3.1 Introduction and background

A key in the competitive development in this century will be, undoubtedly, the development of products with a wide range of functional aspects. In this context, multifunctional materials known as smart materials will play a decisive role (Gandhi and Thompson, 1992). Among many other systems, multiferroic materials stand out for their great applicability.

Ferroic is the generic name for a material that exhibits ferromagnetic, ferroelectric, or ferroelastic behavior. A characteristic spontaneous magnetization, electric polarization, or strain appears, respectively, on those materials (Wadhawan, 2000). A material is called multiferroic when two or more of these ferroic properties coexist in the same phase, namely, magnetoelectricity (ferromagnetism and ferroelectricity) (Bichurin and Wiehland, 2012), magnetostriction (ferromagnetism and ferroelasticity) (Gibbs, 2000), or piezoelectricity (ferroelectricity and ferroelasticity) (Taylor, 1985).

The present increasing need for miniaturization of electronic devices and the great advances in nanotechnology and materials engineering allow us to manufacture multiferroic-based devices at the nanoscale. At this size range, the objects exhibit new physical properties different from the bulk analogs (Stefanita, 2008). These new properties have attracted the attention of the scientific community due to the emergence of new effects that could expand the technological applications. Because of their properties, which are particularly dependent on the dimensionality, several studies of 1-D nanostructures have been performed. Thus, there is an increasing interest in the synthesis of nanowires (He and Tao, 2004), nanorods (Tien et al., 2008), nanotubes (Daub et al., 2007), or nanobelts (Wang, 2004), linked to the study of these low-dimensional systems.

The nanowire is a model in the study of the size-confinement and surface effects on the magnetic, electrical, or optical behavior (Liz-Marzán and Giersig, 2003). For instance, exotic new magnetic and electric polarization states like helical antiferromagnetism can be present in this scale due to geometric frustration and breaking of the long-range coupling (Young et al., 2014). Also, in this scale, novel quantum effects associated with 1-D singularities in the electronic density of states can be observed. On the other hand, 1-D nanostructures have a great appeal in technologies as sensors (Patolsky et al., 2006), high-density magnetic storage media (White, 2000), optics (Agarwal and Lieber, 2006), or electronic technologies (Capasso and Datta, 1990). Furthermore, they can be part of components in devices and nanocircuits (Yan et al., 2014). To fabricate such structures, the template-assisted methods have been revealed to be conceptually simple and effective (He and Tao, 2004). These templates usually contain cylindrical pores, with diameters of tens or hundreds of nanometers, which act as the negative of the desired material. The most used template is probably anodized aluminum oxide (AAO). Its versatility has been proven in the manufacture of materials with different morphologies (nanowires, nanotubes, or antidot thin films) or compositions (inorganic, organic, or inorganic–organic hybrids). Although disordered AAO membranes were used to embed magnetic materials (Keller et al., 1953; Huysmans et al., 1988) a long time ago, the fabrication of nanowires using AAO as a template became common only after Masuda’s group published the conditions to produce a self-ordered regular nanoporous structure in alumina in 1995 (Masuda and Fukuda, 1995). Nevertheless, the achievement of an AAO template had come earlier, in 1989 (Furneaux et al., 1989). After the subsequent use of AAO as filters or masks (Masuda and Satoh, 1996), there has been an increased interest in the tuning of such nanostructures with controlled morphology parameters as pore diameter or interpore distance.

In order to fill the template, several strategies can be employed. An AAO template allows the use of many different techniques to fill up the pores. A widely used technique is electrochemical deposition or electrodeposition. Ferromagnetic nanowires were fabricated by electrodeposition a long time ago for decorative purposes (Wernick et al., 1987), and nowadays there are simple techniques to build inhomogeneous wires modulated along their length in diameter or composition. Also, alloy nanowires can be produced, see for instance (Fei et al., 2007). By means of the sol–gel chemical route, high-purity nanoparticles, in particular oxides of magnetic materials as ferrites or perovskites, have been obtained. The sol–gel technique creates the possibility to fabricate oxide magnetic nanowires (Pechini, 1967), particularly interesting for their multiferroic properties, as was published very recently in de Oliveira and Pirota (2012).

First, this chapter will focus on the preparation of template-assisted heterogeneous ferromagnetic nanowires modulated in geometry or composition by means of conventional electrodeposition filling. Also, sol–gel nanowire production is presented. Finally, we pay special attention to the combination of the two so-called fabrication methods, sol–gel and electrodeposition. This combination allows for the fabrication of core–shell metallic or insulating structures. In these heterogeneous materials, in which metal or insulator components can be placed in both core or shell parts, the individual properties of each constituent can become quite different. Also, possible exchange relations (e.g., exchange bias, exchange spring) or ferroic coupling (e.g., magnetoelectricity induced by mechanical stress) makes them especially interesting. Also, ferromagnetic–multiferroic combinations inside the wire can be produced.

3.2 Nanowire synthesis: A general overview

A general overview of the synthesis of nanowires is sketched in Figure 3.1. Besides top-down lithographic methods, which are out of the scope of this chapter, bottom-up synthetic methods are presented. Although, in some cases, direct reactions will give rise to nanowire structures (Sun et al., 2002; Yu et al., 1997), in a surfactant organized media or positive-template mediated in DNA chains (Mbindyo et al., 2001) or carbon nanotubes (Fullam et al., 2000) in solution, most nanowires are produced by template-assisted methods. The essential features of a good template are chemical stability, the mechanical properties, and homogeneity of the pore ordering, namely, density and diameter. Block co-polymer templates are based on the synthesis of a structure of two noncompatible monomers that tend to be self-organized in macroscopic structures as lamellas or cylinders. In the last case, a selective removal of the cylindrical monomer phase gives rise to a porous polymer template. The most well-known system is the polymethylmethacrylate/polystyrene (Thurn-Albrecht et al., 2000a,b). In the track-etched membranes, a nonporous substrate, mica (Sun et al., 1999, 2000) or a polymer (polycarbonate or polyester) (Ferre et al., 1997), is bombarded with an ion beam in order to create damaged tracks. The membrane formed can be chemically etched to obtain the desired pores (Fleisher et al., 1975). Although the resulting pores have a uniform diameter, the drawback of this technique resides in the lack of porous ordering. The porous alumina membranes are also widely used and will be extensively described in the following text. After the porous template is created, in addition to metals, several materials can be grown inside the pores, for instance, polymers (Martin et al., 1993), semiconductors (Lakshmi et al., 1997), carbon (Manoharan, 2009), oxides (Wang et al., 2007), or intermetallic materials (Huang et al., 2002).

3.3 Advanced heterogeneous ferromagnetic nanowires by electrodeposition

AAO membranes are usually fabricated from a high-purity aluminum foil by the well-known two-step anodization process (Masuda and Fukuda, 1995). The resulting structure is a honeycomb in which a scalloped oxide barrier layer separates the porous zone and the sustaining aluminum. Highly ordered pore arrangement with narrow size distribution is obtained for some optimum values of voltage and temperature (Nielsch et al., 2002) or via previous prepatterning methods such as imprint lithography (Choi et al., 2003a) or e-beam lithography. In order to control the temperature, a specially designed electrochemical cell is usually required.

The field-assisted alumina solution at the electrolyte–oxide interface (bottom of the pore) is compensated by oxide growth at the oxide–metal interface (Parkhutik and Shershulsky, 1992); the pores grow downward, arranging with time in a regular array. Although the true reason for the self-ordering is still unknown, in Jessensky et al. (1998) it was suggested that the mechanical stress, which is associated with the expansion of the aluminum during oxide formation, is the cause of repulsive forces between neighboring pores during the oxidation process and leads to self-organized formation of hexagonal pore arrays. Other explanations have been proposed (Pashchanka and Schneider, 2011; O’sullivan and Wood, 1970; MacDonald, 1993).

In order to reduce the time and manipulation that are required for double anodization, great advances have been achieved using nanoimprint and lithographic techniques before anodization (Asoh et al., 2001) (see Choi et al., 2003b, for substrates different than Al). Nanostructured titanium nitride (TiN) molds have recently been developed (Navas et al., 2007), following the procedure described in Figure 3.2. TiN thin films were grown by reactive magnetron sputtering using a Ti target (99.995% purity) on different AAO membranes as substrates (Figure 3.2a). In all cases, the mean thickness of the plain TiN films is 0.7 μm, controlling the deposition rate. Ti layers of around 1 μm were sputtered to achieve a larger stability, and an aluminum/alumina specimen can be detached by means of selective chemical etching as described in Pirota et al. (2004). The TiN films and the Ti/TiN bilayer were attached to a silicon wafer to facilitate the manipulation (Figure 3.2b).

Prepatterned aluminum can be obtained by applying pressure with the nanoimprint tool (Figure 3.2c). Finally, with this mechanically puckered aluminum, only one anodization is needed to obtain highly ordered AAO membranes without the tedious anodization/removal puckering described in the two-step anodization process. Due to their great hardness and chemical passivation, the TiN molds can be used several times and without corrosion. An atomic force microscope (AFM) image of a typical prepatterned Al foil is shown in Figure 3.2d.

It is quite interesting and easy to produce structures with variable diameters but that have the same distance between wires via wet etching. After the widening process is calibrated by rinsing in a phosphoric acid solution, the later filling gives rise to systems that can be used as a model to evaluate dipolar effects. A typical scanning electron microscopy (SEM) image from a widened AAO surface fabricated from an original 35 nm diameter pore is shown in Figure 3.3.

3.3.1 Geometry modulation

3.3.1.1 By pore widening and third anodization

The combination of the conventional growth of alumina and pore chemical widening (procedure in Figure 3.4) allows researchers to produce templates in order to get double-diameter nanowires, which are important for thermoelectric energy conversion (Zianni, 2010) and for computing purposes (Parkin et al., 2008). Basically, the Al is anodized in two steps, and the resulting template is pore-widened in accordance with the processes explained before (steps a–e). The third anodization (step 5) produces the bottle-neck and the diameter modulation of the pore (f). See an SEM micrograph of a modulated-diameter sample in Figure 3.5. The geometry of the pores can be easily manipulated via the anodization voltage, etching widening time (step 4 of Figure 3.4), and the second/third anodization time ratio (steps 3 and 5 of Figure 3.4).

3.3.1.2 By hard-mild anodization change

Essentially in a conventional anodization, as the electrolyte acid force increases, the applied voltage has to be reduced in order to prevent vigorous fizzing and strong heat emission. When this occurs, the alumina becomes yellow and sequentially darker, giving rise to a disordered and cracked alumina coating (Wernick and Pinner, 1972). This “hard” anodization, used in the industry since the 1960s with a high current density in an electrolyte at zero or negative temperatures, has been recently developed for research purposes by Lee et al. (2006), producing the so-called honeycomb structure. To stabilize the fast growth rate requires (1) the use of a specially designed cell in order to mitigate the large amount of heat generated and (2) the previous growth, before start the hard anodization, of a low-voltage coating alumina. In this system, the growth rate of the alumina layer increases drastically, avoiding the necessity of manipulation and the time-consuming double anodization. In Figure 3.6, we can observe the pore diameter variation with the applied voltage in this system. Samples reveal lower porosity values: 3–4% in contrast to the 10% that is expected using conventional conditions. Interestingly, for the conditions in which the interpore distance is equal in both systems, the hard anodization process produces narrower pores. This fact can be used to modulate the pores exchanging the anodization conditions (Lee et al., 2006).

3.5 Combined techniques of synthesis

It is possible to combine two or more synthesis techniques in order to obtain a complex 1-D nanostructure. In the following, we will highlight the core–shell nanowires, denoted as core@shell. The combination of the production techniques described above (sol/gel and electrodeposition) can result in many different variable geometry and composition materials: metallic/insulating, smooth/modulated profile, or nanowires/nanotubes.

An easy strategy to obtain core–shell nanowires is to oxidize the metallic wires produced by electrodeposition extracted from the template. Using this approach, several different types of core–shell nanowires were produced, for instance, Fe@FeO_x (Lee et al., 2008), Bi@Bi₂O₃ (Li et al., 2006), Si@SiO_x (Lin et al., 2010), and Ni@NiO (Hasan et al., 2012; Kim et al., 2012).

Another way is to synthesize the shell before the core filling. The processes can be performed (1) only by electrodeposition, which has been successful in the fabrication of M@M_xO_y structures (M = metal) or (2) by a combination of sol–gel and electrodeposition. The clear advantage of this last way resides in the versatility of the core–shell nanowire types produced. For instance, D@M nanowires (D = dielectric) can be made by electrodepositing metallic nanotubes inside the AAO template, followed by sol–gel casting of a dielectric inside the tubes (Huang and Hao, 2009). Because the diameter required to grow the core wire is reduced with respect to the AAO pore section, casting the tubes with the sol component involves difficulties resulting in the manufacture of thick-nuclei core–shell nanowires. The inverse M@D nanowires can be synthesized thinner than those of D@M by an inverse procedure: the dielectric shell growth by sol–gel technique and the subsequent metallic electrodeposition of the core. Previously, the template/nanotube specimen had to be annealed at 400–500 °C to get rid of the organic counterpart. Depending on the material, the annealing can result by sintering in the desired phase.

Figure 3.10 shows a TEM image of an M@D nanowire made by our group with this combined technique: A Ni electrodeposited core is wrapped by Fe₂O₃ nanoparticles produced by a modified sol–gel technique (Pechini, 1967).

The combined techniques allow combining the individual properties of the constituent materials to obtain new, fascinating properties. The resulting composite material has effects derived from the core–shell interaction, such as exchange bias (Nogues and Schuller, 1999), exchange spring magnets (Kneller and Hawig, 1991), and magnetoelectric effects resulting from magnetostriction and piezoelectricity (Eerenstein et al., 2006).

3.6 Multiferroic nanowires

A ferroic material presents spontaneous polarization, magnetic or dipole moment for ferromagnetic or ferroelectric materials, respectively, and strain alignment for a ferroelastic material. Every ferroic behavior can be switched by applying a magnetic, electric, or stress field. These materials by themselves already represent a very well studied class with various technological applications. Multiferroics are an interesting class of materials that are attracting considerable attention because of the interesting fundamental physics involved and their promising practical applications. They combine two or more ferroic properties (Figure 3.11) and can be divided into single phase or multiphase.

3.6.2 Multiphase multiferroic synthesis

The difficulties associated with the coexistence of ferromagnetism and ferroelectricity at the same phase have been solved with the manufacture of two-phase multiferroics in which the ferroelectric and ferromagnetic are different phases. In a general view, such materials can be obtained in different shapes and structures: (1) ferromagnetic nanoparticles by a chemical route embedded in a ferroelectric matrix as in Figure 3.12a, (2) thin film ferromagnetic/ferroelectric multilayers (Figure 3.12b), or (3) ferromagnetic nanowires inside a ferroelectric matrix (Figure 3.12c). In these cases, the coupled effect arises from interface effects between the elastic components of the ferromagnetic and ferroelectric phases. An electric field causes strain in the ferroelectric phase, and this strain is transferred to the ferromagnetic phase, causing magnetization. Thus, the magnetoelectric effect will be greater if we increase the coupling between the interfaces. Larger surface areas with a large magnetostrictive or piezoelectric response are desired in these materials. Employing the combined techniques described in Section 3.5, it is possible to fabricate composite multiferroic nanowires using AAO as a template in core@shell morphology.

3.7 Conclusions

In conclusion, the controlled fabrication of multifunctional single- or multiphase nanowires can play an important role in the design of smart nanodevices. In this chapter, we focus on the fabrication methods based on filling anodic aluminum oxide by electrodeposition or sol–gel techniques. An insight of using the combined techniques was also given, creating the opportunity to fabricate multiphase nanowires with desirable properties. As a perspective, in the case of nanowires or nanotubes fabricated using the anodic aluminum oxide template, it will be very important to develop reproducible methods of single nanowire separation. That can allow for handling of isolated nanowires, which is a very important factor, both for technological applications and for the study of fundamental material properties.