applied surface science
Applied Surface Science 65/66 (1993) 131-133 North-Holland
Structural and magnetic studies in Ni/Ti multilayers
M. Porte ‘, H. Lass-i ‘, R. Krishnan ‘, M. Ksabouchi b, M. Msaza b and C. Sella b
‘I Laboraroire de Magnhisme et Mat&au Magnhtiques. CNRS, 92195 Meudon, France h Laboratoire de Physique des MatMaMu, CNRS, 92195 Meudon, France
Received 29 June 1992; accepted for publication 2 October 1992
Structural and magnetic studies have been carried out on Ni/Ti multilayers Qrepared by DC triode sputtering. Both metal layers are crystalline with a (111) fibre structure when they are thicker than 20 A. But for thinner layers one observes a solid solution with an amorphous-like structure. 0 The magnetization decreases with t(Ni) and the analysis of the results at 5 K indicates the presence of a dead Ni layer about 12 A thick. The t(Ni) dependence of the effective anisotropy shows the absence of surface anisotropy contribution.
1. Introduction
Ni/Ti multilayers are particularly interesting for application as neutron mirrors and are being intensely studied. The grazing-angle neutron diffraction studies enables one to characterize the multilayer structure. The fact that Ni is ferromag- netic, a study of the magnetic properties of this system also can bring additional information on the state of the interfaces. Therefore, we have undertaken such a study and describe our results here.
2. Experimental details
The multilayers have been prepared by DC triode sputtering under computer-controlled con- ditions. In thisosystem the layer thickness is con- trolled to & 1 A and the reproducibility is better than 0.1%. Water-cooled glass and NaCl sub- strates have been used. Theodeposition rates of Ni and Ti were 22 and 17 A/min, respectively.
Both layers had the same Lhickness and were varied in the range 10 to 120 A. The total number of bi-layers was ad,justed so as to get a total Ni layer about 500 A thick. The modulation was verified by low-angle X-ray diffraction. Structural
studies were made by transmission electron mi- croscopy (TEM). Magnetization (M > and in-plane M-H loops were obtained using a vibration sam- ple magnetometer and the anisotropy using a torque magnetometer. The above studies were carried out from 5 K to 295 K.
3. Results and discussions
First let us discuss the structure of single lay- ers. The structure of Ni was fee and was inde- pendent of its thickness. The discontinuous na- ture of the diffraction rings indicates that the grain size is of the order of the layer thickness.
The structure of Ti strongly depends on its thick- ness. For 50 5 t(Ti) < 100 A, one has a fee struc- ture. Similar results have been reported [l] also for other refractory metaos such as MO, W, etc.
However for t(Ti) 2 150 A, both fee and hcp are found to coexist. In the cast of multilayers, for metal layers thicker than 40 A, both layers are in fee crystalline form. The diffraction rings are well defined and their intensities indicate a fibre structure with (lll)Ni-(111)Ti parallel to the film plane. But for thinner layers, the diffraction rings of Ti disappear progressively and the width of Ni rings increases, resulting in a Ni-Ti solid solution 0169-4332/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
132 M. Porte et al. / S@uchu-al and magrwric studies in Ni / Ti multilayer.~
Fig. 1. Electron diffraction patterns of the Ni/Ti multilayers where r(Ni) = t(Ti). with metal layer thicknesses (a) 10. (b) 20.
Cc) 60 and (d) 120 A.
with an amorphous-like structure, characterized by three halos whose diameters are very close to the (ill), (220), and (311-222) rings of fee Ni.
Figs. la to Id show Jhe results for t(Ni) = t(Ti) = 10, 20, 60 and 120 A, respectively.
The magnetization decreases strongly with a decrease in Ni layer thickness. Due to the de- crease in the Curie temperature CT,.) with t(Ni), the decrease in M at 295 K is stronger for thin- ner Ni layers. Therefore, let us consider the re- sults only at 5 K which are shown in fig. 2. The general t(Ni) dependence of M could be ex- plained in terms of a dead layer of Ni at each interface, due to alloying effects. The thickness of such a Ni layer is estimated to be in the range 12 to 14 A. The calculated curves are also shown in fig. 2. We have plotted in fig. 3 the product M x t(Ni) as a function of t(Ni) both at 295 and 5 K. The slope which corresponds to the magneti- zation yields 489 and 500 emu crne3 at 295 and 5 K, respectively. It is also seen Lhat the dead Ni layer corresponds to 14 and 12 A at 295 and 5 K, respectively. It is interesting to mention here thtt a total (Ni + Ti) interfacial layer of about 16 A has been reported from grazing-angle neutron reflectometry studies [2].
tNi in A
Fig. 2. r(Ni) dependence of the magnetization at 5 K. The dash-dotted line and the dashed line represent the variation of the magnetization assumjng dead Ni layers of thickness IO
and 12 A. respectively.
The in-plane M-H loops are all rectangular.
No significant anisotropy was found in the film plane. The coercivity first increases slightly from 21 to 26 Oe when t(Ni) decreases from 120 to 80 A, then decreases strongly to 5 Oe when t(Ni) decreases to 40 A. This decrease could be at- tributed to the change in the microstiucturc. TEM studies have shown that for a 40 A layer thick- ness, the width of the Ni diffraction rings in- creases and one has an amorphous like phase.
Since amorphous materials do not have grains and hence no grain boundary, there will be no pinning of the domain wall and hence the coerciv- ity is expected to decrease.
Torque studies yield the effective anisotropy K,,. The t(Ni) dependence of K,, could be analyzed on the basis of the well known phe- nomenological model which predicts the follow- ing relation:
Kcf, = K, + 2K,/t(Ni),
(1)
0
tN1 ,n ii
Fig. 3. Variation of the product M x t(Ni) with I(Ni) at 295 and S K.
M. Porte et al. / Structural and magneric studies in Ni / Ti multilayers 133
Fig. 4. Variation of the product Kcff x t(Ni) with t(Ni) at 5 K.
where the volume anisotropy K, = K,,,, + 2-rrM2 ( Kcwst being the crystalline anisotropy) and K, is the surface anisotropy arising from the surface Ni atoms.
Fig. 4 shows the results at 5 K in order to avoid the effects arising from the lowering of T, for thinner Ni layer samples. It is seen that the linear behaviour predicted by the model ds ob- served only in the range 40 5 t(Ni) I 120 A. For Ni layers thinner than 40 A, even at 5 K, the experimental points deviate strongly and fall be- low the extrapolated straight line. This behaviour shows that the interface structure is strongly per- turbed for these samples, which is in agreement with our TEM studies which also show strong alloying resulting in an amorphous-like phase.
The present results shows that there is no contri- bution to K, from the Ni atoms. This is in agreement with our result on Ni/Ag [3] and that on Ni/Pd by den Broeder et al. [4]. However, recently we have shown that in the Ni/Pt system a strong perpendicular anisotropy is present aris- ing from the surface anisotropy [5]. It is well known that the surface anisotropy arises from the
local crystal fields and hence should not only depend on the state of the interface but also on the electronic structure of the neighbouring metal.
From the slope of the straight line in fig. 4, the volume anisotropy is found to be - 1.4 X lo5 erg cm -j. Knowing M, we can calculate Kcw5t for the samples. For instance, Kcwst values are found to be 5 x 10’ and 4.7 x 10” erg crnp3 for the sam- ples with t(Ni) = 80 and 40 A, respectively. This shows that some uniaxial anisotropy is developed in these samples which could well be due to the stress-induced anisotropy. At this stage of the work it is not possible to infer more details on this aspect. Studying magnetostriction could bring more information and is planned for the future.
In conclusion, we have prepared Ni/Ti multi- layers and studied their structural and magnet$
properties. For layer thickness higher than 20 A, both metal layers are crystalline and showOa (111) fibre structure. However, for layers of 10 A thick, one observes a solid solution with an amorphous- like phase. The magnetization decreases with Ni layer thickness and this attributoed to the presence of a dead Ni layer of about 12 A. No contribution to the surface anisotropy from surface Ni atoms was observed.
References
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