MAGNETOACOUSTIC WAVE IN AMORPHOUS MAGNETIC FILM

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MAGNETOACOUSTIC WAVE IN AMORPHOUS

MAGNETIC FILM

K. Shirae, B. Tsujimoto, H. Miyatake, T. Saito

To cite this version:

K. Shirae, B. Tsujimoto, H. Miyatake, T. Saito.

MAGNETOACOUSTIC WAVE IN

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JOURNAL DE PHYSIQUE

Colloque C8, SupplBment'au no 12, Tome 49, dBcembre 1988

MAGNETOACOUSTIC WAVE IN AMORPHOUS MAGNETIC

FILM

K.

Shirae,

H.

Tsujimoto, H. Miyatake and T . Saito

Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560, Japan

Abstract. - Reduction of large saturation field of sputtered magnetostrictive thin film is inevitable for magnetoacoustic application and was attained by proper selections of annealing temperature and substrate material. Many modes of magnetoacoustic waves were generated by magnetic drive, and transverse wave was found sensitive t o various substances loaded on the film.

1. Introduction

Magnetoacoustic wave generation by piezoelectric drive has been reported [I, 21. Here, we report on magnetoacoustic wave generation by magnetic drive on magnetostrictive amorphous thin film. Properties of the film as a guide of acoustic wave and sensor application of the film are discussed.

2. D e v i c e c o n s t r u c t i o n

The magnetoacoustic wave device consists of magnetostrictive thin strip and two meander coils as shown in figure 1. One meander coil generates magnetoacoustic wave, and another one picks up magnetization change caused by the wave.

Coil(Cu)

\

Film kfrip(~eSiE5)

Fig. 2. - Dependence of magnetoacoustic device gain on bias magnetic field. EA shows easy axis. Hk is anisotropy field.

and easily maximized by the bias field adjustment. Therefore, for a given material, anisotropy control (magnitude and direction) is most effective for higher gain realization.

Fig. 1. - Magnetoacoustic wave device using magnetic field

drive. 3. M a g n e t o s t r i c t i v e t h i n film

If we assume the strip has an uniaxial anisotropy as shown in figure 2, gain from driving current to output voltage can be easily written, by considering the magnetic energy minimum, as follows [3],

V / I = A.

f

. E X ~ M ~ / K ~ . F ( a , H I ) .F

(P,

H 2 ) , where

f

: driving frequency, E : Young's modulus,

X

:

saturation magnetostriction, M : saturation magnetization, and K : anisotropy constant, and a,

P ,

H I , Hz are shown in the figure. F (a, H ) is expressed as sin ( a

-

8) .sin 2 ( a

-

8)

/

[sin 2 8.cot ( a

-

8)

+

2 tan 2 81, where 8 is obtained by solving K sin 2 8 =

M H sin (a! - 8 ) . This function has a peak as shown,

Magnetostrictive thin film (Fe7sSi12B10) on a glass substrate was made using dc quadrupole sputtering. Figure 3 shows

M

-

H curves of 4

x

4 mm2 square samples cut from 4 x 50 mm2 strip. As sputtered film has large saturation field of 20 k ~ / m ~ ( M

-

1 T)

,

however, curves are quite similar for 1

,

2 and any other direction, so the film is isotropic in plane. As a peeled film from the substrate has very low saturation field (b), the origin of the large saturation field is attributed t o isotropic residual stress. Assuming the magnetostriction of 30

x

the residual stress amounts t o 400 ~ / m m ~ : ~ n d u c e d anisotropy by the fo- cusing field of the sputtering apparatus is 0.6 k ~ / m ~ and demagnetization field is only 200 AT/m, so these contributions are negligibly small. Stress relief annealing a t 300 OC reduced the saturation field to one half (c), but the film was also isotropic. The fact that

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C8 - 2036 JOURNAL D E PHYSIQUE @ * ! m X h

r

.

O-+Q Gar x 5 0 m strip

Fig. 3. - Magnetization curves of amorphous magnetostric- tive thin films sputtered on a glass substrate: (a) as sput- tered on 1 mm substrate, (b) peeled film, (c) 300 OC an- nealed, (d) 300 OC annealed, 0.5 mm substrate, (e) 300 OC annealed, 0.15 mm substrate.

the optimum annealing temperature of 300 OC is much lower than the crystallization temperature of 450 OC, is due t o a compromise between the decrease of the residual stress and increase of the stress due to differ-

ential thermal expansion of the film and substrate with increasing annealing temperature. Thinner substrate gave good results as shown in (d) and (e). Satura- tion field of 4 kAT/m was obtained, still isotropic, for 0.15 mm substrate after annealing.

4. Application

Magnetoacoustic wave device (meander coil pitch 250 pm) was made using 0.15 mm substrate. The device is driven by burst pulse current. Bias fields a t drive and pick up were separately adjusted t o maxi- mize the output. They are about 3 kAT/m and corre- spond t o large angle case in figure 2, figure 4 shows the wave forms of the output when the burst frequency is 23 MHz. Three groups of waves A, B, and C are observed (a). Velocities of

A,

B, and C are 5.5, 4.5, and 3.7 km/s respectively. Coil pitch matches t o the wave length of A (240 pm). Wave A is a longitudinal wave excited directly by the coil, because its velocity agrees

Fig. 4. - Output voltage wave forms of a magnetoacous- tic wave device. Burst pulse frequency: 23 MHz, number of meander turns: 10! coil pitch: 250 pm, 1.5 pm amor- phous (Fe7sSi12B10)99Cr1 film on 0.15 mm substrate. (a) Free state, (b) 10 mg water drop loaded; vertical scale 50 mV/div through 40 db amplifier.

well with the calculated longitudinal wave velocities of 5.4 km/s (film) and 5.5 km/s (substrate). Wave C is regarded as a transverse wave, because i t ranges in the calculated transverse velocities of 3 km/s (film) and 3.5 km/s (substrate). Wave B has an intermediate velocity of

A

and C.

When a water drop was added on the surface of the strip, the wave forms changed as shown in (b). Com- paring with (a), wave A is unchanged, wave B has decreased the amplitude t o one half, and the amplitude of wave C decreased to one fourth. The relation of the added mass and wave C amplitude is shown iri figure 5a. The amplitude decreases exponentially with the increase of mass. Minimum detectable mass is about 50 pg. Viscosity changes also the amplitude of wave C (b). Proportionality is observed between am- plitude and logarithm of the product of density and viscosity of a liquid. When a drop of paint or adhesive is loaded, the time variations of wave C amplitude show peculiar behaviours depending on the kind of liquids (c). A sudden decrease in the case of aquapaint is due t o the formation of a creater like in the moon during the hardening.

water drop (mg) log (u p )

E p a i n x

'.

_{--__}

Emxv r a i n adhesive 0 3 0 6 0 time (min)

Fig. 5. - Amplitude change in magnetoacoustic wave de- vice by various loadings. (a) Water drop loading, (b) liquid loading of various viscosity; ( 0 ) from left to right, acetone, water, castor oil, glycerin of constant volume, (c) hardening of paint or adhesive.

5. Conclusion

Large isotropic saturation field of magnetostrictive t h i d film has been reduced by sputterhi on thin substrate and annealing a t low temperature. Many modes of acoustic wave such as longitudinal and transverse waves can be generated simultaneously by magnetic drive through meander coil. The transverse wave was found t o response t o various loadings of liquids, lead- ing to unique sensor application.

[l] Hanna,

S.

M. et al., I E E E h n s . Magn. MAG- 19 (1983) 1802.

[2] Uenaka,

H.

el al., J. Magn. Soc. Jpn 9 (1984)

203.