1222 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 4, APRIL 2005

Obliquely Sputtered TbFe Giant MagnetostrictiveFilms With In-Plane Anisotropy

H. C. Jiang, W. L. Zhang, B. Peng, W. X. Zhang, and S. Q. Yang

College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China,Chengdu 610054, China

We have found that in-plane magnetostriction characteristics at low fields can be greatly improved by an oblique sputtering technique.We report a study of deposition of in-plane anisotropic TbFe giant magnetostrictive films by dc magnetron oblique sputtering, includingthe influences of deposition angle on TbFe film magnetic and magnetostrictive performances. The in-plane magnetization of TbFe filmsat 1600 kA/m is drastically increased with a change of deposition angles from 90 to 15 . Magnetic domain structures explored by mag-netic force microscopy indicate that the easy magnetization directions of the films can be gradually changed from perpendicular to thefilm plane at sufficiently shallow deposition angles. The in-plane magnetostrictive coefficients at 16 kA/m also can be increased bydecreasing the deposition angles from 90 to 15 . The significant variation in the in-plane magnetic and magnetostrictive performancescan be explained by the decrease of perpendicular anisotropy of TbFe films.

Index Terms—Deposition angle, giant magnetostriction, oblique sputtering, TbFe films.

I. INTRODUCTION

RECENTLY, there has been an increasing interest in TbFegiant magnetostrictive films [1]–[4], which can be widely

used in microelectronic mechanical systems (MEMS), such assurface acoustic wave (SAW) [5], mini-motor [6], micro-pump[7], and delay lines [8]. However, because of large anisotropyconstant of TbFe films, a rather high external magnetic field, upto several thousand oersteds (Oe), is needed to obtain obviousdeflection. But, the external magnetic field is usually limited toseveral hundred Oe in microdevices. This is a serious problemfor practical applications of magnetostrictive films, especiallyfor miniaturization and integration film devices. In order to im-prove the low-field magnetostriction of TbFe films, many at-tempts have been made so far. For example, for improving filmmicrostructures and the glass forming ability, a small amountof boron was added into TbFe films, for which 108 ppm deflec-tion at 8 kA/m external magnetic field was obtained [9]. De-positing TbFeB/SmFeB [10] or TbFe/Fe [11] multilayers canalso reduce the magnetocrystalline anisotropy constant and en-hance the soft magnetic properties (large saturation magnetiza-tion, low coercivity). Consequently, the low-field magnetostric-tive performances of the films were improved.

Several methods, such as flash evaporation, ion plating, andmagnetron sputtering, have been reported to prepare TbFe films[12]–[14]. Due to the normal in-plane orientation of drivingmagnetic field, in order to avoid large demagnetization effects, itwas found to be extremely important to realize an in-plane mag-netic easy axis of TbFe films. In this paper, in-plane anisotropyTbFe films were fabricated by dc magnetron oblique sputteringin an effort to reduce perpendicular anisotropy and then to in-crease the low-field magnetostriction of TbFe films.

Digital Object Identifier 10.1109/TMAG.2005.844836

Fig. 1. Schematic diagram of sputtering; � is the deposition angle used in ourdiscussion.

II. EXPERIMENTAL DETAILS

TbFe films were deposited on 20 mm 5 mm 100 m pol-ished monocrystalline silicon (110) substrates by dc magnetronsputtering. Substrates were mounted on a tilted clamp so as tochange the deposition angles (see Fig. 1). The silicon substrateswere cleaned by argon ion beam in Kaufman ion chamber be-fore depositing TbFe films. A TbFe target ( 70 mm) consistingof 40 at% Tb and 60 at% Fe was employed to deposit TbFefilms. By using a pure tantalum target, about 1-nm-thick tan-talum thin layer was deposited on surface of the prepared TbFefilm to prevent it from oxidation. Argon was used as the sput-tering medium at a partial pressure of 0.4 Pa. Base pressure was3 10 Pa. The target-to-substrate distance, sputtering power,and sputtering time was 140 mm, 40 W, and 30 min, respec-tively. The cross section micrographs of the films were analyzedby scanning electron microscope (SEM), and the microstruc-tures were examined by X-ray diffraction (XRD) with Cu K ra-diation. Magnetostriction coefficients were defined and mea-sured by the optical cantilever deflectometer as described in[15]. The magnetic performances were measured by using a vi-brating sample magnetometer (VSM). Magnetic domain struc-tures were explored by magnetic force microscopy (MFM).

0018-9464/$20.00 © 2005 IEEE

JIANG et al.: OBLIQUELY SPUTTERED TbFe GIANT MAGNETOSTRICTIVE FILMS 1223

Fig. 2. SEM images of TbFe film cross sections. (a) � = 90 . (b) � = 60 .

Fig. 3. Typical XRD patterns of TbFe films. (a) � = 90 . (b) � = 30 .

III. RESULTS AND DISCUSSION

Fig. 2 displays the cross section micrographs of TbFe filmsprepared at different deposition angles. At deposition angle

, grain-consisting columnar direction is perpendicular to thefilm plane [Fig. 2(a)]. While decreasing of deposition angle, thecolumnar direction inclines toward the sputtering direction. Thedependence of oriented angle of columnar structures on deposi-tion angle can be described by (1) [16]

(1)

where is the oriental angle of columnar structure toward sub-strates and is the deposition angle.

It can also be seen that the thickness of TbFe films is greatlydependent on deposition angles. When the sputtering angle was90 , film thickness is about 1.8 m, whereas only 1 m for 60deposition angle. In the following discussion, influences of filmthickness on TbFe film magnetic and magnetostrictive perfor-mances are all considered.

Fig. 3 illustrates the typical XRD patterns of TbFe films.There are no distinct diffraction peaks, indicating that the as-de-posited TbFe films are amorphous. But two broad dispersion

Fig. 4. Hysteresis loops of TbFe films at different deposition angles;? denotesperpendicular direction, and // for in-plane direction. (a) � = 90 . (b) � = 80 .(c) � = 70 . (d) � = 60 .

peaks at 25 –35 and 40 –45 are observed when depositionangles are greater than 30 . These evidences may imply thatthe as-deposited TbFe films are separated into two amorphousphases, Tb rich phase and Tb poor phase [17]. It can also beconfirmed by the hysteresis loop experiments in Fig. 4.

The dependences of hysteresis loops of TbFe films on depo-sition angles are shown in Fig. 4. With the decrease of deposi-tion angles, rectangular degree of the TbFe films in perpendic-ular direction is gradually degraded. When deposition angle is

1224 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 4, APRIL 2005

Fig. 4. (Continued.) Hysteresis loops of TbFe films at different depositionangles;? denotes perpendicular direction, and // for in-plane direction. (e) � =45 . (f) � = 30 . (g) � = 15 .

Fig. 5. Dependence of TbFe film in-plane magnetization on deposition anglesat 1600 kA/m.

below 45 , the in-plane magnetization becomes larger than per-pendicular magnetization. The hysteresis loops in the perpen-dicular direction at all angles greater than 30 show two-staged

Fig. 6. Magnetic domain structures explored by MFM of TbFe films atdifferent deposition angles. (a) 90 . (b) 70 . (c) 30 . (d) 15 .

loops, which may imply that the as-deposited films consist oftwo weakly coupled phases as presented in [17]. The reason forsuch phase separation is still under investigation. The depen-dence of TbFe film in-plane magnetization on deposition an-gles is illustrated in Fig. 5. With the decrease of deposition an-gles, in-plane magnetization of TbFe films are greatly enhanced,which indicate that the in-plane soft magnetic performances ofTbFe films are improved by oblique sputtering. At lower de-position angles, the films appear magnetically isotropic whichindicates that the film perpendicular anisotropy caused by themorphology of the grains is reduced. This change can also beconfirmed by MFM, as mentioned below.

Fig. 6 shows magnetic domain structures of the TbFe filmssputtered at different deposition angles. When the deposition an-gles are larger than 30 , maze domain structures are observed[Fig. 6(a) and (b)], meaning that the films possess stronger per-pendicular anisotropy. At lower deposition angles [Fig. 6(c) and(d)], no domain walls are observed. This indicates that TbFefilm obliquely sputtered at shallower deposition angles have re-duced perpendicular anisotropy. The data indicate that the easyaxis can be changed from perpendicular to the film plane at suf-ficiently shallow deposition angles.

Fig. 7 shows the change in in-plane magnetostrictive coeffi-cients of TbFe films at 16 kA/m external magnetic field withthe deposition angles. Upon the decrease of deposition anglefrom 90 to 15 , the in-plane magnetostrictive coefficient in-creases significantly. At 90 deposition angle, is only 35 ppm,whereas at an angle of 15 , can reach 150 ppm. These resultsshow that improved low-field in-plane magnetostriction charac-teristics of the films can be achieved by obliquely sputtering.

JIANG et al.: OBLIQUELY SPUTTERED TbFe GIANT MAGNETOSTRICTIVE FILMS 1225

Fig. 7. In-plane magnetostrictive coefficient � of TbFe films prepared underdifferent deposition angles at 16 kA/m; magnetostrictive coefficients � weremeasured by optical cantilever deflectometer.

IV. CONCLUSION

From the systematic investigation for the influence of depo-sition angles on magnetic and magnetostrictive performancesof TbFe films, it has been found that the in-plane soft magneticperformances of TbFe films become more desirable at lowerdeposition angles. The easy direction of magnetization can bechanged gradually from perpendicular to the film plane at suffi-ciently shallow deposition angles. Improved low-field in-planemagnetostriction characteristics can be achieved by obliquesputtering, which is attributed to the decrease of perpendicularanisotropy.

ACKNOWLEDGMENT

This work was supported by the National Science Fund ofChina under Grant 50271014.

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Manuscript received August 2, 2004; revised January 15, 2005.