(Invited) Controlling Component Distribution in Electrodeposited Multilayer and Alloy Films
Authors: Bakonyi Imre , Péter László 
Published: February 2020

Many advanced materials are produced by electrodeposition nowadays which are mostly alloys or multilayers and contain at least two components. The electrodeposition of some elements and element combinations often requires the use of bath formulations different from the traditional aqueous-based solutions. For the investigation of the properties of such novel electrodeposited materials, it is important to be able to control the deposit composition over the sample volume. For alloy films, this requires ensuring a homogeneous composition profile both laterally and along the growth direction since otherwise the changes of a certain sample property upon varying the deposition conditions may be masked by composition inhomogeneities. For multilayers consisting of two alternating layers of different kind, in addition to the requirement of lateral uniformity, the control of a regular change of composition along the thickness is also necessary.

Over the last two decades, our group was strongly involved in the research activity on the giant magnetoresistance (GMR) effect in electrodeposited (ED) magnetic/non-magnetic multilayer films [1]. During attempts towards a better understanding of layer formation in such multilayers and improving their GMR characteristics, we have identified some factors decisively influencing lateral composition variations and composition modulation in multilayers. We have also succeeded in suppressing the composition variation along the thickness in ED alloy films. In the present talk, the discussion of progress along these lines will cover the following three topics.

 A.      Controlling lateral composition homogeneity of electrodeposits

 Lateral variations can occur due to the well-known edge-effects in the case of electrodeposition in a cell with lateral dimensions much larger than the cathode surface area. Also, the commonly used vertical cathode arrangement usually leads to composition and thickness variations along the cathode surface from the bottom to the top due to the buoyancy effect. The particular sensitivity of GMR to individual layer thicknesses was of great help in revealing macroscopic lateral inhomogeneities of ED multilayers due to both above described effects [2]. The use of a cell with a cathode completely filling the cross-section of the cell and the placement of the cathode with an upward-looking position at the bottom of the cell eliminates these drawbacks [3]. This way one can produce ED films which show a large degree of lateral homogeneity.

 B.       Controlling composition modulation in multilayered electrodeposits

 It turned out from an analysis of possible two-pulse combinations for preparing magnetic/non-magnetic multilayers from a single bath that the best way is the use of a G/P pulse sequence in which a galvanostatic (G) pulse is used for the deposition of the less-noble magnetic metal whereas the more-noble non-magnetic metal is deposited under potentiostatic (P) control. The introduction of a method for establishing the electrochemically optimized potential for the deposition of the non-magnetic spacer layer on the basis of an analysis of current transients during multilayer deposition enabled us to control the composition modulation with a high accuracy. This implies also a proper control of constituent layer thicknesses which was a prerequisite for studying the true dependence of physical properties (e.g., GMR) on layer thicknesses not available beforehand. We have also made attempts to control multilayer formation by a partial anodic dissolution of a thick deposited non-magnetic layer in each period [4] or by intentionally applying a more positive potential for non-magnetic layer deposition than the optimized value and, thus, enabling a partial replacement of the magnetic layer atoms by non-magnetic atoms [5].

 C.       Controlling composition profile along the thickness in electrodeposited alloys

 In magnetic alloys such as permalloy (Ni-Fe), there is an inherent initial composition variation along the thickness when produced by d.c. plating. This occurs due to the interplay of the different deposition preference of the two components and the partial depletion of the solution near the cathode. The initial composition variation may lead to a deterioration of the magnetic properties which can be very pronounced at the small layer thicknesses required in applications striving for miniaturization. Based on depth-profile analysis results, a proper pulse-plating technique could be elaborated to ensure a composition homogeneity even in the initial first few hundred nanometer range [6].

1.      I. Bakonyi, L. Péter: Progr. Mater. Sci.
55 (2010) 107

2.      I. Bakonyi et al., Phys. Rev. B
70(2004) 054427

3.      V. Weihnacht et al., J. Electrochem. Soc.
150(2003) C507

4.      N. Rajasekaran et al., J. Electrochem. Soc.
161(2014) D339

5.      N. Rajasekaran et al., J. Electrochem. Soc.
162 (2015) D204

6.      K. Neuróhr et al., Electrochim.
Acta
103 (2013) 179