An attempt is made to correlate the effective magnetic anisotropy of sputtered CoNi/Pt multilayers to the average stress in the thin film. It was found that compressive film stress decreases the perpendicular magnetic anisotropy

Bijker, M.D.

Drent, W.P. van

Lodder, J.C.

English

van Drent, W.P.

University of Twente Research Information

Journal of Magnetism and Magnetic Materials 156 (1996) 309-310 ~ Journal of - " "  magnetism ud magnetic materials ELSEVIER The effect of stress and magnetostriction on the anisotropy of CoNi/Pt multilayers W.P. Van Drent l, M.D. Bijker *, J.C. Lodder MESA Research Institute, Universio, o f  Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Abstract An attempt is made to correlate the effective magnetic anisotropy of sputtered CoNi /P t  multilayers to the average stress in the thin film. It was found that compressive film stress decreases the perpendicular magnetic anisotropy. 1. Introduction Table 1 Properties of the prepared samples Magneto-Optical (MO) multilayers are considered to be Sample PAr tSeed tCoNi tpt tbilaye, Kefl good candidates for future high-density MO recording (/xbar) (,~) (A) (,~) (A) (kJ/m ~) [1,2]. From other research it is known (e.g. [3]) that there A 5 95 4.9 6.3 11.6 - 50 can be a considerable contribution of stress induced B 8 95 5.0 5.8 10.1 7 anisotropy to the total (perpendicular) anisotropy. To cor- C 11 94 5.1 5.7 10.7 137 relate this effective magnetic anisotropy to the average D 30 95 5.0 6.1 11.5 324 stress in the thin film, a series of sputtered CosoNiso/Pt E 40 95 5.0 6.0 I1.1 376 multilayers was prepared in which only the sputter pres- sure was varied. Each multilayer consists of a Pt seedlayer and a stack of 17 C%oNiso/Pt  bilayers. The experimental details about the deposition are given elsewhere [4]. 2. Sample characterization The properties of the deposited samples are listed in Table 1. The layer thicknesses tSeed, tCoNi and tpt were estimated from the product of sputter time and sputter rate. The bilayer thickness t~ilayer , measured by low angle XRD was found to be in good agreement with the projected layer thicknesses. The largest deviation was found for sample B (7%). Furthermore the saturation magnetization M~ was calculated using the CoNi thickness only and found to be 1060+60  k A / m  for all films. Magnetic torque measurements were performed at room temperature at an applied field of 1330 k A / m .  The effective anisotropies measured in this way, normalised to the mag- netic volume, are also displayed in Table 1. To measure the film stress two different methods were applied: mea- surement of the surface curvature, and XRD measure- ments. The average internal film stress can be calculated * Corresponding author. Email: m.d.bijker@eltn.utwente.nl; fax: + 31-53-430-9547. Current address: Toyota Technology Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468, Japan. from the measured radii of curvature and the lattice spac- ings respectively. The strain free lattice spacing, d o [~,], was determined in the following way: from the curvature measurements we know the pressure range at which the deposited multilayers are in a strain free situation, approxi- mately 3.7 × 10 -2 [mbar]. The corresponding average lat- tice spacing as measured with the XRD was taken as d o and found to be 3.760 + 0.017 ,~. Further experimental details about these methods can be found in Refs. [3,4]. Both methods deliver the results as depicted in Fig. l a and b. Note that if the film stress is isotropic the equation given by Stoney [5] is used. 3. Anisotropy model In order to correlate stress and magnetic anisotropy the following model was derived. For multilayers with inter- face anisotropy a semi-empirical formula [6] was used to determine the interface and bulk contributions: tmag n ' Kefl. = 2 K S + K v •/magn" ( 1 ) The volume anisotropy K v consists of crystal anisotropy K~ and demagnetization anisotropy K D. When a plane stress is homogeneously distributed throughout the mag- netic layers of the multilayer, this will also add to the volume anisotropy by Kstress which is defined as Kstress = (1304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00879-9  310 W.P. Van Drent et al. / Journal o f  Magnetism and Magnetic Materials 156 (1996) 309-310 1 .0  - -  ~" 0.5 o.o == -o.5 E -1.0 g -t.s ¢o - 2 . 0  -2.5 -3.0 10-3 (a) ' . . . .  ! tensile / ~  . . . . . . .  _ . . . . . . . . . . . .  / . . . .  compresstve / . .~I _~ ,&.-.~/// V I -T-" et on R, axis] -~t- (~ on R2 axis / --o-- (r (Stoney) j i i i t l l l l  i i i , i J i  10 -2  10  -1  Argon pressure (mbar) 4 . 0  ~ ,  2 . 0  ; o . o  - 2 . 0  g-4.o -6'0 -8.0 ( b )  . . . . . . . .  ' i_tensi_ le . . . . . . . . . . . .  ~____ compressive ~ ~ /  , r i i J l l l l  i i i i i l l  10-3 10-2 10-1 Argon pressure (mbar) Fig. 1. (a) Stress along the R=, R 2 axes and the stress calculated according to Stoney's equation. (b) Stress measured by XRD as a function of the argon pressure. 3AsO-, with A s the saturation magnetostriction and tr the plane stress in the film. In this case a positive value for tr denotes tensile stress. Together, the effective anisotropy is defined as: 1 2 :~ Kef f = 2Ks/ tmag n + K l - ~/zoM s - 2As o'. (2 )  This equation is schematically shown in Fig. 2. Now the saturation magnetostriction and the surface anisotropy can 2 be calculated as being A s = - ~ .  tan c~ and K s t 2 = ½tmagn(P - K l + ~-/xoM s ) respectively, where P and are defined in Fig. 2. 4. Results  and discussion High angle XRD experiments revealed that the C o N i / P t  multilayers discussed here all had the fcc (111) structure. This corresponds to the fcc structure found in the phase diagram of bulk C%oNiso. Due to the fcc structure, Kj is small and negative: Kj = - 108 k J / m  3 for bulk CosoNis0 [7]. Therefore for the following calculation it is assumed that the K~ of the CoNi layers is - 108 k J / m  3 over the whole argon pressure range. If we now perform the calcu- lation according to Eq. (2), we arrive at A s = - 130 × 10-  o and K s = 0.285 m J / m  2 for the curvature measurements and at A s = - 5 6  × 10 _6 and K s = 0.287 m J / m  2 for the stress measurements based on XRD. We observe that the surface anisotropy values of both methods are very similar. The K s determined in this way compares well with results from magnetic layer thickness series from various authors (e.g. [8]). From this good match we conclude that the choice of the argon pressure at which the multilayer is in the stress-free state is justified. Comparison of the saturation magnetostriction values reveals that the magnetostriction obtained by the surface curvature measurement is more than twice as large as that obtained from XRD results. A probable explanation is that during growth the thin film relaxes on a macroscopic scale, thereby reducing the curvature of the substrate. However, the stress measured with the XRD method gives a more realistic value of A s. With XRD the average lattice spac- ing is measured, which represents the stress state within the crystallites of the layer. As the stress anisotropy is also generated on this local scale, the XRD values are thought to be more representative in this case. In conclusion we can say that both methods indicate that the magnetic anisotropy changes from perpendicular to in-plane with increasing compressive film stress. There- fore compressive film stress, which increases with decreas- ing argon pressure, is considered to be unfavourable for (perpendicular) magneto-optic recording. ' I ' I ' I ' I ' ] ' I ' . . . . .  . // compresswe tensile stress (~ (GPa) Fig. 2. Schematical representation of Eq. (1). The dashed lines indicate the x and y axis. The linear fit with slope ce crosses the y-axis at point P. References  [1] S. Hashimoto, J. Appl. Phys. 75 (1994) 438. [2] Q. Meng, W.P. Van Drent, J.C. Lodder, Th.J.A. Popma, J. Magn. Magn. Mater. 156 (1996) 296 (these Proceedings). [3] S. Honda, N. Morita, J. Ago, Y. Tamura, M. Nawate, J. Magn. Magn. Mater. 121 (1993) 506. [4] M.D. Bijker, W.P. Van Drent, J.C. Lodder, J. Magn. Magn. Mater. 48 (1995) 160. [5] G.G. Stoney, Proc. Roy. Soc. A 32 (1909) 172. [6] F.J.A. Den Broeder, H.C. Donkersloot, H.J.G. Draaisma, W.J.M. de Jonge, J. Appl. Phys. 61 (1987) 4317. [7] R.M. Bozorth, Ferromagnetism (Princeton, NJ, 1951), pp. 277, 570. [8] R. Krishnan, H. Lassri, M. Seddat, M. Porte, M. Tessier, Appl. Phys. Lett. 64 (1994) 2312. 

The effect of stress and magnetostriction on the anisotropy of CoNi/Pt multilayers

https://doi.org/10.1016/0304-8853(95)00879-9

The effect of stress and magnetostriction on the anisotropy of CoNi/Pt multilayers

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