Magnetic and magneto-optic properties of Ni/Au superlattices

3388 IEEE TRANSACTIONS ON MAGNETICS, VOL. 29, NO. 6, NOVEMBER 1993

Magnetic multilayers are interesting both from fundamen- tal and applied points of view and are actively investigated by several laboratories. The properties of these materials are mostly governed by the surface properties and hence the interface plays an important role. We have shown recently that in NilPt mulitlayers (ML) perpendicular magnetization is stabilized for Ni layers thinner than about 17 A, though the Curie temperature is relatively low 1. Uniaxial anistotropy has also been reported in NilPd ML 2* 3. It is intemsting to investigate NilAu system, particularly because, Au and Ni are not miscible and hence the interface could be expected to be sm. Recently Childress et al 4have reported on sputtered NilAu ML. In this paper we describe our studies on NilAu h4L prepared by evaporation under ultra high vacuum conditions.

11. EXPERIMENTAL, DETAILS

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The samples were deposited by electron beam evaporation on to water cooled glass substrates. The layer thickness and the rate of evaporation were controlled individually by pre- calibrated quartz oscillators. The pressure during deposition was Mow 5x1C9 torr. The Ni layer thickness t (Ni) was in the range 6 to 35 A and that of Au was fixed at 15 A. The samples were grown on a Au buffer layer 50 A thick and were over-coated with 15 A thick Au which serves as a protective layer. The number of bi-layers (N) was in the range 10 to 30. The growth parameters will be henceforth designated by [t(Ni), ~(Au)] x N .

The low and high angle X-ray diffraction were studied to characterize the samples. The magnetization and the auiso- tropy were measured in the range 5 to 295 K, using a vibra- ting sample magnetometer and a torque meter respectively.

Both polar Kerr (l‘KR) and Faraday (FR) rotations were measured at a wavelength of 63% A.

111. RESULTS AND DISCUSSIONS

I ” f I I I .

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Fig.1 The low (a) and high angle (b) diffraction in the sample [8, 151 x 2

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Fig. 2 The t(Ni) dependence of the product M x t before annealing, at 0 295 K, at 5K and after annealing at 295K (see text)

All the samples show quazi rectangular in-plane loops indicating the easy plane magnetization. The magnetization decreases with t(Ni). It is known that this variation can be described phenomenologically by the relation.

where, M and Mo stand for the magnetization of the multilayer and the bulk Ni and 8 is the dead layer at each interface. Fig. 2 shows a plot of M x t (Ni) as a function of t(Ni) at 295 and 5 K. It is seen that the experimental points align well on a straight line. The intercept on the abscissa gives the value of 26 which is found to be 4 A at 295K. It is noteworthy that at 5K, however, the intercept is close to zero which indicates that there is apparently no Ni dead layer and therefore the Ni moment is independent of the thickness. This result shows that as expected the interface is quite sharp with no significant intermixing. In contrast. in sputtered NilAu, in ref 4, the dead layer at 5K was found to be about 3A. This shows that the present superlattices prepared by evaporation are of a better quality. The slopes of the strai ht lines give the respectively and which agree well with those of bulk Ni..

We annealed the samples at 300°C in vacuum for periods from 2 to 5 h and then studied their magnetization at 29%.

The result is shown in Fig. 2. It is seen that annealing has practically no effect in all the samples except for the one with t(Ni) = 6.5 A where, M increased from 75 to 116 emu.cm3 upon annealing. This may be attributed to better interface sharpness in the thinnest Ni layer sample due to probable seggregation of the two metals.

M = M g ( l - B l t ) (1)

value of Mo which are 488 and 500 emu/cm-

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Manuscript received February 15 1993

ooi8-9464/93$03.00 @ 1993 IEEE

3389

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Let us discuss now the results on the anisotropy. As men- Both PKR and FR decrease with t(Ni) as shown in Fig. 4.

tioned earlier all the samples show an in-plane The rotations for the bulk Ni are indicated by an arrow for magnetization. The torque studies confirm this. The layer reference. Let us recall that in ColAu a strong increase in thickness dependence of the effective anisotropy Kerf in the FR was reported by Gambino7.The strong decrease observed multilayers can be described by the phenomenological by us in part could as well arise from the decrease in the model as follows : Curie temperature for thinner Ni layers. Low temperature

measurements are necessary to further discuss this result..

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&ff = Kv + 2 Q l t

anismpi=. The bullr anisotropy KV = 2 ~+

cry

where KV and KS stand for the bulk and the surface where the second and third terms designate the crystalline and magneto-elastic anisotropies if any. Fig. 3 shows the t(Ni) dependence of the product Keff x t(Ni) at 295 and 5K.

At 295 K despite a s m a l l dispersion the linear dependence is seen. Results for the annealed samples are also shown at 295 K in Fig. 3 and it is interesting to note that after aunealing there is no change in I Q f but the experimental points are aligned better. This shows that the interface sharpness is improved after annealing. A similar interface refining was reported by den Broeder et a14f0r ion beam sputtered ColAu samples with a consequent enhancement in the perpendicular anisotropy unlike in our case. An extrapolation of the straight line would indicate a small contribution from surface aniso- tropy. However at 5K the situation is quite clear. The effective anisotropy becoma more negative and it remains so for the thickness range studied here. In other words, the surface anisotropy is negative. The analysis of the above results at 5K, yield - 6 . 6 ~ 104 erg.cm3 and -0.1 erg.cm-* for Kv and Q respectively. This shows that the bulk anisotropy arises mainly from the demagnetization energy. This result is in agreement with that for the sputtered samples obtained by childre~s et al5.

8 1

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Fig. 4 t (Ni) dependence of polar Kerr and Faraday rotations at h=6328A

In conclusion NilAu ML have been prepared by evaporation . The analysis of the magnetization at 5K shows the absence of a dead Ni layer at the interface. The Ni moment is conserved for the thinnest Ni layer studied namely

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This work was performed under the BRITE-EURAM con- tract No. BREU 0153 which is gratefully acknowledged.

IV. REFERENCE

1. R Krishnan, H. Lassri, M. Porte, M. Tessier and P. Renaudin, Appl. Phys. Lett, 593650 (1991)

2. N.K. Flevaris. Appl. Phys. Lett, 58 2177 (1991)

3. H. Takahashi, S. Fukatsu. S. Tsunashima and S. Uchiyama, J.

h4agn. Magn. Mater. 93 469 (1991)

4. F.J.A. den Broeder. D. Kuiper, A.P. van der Moesslaer and W.

Having, Phys. Rev. Lett. 60 2769 (1988)

5. J.R. Childress. C.L. Chien and A.F. Jankowski, Phys. rev. B 45 6. R Krishnan, H. Iassri, Shiva Prasad, M. Porte and M. Tessier.

T o appear in J. Appl. Phys.

7. R. J. Gambino and R. R. Ruf, J. Appl. Phys. 67 4784 (1990)

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Fig. 3 The t (Ni) dependence of the product Ken x t(Ni) before annealing, 0 at 295 K. at 5K and after annealing 0 at 295K

In NilPt , we had found the texture to be (1 11) and the sur- face anisotropy was positive6. Though in NilAu also the tex- ture is (1 1 l), the surface anisotropy contribution from Ni atoms is negative. This shows clearly that besides the local symmetry, the specific electronic of the second metal, or in otherwords, the band structure is the determining factor for the anisotropy. It is to be recalled that uniaxial anisotropy has been reported for ColAu ML 4, which means that Au has the appropriate band structure for Co but not for Ni.

Theoretical understanding of the surface anisotropy in the the multilayers is still not complete.