MAGNETIC PROPERTIES OF Fe LAYERS AT Cu AND Ag INTERFACES

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MAGNETIC PROPERTIES OF Fe LAYERS AT Cu AND Ag INTERFACES

W. Keune, J. Lauer, U. Gonser, D. Williamson

To cite this version:

W. Keune, J. Lauer, U. Gonser, D. Williamson. MAGNETIC PROPERTIES OF Fe LAYERS AT Cu AND Ag INTERFACES. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-69-C2-71.

�10.1051/jphyscol:1979223�. �jpa-00218611�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3, Tome 40, mars 1979, page C2-69

MAGNETIC PROPERTIES OF Fe

LAYERS

AT Cu

AND Ag

INTERFACES

* *

W. Keune

,

^J.^Lauer

,

U. ~onser** and D.L. williarnsons*'

*

Laboratorim fiir Angewandte Physik, GesamthochschuZe, 0-4200 Duisburg, Germany

**~achbereich Angewandte Physik, Universitdt des SaarZandes, 0-6600 Saarbriicken, German&

LLI Physics Department, CoZorado Schoo Z of Mines, Golden, CoZorado 80401, U. S. A.

R6suml.- L'effet Mzssbauer appliqub aux couches minces de 5 7 ~ e d'lpaisseurs variabges e$ ddposdes sur Cu st Ag polycristallin conduit 1 des spectres magnltiques bien rdsolus lorsque d > 30 A. Quand d C o 18 A, un comportement superparamagn6tique est observ6. Les spectres des couches minces avec d 7 10 A 1 4 , 2 K indiquent une distribution des champs internes ayant une composante bas-champs et une autre hauts-champs ferromagnbtique. On observe dgalement dans ces couches une anisotropie magnltique per- pendiculaire P leur plan.

Abstract.- The Mgssbauer effect on very thin 5 7 ~ e films of various thicknesses d vacuum deposited on polycrysta1;ine Cu and Ag substrates shows a well-resolved magnetic hyperfine pattern for d 5 30

A.

For d 7 18 A superparamagnetism was observed. The spectra at 4.2 K of films with d 7 10

1

indicate a distribution of hyperfine fields with a low-field component and a large-field ferrowgnetic component. A perpendicular magnetic anisotropy has been observed for these ultra-thin films.

The Mzssbauer effect offers a unique possibility to study microscopically via the hyperfine field (h.f.) the magnetic properties of Fe atoms located at a solid interface /I/. The present paper reports some of our Mzssbauer results on very thin 5 7 ~ e films embedded between Cu or Ag layers.

5 7 ~ e films with average thickness dof 30,18, 10, 5 and 2 have been vacuum deposited between 300

1

thick polycrystalline layers of Cu or Ag and used as Mgssbauer absorbers. Our multi-layer samples (with about 15 5 7 ~ e layers) were prepared by alternately evaporating 5 7 ~ e and Cu (or Ag) onto a room-temperature (R.T.) A1 foil in an oil-free UHV system with

%lo-' torr during evaporation (base pressure %5 x 10-'"0rr). Thickness and evaporation rates (%0.5 A/s) were monitored with a quartz crystal oscillator. The Mzssbauer y-ray direction was normal to the film plane.

Results obtained for Fe films on Cu will be discussed first. At R.T. and below (Fig. 1 a) our 30

1

thick Fe films showed a well-resolved magnetic hiperfine pattern typical for ferromagnetic a-Fe.

We conclude that these films were fairly continuous and homogeneous'. The h

.

^f

.

value at R.T. was only 315

+

3 kOe, i.e. smaller than for bulk a-Fe at R.T., while at 4.2 K the h.f. of 30 Fe on Cu and that of bulk a-Fe were found to be equal. This can be ex- plained by a reduction in the Curie temperature for this film. A small positive isomer shift of +0.03 2

0.01 mm/s (relative to bulk a-Fe at R.T., as all the isomer shift values given in this paper) indica-

tes a slight decrease in s-electron density at theFe nucleus compared to bulk a-Fe.

-L -2 VELOCITY

Fig. 1 : Low-temperature Mgssbauer spectra of thin S 7 ~ e films of various average thicknesses d on Cu substrate.

A weak asyrmnetric broadening of the outer lines in figure la is possibly due to different hyperfine fields at Fe atoms located at the Fe

-

Cu interface

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979223

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c2-70 JOURNAL DE PHYSIQUE

compared to those near the film interior. For 30

1

Fe films the magnetization direction is within the film plane, since the line intensity ratio is about 3 : 4: 1. No paramagnetic absorption line due to ma- gnetically "dead" Fe interface atoms could be detec- ted in figure la.

For Fe films with d ? 18 our spectra indicated typical superparamagnetic behavior which mani- fests the island structure of such ultra-thin films.

At R.T. a central paramagnetic doublet was observed in the spectra, with line positions at

-

0.09 4 0.01 and + 0.23 4 0.03 m / s , respectively. The former value is typical for fcc (y-) Fe at R.T. and leads us to conclude that for part of the Fe atoms in films with d ? 18

1

a structural phase change from bcc to

fcc Fe has occured.

y-Fe could be indentified also in the 4.2 K spectrum of 18 Fe on Cu (Fig. lb), since a computer analysis of this spectrum resulted in two magnetic hyperfine patterns and 2 central, very broad single line with an isomer shift of + 0.033

+

^0.016

mm/s typical for y-Fe at 4.2 K 1 2 1 . The possibility of antiferromagnetic y-Fe thin film formation on (001) Cu has been reported earlier /3/. The two h.f.

values obtained from figure lb are 346

+

5 kOe (being identical within error limits to bulk a-Fe at 4.2 K) and 306

+

8 kOe. We attribute this latter reduced h.f. to the relatively large fraction of Fe atoms located at the Fe

-

Cu interface.

For average Fe thicknesses d < 18

1

the low-temperature spectra change considerably (Fig.

1 c-e). The following observations can be made : (i) tt.e h.f. characteristic of bulk a-Fe has disap- peared. The average h.f. value H obtained from the splitting of the outer broadened lines in figure I c-e is about 290 kOe, i.e. 15 % smaller than for bulk a-Fe at 4.2 K. This smaller h.f. value is characteristic for CL-Fe surface atoms being in close vicinity of Cu atoms of the substrate and covering

layer /4/. This Fe

-

Cu interaction near the interface also results in the observed large line broadening (which indicates a distribution of hyperfine fields) and in a slight positive isomer shift of about + 0.1 to

*

0.2 muds (relative to bulk a-Fe at 4.2 K);' (ii) the central y-Fe line originally present in figure lb shows an increasing splitting with decreasing d (Fig. lc

-

e). The reason is not yet understood. Application of an external field of 50 kG and a h.f, distribution a%alysis of the measured spectra has shown that this central splitting must be of magnetic origin, corresponding to an average h.f. for this low-field spectral component of

'L70 kOe / 5 / . The external field experiment has revea- led also that the large-field component (with %

290 kOe) is clearly ferromagnetic; (iii) in order to obtain a good fit in the h.f. distribution analysis

(full curve in figure Id and e) a line intensity ratio in the basic six-line patterns of about 3 : 0.8 :

1 had to be assumed for the ferromagnetic large- field component, indicating that the magnetization for this phase is preferentially oriented perpendi-

cular

to the plane of 5 Fe and 2 Fe films on Cu. Measurements of H for a 5 f; thick Fe film on Cu at 4.2 K as a function of a transverse external field gave an unusually large value of 12 +_ 3 kG for the critical anisotropy field H necessary to rotate

A the magnetization into the film plane.

The temperature dependence of the spectra of a 5 thick Fe film on Ag also reveals superparamagnetic behavior (Fig. 2).

I

-6 -L -2 0 2 L 6

VELOCITY m m / s

Fig. 2 : Mzssbauer spectra of 5 7 ~ e film of average thickness d = 5 i( on Ag substrate measured at the indicated temperatures.

The observed doublet at 295 K has line positions at

-

^0.017²0.005 mm/s and + 0.394

*

0.005 mm/s (relative to bulk a-Fe at R.T.). Similar to the case of Cu a large h.f. component and a central split component was observed in the spectrum for this film at 4.2 K. A premiminary computer analysis of the outer part of the spectrum in figure 2e resulted in

two

large hyperfi--e fields, i.e. two ferromagnetic Fe states occur with H I s 340 kOe (corresponding to

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bulk a-Fe at 4.2 K), and a 5.3% larger value of H2 % 358 kOe, respectively. Slightly larger hyper-

fine fields (as compared to bulk a-Fe at 4.2 K) have been reported also for thin Fe films of thickness

d > 20 on Ag substrates /6/. The line intensity

ratio of the two six-line patterns in figure 2e is about 3 : 0.45 : 1, indicating a preferred perpendicular magnetic anisotropy also for our ultra-thin Fe film on Ag.

Acknowledgements.- Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

References

/I/ Shinjo, T., Trans. IEEE Mag.

12

(1976) 86 and references quoted therein.

/2/ Williamson, D.L., and Keune, W., Bull. Am. Phys.

SOC.

2

(1974) 1119.

/ 3 / Keune, W., Halbauer, R., Gonser, U., Lauer, J .

and Williamson, D.L., J. Appl. Phys.

9

(1977) 2976); J . Magn. Magn. Mater.

5

(1977) 192.

141 Lauer, J., Keune, W. and Shinjo, T., Physica 86-88B ( 1 977) 1409.

/5/ Keune, W., Williamson, D.L. and Lauer, J., in Proc. Intern. Conf. on Mijssbauer Spectroscopy, A.Z. Hrynkiewicz and J.A. Sawicki eds., (Akade- mia Gorniczo-Hutnicza Im. S. Staszica, Cracow, Poland) 1975, p. 39.

1 6 1 Duncan, S., Semper, R., Chien, C.L. and Ualker,

J.C., "Proc. Intern. Conf. on Mgssbauer Spectros- copy", D. Barb and D. Tarina eds., (Documentation Office, Central Institute of Physics, Bucharest, Romania) 1977, p. 109.