Sensors and Actuators A 106 (2003) 298–301

Magnetostrictive properties of amorphous andcrystalline TbDyFe thin films

A. Speliotis, D. Niarchos∗Institute of Materials Science, NCSR “Demokritos”, 153 10 Ag. Paraskevi, Attiki, Greece

Abstract

A series of amorphous and partially crystalline giant magnetostrictive thin films of the composition (Tb0.3Dy0.7)40Fe60 have beenprepared by DC magnetron sputtering. The sputtering conditions were the same for all samples apart from the substrate temperature,Ts,which varied between 330 and 510◦C. The crystalline state, the magnetic and the mechanical properties of those films were investigatedin relation to the substrate temperature. Films deposited at 330–400◦C were amorphous. Crystallization started atTs = 425◦C. Themagnetostrictive coefficientλ, at 4 kOe and at room temperature increased with increasingTs from 185 to 750 ppm atHmax. This wasrelated to an increase of the Curie temperature,TC, from 35 to 315◦C. The hysteresis loops of the amorphous samples presented close tozero coercivities. These samples showedλ ∼= 300 ppm in a field of 1000 Oe. The direction of the magnetic moments in the amorphousstate films changed gradually from perpendicular to parallel to the film plane with increasingTs. The samples were deposited atTs ≥425◦C showedλmax ∼= 350–750 ppm but the magnetostrictive curve was much broader (a non-linear butterfly shape) and shifted to thevalue ofHc.© 2003 Elsevier B.V. All rights reserved.

Keywords: Magnetostrictive properties; Terfenol-D; X-ray powder diffraction

1. Introduction

The giant magnetostriction of Terfenol-D in the formof thin films is attracting attention for the use in mi-croactuation and sensing, in the area of miniaturizationof magnetic technologies[1]. Crystalline Terfenol-D filmsexhibit magnetostriction hysteresis loops and their appli-cation is difficult due to a large remanence and coercivity[2,3]. In contrast, amorphous Terfenol-D films show asharp increase of the magnetostriction at low fields, a veryhigh value of the slope∂λ/∂M perfectly reversible and theabsense of coercivity make them technologically advanta-geous[4]. This paper presents the results of investigationsof the magnetic and magnetostrictive properties of amor-phous sputter-deposited ternary TbDyFe films. We reportthe influence of substrate temperature,Ts, during deposi-tion. It is shown that by varyingTs amorphous or crys-talline films are obtained, which directly reflects changesin the magnetic and magnetostrictive properties of thematerials.

∗ Corresponding author. Tel.:+30-10-6503385; fax:+30-10-6533706.E-mail address: qmds@ims.demokritos.gr (D. Niarchos).

2. Experiment

The samples were prepared by DC magnetron sputtering.The target was a 5 cm diameter Tb0.3Dy0.7 cast compos-ite plate having 25 holes with 5 mm diameter occupied byFe inserts that allows the adjustment of the composition.The chamber evacuated at 5× 10−7 Torr pressure and thesputtering conditions used were following. Argon pressure0.5 Pa, DC power 120 W, and substrate to target distance5 cm. The substrate temperature varied between 330 and510◦C. The samples were deposited on Si[1 1 1] substrates,270�m thick. The sputtering rate was typically 0.1�m/minand the film thickness was 1.5�m. The measurement ofthe magnetostrictive coefficientλ at room temperature wasdone by an optical cantilever method in a maximum fieldof 3.5 kOe applied parallel to the film plane. To calculateλ

from these measurements the formula proposed in[4] and aYoung’s modulus value,Ef , of 50 GPa were used. The sam-ples were characterized by means of X-ray powder diffrac-tion (XRD) using Cu K� radiation and electron microprobe(EDAX). The Curie temperature of the samples was mea-sured by thermogravimetric analysis (TGA). Room tem-perature magnetic measurements were done in a QuantumDesign SQUID magnetometer. The film surface morphol-ogy and domain structures are recorded using atomic force

0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0924-4247(03)00188-2

A. Speliotis, D. Niarchos / Sensors and Actuators A 106 (2003) 298–301 299

microscopy (AFM), a Digital Dimension 3000 equippedwith a magnetic tip.

3. Results and discussion

The substrate temperature was varied in order to studyits influence on magnetic and magnetostrictive properties ofthe films. The crystalline state of the samples depends onthe substrate temperature. The samples sputtered atTs =330–400◦C were XRD amorphous with a chemical com-position (Tb0.3Dy0.7)40Fe60 and Curie temperature valuesTC < 100◦C [4]. Due to the different thermal expansioncoefficients� of the substrate and the film (Terfenol-D:α = 12 × 10−6/K and Si: α = 2.5 × 10−6/K), thermalstresses are induced during the cooling of the samples fromthe deposition temperature to room temperature. As we haveshown in[4] the magnetoelastic properties of the films de-pend on the substrate temperatureTs, during deposition. Thefilms deposited at lower temperatures,Ts = 330–360◦Cpresent compressive stresses, whereas those deposited atslightly higher temperatures,Ts = 375–400◦C present ten-sile stresses[4]. Samples deposited at 330–360◦C showa perpendicular magnetic anisotropy. In contrast, films de-posited at 375–400◦C show in plane anisotropy (easy axis ofmagnetization parallel to the film plane). The change fromthe perpendicular to in-plane magnetic anisotropy caused bythe different stresses of the films originates from the mag-netoelastic coupling energy. Any magnetostrictive materialtries to compensate the external or internal stresses by rota-tion of the spins.

The samples sputtered atTs = 425–510◦C showed crys-talline peaks that can be attributed to [1 0 0] oriented TbFe2phase plus some diffraction lines of pure rare earth. Thecompositions of the films, as determined by EDAX, was also(Tb0.3Dy0.7)40Fe60 and did not vary considerably withTs.

In Fig. 1 the magnetoelastic properties of the film de-posited atTs = 400◦C as measured parallel and perpendic-ular to the long geometrical axis of the film, which is in acantilever shape, and with the magnetic field applied parallelto the film plane is presented.

The magnetoelastic coefficientb of the film is given bythe formula[5]:

b = α

L

h2s

hf

Es

6(1 + νs)

whereα is the sample’s deflection angle as a function ofthe applied magnetic field,L is the length of the sample,Esandνs are the Young’s modulus and the Poisson ratio of thesubstrate, respectively, andhf , hs the thickness of the filmand the substrate, respectively. The relationship betweenλ

andb is

λ = −b(1 + νf )

Ef

Fig. 1. Magnetostrictionλ and magnetoelastic coefficientb measured atRT parallel to the film plane vs. external field with the long geometricalaxis of the film parallel and perpendicular to the field, of a TbDyFe filmwith tensile internal stresses.

whereEf andνf are the Young’s modulus and the Poissonratio of the film, respectively[5]. The intrinsic, materialdependent, parameter is the differencebγ,2 = b|| − b⊥ orλγ,2 = λ|| −λ⊥. In Fig. 1optimised sample exhibited a highmagnetostriction at low fields corresponding to a very highvalue of the slope∂λ/∂M = 4 × 10−3 ppm T−1, perfectlyreversible.

The relationship between magnetostriction and magneti-zation was studied for two different amorphous films pre-senting compressive and tensile stresses, respectively. Schatzet al.[6] have reported a phenomenological approach for theinfluence of the magnetic anisotropy on the magnetoelasticproperties of amorphous TbDyFe films. As described in thatwork, in materials with positive magnetostriction and ten-sile stresses the magnetic moments lie in the film plane. Inthe case of isotropic alignment of the magnetic moments inthe plane and of a random distribution of the magnetic do-mains, according to the model of random anisotropy, whenthe magnetic field is applied parallel to the plane, the mag-netization process is done in two steps: first, displacementof the 180◦ domain walls resulting in zero magnetostrictionand magnetization equal toMmax/2 and second, rotation ofthe 90◦ domain walls as the magnetic moments rotate to-wards the direction of the external field resulting in a rapidincrement of the magnetostriction[7]. Thus, films with themagnetic moments parallel to the film plane fulfill the re-quirement for large magnetostriction at low fields.

λ(H)

λmax= 0, for 0 <

M(H)

Mmax< 0.5;

λ(H)

λmax=

[2M(H)

Mmax− 1

]3/2

, for 0.5 <M(H)

Mmax< 1

On the other hand, in materials with positive magnetostric-tion and compressive stresses the magnetic moments lie

300 A. Speliotis, D. Niarchos / Sensors and Actuators A 106 (2003) 298–301

Fig. 2. Normalized magnetostriction vs. normalized magnetization for aTbDyFe film with the magnetic moments parallel to the film plane.

perpendicular to the film plane. In this case when the fieldis applied parallel to the film plane strong fields are neededto saturate the samples resulting in a slow increment of themagnetostriction at low fields.

λ(H)

λmax=

[M(H)

Mmax

]2

In Figs. 2 and 3the normalized magnetostriction versusthe normalized magnetization for films with the magneticmoments parallel to the film plane (Fig. 2) and perpendicularto the film plane (Fig. 3) are plotted.

In the first case the magnetization reaches 50% withoutany significant increase of the magnetostriction. For furtherincrease of the magnetization the magnetostriction increasesrapidly. For the film with the magnetic moments perpendic-

Fig. 3. Normalized magnetostriction vs. normalized magnetization for aTbDyFe film with the magnetic moments perpendicular to the film plane.

Fig. 4. MFM photograph of a TbDyFe film with tensile internal stresses.

ular to the film plane, a parabolic dependence of the mag-netostriction over the whole range of the magnetization isobserved. Such a behaviour is consistent to what is reportedin [6].

In Fig. 4, the magnetic image by means of MFM of thesample deposited atTs = 400◦C is shown. The observedcontrast is due to the presence of domains, rather than do-main walls, since it disappears when the sample reachessaturation. Some of the domains end in the materials ma-trix and some of them on the materials surface resulting inthe bright regions which would not be the case if the con-trast was due to the presence of domain walls. The domains

Fig. 5. The magnetostriction curves of partially crystalline and polycrys-talline films deposited at temperatures 425, 485 and 510◦C.

A. Speliotis, D. Niarchos / Sensors and Actuators A 106 (2003) 298–301 301

are curved building a “magnetic labyrinth” which was at-tributed to the giant magnetoelastic energy of TbDyFe, thatis, the large elastic energy contribution in the domain wallsenergy.

For substrate temperature up to 400◦C the magnetostric-tion is increased up to about 750 ppm but the samples de-veloped coercivities larger than 1 kOe and the shape of themagnetostrictive curve become broader.

In Fig. 5, the magnetostriction curves of partially crys-talline and polycrystalline films deposited at temperatures425, 485 and 510◦C are shown. The coercivities in thosefilm are high and varies from 0.55 to 1.4 kOe.

4. Conclusions

Amorphous TbDyFe films deposited by DC sputteringtechnique on heated substrates and using a mosaic tar-get present giant magnetostriction of 300–400 ppm andvery low coercivity and high value of the slope∂λ/∂M4 × 10−3 ppm T−1. The substrate temperatureTs, duringdeposition, affects the microstructure magnetic and magne-tostrictive properties of these materials. A detailed analysisof magnetization behaviour of amorphous TbDyFe thinfilms has been presented. The direction of the magneticmoments, with respect to the film plane, depends also onsubstrate temperature. Deposition at higher substrate tem-

peratures results in the formation of partially crystalline andpolycrystalline films with a coercive field at 1.4 kOe andshowing a magnetostriction of more than 700 ppm.

Acknowledgements

This work was partially supported by the B/E CT93-536project of the EU. The authors would like to thank Prof. D.G.Lord from the Joule Laboratory, Department of Physics, Uni-versity of Salford, UK for carefully taken the MFM photo-graph.

References

[1] T. Honda, K.I. Arai, M. Yamaguchi, J. Appl. Phys. 76 (1994) 6994.[2] A.E. Clarck, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol.

1, North-Holland, Amsterdam, 1980, p. 531.[3] P. Farber, H. Kronmüller, J. Magn. Magn. Mater. 214 (2000) 159–

166.[4] A. Speliotis, O. Kalogirou, D. Niarchos, J. Appl. Phys. 81 (1997)

5696.[5] E. Tremolet de Lacheisserie, J.C. Peuzin, J. Magn. Magn. Mater. 136

(1994) 189.[6] F. Schatz, M. Hirscher, M. Schnell, G. Flick, H. Kronmüller, J. Appl.

Phys. 76 (1994) 5380.[7] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley,

Reading, MA, 1972.