The temperature dependence of the magnetization of magnetic multilayers

Journal of Magnetism and Magnetic Materials 93 ( 1991 ) 105-108 North-Holland

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The temperature dependence of the magnetization of magnetic multilayers P.J.H. B l o e m e n a, W.J.M. de J o n g & and F.J.A. d e n B r o e d e r b ~Department of Physics, EindhoL'en Unirersity of Technology, 5600 MB Eindhocen, The Netherlands t~Philips Research Laboratories, 5600 JA Eindhocen, The Netherlands

The magnetization of C o / P t and C o / A u multilayers has been measured a a function of temperature. For the C o / P t multilayers with fixed cobalt sublayer thicknesses of 4, 6 and 8,~, the magnetization decreases faster with temperature as the platinum layers are made thicker. In contrast the magnetization of the C o / A u multilayers is nearly independent of the Au thickness in a range from 8 to 40 A with a Co thickness of 6 A and remains almost constant up to 500 K. Interlayer coupling and interdiffusion are discussed as possible causes for the observed behaviour.

Using modern ultrahigh vacuum deposition techniques thin films and multilayers can be grown with unique structural and magnetic properties both from the standpoint of basic research as well as from the technological point of view. In that respect, it has been argued [1] that interlayer couplings of the magnetic layers via nonmagnetic layers should be present and could be modelled in.magnitude by variation of the spacer layer. The existence of such interactions are of technological importance when multilayers are used as storage medium. C o / P t multilayers with perpendicular anisotropy, for instance, have recently been shown to be candidate materials for magneto-optic (MO) recording [2]. In this application the shape of the magnetization versus temperature M(T) curve and the Curie temperature Tc are extremely important in MO writing and recording processes. The Curie temperature has to be sufficiently low in order to switch the magnetization with the available laser powers and the shape of the M(T) curve has to be as square as possible, i.e., a sharp drop close to T and a slow decrease at low temperatures in order to retain the maximum moment and associated Kerr effect at room temperature. In view of this, we investigated the temperature dependence of the magnetization of C o / P t

and C o / A u multilayers with perpendicular anisotropy. From the effect of the thickness of the nonmagnetic spacer layers on the temperature behaviour one may obtain information about a possible interlayer coupling. The M(T) behaviour and the Curie temperature will of course not only be determined by an interlayer coupling but by the interplay between anisotropy [3], size effects [4] and coupling effects. Furthermore, interdiffusion causing graded interfaces and disorder resulting in a distribution of exchange interactions, can play an important role. Especially when the magnetic sublayer thickness is in the monolayer regime the latter two factors might dominate the temperature dependence. C o / P t multilayers were grown on silicon and glass substrates at room temperature by vapor deposition in UHV with deposition rates of 1 A / s . The substrates were covered with a baselayer of 1000,~ Pd to ensure growth in a [111] texture. The thickness of the Co layers was fixed at 4, 6 and 8,~ while the platinum layer thickness was varied from 9 to 63,~. The C o / A u multilayers were deposited under HV conditions with Co layers of 6,~. The Au layer thickness was set at 8, 12, 16 and 40A. Again silicon and glass substrates were used, in this case covered with a 1000.~ Au baselayer.

0304-8853/91/$03.50 © 1991 - E l s e v i e r Science Publishers B.V. (North-Holland)

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P.J.H. Bloemen et al. / Temperature dependence of the magnetization of muhilayers

X-ray diffraction profiles, taken in reflection geometry at both low (20 < 12°) and high (34 ° < 20 < 5(1°) scattering angle, confirmed the modulated structure and shows a [111] texture for all the multilayers. The temperature dependence of the magnetization was measured with a Faraday balance. The saturation magnetization M s, at a given temperature T, resulted from extrapolation to zero field ( H = 0) of the high field part (700-1200 k A / m ) of the easy-axis M( H )-curves. Results for the C o / P t multilayers are shown in fig. 1. The curves are reversible up to 450K. Above 500K slight irreversible changes were detected (not shown here). Several features in the temperature dependence of M are worth noting. First of all it is clear that the temperature dependence is affected by the thickness of the Pt layers, yielding a substantial decrease of Tc with increasing Pt thickness. This effect is observed for all the samples, so for too = 4, 6 and 8,~. In principle this may reflect a Co interlayer coupling mediated by the Pt layers which increases with decreasing Pt thickness. The interaction enhances, so to say, the ordering p a r a m e t e r of the layers at a given temperature and induces a gradual cross over from 2d layer behaviour to 3d behaviour. Qualitatively similar behaviour has been reported for C o / P d [5], F e / A g [61 and F e / V [7] multilayers although the data in the latter case were restricted to temperatures far below Tc. However, the dependence o n tpt could also be explained with a totally different reasoning. As mentioned before, it is important to note that in case of ultrathin magnetic layers disorder and interdiffusion could drastically influence the temperature behaviour. Disorder might cause a flattening of the magnetization curve [3, 8] whereas interdiffusion lowers the Curie temperature. Assuming, as a result of interdiffusion during deposition, a concentratoin profile for cobalt with a decay length of 4 monolayers into the Pt, which seems not unrealistic, one has different concentration profiles depending on the Pt layer thickness. At large t m ( _> 27,~) the tails of the concen-

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Fig. l. The temperature dependence of the magnetizanon of C o / P t multilayers (a) with Co sublayers of 4 A and (b) with 6 A Co layers. The drawn curve corresponds with bulk fcc Co. The data are normalized to 1 at 77K. The 8 A Co multilayers show similar behaviour.

tration profiles of neighbouring Co layers do not overlap while in multilayers with thinner Pt layers, the tails certainly do have some overlap, the amount of which increases with decreasing Pt thickness. So, decreasing the Pt thicknesses will lead to increasing Co concentrations in the Pt layer and corresponding increasing Curie temperatures and magnetizations in accordance with the observed behaviour. Our suspicion that this explanation is valid becomes stronger when we consider our data

P.J.H. Bloemen et al. / Temperature dependence of the magnetization of multilayers

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obtained for the C o / A u multilayers with fixed Co thickness of 6 A and Au thicknesses varying from 8 to 40,~. The M ( T ) curves displayed in Fig. 2 show almost no dependence on the Au thickness. Realizing that Co and Au are mutually insoluble whereas Co and Pt or Pd and Fe and V do form solid solutions, the apparent contradicting thermomagnetic behaviour between C o / P d [5], C o / P t , F e / V [7] with probably graded interfaces and the C o / A u multilayers with chemically sharp interfaces can in principle be understood. On the other hand the behaviour of the F e / A g multilayers reported by Gutierrez et al. [6] cannot be accounted for in this way since Fe and Ag are insoluble. A long range interaction is more plausible in this case. It is surprising that the magnetization of the C o / A u multilayers with these very thin Co layers remains almost constant and that the behaviour is unaffected by the thickness of the Au layers. A possible explanation could be that Co layers of 6 A and larger thicknesses display 3d behaviour in the investigated temperature range. If this is the case an interaction between the Co layers can of course not enhance the magnetization anymore

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and consequently leads to the observed Au thickness independence. Recent observations by de Miguel et al. [9] are in conflict with this view. They found for 2.5 ML Co on Cu(100) a Tc of about 570 K. One should realize however that for ordering phenomena in ultrathin layers, the anisotropy is a very relevant p a r a m e t e r [3]. A major source for additional anisotropy in multilayers and adlayers such as the present ones, is brought about by the interface. Thus, the different nature of the interface could easily account for the conflict. On the other hand Pescia et al. [10] reported no variation at all for the magnetization of Co monolayers on Cu(100) up to 400K whereas De Miguel et al. found a Curie temperature of 170K for the same system. This clearly reflects the importance of the deposition conditions causing the conflicting results and preventing us from giving a conclusive interpretation of the temperature dependence. In conclusion, the temperature dependence of the magnetization of C o / P t and C o / A u multilayers has been investigated for fixed thin Co layers as function of the Pt and Au layer thicknesses. The magnetization of the C o / P t multilayers is markedly affected by the Pt thickness. The thinner the Pt layer the larger the magnetization. The M ( T ) curves of the C o / A u multilayers are almost independent of the thickness of the Au layers. In the case of C o / P t the temperature dependence could be explained by assuming an interlayer coupling but the existence of graded interfaces as a result of interdiffusion during the deposition process seems more likely to cause the observed behaviour. The origin of the behaviour of the C o / A u multilayers is not clear.

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P.J.H. Bloemen el a L / Temperature dependence qf the magnetization of multilayers

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