, such as a high-temperature shape-memory

effect.

The in-plane and out-of-plane hysteresis loops are

shown in Fig. 4. These loops show a significant difference

in the values of saturating magnetic fields. The shape

anisotropy alone is not enough to explain the difference in

saturating field. The estimated value of the shape anisot-

ropy field is about 0.5 T, whereas the actual saturating field

is about 1.1 T (Fig. 4). Thus, the difference must be

explained by the contribution of the magnetocrystalline

anisotropy of the film. This result agrees with the structural

characterization, which assumes the b axis (hard) to be out

of plane. The in-plane magnetic anisotropy was studied

in detail in Ref. [18]. The coercive field of the film was

determined from these loops as 0.02T for in-plane case,

and 0.03T for out-of-plane case. These relatively high

values of coercive field are associated with the films

imperfections, such as high-density twin boundaries, con-

strains, etc. Since the twin boundaries are fixed by the

substrate, they could only be activated by the application of

a magnetic field if the substrate is removed, and free-

standing films are produced.

Conclusions

We have found that the chemical composition of the

Ni–Mn–Ga films epitaxially grown onto MgO(001) substrates

heated at 500 �C depends mainly on the sputtering argon

pressure, enriching in Mn and Ga and depleting in Ni

Fig. 3 (Color online) Magnetization at 0.05 T and electric resistivity

as a function of temperature. The thermomagnetization curve shows

the TC. The resistivity abrupt change indicates the MT

Fig. 4 (Color online) In-plane (1) and out-of-plane (2) hysteresis

loops at room temperature

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contents as Ar pressure increases so, the transformation

behavior of the these films can be mainly tailored by

changing the Ar pressure. Deposition power in this case is

of minor influence. The relative influences of Ar pressure

and power are just opposite in the case of sputtering onto

cold substrates [15].

One of the representative Ni–Mn–Ga films epitaxially

grown on MgO(001) exhibits MT accompanied by sharp

changes in the lattice parameter and resistivity above the

TC. X-rays measurements showed an orthorhombic crystal

structure of the martensite, with the b axis out of plane,

which means, in average, ac twinning in the film plane with

boundaries perpendicular to it. Previous SEM observations

had shown evidences of out-plane-plane ac twinning as

well. Probably, a mixture of differently oriented twin

packets, with predominant in-plane ac twinning, better

represents the highly relaxed structure, self-accommodated

in the mechanical constraints imposed by the substrate. The

observed high transformation temperature and film mor-

phology are prerequisites for advanced multifunctional

behavior, such as high-temperature shape memory and

magnetic field actuation, respectively. A large resistivity

change (about 25%) measured at MT is also important, as it

can allow the sensorless mode of control for the actuation

of this film [22].

Acknowledgements The financial support from the Department of

Education, Basque Government (Project ETORTEK No. IE10-272),

and the Spanish Ministry of Science and Innovations (Project No.

MAT2008 06542-C04-02) is acknowledged. The discussions with

Prof. G. Jakob about twinning crystallography are very much appre-

ciated. Authors are very grateful to M. Pini and G.Carcano for

technical support.

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