Magnetic and magneto-optical studies in Co–Tb/Pt multilayers

MAGNETIC AND MAGNETO-OPTICAL STUDIES IN Co–Tb/Pt MULTILAYERS H. Hamouda,

H. Lassri,

* R. Krishnan

and D. Saifaoui

a

Laboratoire de Physique The´orique, Universite´ Hassan II, Faculte´ des Sciences, B.P. 5366 Maaˆrif, Ain Chock, Route d’El Jadida km-8, Casablanca, Morocco

b

Laboratoire de Physique des Mate´riaux et de Microe´lectronique, Universite´ Hassan II, Faculte´ des Sciences, B.P.5366 Maaˆrif, Ain Chock, Route d’El Jadida km-8, Casablanca, Morocco

c

Laboratoire de Magne´tisme et d’Optique de l’Universite´ de Versailles, CNRS, URA 1531, 45, Avenue des Etats-Unis, 78035 Versailles cedex, France

(Received 19 July 1997; in revised form 15 April 1998; accepted 3 August 1998 by P. Burlet)

We have prepared Co

Tb

/Pt multilayers by r.f. sputtering and studied their magnetic and magneto-optical properties. As the Co–Tb layer thickness (t

) decreases below 200 A ˚ the saturation magnetization increases, the rectangularity of the M–H loops and the coercivity decrease.

The effective anisotropy K

of the multilayers was determined by a torque magnetometer. The product K

⫻ t

shows a linear dependence with t

as normally found for the superlattices yielding the bulk and surface anisotropies of 1 ⫻ 10

erg cm

and ¹ 0.2 erg cm

, respectively. These results are explained in terms of an interfacial Co–Pt layer. The polar Kerr rotation for the multilayers is found independent of layer thickness and is equal to 11.5 min, which is higher than the single thick layer. 䉷 1998 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION

Amorphous films of transition metal–rare earth alloys such as Co–Tb are of interest from both the fundamental and the application points of view. As disordered systems they offer rich possibilities for exploring some fundamental aspects in magnetism such as anisotropy and spin structure. From the practical point of view it is now well established that these alloy films could be used for magneto-optical information storage [1, 2]. The main characteristics of these films, generally obtained by deposition, are the presence of an uniaxial anisotropy (K

) which makes the film normal the easy axis of magnetization, a rectangular hysteresis loop and coercivities on the order of a few kilooersteds [1–4].

These layers are chemically very reactive and hence have to be protected either by a dielectric or a metallic layer such as Al or Pt. It is interesting to study the effect of Co–Tb/Pt interfaces. In order to amplify interface effects

it is preferable to study Co–Tb/Pt multilayers. We had shown previously in Fe–Tb/Pt multilayers that with a decrease in the Fe–Tb layer thickness, a negative surface anisotropy K

¼ ¹ 0 : 73 erg cm

is observed which arise from the surface atoms and which favours the in-plane easy direction [5]. In this work we describe the results of our studies in Co–Tb/Pt multilayers prepared by r.f. sputtering.

2. EXPERIMENTAL DETAILS

Multilayers of Co–Tb/Pt were prepared by sequential r.f. sputtering. The system was pumped down using a turbomolecular pump to a pressure of 1–2 ⫻ 10

Torr and baked overnight at a temperature of about 343 K.

High purity argon (5N) was used as the sputter gas and its pressure was fixed at 6 ⫻ 10

Torr. The r.f. power was set at 80 W. Water cooled glass substrates were used.

The Co–Tb layer thickness (t

) was varied from 15 to 250 A ˚ and that of Pt was fixed at 20 A˚. The number of bi-layers (n) was in the range 4 to 10. The top layer in all cases was 50 A ˚ of Pt, which also served to protect the 451

Pergamon

PII: S0038–1098(98)00369-X

* Corresponding author.

magnetic layer from oxidation. The thickness of the layers was measured in situ by a quartz oscillator which had been calibrated previously against a ‘‘Talystep’’.

The composition of the magnetic layer was determined by electron probe microanalysis of a single layer of Co–Tb about 1500 A ˚ thick and was found to be Co

Tb

. We chose this composition because the compensation temperature for this well below room temperature and therefore will not interfere with the properties (M, H

) that we are interested. X-ray diffraction studies confirmed the amorphous character of the samples.

As a routine characterization a vibration sample magnetometer (VSM) was used to study the M–H loop along the film normal and the polar Kerr loops (PKR) were taken at the laser wave length of 6328 A ˚ . The effective anisotropy (K

) and also the saturation magnetization (M) were measured at 300 K, using a torque magnetometer. There was a good agreement with M values obtained from VSM.

3. RESULTS AND DISCUSSION

The magnetic properties of the multilayers were found to be very dependent on t

. Figure 1 shows the PKR loops for the single layer sample and four Co–Tb/Pt multilayers with t

¼ 15, 60, 160 and 200 A ˚ , respectively. The hysteresis loops loose their rectangularity as t

decreases; the magnetization was relatively easily saturated in a direction perpendicular to the film plane. In the worst case this required a field of approximately 3.5 kOe. The t

dependence of the coercivity H

is shown in Fig. 2. It is seen that the coercivity decreases strongly with a decrease in t

. Correspondingly, the saturation magnetization also starts increasing with a decrease in the Co–Tb layer thickness.

Figure 3 shows at 300 K the t

dependence of the saturation magnetization (in emu cm

). These results would indicate that the Co sublattice magnetization has increased. A similar conclusion was drawn by Fumiyoshi Kirino et al. [6] who observed that the signe of PKR loop was reversed when 10% Pt was alloyed to the amorphous TbFeCo film, indicating that the transition metals sublattice moment has increased. However no reduction either in remanance or in H

was found. Multilayers are quite different from alloy films. So our present results lead us to conclude that some interdiffusion has occured and Pt has alloyed with Co as to be discussed below. Some alloying of Pt with the magneto- optical (MO) layer is also possible but it seems to be negligible. However Taylor et al. [7] concluded that Tb diffusion in Pt was very much smaller than that of the transition metals which supports this conclusion.

The exchange coupling force between this interfacial Co–Pt layer and the Co–Tb MO layer assists the magnetization reversal in the MO layer at a low external field. It is hard to think that the exchange coupling force from the ultrathin interfacial Co–Pt layer helps the reversal of magnetic moments in the deep inside of the thick MO layer. As a hypothesis, the magnetization reversal at only top surface region of the thick MO layer may take place in the form of nucleations as a first step of domain formation. The interfacial Co–Pt layer may reduce a coercivity of the surface nucleation through the exchange coupling between two magnetic layers.

The saturation magnetization in multilayers can be expressed by the phenomenological model follows:

M ¼ M

þ 2d ⫻ M

= t

(1)

where M, M

and M

are the magnetizations of the

multilayer, the bulk material and interface material,

Fig. 1. The PKR loops for the single layer (a) and

(Co–Tb/Pt)n multilayers with t

(b) 200, (c) 120, (d)

60 and (e) 15 A ˚ (with n ¼ 8) at 300 K.

respectively. d is the interface layer thickness. Plotting M ⫻ t

as a function of t

yields a straight line, whose slope gives M

and the intercept on the ordinate axis gives the product 2d ⫻ M

. For instance Fig. 4 shows such a plot at 300 K. By analysing the data, we calculated both M

and the product 2d ⫻ M

. We find that M

¼ 240 emu cm

in agreement with the value obtained on the single-layer thick film. Of course, M

can be determined only if d is known. By the way it is also known that some transition metals induce a ferro- magnetic moment on Pt as has been observed for the

Co/Pt multilayers [8]. This also could contribute to the magnetic moment.

Let us now discuss the anisotropy in these multi- layers. The measured effective anisotropy K

, based on the well experimented phenomenological model, can be expressed as

K

¼ K

þ 2K

= t

; (2) where, K

and K

are the volume and surface anisotropies.

The former can be written (for the case where the film normal is the easy axis of magnetization) under the usual convention, as

K

¼ K

¹ 2pM

(3)

where K

is the intrinsic uniaxial anisotropy.

In our case eventhough we have an amorphous film, there is a strong uniaxial anisotropy K

induced by the sputtering process. This involves a strong dependence of the anisotropy on the deposition parameters controlling the structure of the film and of the average anisotropy contribution of the rare earth and transition metal atoms.

Different origins have been discussed to account for the observed uniaxial anisotropy such as anisotropic micro- structures [9], dipolar interactions [10], stress-induced anisotropies [11, 12], pair ordering [13], anisotropic exchange [14], bond-orientational anisotropies [15] and single-ion anisotropy [16–19]. However, the correlation between deposition parameters and film structure on the one hand and the magnetic properties on the other hand is still not yet explained satisfactorily [20].

Figure 5 shows the variation of the product K

⫻ t

as a function of t

. A linear decrease Fig. 2. The t

dependence of H

(with n ¼ 8) at 300 K.

SL indicates single layer sample.

Fig. 3. The t

dependence of M at 300 K.

Fig. 4. The t

dependence of the product M ⫻ t

at

300 K.

with t

is found as predicted by the model. For the thickness below 60 A ˚ , the experimental data deviate from the fitted line obtained. Normally in metallic super- lattices, for magnetic layers thicker than 15 A ˚ or so, K

is negative due to a large demagnetization energy. But in some systems [21], for layers thinner than about 10 A ˚ , it could, under the effect of the surface anisotropy change sign become positive. In our present case, thicker Co–Tb layers show a positive K

and it decreases with the decrease in t

. The extrapolation of the straight line yields a negative (in-plane) surface anisotropy K

¼ ¹0 : 2 erg cm

. This value is much smaller than what had been observed by us in Fe–Tb/Pt [5]. From the model, the slope of the straight line in Fig. 4 gives the volume anisotropy K

which is found to be 1 ⫻ 10

erg cm

. Knowing M

¼ 240 emu cm

, we calculate the demagnetizing energy to be

¹0 : 36 ⫻ 10

erg cm

. We hence find that the uniaxial anisotropy K

¼ 1 : 36 ⫻ 10

erg cm

. It is interesting to note that this value agrees with the value of 1 : 6 ⫻ 10

erg cm

that we measured on the single thick layer of Co–Tb and the values we had obtained in the case of Tb–Fe films [4].

In general we assume that the surface anisotropy energy constant K

could be treated as originating from several effects which alter the surface spins at the interfaces such as misfit strain anisotropy [22], surface roughness [23, 24], Ne´el’s anisotropy [25] and from the magnetic polarization of interfacial Pt atoms [26]. When there is some interdiffusion between the magneto-optical layer and the Pt layer, roughness effects may greatly alter the magnetic surface anisotropy.

Figure 6 shows the variation of the specific Faraday rotation (SFR) in Co–Tb/Pt multilayer with t

for

t

¼ 20 A ˚ . It is seen that the FR increases when t

decreases. This result could be explained as follows.

The SFR has been calculated with respect to total Co–Tb layer thickness. However interdiffusion occurs which leads to the mixing between Co and Pt and to the formation of an alloy layer which could be magnetic. So this alloy layer also would contribute to the FR measured.

Therefore in order to calculate the SFR the thickness of these interfacial alloys has to be taken into account too.

So the observed FR can be expressed as:

v

⫻ t

¼ v

⫻ ðt

¹ 2dÞ þ v

⫻ 2d (4) where v

is the measured FR; v

and v

are the SFRs of the Co–Tb and the interfacial layer, respectively. The v

and the product v

⫻ 2d are calculated by the fit of our result (Fig. 6) to be 3 : 1 ⫻ 10

deg cm

and 15 ⫻ 10

deg, respectively. The SFR of the Co–Tb layer is found to be in agreement with the value obtained on the single-layer thick film.

Finally the polar Kerr rotation v

for the multilayers is found to be almost the same and is equal to 11.5 min for all t

. However PKR in the single layer sample is lower and 9.3 min. The increase in PKR for the multi- layers is mainly due to the effects of multiple reflection.

This increase is of interest for application in the informa- tion storage. The study of the spectral dependence of the PKR spectra could perhaps reveal interesting features arising from the interfacial Co–Pt layer.

4. CONCLUSION

In conclusion, we have prepared and studied both single thick layer of Co–Tb and Co–Tb/Pt multilayer Fig. 5. The variation of the product of K

⫻ t

as a

function of t

(with n ¼ 8) at 300 K.

Fig. 6. The t

dependence of the specific Faraday

rotation v

at 300 K.

films. The product K

⫻ t

shows a linear dependence with t

as normally found for the superlattices yielding the bulk anisotropy of 1 ⫻ 10

erg cm

. The increase of the FR with t

has been attributed to the presence of the interfacial alloy formed due to mixing. The polar Kerr rotation v

for the multilayers is found to be 11.5 min and is independent of t

.

Acknowledgements—We wish to thank Mr. Tessier for assistance in samples preparation and Mr. Porte for the careful measurements of the anisotropy.

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