SURFACE ACOUSTIC WAVE ATTENUATION IN THIN FERROMAGNETIC FILMS

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SURFACE ACOUSTIC WAVE ATTENUATION IN

THIN FERROMAGNETIC FILMS

R. Wiegert, H. Yoshida, Keun-Jen Sun, M. Levy, H. Salvo, Jr Mcavoy, B.

Mcavoy

To cite this version:

R. WIEGERT, H. YOSHIDA**, KEUN-JEN SUN, M. LEVY, H. SALVO, Jr' and

B.R.

McAVOY"

Physics Department, University of Wisconsin-Milwaukee. Milwaukee, W I 53201, U.S.A.

'Westinghouse Defense & Electronics Center, Baltimore, MD 21203, U.S.A.

++Westinghouse R & D Center, Pittsburgh, PA 15235, U.S.A.

Abstract

-

Experimental results on the thickness and composition dependence

of surface acoustic wave attenuation in thin films of nickel and nickel-iron

alloys are discussed.

Introduction

The phenomenon of magnetic field dependent high frequency surface acoustic wave

(SAW) attenuation in thin films of Ni and Ni-Fe films /1,2,3.4/ provides a method

for characterizing the magnetic parameters of such films and offers the possibility

of important technological applications. In single domain magnetic films which may

be characterized by an in-plane magnetization

M

related t o a uniaxial anisotropy

field HA; i.e. films in which the Stoner-Wohlfarth model

/ 5 /

for coherent magne-

tization reversal is applicable, the SAW-film attenuation mechanism may be modeled

as being essentially due t o a variation in M via a SAW-induced (through inverse

magnetostriction) variation AHA of the films anisotropy field HA. Solution of a

linearized form of the Gilbert damping equation for small variations of

M

about

its equilibrium position as determined by a free energy equation appropriate for a

single-magnetic-domain uniaxially anisotropic film leads to an imaginary component

of magnetic susceptibi'lity and hence provides a mechanism for SAW energy dissipa-

tion, E~(HA, H,

M, 6).

where H is an externally applied in-plane magnetic bias

field and G is the Gilbert damping parameter /2/. For the case of either H

II

k

or H 1

k where k is t h e SAW propagation vector, Ed the energy dissipation per unit

volume per cycle is given

.($

sin 2e0l2 [g2

+

nS2

(1

+

G ~ ) ]

H ~ M

E =

[AB - ~

~

+

~

G ~ ) ] ~

(

+

1

[GHs(A+B)] 2

' ~ e s e a r c h supported by NSF Grant n o ECS-8001773

'*present address : The Research Institute for Iron, Steel and other Metals, Tohoku University, Sendai 980, Japan

C10-738 JOURNAL

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PHYSIQUE

where A = HACOS 2(e0-+)

+

Hsine, B = Hsine,

+

4nM

e0 = e q u i l i b r i u m p o s i t i o n angle o f H w i t h r e s p e c t t o k

+

= Angle o f t h e f i l m ' s u n i a x i a l a n i s o t r o p y a x i s w i t h r e s p e c t t o k Hs = a / y where i s t h e a n g u l a r frequency o f SAW, and y t h e magneto-

mechanical r a t i o

AHA = ne where n = magnetoelastic c o u p l i n g c o e f f i c i e n t and e = s t r a i n due t o SAW

Then t h e SAW a t t e n u a t i o n p e r c e n t i m e t e r i s g i v e n - b y

where E, F, and V are, r e s p e c t i v e l y , t h e t o t a l energy d e n s i t y , frequency and v e l o c i t y o f t h e SAW.

ti COERSTEDS)

Fig. 1

- T y p i c a l a(H) curve f o r a 250g n i c k e l f i l m a t 618 MHz.

The s o l i d curve i s t h e t h e o r e t i c a l f i t , i n a r b i t r a r y a t t e n u a t i o n u n i t s , t o t h e experimental c u r v e (dots) generated by using HA = 40 Oe,

+

= 53.3: and G = 0.11 i n Eq. 1.

As shown i n F i g u r e 1, i t i s p o s s i b l e t o q u a l i t a t i v e l y f i t many experimental d a t a by u s i n g reasonable values f o r t h e i m p o r t a n t f i l m parameters HA. G, and

+

(and t h e b u l k value o f H) i n Eq. 1. Experimentally i t has been found t h a t a i s s t r o n g l y dependent on f i l m t h i c k n e s s /2,3,4/, composition /3/, temperature /2,6/ and in-plane s t a t i c t e n s i l e s t r a i n '/6/ which i n t h e c o n t e x t o f Eq. 1 may i m p l y corresponding dependencies o f HA, G, and

+

on these e x t e r n a l v a r i a b l e s . Depar- t u r e s f r o m t h e i d e a l i z e d s i n g l e domain s t r u c t u r e i.e., t h e occurence o f magnetiza- t i o n r i p p l e and even m u l t i p l e domain s t r u c t u r e may be r e s p o n s i b l e f o r t h e behavior o f t h o s e samples whose response can o n l y p o o r l y be f i t by Eq. 1.

Thin films of Ni o r Ni-Fe alloys were deposited between

SAW

interdigital

transducers on quartz substrates by evaporation from tungsten sources in a vacuum

of 5

x

torr.

A

Matec ultrasonic attenuation system was used t o perform in

situ attenuation measurements during the process of film deposition while film

thickness was registered on a digital thickness monitor. Thus u(H

=

0) vs

thickness, T, curves such as that of figure

2

were generated during formation of

the samples.

20

. ..

.

..

T

(ANGSTROMS)

Fig.

2

- Typical

u

vs T curve for Ni at 700 MHz.

Results and Discussion

Figure

2

shows a typical

u

vs T curve for a 100% Ni film at 700 MHz. The

initial sharp peak in

u

at about 201 thickness are thought to be due t o coupling

of the

SAW

t o the film through piezoelectrically induced film currents /7/.

For

each film a broad peak in the

a

vs T curve is observed; for 100% Ni the maximum

T (ANGSTROMS)

C10-740 JOURNAL

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PHYSIQUE

a t about 2008 corresponds t o t h e f i l m t h i c k n e s s f o r which maximum magnetic f i e l d response i s observed. As f i l m composition becomes more i r o n r i c h t h e p o s i t i o n s o f t h e broad peaks s h i f t t o g r e a t e r thicknesses w h i l e f o r Ni-Fe 85:15 wt. % ( n o t shown) a c o n t i n u a l l y increases w i t h thickness and no peak i s observed /4/. I f one may assume t h a t t h e f i l m s have a s i n g l e domain s t r u c t u r e a t l e a s t w i t h i n some range o f thicknesses then, i n t h e c o n t e x t o f o u r model, i t i s reasonable t o assume t h a t should be p r o p o r t i o n a l t o t h e f i l m t h i c k n e s s when t h e thickness i s much s m a l l e r t h a n t h e wavelength o f t h e SAW

/a/.

Thus t o see t h e i n t r i n s e c behavior o f t h e f i l m e x c l u d i n g t h i s l i n e a r t h i c k n e s s dependence we have p l o t t e d a/T vs T i n F i g u r e 3. The r e s u l t i n g curve f o r 100% N i may be f i t by a Gaussian; t h e curve f o r Ni-Fe 90:lO appears t o be a s t e p f u n c t i o n w h i l e t h a t o f Ni-Fe 95:05 appears t o be some combination o f t h e f i r s t two. It may be p o s s i b l e t h a t magnetization r i p p l e e f f e c t s i n c r e a s i n g l y c o n t r i b u t e t o d e v i a t i o n from t h e i d e a l Stoner-Wohlfarth t y p e f i l m behavior as t h i c k n e s s i s increased, however t h e magnetization r i p p l e models o f Hoffmann /9/, Harte

/lo/

and F e l d t k e l l e r /11/ do n o t p r o v i d e f e a t u r e s i n t h e i r t h i c k n e s s dependences /12/ which c o r r e l a t e w i t h t h e experimental curves o f F i g u r e 3. Another p o s s i b i l i t y could be t h a t t h e f i l m t h i c k n e s s corresponding t o t r a n s i - t i o n s between Neel w a l l s , c r o s s - t i e w a l l s and Bloch w a l l s m i g h t c o r r e l a t e w i t h t h e p o s i t i o n s o f t h e maxima i n t h e i n s e r t i o n l o s s curves. C a l c u l a t i o n s by Middelhoek /13/ on permalloy (Ni-Fe 80:20) f i l m s i n d i c a t e a c r o s s - t i e t o Bloch w a l l t r a n s i t i o n a t about 9008. F u r t h e r refinement o f t h e domain w a l l model y i e l d s a Neel t o cross- t i e t r a n s i t i o n a t 2008. We have repeated t h e c a l c u l a t i o n f o r a f i l m o f 100% Ni and f i n d a t r a n s i t i o n from c r o s s t i e t o Bloch w a l l a t about 21001(, w h i l e through e x t r a p o l a t i o n o f Hiddelhoek's r e s u l t s f o r t h e energy o f c r o s s - t i e w a l l s a t small thicknesses we f i n d a t r a n s i t i o n from Neel t o c r o s s - t i e w a l l s a t 210A. Unfortu- n a t e l y these t r a n s i t i o n s appear t o go i n t h e opposite d i r e c t i o n w i t h a l l o y i n g com- pared t o t h e p o s i t i o n s o f t h e maxima i n t h e i n s e r t i o n l o s s curves.

It i s seen t h a t a t present, an adequate model f o r e x p l a i n i n g t h e thickness dependence o f SAW a t t e n u a t i o n i n magnetic t h i n f i l m s has y e t t o be developed; as t h i s paper i n d i c a t e s i t would seem t h a t n e i t h e r domain w a l l t r a n s i t i o n s n o r magne- t i z a t i o n r i p p l e (as given by t h e standard t h e o r e t i c a l treatments) p l a y a d i r e c t r o l e i n t h e observed t h i c k n e s s dependence. C u r r e n t l y we a r e modifying o u r experi- mental setup i n o r d e r t o be a b l e t o check t h e hypothesis t h a t f i l m s u b s t r a t e stresses may p r o v i d e t h e dominant mechanism f o r determining t h e e f f e c t . References

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