Compositionally modulated multilayer

Sato, K. Kida, H. (1988). Calculations of magneto-optical spectra in compositionally-modulated multilayered films. Journal de Physique, Colloque C8, C8-1779-80. 

CeUs J., Van Acker K., CaUewaert K. and Van Houtte P. (1995), Residual stress measurements in electrolytic copper-nickel compositionally modulated multilayers , J. Electmchem. Soc., 142,70-4. 

The use of UPD layers can in principle generate deposits with composition modulated on the atomic scale, and Pauling et al. have produced what they call hetero-structured ultra-thin films containing Ag, Pd and T1 by this method [158], Stickney and coworkers have assembled multilayered deposits of CdTe and GaAs by addition of one atomic layer of the individual components at a time, a process they call electrochemical atomic-layer epitaxy [159 162], The essential controlling feature in the UPD mechanism is that the deposited layers are allowed to reach equilibrium. Hence, the process represents an extreme of local, reversible control. 

Electrodeposition of composition-modulated films was first performed by Brenner in 1939 (1) by employing two separate baths for the two components and a periodic immersion of the deposit in the two baths. This is too cumbersome a method to be adopted in practice. Deposition from a single bath with the presence of salts of the two components of the multilayer is what is desired, but there was a serious problem with the deposition of two metals from one bath. Specifically, whereas a layer of the more noble member can be deposited by choosing the potential to be between the reduction potentials of the two metals, one can expect that when the potential is set to a value appropriate for reduction of the less noble member, both will be deposited, resulting in an alloy layer rather than a pure metal. Thus, to nobody s surprise, even as recently as 1983, Cohen et al. (2), were able to deposit only a layered structure of alloys rather than pure metals. In addition they cast doubt on the possibility that a modulation cycle (the thickness of the basic layers, the periodic repetition of which... 

The actual deposition of the multilayered composite can be carried out by either current or potential control. Clearly, a pulsed polarization curve has to be constructed for the former case. The actual composition modulation cycle would be controlled coulometrically by fixing the amount of electric charge delivered while at point A in Figure 17.2 Qp and the amount of charge delivered while at point B in Figure 17.2 (2b) via suitable input to the unit regulating the pulsing. 

The choice of materials for metallic systems is still expanding and at present various examples of combinations with different atomic radii are being prepared. Here multilayered techniques also show possibilities for new material syntheses. In contrast to materials prepared by chemical procedures, supedattices are made far from equilibrium. The various possibilities for layering the artificial supedattice materials are given in Figure 16c—f. Most of the stacked layers (c, d, f) have more or less sharply defined boundaries and some have a noncrystalline structure in the individual layers (c) or one of the layers is noncrystalline (d). In such situations the structural information is not transmitted between adjacent layers and therefore, stricdy speaking, no supedattice is formed. In the case of an unsharp boundary (e), compositionally modulated alloy-layered structures have been made. The amplitude of composition modulation in the center of a layer can be in the range of 0 to 100%. Supedattices can also be formed with sharp boundaries (<5% of the thinnest layer) between the two components. 

Electrodeposition of composition-modulated films was first performed by Brenner in 1939 (1) by employing two separate baths for the two components and a periodic immersion of the deposit in the two baths. This is too cumbersome a method to be adopted in practice. Deposition from a single bath with the presence of salts of the two components of the multilayer is what is desired. 

The variation of hardness with multilayer wavelength in a range of different types of structures. These include multilayers of (a) isostructural transition metal nitrides and carbides, which show the greatest hardening (b) nonisostructural multilayer materials, where slip cannot occur by the movement of dislocations across the planes of the composition modulation, because the slip systems are different in the two materials and (c) materials where different crystal structures are stabilized at small layer thicknesses, such as AIN deposited onto TiN. 

P. Ganesan, S.P. Kumaragura, B.N. Popov, Development of compositionally modulated Zn-Ni multilayer deposits as replacement for cadmium. Surf. Coat. Techol. 201 (2007) 7896-7904. 

Fig. 3. Schematic diagrams of magnetic structure for (a) thick (Int. = interface regions) and (b) thin R/T multilayers (c) compositional modulation for the thin case (after Shan and Selln fer 1990b).

In this section we give a brief review of the experimental results including the layered or compositionally modulated structure for R/T multilayers, the layer thickness and temperature dependencies of magnetic properties for Dy/Fe, Dy/Co and Tb/Fe multilayers. We will not provide a comprehensive review of all recent work in this field. Rather, our aim is to focus on discussing die above fimdamental properties, and only limited references which are closely related to this discussion are dted. The role that the interfacial magnetism plays in determining magnetic properties will be emphasized. 

It is concluded from table 1 that for most of the samples only the first diffraction peak, which implies sinusoidal composition modulation, is observed for the bilayer thicknesses A <30 A. This implies that R/T multilayers with thin layers have diffuse interfaces and... 

Figure 23.18. Pourbaix diagram of Co-water system [84]. (Reprinted by permission of ECS—The Electrochemical Society, from Balirololoom ME, Gabe DR, Wilcox GD. Development of a bath for electrodeposition of zinc-cobalt compositionally modulated alloy multilayered coatings.)...

Zr—Fe/TbFe)n amorphous multilayers. Katayama et al. (1987) have studied the variation of saturation magnetization as a function of the composition modulation period. As the multilayers becomes thinner, increases. This is explained by the fact that magnetization of Fe within the ZrFe film is induced by the TbFe film. 

In a multilayer structure built by successive depositions, it can happen that the structures at the top and bottom interfaces of each layer have different chemical profiles. In other words, the compositional modulation may have a unidirectional profile with respect to the film growth direction. Mossbauer spectroscopy is a unique method to clarify the difference of interface chemical profile in two interfaces top and bottom. In general, if a Mossbauer probe is located at the interface of magnetic layer, observed hyperfine field distribution is useftd information for the estimation of degree of intermixing at the interface. 

The principles described have been used to solve an interesting case of galvanic corrosion observed in compositionally modulated metal multilayers (CMMs) and alloys. The electrodeposition of copper and nickel alloys or metal multilayers has received attention due to their practical importance for corrosion protection (Jensen et al, 1998 Fei arrd Wilcox, 2006) as well as rrtagnetic (Foecke and Lashmore, 1992 Lenczowski et al, 1995 Miyazaki et al, 1999 Bakor i et ai, 2002 Spray and Nowak, 2006) and mechanical (Tsalakos arrd Jarrkowski, 1986) applications. Details of the rrses and fabrication are elaborated elsewhere (Roy, 2009). 

Roy, S. (2009), Electrochemical Fabrication of Nanostructured, Compositionally Modulated Metal Multilayers (CMMMs) , in Schmiiki, P. and Virtanen, S. (eds) Electrochemistry at the Nanoscale. Berlin Springer-Verlag 349-76. 

Alternating layers of two different materials with different laminate thickness in the range of nanometers can be fabricated by electrochemical deposition. This technique demonstrates the capabiUty of the relatively inexpensive process of electrodeposition for the production of advanced materials with ultrafine microstmcture (Schwarzacher et al, 1997). Electrodeposition of multilayers can be carried out using either a single or dual bath approach. In dual bath electrodeposition the substrate is transferred to suitable electrolytes during the deposition of each layer. A single bath is a more desirable process and compositional modulation of the layers can be achieved by prdsing the current or potential. 

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