Magnetic properties of amorphous Fe . . Si compositionally modulated thin films

The B coefficients for glassy ferromagnets are of about 10-K -3/2 and a factor 10 smaner for the crystalline case. The C coefficients are comparable in both cases. 1 The properties of the collective excitations in superlattices have been a subject of increasing interest in recent years. In the case of magnetic superlattices, in which one has a set of magnetic films separated from each other by a nonmagnetic material, the spin waves appearing as T = 0 K in the different magnetic films may couple either through the long-range dipole fields, which accompany a spin wave or through exchange coupling. For separations greater than 20 A the dipolar fields alone allow the spin-wave existence, and, when the magnetic films are close, the exchange interactions existing between spins of different materials explain the spin waves presence. Many theoretical attempts have been recently published with the aim to study the magnetostatic spin-wave modes for ferromagnetic multilayers. 5


INTRODUCTION
For both crystalline and glassy ferromagnets, the magnetization as T = 0 K varies as The B coefficients for glassy ferromagnets are of about 10--4 K -3/2 and a factor 10 smaner for the crystalline case.The C coefficients are comparable in both cases. 1 The properties of the collective excitations in superlattices have been a subject of increasing interest in recent years.In the case of magnetic superlattices, in which one has a set of magnetic films separated from each other by a nonmagnetic material, the spin waves appearing as T = 0 K in the different magnetic films may couple either through the long-range dipole fields, which accompany a spin wave or through exchange coupling. 2For separations greater than 20 A the dipolar fields alone allow the spin-wave existence, and, when the magnetic films are close, the exchange interactions existing between spins of different materials explain the spin waves presence.Many theoretical attempts have been recently published with the aim to study the magnetostatic spin-wave modes for ferromagnetic multilayers. 3

EXPERIMENT
The Fe-Si compositionally modulated thin films were prepared in a special triode sputtering system on glass sub- The amorphous character of the samples was verified by means ofx-ray diffraction and, using low angle x-ray scattering, we tested the modulated structure of the samples.
In Table I we summarize the thickness and composition of the samples.The magnetization measurements were recorded using a SQUID magnetometer in applied magnetic fields up to 5.5 T working in the temperature range between 1.8 and 300 K.

RESULTS AND DISCUSSION
In Figs. 1 and 2 we show the magnetization dependence, at constant temperature T = 4.2 K, on the external field for the easy direction contained in the substrate plane, M II ' and in the perpendicular direction to the substrate, Ml .In both cases the curves are dearly not saturated at external applied fields, H, of 5.5 T. This can be correlated with the existence of paramagnetic iron clusters as has been deduced by means of conversion electron Mossbauer spectroscopy. 6The magnetization values at each H, increase as the characteristic modulation length, A. 6 In fact the M(A) dependence reflects the increase of both the exchange interaction and the Curie temperatures when /t increases.
We have also measured the .f;1(ndependence, in the easy direction of magnetization, for all the samples in the low-temperature regime.In this case we measured the magnetization applying an external field of 5.5 T when the samples were at T = 1.8 K. Then we changed the temperature maintaining constant the applied magnetic field.Our data are shown in Fig. 3.These data can be well fitted by using Eq.M(O) is the extrapolated value of M(T) as T=O K.The values of lJ and C can be determined by plotting(M IM(O) VT 3 !2vs T (Fig. 4) since Eq. ( 2) can be rewritten as (3) The Band C values obtained from our fit procedure are tabulated in Table H.
Our experimental findings can be briefly summarized as follows: (0 The leading ]'3/2 term dominates in the temperature range 0<: T<: 12 K. (ij) The value of both lJ and Care anomalously large implying a much higher density of states at low E in the superlattice.(iii) The spin-wave stiffness constant D-(I/B 2/3) decreases when A increases.
Because of the existence of a zone of interfacial iron atoms with an extension of about 6 A as has been demonstrated by means of electron conversion Mossbauer spectroscopy,6 our superlauice for the samples 3 and 4 can be represented by a set of two magnetic films corresponding to the bulk and interfacial iron atoms, respectively, separated by a nonmagnetic space (Fig. 5), For samples 1 and 2 we can consider that all the magnetic layer is interface because of the interlayer diffusion.
The spin-wave spectra of the different samples should depend on the distances d!. d z , and d J as wen as on the number of layers.The experimental fact that the highest tern-   perature at which the r 3/Z dependence on M( T) does not hold any more is the same for all the samples may suggest: ( 1) that the frequency of the mode is independent of the ratio Cd l + d z )/d 3 which can be interpreted in terms of sur-

FIG. 2 .
FIG. 2. Ml dependence on the external applied field at constant T = 4.2 K for the different samples.
FIG. 3. Fractional change of magnetization vs r 3/2 for the different sam• pIes.
FIG. 5. Sample geometry.One has a stack of two different magnetic films of thickness d, and d 2 separated by a nonmagnetic film of thickness d,.

TABLE I .
Thickness and composition of the different samples.
strates held at room temperature.The thin films are formed by single layers of an amorphous alloy of composition Fe 75 Si 25 with different thicknesses separated by amorphous layers of Si.The total thicknesses of the samples, detennined by using a Tolansky interferometer, are about 1000 A.

TABLE II .
Coefficients Band C for the different samples.