TORQUE MEASUREMENTS IN Ni-Ag MULTILAYERS

Torque measurements were carried out on Ni-Ag multilayers in the temperature range 6 to 300 K. From the analysis of the torque curves both magnetization (M) and anisotropy (K, ) were obtained. The K, values thus obtained for t ~ j > 4 nm are in good agreement with FMR data. For t ~ i < 3 nm, a small in-plane anisotropy of the order of 1 x lo6 erg.cm-3 is obtained at 6 K. We had recently reported on our magnetization and FMR studies on Ni-Ag multilayers [I ] . The essential results were as follows. The magnetizationM remains relatively high even for Ni sublayer thickness ( t ~ i ) of 1 nm. FMR measurements yield the effective magnetization 4nM' which is equal to 4rM HA where HA is the uniaxial anisotropy. For t ~ i > 6 nm, HA was strongly positive mainly arising from stress induced effects whereas for t ~ i < 4 nm, HA becomes negative. This latter result was shown to be difficult to reconcile with since it needed a change in the sign of either the stress or the magnetostriction (A) . It was tentatively concluded that HA was not present and 4nM' = 4rM [ I ] . We since measured X and found our conclusions to be right [2]. It was hence decided to carry out torque measurements to get more insight into this problem of anisotropy and we describe our results here. Experimental details The Ni-Ag multilayers used in this work are the same as those in reference [ I ] and had been prepared by sequentiel evaporation in ultra high vacuum using dual electron guns. The growth parameters are as follows: 0.8 < t ~ i < 8.9 and t ~ , = 5 nm. The total number of bi-layers was adjusted to give an effective Ni layer thickness of about 120 nm. Torque measurements were performed in the range 6-300 K with a home madetorque meter with a sensitivity of 0.5 dyne.cm. Torque was measured as a function of angle 8 (measured from the film plane) at every 6' interval through 360'. The angle setting was better than 0.1'. Studies were made also as a function of field in the range 1 to 12.5 kOe. The torque results were analysed to get M and the anisotropy Ku, by both the methods proposed by Neugebauer [3] and Niyajima et al. [4]. Let us note that the results from both these methods agree to within 20 % and we recon here the results obtained by Neugebauer method [3]. ter measurements reported by us in reference [I]. Let us discuss the imisotropy K,. The measured anisotropy (K) and the intrinsic uniaxial anisotropy Ku are related by the equatio? Ku = K + 2 7 ~ ~ ~ . So if K , = 0, K = 2 n ~ ~ , namely, the demagnetization energy, or called the form anisotropy arising from geometrical effects. Fig. 1. Angular dependence of torque curves for different fields for the sample with t ~ i = 7.4 nm. Figure 1 shows the angular dependence of the torque curves taken at different fields 1.0 < H < 12.5 KOe for the sample with t ~ i = 7.4 nm. Analjrsis of the field dependence of the torque (L) for 8 = 45' yields the magnetization and the anisotropy of the sample. The results for t ~ ; > 4 nm, are quite clear and interpretation is straightforward and hence will first be discussed. Figure 2 shows the tN; dependence of K , at 290 and 6 K obtained from torque measurements. A board peak is observed neat t ~ i = 6 nm which could be explained as follows. The stress induced anisotropy Results and discussion [ K , > 01 [ I ] increases at t ~ i decreases may be due to an increase in the stress. However Ku vanishes near t ~ i = The magnetization ( M ) values obtained from torque 4 nm and changs sign for smaller Ni layer thickness as measurements agree well with those from magnetomeexplained in the following paragraph. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888797 C8 1752 JOURNAL Dl3 PHYSIQUE However, we are faced with a serious difficulty if we want to correct for the possible presence of dead layers. Taking this to be between 0.5 to 1.0 atomic layer of Ni, the whole reasoning would change. It is possible then to show that for an appropriate value of dead layer thickness KM z 2 x ~ ' . At present, we are unfortunately not in a position to decide, though we are tempted to conclude rather that the surface anisotropy could be negligibly small and that M 'values, higher t h b measured, in the same trend as we proposed in our earlier work [I]. 0 5 10 t,i (nm) Fig. 2. t ~ i dependence of K, at 290 K and 6 K obtained from torque measurements. Samples with t ~ i > 4 nm. Now let us discuss the results on anisotropy for samples with t ~ i < 3 nm. As we had mentioned earlier if we consider FMR data (47 r~ ' ) along with 47rM measured with a magnetometer then an in plane anisotropy namely K, < 0, would result [I]. For these samples we measured the angular dependence of the torque curve for 0 < 8 < 180" and calculated the anisotropy from the maximum value of torque. Then from this measured value of anisotropy KM we obtained the intrinsic anisotropy Ku from the relation Ku = KM 27r~ ' . For M we took the values from magnetometer measurements. Figure 3 shows the results at 290 and 6 K where K, is plotted as a function of inverse Ni layer thickness. It indicates that for t ~ i N 4 nm, K, tends to zero. If this were to be the surface anisotropy arising from the surface Ni atoms (due to the local symmetry) [5], then its dependence on ( t ~ i ) ' should be linear which is not the case. Also our values (a few kOe expressed as field) are much smaller than what is usually found. For instance for Ni(ll1) on Cu, Gradmann reports surface anisotropy of 40 kOe 161. The anisotropy field obtained from the relation 47rMf = 4nM HA, also is in good agreement with the present torque measurements (Ku = 2 H n l ~ ) (1). 0 0.2 0.4 0.6 0.8 (+,;I ) [nm-'1 Fig. 3. Anisotropy Ku (in-plane) as a function of 1 / t ~ ; at 290 and 6 K. [I] Krishnan, R., Tessier, M., Frey, Th. and Jantz, W., Trans. IEEE Magn. MAG-23 (1988) 3713. [2] Zuberek, R., Szymczak, H. and Krishnan, R., ICM-88. [3] Neugebauer, C. A., Phys. Rev. 116 (1959) 1441. [4] Miyajima, H., Sato, K. and Mizoguchi, T., J. Appl. Phys. 47 (1976) 4669. [5] Neel, L., C. R. Acad. Sci. 237 (1953) 1468. [6] Gradmann, U., J. Magn. Magn. Mater. 54-57 (1986) 733.