Perpendicular magnetization in Ni/Pt multilayers
R. Krishnan, H. Lassri, M. Porte, M. Tessier, and P. Renaudin
Citation: Applied Physics Letters 59, 3649 (1991); doi: 10.1063/1.105608 View online: http://dx.doi.org/10.1063/1.105608
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/59/27?ver=pdfcov Published by the AIP Publishing
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Perpendicular magnetization in Ni/Pt multilayers
R. Krishnan, H. Lassri, M. Porte, M. Tessier, and P. Renaudin
Laboratoire de Magnttisme et Mathiaux Magnitiques, C.N. R.S. 92195 Meudon. France
(Received 30 August 1991; accepted for publication 8 October 1991)
Ni/Pt multilayers have been prepared by electron beam evaporation under ultrahigh vacuum conditions. All the samples were grown at 300 K. The t(Ni) was in the range 0.7-2.1 nm and t(Pt) was fixed at 2 nm. The magnetization and the Curie temperature for these samples are lower than that of bulk Ni. For the samples with t( Ni) < 1 nm perfect
rectangular hysteresis loops are obtained along the film normal in the low temperature range (5 to 70 K) indicating the presence of a strong uniaxial anisotropy. The coercivity for the sample with t(Ni) = 0.7 nm at 5 K is as high as 4.7 kOe.
Recently much attention has been paid to multilayers based on Pt such as, Co/Ptls3 and Fe/Pt4 which show perpendicular anisotropy for transition metal layers thin- ner than about 1 nm. Besides the fundamental interest, these materials are attractive as magneto-optical storage media of the future generation. However until now, to our knowledge, there has not been any report on Ni/Pt system and hence we have carried out a systematic study of this system. In samples with Ni layer thickness t(Ni < 1 nm, at low temperatures a perpendicular magnetization has been observed for the first time. We describe here our magneti- zation studies.
Ni/Pt multilayers (ML) were prepared by sequential evaporation in ultrahigh vacuum under controlled condi- tions and the pressure during the film deposition was less than 5 X 10 - ’ Torr. Samples were directly deposited on water cooled glass and silicon substrates. The rate of dep- osition (about 0.03 rim/s)) and the final thickness were monitered by quartz oscillators, precalibrated by using a PROFILOMETER for Pt and magnetization measure- ments for Ni layers, respectively. The Ni layer thickness t(Ni) was varied from 0.7 to 2.1 nm and that of t(Pt) was 2 nm. The number of bilayers in the range 10 to 32 was adjusted to get a total t(Ni) of about 20 nm. All the sam- ples were overcoated with a protective Pt layer 3-nm thick.
Low angle x-ray diffraction studies were made to check the periodicity and the thickness of ML. Magnetization
(M), and the M-H loops were measured with a vibrating sample magnetometer (VSM) in the temperature range 5 to 295 K under a maximum field of 15 kOe field, applied both in the film plane and normal to it.
Low angle x-ray diffraction of all the samples revealed peaks typical of the modulated structure and the thickness calculated from these peaks agree within 3% with that obtained from the quartz oscillator after calibration.
The samples with t( Ni) < 1 nm were not magnetic at 295 K, and the magnetization for the samples with t(Ni)
= 1.33 and 2.1 nm was lower than that of the bulk Ni.
However at 5 K all the samples studied are magnetic. The magnetization was calculated with respect to total Ni vol- ume and Table I shows the result at 5 K. The relatively lower M values and the decrease in M with decreasing t(Ni) indicate the formation of the Ni-Pt alloy at the in-
terface. However, it appears that in Ni/Pt, the polarization
of the Pt is not as effective as in Co/Pt ML, where actually an increase in M has been observed.2 Table I also shows that the Curie temperature for the samples three is low.
In order to find out if a uniaxial anisotropy is present in these samples we examined the M-H loops in the tem- perature range 5 to 295 K. At 295 K for t(Ni)> 1.33 nm the in-plane (parallel) M-H loops are rectangular with a high remanence ratio (R=M”/M,,). This shows that the easy axis lies in the film plane. But as the temperature is decreased in-plane R starts decreasing and the M-H loop along the film normal (perpendicular) starts showing some remanence. This indicates that the perpendicular anisot- ropy starts increasing in the low temperature range. How- ever for the samples with t( Ni) < 1.05 nm, on the contrary, the in-plane loops are typical of the hard axis one, with practically no remanence at all and one observes rectangu- lar M-H loops along the film normal indicating the pres- ence of a strong uniaxial anisotropy. Let us discuss the results at 5 K. Figure 1 shows both the parallel and per- pendicular M-H loops for the samples with t(Ni) = 2.1 nm at 5 K. Figures 2 and 3 show at the perpendicular loop for t(Ni) = 1.05 and 0.7 nm, respectively, and it is note- worthy that the loops are perfectly rectangular with R = 1 and the coercivity (HJ of the thinnest sample is as high as 4.7 kOe. The range of temperature in which the loop re- mains rectangular of course depends on the Curie temper- ature of the sample. For example, for the sample with t(Ni) = 0.7 nm for which Curie temperature is 200 K the remanence ratio R starts decreasing for T > 50 K. This decrease in R as is well known is due to the fact that the uniaxial anisotropy decreases faster with increasing tem- perature than the magnetization. The perpendicular and parallel coercivities, the remanence ratio R and the Curie temperature are all shown in Table I. The above results
TABLE I. Some magnetic properties of Ni/Pt multilayers at 5 K.
M (W, Wc)II
No t(ni)nm emu/cm3 kOe kOe (R)L (RI,, T,(K)
1 21 380 0.20 0.14 0.1 0.57 ,300
2 1.33 315 0.22 0.34 0.07 0.30 ,300
3 1.05 400 1.7 ... 1.0 0 320
4 0.7 295 4.7 ‘.. 1.0 0 200
3649 Appl. Phys. Lett. 59 (27), 30 December 1991 0003-6951/91/523649-02$02.00 @ 1991 American Institute of Physics 3649 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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I I 1 I 1 I I
-a -2 -1 D 1 2 3
H (kOe)
FIG. 1. In-plane and perpendicular M-H loops for the sample with t(Ni)
=2.1 nm at 5 K.
show that for t(Ni)<l.OS nm, at low temperatures, the easy axis is normal to the film plane and it can therefore be concluded that the surface Ni atoms do contribute to the uniaxial anisotropy via the surface anisotropy (K,V> as in the case of Co/Pt.2 In our opinion, this is the first report of such a perfectly rectangular perpendicular loop with large coercivity and a strong perpendicular anisotropy for Ni based ML. We have investigated Ni/Ag ML before and found that K, was absent in the temperature range 5 to 295 K.5 Similar result was also reported on Ni/Pd ML by den Broeder et aL6 However, it is interesting to note that Plevaris’ has reported recently, by a study of polar Kerr
:’ -
I I I I 4
-3 -f.S 0 1.5 3
H (we)
FIG. 2. Perpendicular M-H loop for the sample with t(Ni) = 1.05 nm at 5 K.
15 _) I
-10 I
-5 I r I 4
0 5 10 15
H (kOe)
FIG. 3. Perpendicular M-H loop for the sample with t(Ni) = 0.7 nm at 5 K. The negative slope in the magnetization is due to the diamagnetic signal from the substrate.
loops, the presence of a perpendicular anisotropy in Ni/Pd ML for t(Ni) of the order of 0.5 nm.
In order to evaluate the surface anisotropy KS quanti- tatively, it is necessary to carry out magnetic torque study in the temperature range 5-295 K, which is in progress. A detailed account of these studies will be published in the near future.
This work was performed under the European action BRITE-EURAM Contract No. BREU-0153-C which is gratefully acknowledged.
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