Solid State Communications, Vol. 84, No. 4, pp. 413-415, 1992.
Printed in Great Britain.
0038-1098/92$5.00+.00 Pergamon Press Ltd
Magnetic and Magneto-optical Kerr studies in Compositionally Modulated Fe/Bi Films R. Krishnan* and H. Lassri
Laboratoire de Magnetisme et MatrZriaux Magnttiques, C.N.R.S. 92195 Meudon, France (Received 20 may 1992 by P. Burlet)
ABSTRACT
Compositionally modulated Fe/Bi films have been prepared by evaporation in ultra high vacuum and their magnetic and magneto optic properties studied. Magnetization data indicates that the Fe dead layer is about 0.5 nm thick. There is evidence for oscillatory coupling between Fe layers when Bi layer thickness is varied. An enhancement in the Kerr rotation is observed for thin Bi layer separation and attains a value of 0.32 deg for t(Bi) = 1 nm.
The field of multilayers is ever active and an intense search is being made for new materials. In such an endeavour we have prepared compositionally modulated (CM) Fe/Bi films and studied their properties. Bismuth has been chosen for the following reasons: it is diamagnetic and due to strong spin-orbit coupling in the excited state it can modify the magneto-optical rotation. For example the enhancement of magnetooptical rotation, due to addition of Bi, in garnets 1 and later in amorphous rare earth transition metal alloy films2 has been known and well documented.
Besides, bismuth and iron metals are known to be practically insoluble 3. So we wanted to study the properties of the multilayer system based on Bi.
The CM films were prepared by evaporation using two e-guns fitted in a ultra high vacuum chamber with a base pressure on the order of 1.10-10 torr. The pressure during the film deposition was in the range 5-8. 1O-9 torr. The rate of evaporation ( -0.05 nm.s-1 ) and the layer thickness were controlled individually by quartz monitors which were calibrated, for Fe by using the magnetization value of a single layer Fe about 30 nm thick and for Bi by measuring with a profilometer. The thickness values reported here are accurate to about f 5 %. The CM films were grown on a Bi buffer layer 10 nm thick on water cooled glass and silicon substrates, Two sets of samples were made where in one, the thickness of the Bi layer t(Bi) was fixed at 2 nm and that of Fe t(Fe) was varied from 1 to 4 nm and the in the other, t(Fe) was fixed at 3.4 nm and t(Bi) varied from 1 to 5 nm . This second set was to check the possible influence of t(Bi) on the interlayer coupling and also on the magneto optical rotation.
Low angle x-ray diffraction studies were made to check the periodic structure. Magnetization (M) and in-plane M- H loops were measured using a vibration sample magneto meter (VSM). The magneto-optical polar Kerr rotation (PKR) were measured at the laser wavelength of 623 nm using an experimental set up capable of measuring a rotation of the order of 10-4 deg.
In all we prepared twenty samples. Three of them did not show the metallic lustre and had a matty surface. The reason for this is not clear and we negelcted these samples.
Low angle x-ray diffraction indicated broad peak of low intensity indicating that the interfaces are not sharp. It is quite possible that there is some diffusion of Bi into Fe layer because it has a fairly low melting point.
* To whom all correspondance should be addressed
The in-plane loops were generally rectangular though the remanence ratio R = M,/Ms and the saturation fields showed large depepndences on the layer thickness of both Fe and Bi. However it is more interesting to examine the evolution of these loops as a function of t(Bi) for a given t(Fe). Fig. 1 shows a typical example when t(Fe) is fixed at 3.4 nm and t(Bi) = 1 and 2 nm. It is seen that for t(Bi) = 1 nm, the field for saturation is lower and R is higher as compared to the sample with t(Bi) = 2 run. The difficuly to saturate would indicate that the coupling between Fe layers is antiferomagnetic like. This evolution is better illustrated in Fig. 2 where we have plotted the variation of the field for
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1. The in-plane M-H loops for samples with fixed t(Fe) of 3.4 nm and (a) t(Bi) = 1 and (b) 2 nm respectively.
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2. The oscillatory nature in the t(Bi) dependences of the saturation field Hs and the remanance ratio R for the samples with t(Fe) = 3.4 nm. Notice the large period of 2 nm.
saturation Hs and the remanence ratio R as a function of t(Bi). It is seen that Hs sharply increases from 1 kOe to 2.3 kOe when t(Bi) increases from 1 to 2 nm and in this same range of t(Bi), Ralso decreases from 0.8 to 0.5. This oscillation continues with some damping as t(Bi) is increased. This in our opinion is clear evidence that as t(Bi) increases the coupling between the adjacent Fe layers changes from ferromagnetic to antiferromagnetic like.
Similar result has been observed for several multilayers which is attributed to several much debated mechansims such as, RKKY-type (Ruderman-Kittel-Kasuya-Yoshida) interaction, spin -pump effect etc., and is an active field of research 4-g. However the period in our case is about 2 nm which is twice that normally found in other cases, Cu for example 6. Similar large period was recently reported for Fe/AI/Fe and Fe/Au/Fe superlattices 9.The reason for this is not clear though the period should depend on the electronic structure of Bi.
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