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Difference in the behaviour of interfacial Co and Ni atoms in CoxNi1-x/Pt multilayers: an explanation
View the table of contents for this issue, or go to the journal homepage for more 2000 J. Phys. D: Appl. Phys. 33 1662
(http://iopscience.iop.org/0022-3727/33/14/305)
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Difference in the behaviour of interfacial Co and Ni atoms in Co x Ni 1 − x /Pt multilayers: an explanation
M Seddat
†
, M Tessier†
, R Krishnan†
, H Lassri‡
, ˘S Vi ˘s ˘novsk ´y§
, S K Kulkarniand M Vedpathak† Laboratoire de Magn´etisme et d’Optique, UVSQ - CNRS, 45, Avenue des Etats Unis, 78035 Versailles C´edex, France
‡ Laboratoire des Mat´eriaux et de Micro´electronique, Facult´e des Sciences, Universit´e Hassan II, Ain Chock, Casablanca, Morocco
§ Charles University, Faculty of Mathematics and Physics, Institute of Physics, Ke Karlovu 5, 12116 Prague 2, Czech Republic
Centre for Advanced Studies in Material Science and Solid State Physics, Department of Physics, University of Poona, Pune 411 007, India
E-mail:[email protected]
Received 31 January 2000, in final form 8 May 2000
Abstract.Magnetization at 5 K and polar Kerr spectra in CoxNi1−x/Pt multilayers were studied as functions of Co content in order to better understand the difference in the behaviour of interfacial Co and Ni atoms. While the magneto-optic enhancement in the Kerr spectra is observed for the whole range of 0< x <1 the enhancement in the magnetization arising from the spin polarization of Pt occurs only forx >0.3 indicating that Pt atoms are not polarized by the Ni atoms. The x-ray photoelectron spectroscopy experiments indicate that there is charge transfer from Pt to 3d bands of Ni at the interface, leading to the loss of ferromagnetism in Ni. Therefore, non-magnetic Ni is not able to polarize Pt and induce a moment. In contrast, in the case of Co, no charge transfer is seen and Co remains ferromagnetic and induces a moment on Pt by polarization effects.
1. Introduction
Metallic multilayers (MLs) are artificially prepared materials that present novel properties. The possibility of obtaining MLs with uniaxial anisotropy and good magneto-optical (MO) properties opened up intense research to optimize MLs for application of MO information storage. One of the most studied of such MLs is the Co/Pt system. Since the first report of Co/Pt, several laboratories have intensified this research on this type of ML [1, 2]. Also, this material was characterized using various techniques in order to obtain information on the structure of the film, the interface, etc. For instance, the presence of an alloy at the interface has been identified using NMR spin echo techniques [3]. Since the uniaxial anisotropy in a ML arises from the surface atoms, which see a break in the symmetry, one would expect to have an interface without any interdiffusion. Therefore, it is interesting to observe such an anisotropy where an alloy layer is present. In addition, it was seen that the magnetizationM increased above that of bulk Co for MLs with Co layers thinner than about 1 nm [4]. This was explained in terms of the magnetic contribution arising from Pt atoms at the interface which acquire a ferromagnetic moment due to polarization by the first-neighbour Co atoms
[5]. Neutron diffraction experiments confirmed the presence of magnetic moments of a few tenths of aµBon Pt. It was also proposed that not just one monolayer of Pt is polarized, but up to three of them [6]. The overall effect of Pt polarization on magnetization and the MO properties depends on the degree of roughness, intermixing and alloying at the interfaces.
In the Ni/Pt system perpendicular anisotropy is also present. The surface anisotropy (KS) was found to be +0.15 erg cm−1, which is smaller than that found for Co in Co/Pt [7]. Another characteristic of Ni/Pt was thatMdoes not show any increase for thin Ni layers in contrast to the case of Co/Pt. On the contrary,Mdecreased as the Ni layer thickness decreased. This leads us to the conclusion that in Ni/Pt, the Pt atoms do not acquire any moment and they are not polarized by the Ni atoms.
However, as regards the MO effects, a different picture is obtained. The polar Kerr effect spectra of Co/Pt have been widely studied by several authors, including ourselves [8–
10]. It is observed that the Kerr rotation shows a broad peak centred near 4.3 eV which is absent in bulk Co. As a result, the magnitude of the Kerr rotation above 3.7 eV exceeds that in Co. This has therefore been attributed to the large spin–
orbit coupling of the excited states of Pt, which induces an
Behaviour of Co and Ni in CoxNi1−x/Pt multilayers enhancement on the MO properties of neighbouring Co atoms
[11]. This explanation is further supported by the fact that when the Co/Pt ML is annealed at temperatures near 400◦C, the peak height increases considerably [12]. This is under- stood by the increase in the number of Pt neighbours around a given Co atom, which is brought about by the diffusion at the interface. The Kerr rotation spectrum of Ni/Pt shows a similar peak centred at a lower energy, i.e. near 3.9 eV, which again can be attributed to the effect of Pt atoms as in the case of Co/Pt mentioned above. So it is seen that while MO effects from Pt are observed both in Co/Pt and Ni/Pt, the increase in Mfor thin magnetic layers arising from the polarization of Pt atoms is observed only in the case of Co/Pt.
The CoxNi1−x/Pt system has been found interesting because in this system, by adjusting the relative concentrations of Co and Ni, the Curie temperature (TC) could be tailored to suit MO applications without trading off the uniaxial anisotropy and the Kerr rotation [13]. This system also offers the interesting possibility of studying the evolution of the enhancement in the magnetization and MO properties discussed earlier as a function of the Co content. We therefore prepared samples with 0< x < 1, and investigated the magnetization and the Kerr rotation as a function of the Co content for various magnetic layer thicknesses. As we have published some results on the magnetic and MO properties of this system we will describe here only those results which are relevant to the problem of Pt polarization and MO enhancement. In order to have some information on the hybridization between Co, Ni and Pt we also studied the Co/Pt and Ni/Pt interfaces as functions of the magnetic layer thickness using x-ray photoelectron spectroscopy (XPS).
2. Experimental details
The Co/Pt and Ni/Pt multilayers were prepared under an ultra- high vacuum under controlled conditions. The details can be found in our earlier publication [14]. For the CoxNi1−x/Pt series prepared by the same technique a Pt buffer layer 10 nm thick was deposited first on the glass substrate and then the ML was grown. The magnetic CoxNi1−xlayer thickness was varied from 0.5 to 5 nm, but that of Pt was kept fixed at 1.5 nm. The top layer was a 2.0 nm thick Pt layer. The total number of bilayers varied from 5 to 20. The magnetizationM was measured in the range from 5 to 300 K using a vibrating sample magnetometer (VSM). We express the moment per unit volume of the magnetic layer alone and do not include the Pt layer. The MO polar Kerr rotation spectra were measured in the energy range of 1.5–5.2 eV.In situ Co/Pt and Ni/Pt interfaces were studied using an Escalab MK II spectrometer (VG Scientific, UK), with Al Kα(hν = 1486.6 eV) x-ray source. A concentric hemispherical analyser (CHA) was used with 50 eV pass energy, which gives a total energy resolution of about 0.9 eV. Au 4f 7/2 at 84±0.2 eV served as the external reference.
3. Results and discussion
The Curie temperature (TC) of the Ni-rich samples is relatively close to room temperature. Therefore, in order
Figure 1.The magnetic layer thickness dependence of the magnetizationMat 5 K forx=0, 0.04 and 0.1.
Figure 2.The magnetic layer thickness dependence of the magnetizationMat 5 K forx=0.3, 0.5 and 1.0. The data for x=1.0 are taken from [4].
Table 1.Magnetic moment of Pt atoms as a function of the Co content (x) at 5 K.
x
0 0.04 0.1 0.3 0.5 1.0
µP t(µB) 0 0 0 0.2 0.3 0.3
to avoid the effect ofTCon the properties, it is necessary to discuss the results on magnetization at 5 K. We focus on the magnetic layer thickness,t, below 3 nm where the interface effect is clearly manifested. Figure 1 shows the magnetic layer thickness dependence of M at 5 K for x = 0, 0.04 and 0.1. Figure 2 shows the same dependence forx=0.3, 0.5 and 1.0. In Ni/Pt (x = 0),Mdecreases strongly when decreasing the magnetic layer thickness. However, when the Co concentration increases, as shown in figure 2, M for x 0.3 no longer decreases for thin layers, but in 1663
Figure 3.The plot ofMmltas a function oftfor CoxNi1−x/Pt withx=0.5.
fact it increases with decreasing magnetic layer thickness.
For Co/Pt the increase inM for thin layers is remarkable, as we mentioned in the introduction. The layer thickness dependence ofM in this case can, by a phenomenological model, be written asMml=Mb+2δMi/twhereMml,Mband Mi indicate the magnetization of the ML, the bulk material and the interface region, respectively. The thickness of the interface region, which contributes to the enhancedM, is denoted asδ. So if one plotsMmlt as a function of t one obtains a straight line and the intercept on the ordinate axis gives the productMi2δ. The data up to t = 5 nm were considered in order to establish the linear variation ofMmlt against t. Figure 3 shows such a plot forx = 0.5 as a typical example. Similar plots were obtained for other values ofx greater than 0.3. If one knows δ then the interface magnetization can be obtained. In our case we have assumed that two atomic layers of Pt close to the Co layers are magnetic and then calculated the magnetic moment of Pt. The results are shown in table 1. It is seen that only whenx=0.3 does the polarization of Pt by Co atoms occur, inducing a moment of 0.2µB. This value then increases with an increase in the Co content (x) to reach a saturated value of 0.3µB. It is seen that Ni atoms do not induce any magnetic moment on Pt, but the presence of Co atoms even when alloyed with Ni can do so.
Let us now turn to the MO Kerr spectra for some of the samples. We will discuss them only briefly here insofar as they are relevant to the objectives of this study, as we have reported these results elsewhere. Figure 4 shows the spectra for Ni/Pt, Co0.3Ni0.7/Pt and Co/Pt MLs. The spectrum for Ni/Pt was taken at 80 K in order to avoid the effects of the relatively lowTC. The polar Kerr rotation shows an increase towards higher energy and the peak for Co/Pt occurs at an energy close to 4.3 eV. This increase in the Kerr rotation in MLs has been discussed before by several authors [8, 9]. This is generally attributed to the effect of Pt neighbours on Co atoms. In fact, this can be demonstrated as follows. If the substrate temperature is increased during Co/Pt deposition it induces some interdiffusion. This then increases the number of Co atoms having Pt as first neighbours. Therefore, one
Figure 4.Polar Kerr rotation spectra of Ni/Pt, Co0.3Ni0.7/Pt and Co/Pt MLs. For Ni/Pt the spectrum was taken at 80 K. The spectra of Co and Ni are shown for comparison.
Figure 5.Polar Kerr rotation spectra of Co/Pt MLs deposited at 300 and 420 K and annealed at 720 K. The thickness of Co and Pt layers in the three samples was fixed attCo=1 nm and
tP t=1.6 nm and the number of bilayers was 18. The spectrum of Co is shown for comparison.
would expect higher rotation from such samples. Indeed this is verified experimentally as shown in figure 5 where the Kerr spectra are shown for samples deposited at 300 and 420 K. It is seen that the peak height of the latter is higher. We also annealed the samples deposited at 300 and 720 K under ultra-high vacuum [12]. This treatment leads to further interdiffusion and the formation of a Co–Pt alloy at the interface. Figure 5 shows that for this sample the peak height is remarkably large and increased by almost 40% from the initial state before annealing. The effect of annealing was recently studied on a trilayer system Pt/Co/Pt by Train et al [15] who confirmed this result. So, it is seen that the Kerr rotation enhancement occurs in Co/Pt and similar modifications are also seen in the Kerr rotation spectra of Ni/Pt, unlike in the case of magnetization.
Behaviour of Co and Ni in CoxNi1−x/Pt multilayers
Figure 6.Core level spectra for the Ni/Pt sample as a function of the Ni layer thickness.
In order to obtain more information on this difference in the behaviour between Co and Ni we performed some XPS studies [16]. As we could not prepare the MLs and study XPS in situ, we adopted the following procedure. We deposited a layer of either Co or Ni on the covering Pt layer of the ML samplein situand observed the x-ray photon spectra as a function of the layer thickness, which was varied in the range from 0.1 to 2.0 nm.
Neither in the case of Co nor in the case of Ni deposition was any change in the valence band of Pt observed. As the thickness increased, the intensity of the peak near the Fermi level also increased. This is expected, as the 3d band lies close to the Fermi level. However, in the case of Co deposition, the Co 2p levels do not show any shift with the layer thickness.
Also, no shifts in the Pt 4f levels were observed. These results indicate that there is no detectable charge transfer from Co to Pt or vice versa in the case of a Co/Pt interface. However, the situation is totally different for Ni deposition.
Figure 6 shows the variation observed in Ni 2p core levels as a function of the Ni layer thickness. The spectrum of pure Ni is also shown for comparison. With the deposition of a Ni layer, 0.1 nm thick, the peak shifts to a lower binding energy of 852.2 eV. With the increase in Ni layer thickness it slowly shifts to a higher binding energy and reaches the Ni metallic value of 853.2 eV. The absolute values of the photon energies in Ni are in agreement with those by Fuggle and Martensson [17]. Thus there is nearly a 1 eV binding energy shift observed at the Ni/Pt interface. For a Ni layer that is 2.0 nm thick the Pt 4f peak also shifts from its metallic value of 70.9 eV [17] to 71.5 eV, which is quite large. These results suggest that there is a charge transfer from Pt to Ni, which means that the Ni 3d bands are being filled gradually.
Nickel, which already has a low magnetization and a lower TC, then begins to lose the moment quickly and the nickel at the interface becomes non-magnetic. Non-magnetic Ni is not able to polarize the Pt atoms and induce any moment on Pt, unlike in the case of Co.
In conclusion, XPS studies indicate that in Co/Pt there is no charge transfer between the Co and the Pt. In Ni/Pt, on the other hand, the charge transfer from Pt to Ni leads to non-magnetic Ni atoms at the interface. Therefore Ni atoms do not polarize the Pt atoms and no moment is induced on them. Such a situation does not exist for Co. This is in agreement with the results on magnetization studies.
However the enhancement observed in the Kerr rotation arises from a totally different mechanism, namely the large spin–orbit coupling in the excited states of Pt. Therefore one observes this both in Co/Pt and Ni/Pt.
Acknowledgments
The paper was prepared during the stay of SV at Universit´e de Versailles and their financial assistance is gratefully acknowledged. The research was partially supported by the Grant Agency of Czech Republic (grant numbers
#202/97/1180 and #202/00/0761).
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