Magnetostriction and magnetic properties of Sm-Fe-B and Tb-Fe-B thin films and multilayers

Journal of

ALLOYS AHD COMPOUNDS

ELSEVIER

Journal of Alloys and Compounds 258 (1997) 149-154

Magnetostriction and magnetic properties of Sm-Fe-B and Tb-Fe-B thin films and multilayers Toshiyuki Shima*, Hiroyuki Yokoyama, Hiroyasu Fujimori b~stitute for Materials Research, Tohoku University, Sendai 980- 77, Japan Received 7 November 1996

Abstract Magnetostriction and magnetic properties of amorphous Sm-Fe-B and Tb-Fe-B thin films and Sm-Fe-B/Tb-Fe-B multilayers have been investigated. From a systematic investigation on Sm-Fe-B alloy thin films, it was found that the film with B content of 3.5 at.% exhibits a low coercivity of about 50 Oe and a large magnetostriction of about -340× 10.6 at 100 Oe. In the case of multilayers, magnetostriction varies with the thickness ratio of the two layers. For the magnetostriction of the multilayers it was found that the magnetostriction is sensitively affected by Young's modulus, Poisson ratio and the thickness of the constituent layers. The present result may indicate that multilayering is a suitable route to achieve good magnetic softness even in this kind of R-Fe thin film. © 1997 Elsevier Science S.A. Keywords: Giant magnetostriction; Sm-Fe thin films; Tb-Fe thin films; Multilayers; B addition; magnetic softness

1. Introduction The cubic Laves phase compounds RFe 2 (R; rare earth) are well known to possess giant magnetostriction [1]. Among these compounds, SmFe 2 and TbFe 2 exhibit the largest negative and positive magnetostrictions at room temperature, respectively. However, a large magnetic field is usually necessary to achieve large magnetostriction due to large magnetocrystalline anisotropy of the compounds. In order to reduce the magnetocrystalline anisotropy and hence to obtain large magnetostriction at low fields, several approaches have been made. One effective method is to reduce the magnetocrystalline anisotropy by combining two rare earth elements having positive and negative signs in the magnetocrystalline anisotropy constant, as demonstrated successfully in T b - D y - F e alloys (so called Tefenol-D) [2,3]. Another method is the amorphization which is "known to reduce the magnetic anisotropy although local anisotropy remains [4]. In many cases, however, the magnetostriction at low fields is not greatly improved even with the amorphization. One possible reason for this is the poor integrity of the amorphous phase, for example, the presence of very small clusters. Since B is known to stabilize the amorphous state effectively, this problem may be solved by the addition of B to *Corresponding author. Fax: +8I [email protected].~p

22 215 2096; e-marl:

0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 8 3 8 8 ( 9 7 ) 0 0 0 5 8 - 3

the alloys. Previously, we reported that in the amorphization, the addition of B to Sm-Fe or Tb-Fe amorphous alloys is very effective in reducing the saturation magnetic field while maintaining large magnetostriction [5]. This is considered to result from the large reduction in effective anisotropy due to the improvement in the integrity of the amorphous phase with the B addition. The results, however, were obtained, in bulk samples. In order to apply these giant magnetostrictive materials to highly sensitive magnetic sensors, small actuators, micro pumps and magnetic SAW devices, materials in the form of thin films are required. In this study, we investigate the effect of B element on the magnetostriction and other magnetic properties of SmFe and Tb-Fe thin films. S m - F e - B / T b - F e - B multilayers are also investigated and a simple model is devised to explain the magnetostriction of the multilayers.

2. Experimental procedure The samples were prepared by RF sputtering using a target of Sm-Fe or Tb-Fe alloy with 50 mm diameter. These alloy targets were prepared by arc melting under an Ar atmosphere. Fe9oBlo (at.%) chips were used to introduce B to the Sm-Fe and Tb-Fe thin films. The thin films of 1 Ixrn thickness were deposited on glass substrates. The

T, Shima et al. I Journal of Attoys and Compounds 258 (1997) 149-t54

150

base pressure was below 3.0X10 -7 Torr and the Ar gas pressure during sputtering was varied from 15 to 40 mTorr. The structure was observed by XRD using Cu Koe radiation. Coercivity and saturation magnetization were measured using a VSM at room temperature in applied fields up to 15 kOe. Magnetostriction was measured by the cantilever method in rotating in-plane fields up to 5 kOe. The film composition was determined by ICP spectroscopy and EDS analysis.

0

.......

, . . . . . . . . .

-100 -200 -300 O~

B=7.0 l,,i , , ,,I,

. ,l~lqt,,IJp,li,,l~l~,,,lJ,,,I

-I00

,

, . . . . . . . . .

s

:

•

i

.,.

,'

1

....

' '

=o

B=I

B4

-40O -500

¢~

3. Results and discussion

B=3.5

,,,)

....

I ....

i ....

I ....

( ....

i ....

) ....

I ....

.........

I

1

6 0 ~" (a)

,0

'

I

I

'

]

f--

-20

'

t

(b)

,.

20

,~

1

/.---

:

I

i

1

r

r

1

'

l

,

~

60 ..- (c)

t

i

I

-

I

i

I

I

'

I

(d)

t

r

i

l

l

% 2o '~ 0 g -2o

-4o -~O 1~

r

,

,

.

,I "

-T.--T'-r""~t

t

,

,

,

6 0 ..- (e) 402O 0

-40 .-60

,,,,

0'

1

and T b - F e - B thin films

Fig. l(a)-(f) shows the magnetization hysteresis loops of S m - F e based thin films for the B contents of 0, 0.7, 1.4, 3.5, 7.0 and 8.8 at.%, respectively. From these figures, the B-free S m - F e thin film has the largest coercivity of about 650 Oe but the coercivity is reduced significantly by the addition of B. Particularly, Sm2,2.gFe73.6B3,5 thin film shows the smallest coercivity of 54 Oe. The reason for the reduction of Hc by B addition is considered to be due to the improvement of the integrity of the amorphous state. Fig. 2 shows the field dependence of magnetostriction for S m - F e and S m - F e - B thin films. The B-free S m - F e

-6o

f,,,

-300

'-600

I

B=8 ,,,11

-200

-go

3.1. S m - F e - B

, . . . . . . . . .

,

,__.a...~,

,

-4

-2

2

r 4

Magnetic

, -4

-2

0

I 2

,

, 4

field, H ( k O e )

Fig. 1. The magnetichysteresisloops of Sm-Fe and Sm-Fe-B thin films. The compositions of the films are; (a) Sm~.~3F%~.7, (b) Sm2o,TFevs,~Bo7, (c) Sm,~2Fev~.4B1.4, (d) Sm22.gFe73:6B3:5, (e) Smi~.oFevs.oBT.0,and (f) Sm~v.~Fe7~9B8.~ at.%.

........

-400

-500 ?rn-Fe-B th'mfilm

-600 -700 0

1

B

B:O.

2. The

field dependence

....

2 Magnetic

Fig.

.

,,,l~,,,I,~,l,,,,l~,,,~,,,I,,~w,,,,I,,,,I 3

4

I 5

fietd, H (kOe)

of magnetostriction

for Sm-Fe

and

SIn-Fe-

thin films.

thin film shows a giant negative magnetostriction of about - 5 8 0 × 10 -6 at 5 kOe but magnetostriction at low fields is poor and saturation is not reached even at 5 kOe. However, the addition of B makes it easy to saturate the magnetostriction. The addition of 3.5 at.% B leads to good soft magnetostrictive characteristics: magnetostriction of -3605<10 .6 is achieved at an applied field as low as 200 Oe. However, with the further addition of B, the magnetostriction value is reduced and these alloys with high B contents are not useful for applications. Fig. 3(a)-(f) shows the magnetization hysteresis loops of T b - F e based thin films for the B contents of 0, 0.3, 1.0, 1.0, 7.3 and 3.8 at.%, respectively. On the left hand side ((a), (c) and (e)), the films having a strong perpendicular magnetic anisotropy are shown, and on the right hand side ((b), (d) and (f)) the films with in-plane anisotropy are shown. From these figures, the behavior is different from that in the S m - F e based thin films, since a strong perpendicular magnetic anisotropy is observed in some cases. It is noted that whether the perpendicular magnetic anisotropy appears or not depends on the deposition condition, rather than the B-content. Schatz et al. reported on the influence of internal stress on the magnetic anisotropy and magnetostriction in sputtered T b - D y - F e films [6,7]. It was concluded that induced stress is an important factor for the magnetic anisotropy of these films. Our results on the behavior of different anisotropy with similar composition are thought to have originated from the different induced stress during the fabrication. Fig. 4 shows the field dependence of magnetostriction for T b - F e and T b - F e - B thin films. The upper figures are

15I

T. SMnm et al. / Journal of Alloys and Compounds 258 (I997) 149-154

'

60

I

I

'

'-

•'

(a)

'

I

I

(b)

40 20 0 -20 40 -60

I

1

I

[

' I

60

I I

~ '

1 '

i'r

(c)

I

I

'

l

I

,

I

e

z

s

s

[

I

r n

I

'

'

(d)

40

20 o

.~ -20 .~ -40

-60

r

I

~

T ~ m

19rrl~t

"Ili1911

I ' '

(e)

40! 20 o

-20 -40 -60

1

-10

9

I

0

~

I

I

~'

"1

I

I

:

,

,

10 -10 Magnetic field, H (kOe)

,

0

~0

Fig. 3. The magnetic hysteresis loops of T b - F e and T b - F e - B thin films. The compositions of the films are; (a) T b ~ ~Fer~,, (b) Tbn~F%~=Bo ~, (c) Tb~.~/F%4.oBI. o, (d) T b ~ F e ~ 2 ~ B I o, (e) Tb~a~FeroiB2~, and (f) T b ~ , , F e ~ , ~ B ~ . On the left hand side, the films having perpendicular magnetic anisotropy are shown, and on the fight hand side those with in-plane anisotropy are shown.

for the magnetostriction of the films with in-plane anisotropy as shown in Fig. 3(b), (d) and (f) and the lower ones for out-of-plane anisotropy as shown in Fig. 3(a), (c), (e). The different behavior of the magnetostriction between the films with the in-plane and the perpendicular anisotropy is obviously due to the fact that films with in-plane anisotropy saturate easily in the present magnetic field range but those with perpendicular anisotropy do not. Magnetostriction of thin films with in-plane anisotropy is about +210× 10 .6 at 5 kOe. It is noted that the S m - F e - B thin films have only in-plane anisotropy but T b - F e - B thin films have both in-plane and perpendicular anisotropy. The reason for this is considered to be caused by the different signs of magnetostriction, from induced stress during the sputtering or restraint stress from the substrate. Further study is to be performed in order to further improve the magnetostrictive properties, with particular emphasis being placed on the control of induced stress. Fig. 5 shows the Comparison of the present data with those reported in the literature. The T b - D y - F e Laves phase compound shows giant magnetostriction, but a large magnetic field is needed. In the case of Sm-Fe systems with B addition, both thin films and bulk sample exhibit a large magnetostriction at low fields. While, in the case of Tb-Fe systems, the magnetostriction of thin films is not satisfactory when the value is compared to that of bulk

' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

/

1000

,f /

o 300

, , , i , , , , l , , , , i , , , , l , , , ~ l , , , , i , , , , i , , , , i , , , ~ l , L ~ ,

250

'

Th-Fe-B thin film

,'

lin'planel ~4~

~-

200

~

t50

1

!

!

I

o=

i

500

/

/

. ..............

-

~,....... """".~ .......................................

"ff~o

.2

!

"

•'~

100

0

ID

~' '

;~

,

O

5o

"t

~N

120

perp. to the plane

"'"

5

40

o ~,,~,,

.

~

~,

, i , ' , , , ~ . . . . ? . . . . , , , , , I , , , , , . . . . , . . . . ~,,,,~ 1 2 3 4 5 Magnetic field, H (kOe)

Fig. 4. The field dependence of magnetostriction for T b - F e and T b - F e B thin films. Upper figures indicate the magnetostriction of the films with in-plane magnetic anisotropy and lower figures indicate the films with

out-of-p~anemagnetic anisotropy.

.

-t,

-500

8O x

"~- ...........................

0

T

T

I

I

1

5

F

I

F

I

2

I

T

T

f

1

F

~

3

T

I

I

4

f

I

T

T

5

Magnetic field, H (kOe)

Fig. 5. The field dependence of magnetostriction for S m - F e - B and T b - F e - B thin films and other magnetostrictive materials: - • - , SmF e - B (amorphous, film); - (3 - , S m - F e (amorphous, film); - • - , S m F e - B (amorphous, bulk); - / ~ - , S m - F e (amorphous, bulk); - • - , T h - F e - B (amorphous, film); - [ ] - , T b - F e (amorphous, film); - • - , T b - F e - B (amorphous, bulk); - - - , T b - D y - F e (Laves phase compound, bulk).

152

T. Shima et al. / Journal of Atloys and Compounds 258 (1997) 149-154

alloys. The difference between the behavior of magnetic properties with thin film and bulk samples is considered to result from the facts that induced stress originated from the restraint from the substrate, composition and deposition condition are very sensitive to the magnetic anisotropy and magnetostriction. However the details of this cause are not yet clear.

3.2. S m - F e - B / T b - F e - B

400 Tb~.s Fes6.aB0.a

10% 200

20% 30% 40%

multilayers

60%

o

"5

It was found that S m - F e - B and T b - F e B thin films are promising materials for many useful applications at low magnetic fields. It would be of more benefit to further reduce the saturation magnetic field of these thin films. One method is to reduce the magnetocrystalline anisotropy directly and the other is to increase the magnetization [8]. From these considerations, ferromagnetic materials with high saturation magnetization (e.g. pure Fe, Fe-Co alloys...) can be used to form multilayers with S m - F e - B or T b - F e - B alloys. In this work, before confirming the effect of multilayering with high magnetization materials, we have investigated the SmFeB/TbFeB multilayers with negative and positive magnetostriction to verify our simple model. The essence of this simple model is to see whether strain and stress are transferred effectively at the interface. We prepared ( S m - F e - B / T b - F e - B ) multilayers by changing the ratio of S m - F e - B and T b - F e - B layer thickness. The total thickness of 1 ~m was fixed but the thicknesses of S m - F e - B and T b - F e - B were varied from 10 to 90 nm and 90 to 10 nm, respectively. The S m - F e - B layer thickness ratio was given by, Sm - Fe - B layer thickness ratio =

fSm-Fe-B t S m _ F e _ B + tTb_Fe_ B

(1) By using this formula, we considered the relation between the thickness ratio and magnetostriction. Fig. 6 shows the field dependence of magnetostriction for T b - F e - B and S m - F e - B single layers and several multilayers with the thickness ratio ranging from 10 to 90%. The behavior of the changing magnetostriction is quite reasonable. With increasing S m - F e - B layer thickness, the value of the magnetostriction changes from positive to negative. From these results, we propose to calculate the magnetostriction of multilayers by using a simple two dimensional model Fig. 7 illustrates a two dimensional model of simple dynamics. First of all, we assume that each layer ( S m - F e B or T b - F e - B ) has only one phase and has no magnetic anisotropy. Without this assumption, the mechanism is much more complicated. The magnetoelastic energy (Eme) is a linear function of strain (e) and the elastic energy (E~]) is a quadratic function of the strain [9] so that equilibrium

70%

-200

80% -400 90% -600 ~

-~

0

2

4

6

8

Sm2o.7Fers.sBo.7

10

12

14

Magnetic field, H (kOe)

Fig. 6. The field dependence of magnetostriction for Tb-Fe-B and Sm-Fe-B single layers and several mulfilayers.

is attained at some finite strain. The condition for this is to minimize the total energy, E = Eme + Eel

(2)

Since the value of strain and magnetostriction is smalt and the elastic modulus does not change, the problem of minimizing the total energy will become the simple dynamics problem. The parameters were: film thickness (t); magnetostriction (A); Young's modulus (E); Poisson ratio (v); strain (E); compression or tensile stress (W); elongation (l As). In the following, the superscripts denote the signs of the magnetostriction and the subscripts denote the (x, 3') direction. From the function between stress and strain, the following equation is obtained; o-x - ~ ( % - o-~) ~

-

E

,

o-z = 0

(3)

The elongation observed in the direction of (x, y) is given by;

<17 i LX Fig. 7. Two dimensional model of simple dynamics for the multilayer.

T. S~.ima et a[. f Journal of Alloys and Compounds 258 (1997) 149-154

+

+

+

o"x - v O'y AI., = I~A+ +

drx - . v

E+

• I~ = l x A - + +

+

O's

E-

+

600

E+ P

50

700, " lx

cry -- v O"x

O"y-

153

40

• ly

5001

dr x

I

E-

.ly

(4)

0

30

///

There is no change of volume fraction due to magnetostriction, so that the value of magnetostriction in the y direction could be assumed to be - l / 2 . Considering the balance of force, the stress is given by

.e 400(

~" 300

(5)

200

Thus, the magnetostriction which was assumed in this

lOO

+-

o'~

% , o - 2 - l y%t -

l yt+

+=

' O'y

-

l/+

,o-.;-- l ~%t -

20

10

two-dimensional model may be expressed in terms of Poisson ratio, Young's modulus and thickness of each sample and it is given by;

As-

As+ E + (1 - v - ) t + + A s E - ( 1 - v + ) t E+(1 - z,=)t + + E - ( 1 - v + ) t -

(6)

The variations of the magnetostriction of S m - F e - B / T b - F e - B multilayers with the ratio of S m - F e - B layer thickness to the total thickness are shown in Fig. 8. The circles and the solid line denote the measured values and

200

"%, ©

°_9_

%,

"%,,

o

"%

•

.o "6

"•',,%,

-200

•',,, %,• [SmFeB/TbFeB]I o

"",,,,,

-400

0

20

40

60

80

-i F I I ~ F 1 1 r [

)

,llt

20

[SmFeB/TbFeB]lo llr

ill

lIT lit

l~l

IlI

111 , 1 1 1 I l l

40 60 Thickness ratio (%)

80

llIt

f

0

100

Fig. 9. The change in the magnetization and the coercivity of S m - F e - B / T b - F e - B multilayers as a function of the ratio of S m - F e - B layer thickness to the total layer thickness. The open and closed circles denote the magnetization and the coercivity, respectively.

the calculated values from Eq. (6), respectively. The straight dotted line connects the magnetostriction value of the T b - F e - B and S m - F e - B single layer films. The calculated results were obtained by using the following parameters; A, + = 2 3 0 X 10 -6, '~s- ---- - - 4 4 0 M 10 -6, E + = 76 GPa, E - = 4 0 GPa and v + = u- = 0.3. Several measured data are in good agreement with this calculation. From the results, magnetostriction of the multilayers is greatly influenced by Young's modulus, the Poisson ratio of the material and each layer's thickness. The present model may be used as a guideline to control the magnetostrictive materials. Fig. 9 shows the change in the magnetization and the coercivity of S m - F e - B / T b - F e - B multilayers as a function of the ratio of S m - F e - B layer thickness to the total layer thickness. The magnetization in the multilayers lies nearly on the straight line connecting the saturation value of the T b - F e - B and S m - F e - B single layer films. But coercivity exhibits a minimum of 80 Oe for the [ S m - F e - B (20 n m ) / T b - F e - B (80 nm)]~o multilayer. From these results, multilayering may be a suitable way to obtain good magnetic softness.

100

Thickness ratio (%) Fig. 8. The change in the magnetostriction of S m - F e - B / T b - F e - B multilayers as a function of the ratio of S m - F e - B layer thickness to the total thickness. The circles and the solid line denote the measured values and the calculated values from Eq. (6), respectively. The straight dotted line connects the magnetostriction value of the T b - F e - B and S m - F e - B

single layer films.

It"

4. Conclusions The magnetostfiction and magnetic properties of S m Fe-B, T b - F e - B thin films and multilayers have been investigated. From the systematic investigation of S I n - F e B alloy thin films, it was found that S m - F e - B alloy thin

I54

T. Shima et at. / Journal of Alloys and Compounds 258 (1997) 149-154

film with the B content of 3.5 at.% exhibited a fairly low coercivity of about 50 Oe and, as expected, this film showed a large magnetostriction of about - 3 4 0 × 10 .6 at a low magnetic field of 100 Oe. It is considered that this S m - F e - B thin film is a promising material for magnetomechanical thin-film devices. In the case of T b - F e - B thin films, however, the giant magnetostriction at low fields was not obtained even in the films with in-plane anisotropy. In the case of multilayers, magnetostriction was varied with the ratio of each layer and it was found that the Young's modulus and Poisson ratio of the material and each layer's thickness are very influential in the model calculation. Multilayering may be a suitable way to obtain good magnetic softness.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

A.E. Clark, AIP Conf. Proc. 18 (1974) 10t5. C.M. Williams, N.C. Koon, Solid State Commun. 27 (1978) 81. K. Mori, J. Cullen, A. Clark, IEEE Trans. Mag. 19 (1983) 1967. D. Forester, C. Vittoria, J. Schelleng, R Lubitz, J, Appl. Phys. 49 (1978) 1966. H. Fujimori, J.Y. Kim, S. Suzuki, H. Morita, N. Kataoka, J. Magn. Magn. Mat. 124 (I993) 115. F. Schatz, M. Hirscher, M. Schnell, G. Flik, K. Kronmuller, J. AppI. Phys. 76 (1994) 5380. F. Hellman, E.M. Gyorgy, Phys. Rev. Lett. 68 (1992) 139I. T. Shima, N. Kataoka, H. Fujimori, IEEE Trans. J. Magn. Jpn 9 (1994) 55. S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964.