INTERFACIAL COUPLING BETWEEN A MAGNETIC THIN FILM AND A NORMAL METAL

(1)

HAL Id: jpa-00229031

https://hal.archives-ouvertes.fr/jpa-00229031

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

INTERFACIAL COUPLING BETWEEN A

MAGNETIC THIN FILM AND A NORMAL METAL

H. Hurdequint, G. Dunifer

To cite this version:

(2)

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, decembre 1988

INTERFACIAL COUPLING BETWEEN A MAGNETIC THIN FILM AND A

NORMAL METAL

H. Hurdequint ( I ) and G. Dunifer (2)

( I ) Laboratoire de Physique des Solides, Universite' Paris Sud, 91405 Orsay, France

(2) Physics Dept., Wayne State Univ., Detroit, M I 48202, U.S.A.

Abstract.

-

FMR results, at E band and X band, were obtained for an iron layer (sandviriched by Ag and deposited on mica and bulk silver substrates), TESR measurements being also performed for the silver sample. Various effects induced by the coupling at the (Fe/Ag) interface are evidenced in the spectra and are to be dominantly associated to the diffusion of the conduction electron magnetization.

We have undertaken [I] a systematic study of the magnetic interfacial coupling between a metallic ferro- magnetic film and a normal metal in a layered struc- ture sample geometry. Specifically the present work is concerned with the study of crystalline iron films in contact with pure silver. In order to investigate such proximity effects induced by the interfacial coupling, we carried out a series of ESR measurements looking at the effects induced by this coupling, both on the FMR resonance spectrum of the magnetic film and on the conduction electron spin resonance (C.E.S.R.) spectrum of the pure metal substrate (through trans- mission experiments: T.E.S.R.) for the samples corresponding to a bulk silver substrate.

All our films are constituted of (Aglmagnetic film/Ag) sandwiches deposited on different substrates with a base Ag layer 400

a

thick and a top Ag layer 200

A

thick. The films are deposited by sputtering on heated substrates

(T.

x 160 OC)

.

The conditions of deposition used enable one to get on the cleaved mica substrates a polycrystalline Ag base layer with crystal- lites of large lateral size and a preferred (111) orienta- tion perpendicular t o the film. On these (111)-face Ag crystals, (110)-face a-Fe crystals are expected [2] to grow subsequently. The two samples whose results are presented here consist of films deposited (same sput- tering run) on a mica substrate and on a very pure polycrystalline silver foil (25 pm thick), previously studied [3] by T.E.S.R. a t X band, and correspond to a single "thick" layer of Fe (x 650

A).

These two samples will be called, hereafter, the mica sample and the silver sample.

Let us first discuss the results obtained in reflection experiments performed at E band ( x 79 GHz), in the perpendicular geometry. On the mica sample

two well-resolved resonance modes are observed: the principal mode and a lower-field mode corresponding to a volume standing spin wave. On the silver sample the resonance spectrum is composed of three lines the more-intense central peak corresponds to the principal mode, the lower-field one to a volume spin wave and the higher-field one may be identified as a surface

mode, as described theoretically by Puszkarski [4]. Fig-

ure 1 displays the variations with temperature of the main characteristics of these resonance spectra: (a)

= a

_-

_I

. o s ! I silver sample' 135 B s g

'

2

f mica sample

-

.

. . .

.

wlume spin wave

\

.

6 .

.

. .

+1

Fig. 1. - Variations with temperature, at E band, in the perpendicular geometry, of the FMR spectrum: (a) Hres of the different modes observed and (b) the linewidth AH of the principal mode in these two samples.

the field for resonance Hres of the different modes observed for the mica and the silver samples and (b) the linewidth A H of the principal mode in these two samples. The observed field position of these modes may be discussed in terms of the following expression for the resonance condition:

w 2A

-

= (H

+

HA

-

~ Z M )

+

-lc2

Y M (1)

where, for the frequency w used and with g = 2.090 for pure iron, w

/

7~~ = 26.99 kOe. M designates the satu- rated equilibrium magnetization, A the exchange stiff- ness constant and HA a uniaxial anisotropy field (along

fi

normal to the film plane). For the volume spin waves the condition on the wave vector k for standing waves reads: k = n r l L , where L is the film thickness. If we take for our "thick" iron film (L

=

650

A)

standard values for the Fe parameters: 47rM =

21.55 kOe and A = 2 x erg/cm, the folloying information can be drawn from the comparison of our results to equation (1):

i) from the position of the principal uniform mode there is evidence of the presence of a rather strong

(3)

C8

-

1718 JOURNAL DE PHYSIQUE

uniaxial anisotropy field whose value, in the high temperature range (150-220 K), is HA x 3.645 kOe for the mica sample and a larger value HA M 8.436 kOe for

the silver sample, which shifts the line to lower field; ii) for the mica sample, the field splitting (R 2 k0e) of the observed volume spin wave corresponds closely t o the one expected for the n = 2 order spin wave;

iii) for the surface mode (k = ik.) observed in the silver sample, the field splitting with respect to the uniform mode is described by:

2A H:::f

-

Ho = - - k c

M (2)

If the presence of such a mode were to be inter- preted solely in terms of a su~face anisotropy (E,,,f =

-K, cos2 0, with 0 = (N, M)

,

taking [5] thus k,

=

-Ks

/

A, one would deduce at T = 218 K a value K, = 1.28 erg/cm2 characterizing the (Ag/Fer interface. Moreover the observed temperature variation of this field splitting would lead to a monotonic strength- ening of

K,

with decreasing temperature.

On the same two samples we performed reflection experiments at X band (9.2 GHz) and figure 2 displays the variation with temperature of the characteristics of the FMR principal mode observed in the parallel geometry: (a) the field for resonance Hres and (b) the linewidth AH. If we compare first the results obtained for HIes, in the range (160 K-270 K), to the condition for resonance:

we see, using equation (3) and w

/

y ~ , = 3.123 kOe, that the value of Hres expected from the value ofHreS

$ 1

o 1

1.

silver sample 2 - .

r

L

-

--

= 0 Temperature Tikl 50 100 150 200 250

Fig. 2. - Variations with temperature, at X band, in the parallel geometry, of the FMR resonance parameters: (a) HreS and (b) the linewidth AH of the principal mode.

expected from HA (whose value was deduced above from the E band data) is 529 Oe for the mica sample and 706 Oe for the silver one, i.e. slightly lower but close to the observed values 562 Oe and 750 Oe, respectively. This finding indicates that, at high tem- peratures, the position of the principal mode can be accounted for, in a consistent way, by essentially the presence of a strong uniaxial perpendicular anisotropy. Below typically T

-

150 K, for both samples, Hres monotonically shifts to lower field, this behavior being very pronounced for the silver sample and concomitant with a large linewidth increase.

Finally, the T.E.S.R. measurements performed at E band on the silver sample which constitutes a bimetallic system give the following results for the C.E.S.R. observed at low temperature (4 K-45 K): the line po- sition corresponds to a factor g = 1.986 f 0.002 (very close to the pure silver one), the line is very much broadened ( A H x 500 Oe at T = 28 K) compared to pure silver and its intensity is enhanced compared to the C.E.S.R. of the silver substrate alone. All these features are qualitatively the same as those observed previously [6] through T.E.S.R. measurements

in bimetallic samples composed of a normal metal substrate coated by a ferromagnetic film. They imply an interfacial coupling mechanism due t o the diffusion of

the microwave induced magnetization of the conduc- t i o n electrons from one metal t o the other and can be

accounted for within a phenomenological model which includes the appropriate boundary conditions [3, 61, at the interface, for the thermodynamic "magnetization potential" and magnetization current of the conduction electrons.

To conclude, the FMR results of the Fe layer presented above show effects induced by the interfacial coupling (at the Fe/Ag interface) on the features of the resonance spectrum as clearly evidenced in the observed behavior versus temperature. A detailed understanding of all the observed effects calls for a more complete theoretical description of the FMR in a metallic ferromagnet and we are presently working on such a description.

This work was supported in part by grants from Nato N.86/0775 and N.S.F. Grant DMR 8414488.

[I] Hurdequint, H. and Dunifer, G., presented at Conf. M.M.M., Chicago (nov. 9-12 1987). [2] Krebs, J., Vittoria, C., Jonker, B. and Prinz, G.,

J. Magn. Magn. Mater. 54-57 (1986) 811.

[3] Hurdequint, H., These d'Etat, Universite Paris Sud, Orsay (1981).

[4] Puszkarski, H., Pmgr. Surf. 9 (1979) 191. [5] Maksymowicz, L. J. and Sendorek, D., J. Magn.

Magn. Mat. 37 (1983) 177.

[6] Silsbee, R. H., Janossy, A. and Monod, P., Phys.