Magnetic properties of Ni/Pt multilayers

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R. Krishnan, H. Lassri, Shiva Prasad, M. Porte, and M. Tessier

Citation: J. Appl. Phys. 73, 6433 (1993); doi: 10.1063/1.352623 View online: http://dx.doi.org/10.1063/1.352623

View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v73/i10 Published by the American Institute of Physics.

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Magnetic properties of Ni/Pt multilayers

FL Krishnan and H. Lassri

Magt&istne et Mat&iaux MagnL;tiques, CNRS, 92195 Meudon, France

Shiva Prasad

Physics Department, Indian Institute of Technology, 400076 Bombay, India

M. Porte and M. iessier

Magnkisme et iliatC;riaux Mugnetiques, CNRS, 92195 Meudon, France

We have grown Ni/Pt multilayers (ML) with and without Pt buffer layer, by evaporation under UHV conditions on glass and Si substrates maintained at 30 and 200 “C!. The magnetization decreases with t(Ni) and no induced moment is found on Pt from polareation effects even at 5 K. The perpendicular magnetization loops for samples with t(Ni)<l7 A are rectangular and coercivities as high as 4.7 kOe is obtained for t(Ni) =9 A at 5 K. For the samples deposited at 20 “C, due to dispersion in the results, KS could not be calculated with accuracy though it is positive. But for the samples deposited on a Pt buffer layer at 200 “C, KS is found to be $0.17 erg cm -’ at 5 K and the volume anisotropy is also higher with respect to those deposited at 30 32.

INTRODUCTION

Recently much attention has been paid to multilayers based on Pt, such as Co/I?, which show perpendicular anisotropy for Co layers thinner than about I5 A.‘-s Be- sides the fundamental interest, these materials are also promising candidates for magnetooptical storage media ca- pable of performing at shorter wavelengths. Recently perpendicular anisotropy was reported also in Fe/Pt multilay- em4 However, reports on Ni-based systems are relatively very few. We had reported that in Ni/Ag (Ref. 5) multilayers there was no contribution to the surface anisotropy, even though a contribution to the surface magnetostriction from the surface Ni atoms was found.6 Therefore it is interesting, from the fundamental point of view, to investi- gate if surffce anisotropy could be found in other Ni-based systems. Our preliminary investigations indeed showed that perpendicular magnetization with a remanence ratio of one could be obtained in Ni/Pt multilayers for relatively thin Ni layers.7 Earlier, rectangular hysteresis loops had been observed also for Ni/Pd multilayers even though the remanence ratio was much smaller than one.8p9 In this pa- per we describe our study of the magnetization and the anisotropy in Ni/Pt multilayers. We show that the samples with Ni layers thinner than about. 15 A show distinctly different behavior from those with thicker Ni layers. The magneto-optical Kerr speetroseopy of these samples is reported elsewhere.‘”

deposited on glass substrates at 30 “C. Samples with Ni layer thinner than about 15 A were also deposited on a platinum buffer 100 A thick at 200 “C for the reasons ex- plained later. The top layer in all the samples was Pt 20 A thick. The growth parameters would be designated as [r(Ni),r(Pt)]Xn, where it indicates the number of Ni layers.

Low-angle x-ray diffraction studies were made to check the periodicity and the thickness of the bilayer. Mag- netization (M) and the M-H loops were measured with a vibrating sample magnetometer (VSM) and the anisotropy with a torque magnetometer, in the temperature range 5-295 K under a maximum field of 15 kOe.

RESULTS AND DJSCUSSION

Low-angle x-ray diffraction of ah 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. Fig- ures l(a) and l(b) show the low- and high-angle x-ray diffractions for a sample [9,20] x 32 grown at 200 “C on a

lOO-A-thick Pt layer. The (111) peak from the Pt buffer

EXPERIMENTAL DETAILS

NiPPt multilayers (ML) were grown by evaporation in ultrahigh vacuum under controlled conditions, and the pressure during the film deposition was maintained in the range 3-5~ lOem Torr. The rate of deposition (about 0.3 A/s) and the flnal thickness were monitored by precali- brated quartz oscillators. The Ni-layer thickness t( Ni) was varied from 6 to 40 A and that of t(Pt) was kept fixed at 20 A. The number of bilayers in the range 10-32 was ad- justed to get a total t(Ni) of about 250 A. Samples were

0 4 8 -40 50 60

‘20, IUi)

FIG. 1. Low-angle (a) and high-angle (b) x-ray diffraction patterns for the sample [9,20] ~32 deposited on IO@k-thick Pt buffer layer and at 2CXfT.

6433 .I. Appi. Phys. 73 (IO), 15 May 1993 0021-6979/93/106433-03$06.00 @ 1993 American institute of Physics 6433

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500 t

I I : t , , , .

0 10 20 30 40 50

tpJi (A)

FTG. 2. The t(Ni) dependence of the magnetization at 295 and 5 K.

layer and the satellite peaks are clearly seen in Fig. 1 (b). It can be concluded that Ni layers have a (111) texture.

Figure 2 shows the t(Ni) dependence of the magnetization both at 295 and 5 K. At 295 K the samples with f(Ni) < 15 A are very weakly magnetic because the Curie temperature for these samples is close to 300 K. The relatively lower M values and the decrease in M with decreasing r(Ni) indicate the formation of the less magnetic or nonmagnetic Ni-Pt alloy at the interface. Assuming a dead layer at the interface, one can express the magnetization in the multilayers M as M=Me( 1 - 26/t), where Ma stands for the magnetization of bulk Ni and 6 is the dead-layer thickness at each interface. An analysis of a plot of MXt(Ni) as a function of t(Ni) at 5 K gives Me= 500

~20 emu/cm3 which is close to the value of 520 emu/cm3 of bulk Ni and 2S in the range S-6 A. This means that at each interface about a monolayer of Ni is nonmagnetic. It seems, therefore, that in Ni/Pt system, Ni atoms do not induce any moment on Pt. It is recalled that in Co/Pt multilayers Co has been shown to induce a moment on neighboring Pt atoms. ‘*

For the samples with t(Ni) 220 A the in-plane (H applied parallel to the tllm plane) M-H loops at 295 K show a high remanence ratio (R =MR/MS), indicating that the magnetization lies in the film plane. The in-plane coercivity increases at low temperatures. For example, the coercivity of the sample [28,20] X 11 deposited at 300 K on glass increases from 17 to 180 Oe as the temperature is decreased from 300 to 5 K. On the contrary, for the samples with f( Ni) d 17 A, the in-plane loops are typical of the hard axis one, with practically no remanence at all and one observes rectangular M-H loops along the film normal indicating the presence of a strong uniaxial anisotropy. Fig- ure 3 shows the perpendicular M-H loop at 5 K for the sample [9,201X 32, deposited on lOO-A-thick Pt buffer layer and at 475 K. It is noteworthy that the loop is rectangular with R = 1 with a coercivity as high as 4.7 kOe. As the temperature is increased, the remanence ratio and the coercivity start decreasing.

Let us now discuss the results on anisotropy. It is well known that the layer thickness dependence of the anisotropy in multilayers can be described, based on the phenom- enological model, by the equation Kerr= K V+ 2&/r, where,

6434 J. Appl. Phys., Vol. 73, No. 10, 15 May 1993

M L

H

// / F

5 koe

FIG. 3. The perpendicular loop at 5 K for the sample, [9,20] X 32, deposited on E&&-thick Pt buffer layer and at 200 ‘C.

&,, is the measured anisotropy, and KY, K, are the volume and the surface anisotropies. The term K, consists of three contributions: ( 1) the demagnetizing energy 2?rM”, (2) the intrinsic crystalline anisotropy Kerr, and (3) the magnetoelastic anisotropy KhllE, which arises from the interac- tion of the magnetos&i&ion d with the stress in the film.

This is given by the term KME= - f aA, where (z is the stress. In polycrystallme layers in the absence of any crystalline anisotropy, one can expect a contribution to KY from KCYLp We had mentioned that in our samples some ( 111) texture is observed. Therefore the film normal cor- responds to [l 1 I] direction, which is the easy axis for bulk nickel. So one might argue that this could lead to the observed perpendicular anisotropy. But this is not so. For instance, Takahashi er aL9 found in Ni/Pd multilayers that perpendicular anisotropy was observed only for the [lOO]

orientation and not for the [Ill]. Therefore we consider that the intrinsic anisotropy observed here arises only from the magnetoelastic interactions.

The product of K&X t(Ni) versus t( Ni) at 5 K is plotted for the samples with 6 < t( Ni) < 40 A, deposited at 30 “C in Fig. 4 and one finds that for t(Ni) < 10 A Keff is positive. It is seen that while the experimental points align well in a straight line for t(Ni)> 12 A, those for thinner layers show some dispersion, though the deposition param-

t

0.3 1 -. _\,

‘.

*\

‘\

t -.

03 0 ‘1 l

CJJ

3 5 . .._ . aJo

0.1.

‘._

Y 0 2 ^o ^.

l---

~~-.

i r.,,,

3 -...ia<o JbtNIIA) &+

- 0,s - \-.

0,s J----Y

FIG. 4. The t(Ni) dependence of the product K,,x f(Ni) for the samples deposited both at 30 “C (open circles) and at 200 “C on LOO-A-thick Pt buffer layer (closed circles).

Krishnan et al. 6434

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eters were carefully controlled. Nevertheless, one can re- mark that there are two distinct straight lines with two different slopes, the one for t ( Ni) < 10 8, being higher. This suggested that the change in the properties observed for thinner layers could be either due to a change in the structure or to some coherency strains. The extrapolation of the data for samples with t(Ni) > 10 A shows that there is a small contribution from the surface anisotropy with KS > 0.

As it was difficult to calculate the anisotropies for the thinner Ni layers with any accuracy, we tried to deposit the samples at a higher temperature, hoping that this could stabilize the changes that occurred for the thinner layers.

Figure 4 also shows the results at 5 K for the samples deposited at 200 “C on a Pt buffer layer 100 A thick. It is seen that the experimental points are now aligned better and the &is positive for all the samples (9 to 17 A thick).

The extrapolation of the straight line gives KS= 3-0.17 erg cm ‘. This is certainly smaller than l-O.6 erg cmd2 found for Co/Pt multilayers. Nevertheless this is the first time such a positive surface anisotropy has been reported for N&based multilayers.

The slopes of the straight lines in Fig. 4 confirm the two dist.inct behaviors both for the samples deposited at 30 and 200 “C. RV values of -1.9X 10’ and -0.9X lo6 erg cm -3 are found for the thinner and thicker Ni layers, respectively. This increase in K, for the thinner samples [deposited both at 30 and 200 “C) with respect to the thicker ones suggest that KME contributions are not the same in both the cases.

In order to calculate KkjE it is necessary to know the magnetostriction constants (A) in these materials. We have shown that 3, is also strongly dependent on Ni layer thickness as, for example, in Ni/Ag, where we have shown” that the absolute value of A decreased with r(Ni) and tended to become posit.ive due to the contribution from surhace magnetos&i&ion. This would mean that even if the stress (LT) in the tilms remains the same (as a first approx- imation) XhiE could still be different for sufficiently thinner Ni layers. But on the one hand, the t dependence of/z in Ni/Pt could be different from that observed in NVAg, and on the other hand, o also could be quite different for very thin layers. In the absence of any measurements of R in NVPt, it would be a speculation to discuss any further.

Nevertheless our results do show the important role played by the magnetostriction in multilayers, which is often ne- glected and one assumes the bulk value of L to interpret the anisotropy in very thin layers. Magnetos&i&ion measurements in Ni/Pt are necessary to understand our results further and work is in progress. It is also interesting to study the influence of the substrate temperature on the anisotropy.

In conclusion, we have prepared Ni/Pt multilayers by evaporation under ultrahigh vacuum conditions and char- acterized them. The effective anisotropy for layers thinner than about 1.5 A deposited at 30 “C is positive but show a dispersion. On the contrary, for the samples with Ni layers thinner than 15 A deposited at 200 “C on Pt buffer layer it is found clearly that the surface anisotropy at 5 K was j-O.17 erg cm-z. The perpendicular loops for these thinner samples show a remanence ration of 1.0 and the coercivity for the samples with t(Ni) =9 A was as high as 4.7 kOe at 5 K.

The partial supports of this work from Brite Euram Contract BREU-0153 and from Indo-French Center for the Promotion of the Advanced Research (Centre Franco- Indien pour la Promotion de la Recherche Avancee) are gratefully acknowledged.

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6435 J. Appl. Phys., Vol. 73, No. 10, 15 May 1993 Krishnan et al. 6435