MAGNETIC AND MICROWAVE PROPERTIES OF Fe/Ag(001) SUPERLATTICES CONTAINING ULTRATHIN Fe LAYERS

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MAGNETIC AND MICROWAVE PROPERTIES OF

Fe/Ag(001) SUPERLATTICES CONTAINING

ULTRATHIN Fe LAYERS

J. Krebs, B. Jonker, G. Prinz

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplt5ment au no 12, Tome 49, dkcembre 1988

MAGNETIC AND MICROWAVE PROPERTIES OF Fe/Ag(001) SUPERLATTICES

CONTAINING ULTRATHIN Fe LAYERS

J. J. Krebs, B. T. Jonker and G. A. Prinz

Naval Research Laboratory, Washington, D C 20375, U.S.A.

Abstract. - Magnetization and X-band magnetic resonance studies were carried out on Fe/Ag(001) superlattice samples with Fe layers only a few monolayers (ML) thick. The magnetization data show that for Fe layers

<

2.5 ML thick their low temperature moments are normal to the film. The in-plane and perpendicular magnetic resonance fields both decrease strongly at low temperatures.

The ability to grow good quality monolayer (ML) epitaxial films of Fe on single crystal Ag(001) [I] has led to extensive experimental [2-61 and theoretical [7-81 investigations aimed at elucidating the ultrathin mag- netic film regime in which surface and interface effects become dominant in governing the magnetic behav- ior. In this paper, we discuss our recent work on the magnetic and microwave resonance properties of Fe/Ag(001) superlattices containing Fe layers only a few ML thick.

The Mossbauer data on the same samples [4] showed that for samples containing Fe layers less than 3 ML thick: 1) the Fe moments were predominantly perpen- dicular to the film a t low temperatures, 2) there was no net Fe hyperfine field at room temperature, and 3) the linewidths were dependent on both temperature and the Fe-layer superlattice thickness, suggesting rapid

(<

s) relaxation effects even for T

<

100 K. By contrast, the 5.5 Fe ML superlattice showed a well re- solved hyperfine-split spectrum at room temperature with clear evidence that the magnetization was in the plane of the film.

The ~e~~

/

Ag(001) superlattice samples were grown by molecular beam epitaxy on single crystal Ag(001) base layers which were in turn grown on GaAs(001) substrates. Details of the growth methods and the in

situ Auger and RHEED characterization were given earlier [4, 61. The principal samples studied had Fe layers 0.9, 1.8, 2.4, or 5.5 ML thick. Because of space limitations only the 2.4 Fe ML sample which consisted of 20 perods of 2.4 ML of Fe and 5.6 ML of Ag, is treated in detail here, although the 0.9 ML and 1.8 ML of Fe samples behave similarly.

Suitable parts of the sample were used for the SQUID magnetometer (0-40 kOe, 10-300 K) measure- ments and the 9.3 GHz FMR studies (4-300 K). In both cases data were taken with the mabetic field H applied in-plane (11) or perpendicular (1) to the film plane and the sample was cooled by a stream of He gas. For the FMR, the microwave field was always in the plane of the film.

Fig. 1. - Temperature dependence of the magnetization of a Fe/Ag superlattice sample with Fe layers 2.4 ML thick. The magnetic fields shown were normal to the film.

The net superlattice magnetization data for the 2.4 Fe ML sample are given in figure 1 for several selected fields perpendicular to the film. Note that 1) there is a strong temperature dependence to the 30 kOe saturation magnetization M,, 2) the low tem-

perature M, value is close to that of bulk Fe, and 3) M,

shows a linear rather than a quadratic temperature de- pendence at low temperatures. All of these properties are consistent with two-dimensional magnetic behav- ior. Also in figure 1, one sees a large low-temperature, low-field permeability. In fact, it is easier to satu- rate the sample with H applied perpendicular than in-plane. Finally, there is a remanent magnetic mo- ment along the film normal .for the 2.4 ML superlat- tice, but the 5.5 Fe ML sample clearly has its easy magnetization axis in-plane.

All of the above results are in agreement with the Mossbauer conclusions. Note that the remanent mag- netization is only a small fraction of M, and there is

C8 - 1708 JOURNAL DE PHYSIQUE

Fig. 2. - Temperature dependence of the in-plane ( o ) and

perpendicular ( x ) magnetic resonance fields at 9.3 GHz. Horizontal line shows g = 2.

negligible remanence above 50 K. The zero-field Moss- bauer data indicate a clear-cut local moment normal to the film for T

<

100 K. These facts suggest [6] that in zero field, the sample consists of many up and down domains with M perpendicular to the film. Recently, Yafet [9] has shown that such a domain configuration is in fact the lowest energy state of an ultrathin ferro- magnetic film when surface anisotropy has the correct sign and magnitude t o overwhelm the demagnetization energy.

Turning to the FMR results, the very unusual tem- perature dependence of the magnetic resonance fields H,,, for the 2.4 Fe ML sample is shown in figure 2 for both the

11

and

I

resonance cases. The arrows along the baseline in figure 2 indicate the temperatures below which there is a Mossbauer hyperfine splitting (Tm) or a constant linewidth (T,)

.

For T

>

Tm, although there is no hyperfine field and hence no time-averaged mag- netic moment in zero field, at H,,, there is an induced moment (Fig. 1). For such a magnetic film, the reso- nance equations can be approximated by

where MII ( M l ) is the magnetization induced by

H ~ L ,

(HA,)

and Ho = w

/

7. These equations are valid if 27rM <C Ho and demagnetizing effects are the only source of anisotropy. While equation (1) has the proper qualitative behavior above T,, the deduced val- ues of MI, and ML are much smaller than expected from figure 1. For T

<

Tm,

H ~ L ,

and

HA,

both begin to decrease with decreasing temperature, falling rapidly at the lowest temperatures. Simultaneously, the FMR linewidths increase significantly. The temperature at

which this behavior occurs is found to decrease with decreasing Fe-layer thickness.

As yet, we have no satisfactory explanation for this puzzling phenomenon below T,. We note, however, that very similar behavior was found in the magnetic resonance spectra of films of the concentrated spin- glass material Gd0.37A10.63 [lo]. In that case, the shifts were interpreted in terms of a temperature-dependent effective anisotropy field arising from local demagne- tizing effects in a magnetically inhomogeneous system. The downward shift of the resonance lines occurred just below the paramagnetic Curie temperature. It is difficult to see how the type of inhomogeneity as- sumed in Gd-A1 (randomly-shaped ferromagnetic clus- ters imbedded in a weakly ferromagnetic sea) can be applied to the present case where we have well-formed ultrathin Fe layers.

In order to clarify this situation, we are planning additional experiments at high microwave frequencies to probe the nature of the "effective anisotropy" in these superlattice samples.

Acknowledgments

This work was supported by the Office of Naval Re- search. We wish to thank F. A. Volkening for useful discussions.

[I] Smith, G. C., Padmore, G. A. and Norris, C., Surf. Sci. 119 (1982) L287.

[2] Jonker, B. T., Walker, K. H., Kisker, E., Prinz, G. A. and Carbone, C . , Phys. Rev. Lett. 57 (1986) 142.

[3] Heinrich, B., Urquhart, K. B., Arrott, A. S.,

Cochran, J. F., Myrtle, K. and Purcell, T. S., Phys. Rev. Lett. 59 (1987) 1756.

[4] Koon; N. C., Jonker, B. T., Volkening, F. A., Krebs, J . J. and Prinz, G. A., Phys. Rev. Lett. 59 (1987) 2463;

Volkening, F. A. et al., J. A p p l . Phys 63 (1988) 3869.

[5] Stampanoni, M., Vaterlans, A., Aeschlimann, M. and Meier, F., Phys. Rev. Lett. 59 (1987) 2483. [6] Krebs, J. J., Jonker, B. T. and Prinz, G. A., J.

A p p l . Phys. 63 (1988) 3467.

[7] Fu, C. L., Freeman, A. J . and Oguchi, T., Phys. Rev. Lett. 54 (1985) 2700.

[8] Gay, J . G. and Richter, R., Phys. Rev. Lett. 56 (1986) 2728; J. A p p l . Phys. 61 (1987) 3362. [9] Yafet, Y., private communication.