Journal of Materials Science and Nanomaterials
Open Access

Thin films of numerous materials possess controlled physical, chemical and technological properties that make them extraordinarily useful in a very large number of devices [1-4]. The techniques used for their manufacturing must be precisely controlled. Among these, the Pulsed Laser Deposition (PLD) technique allows the deposition of films of one or several elements as well as of compounds, both metallic and especially non-metallic, with thicknesses between a few and hundreds of nanometers [5-9]. The manufacture of these films formed by a monolayer or by multilayers of the same element or alternatively or consecutively of several elements, is also feasible [10-12]. The enormous variety of physical and chemical properties of these films, that are associated with their shape and size, are being used in multiple devices in fields of science and technology, such as Physics, Chemistry, Biology, Medicine, Electronics and Computing [13-17]. All this, together with the accessible opportunity of great packaging of these films, favored by their controllable dimensions, make them essential for the vast majority of devices and tools that our society uses every day in virtually all fields of activity based on micro and nanoelectronics [18,19].

Numerous parameters are involved in the special manufacturing process of thin films. In particular, it is known that oblique deposition techniques [20], along with PLD [4,7-12,21], such as "molecular beam epitaxy " [22] or evaporation [23] for example, are used for the generation of physical anisotropies in different materials. Specifically, the nature, shape, relative arrangement and the mechanical-kinetic state of the primary parts required for the PLD technique (the laser, the target on which the incidence of the laser is produced, the plasma generated and the substrate on which the film grows) have allowed us to be pioneers in the fabrication of thin films by PLD with special and controlled magnetic, electrical, electromagnetic, ferromagnetic resonance, optical and mechanical properties [9-13,24-28]. Indeed, we have developed a specific technique of oblique incidence by PLD [29, 30] thanks to which the growth of nano-films is achieved due to the particular position of our substrates during the PLD: they are located such that the perpendicular to their plane forms a constant angle with the direction of the plasma (the angle of oblique deposition) and its axis of rotation is always parallel to the direction of the plasma during PLD. This causes that the direction of growth of the films (always towards the source of the plasma) maintains a constant angle with respect to the plane of the substrate [29]. So, the growth of thin films constituted by a superposition of oblique nano-sheets with ≈ 300 nm in length, between 3 and 70 nm in width and ≈ 4-5 nm in thickness with separation between them ≈ 1.5 nm was achieved for the first time. In addition, its angle of inclination could be varied between ≈ 40° and ≈ 0° with respect to the perpendicular to the plane of the thin film [29,30]. This special nano-morphology gives the films a whole series of anisotropic properties, which are often desirable for numerous devices as well as for the fundamental study of thin films with a nanomorphology unknown until now.

In this way, PLD Co films (thicknesses between 60 and 90 nm) were produced with uniaxial in-plane magnetic anisotropy fields of ≈ 500 Oe [29,30]. Considering magnetostatic energy interactions and due to the shape of the nano-sheets described above, the easy direction of magnetization in each nano-sheet was the longitudinal one (perpendicular to the incidence plane of the plasma). Besides, studies of the surface topography showed also a nano-morphology with surface nano-strings of average width of 10-30 nm and length of 200-300 nm, depending on the manufacturing conditions [8,24,29,30].

The effects of this special nano-morphology on other physical properties were also relevant. These Co films exhibited an anisotropic resistivity in the film plane: when an electric current flowed along the perpendicular direction of the nano-sheets, the resistivity was approximately 30% higher than the resistivity measured parallel to the nano-sheets [30]. Besides, the optical properties of these thin films were also affected by this nano-morphology: the transmission of polarized light through the sample was in fact anisotropic: the intensity of the light transmitted through the sample depended on the angle between the direction of the plane of polarization of the light and the direction of the nano-sheets in the Co film [30]. Furthermore, the anisotropic elastic behavior of these Co films was also evidenced: it was determined by measuring the changes in the resonant frequency of Si micro-cantilevers coated with these Co oblique nano-sheets [24,31]. The selected position of each micro-cantilever during the coating process created longitudinal or transverse nano-sheets: for microcantilevers coated with longitudinal nano-sheets, the frequency of the mechanical resonance decreased ≈ 1.5% with respect to the non-coated ones, while it decreased ≈ 3% for micro-cantilevers coated with transversal nano-sheets. This differential procedure allowed determining the difference between the Young’s modulus of the different films based on the different direction of the nano-sheets. This difference was determined to be, at least, the 20% of the Young’s modulus of the bulk Co [31].

In addition, studies in thin films of Co-metal compounds, Co-MT (MT=V, Cr, Cu, Zn, Cd, Hf ...) also showed the presence of high magnetic uniaxial anisotropies, whose values (≈ 500 to 900 Oe) depended on the nature of the metal that accompanied the Co [29].

The behavior of all these materials after being subjected to thermal treatments was also of interest; this was of importance in terms of the stability of the material, even in terms of its quality as a magnetically hard material, by allowing to obtain a higher value of the uniaxial magnetic anisotropy field. In other cases, this study revealed that following certain thermal annealing an initial approach and a later coalescence of the nano-sheets took place, which allowed explaining for example the loss of the anisotropic magnetic, transport and optics properties of the Co films [29,30].

On the other hand, the existence of the free spaces (≈ 1.5-2.0 nm) between the nano-sheets of these materials can make possible the inclusion of other elements in their volume, producing the modification of their magnetic behavior allowing, perhaps, the increase of their coercive field or the magnetic anisotropy. In any case, the possibility of inserting a layer of non-ferromagnetic material between two deposits, whose thickness would allow modifying the "exchange" between the magnetic layers, would enable obtaining magnetic entities with a high anisotropy, isolated or magnetically controlled, linked by the exchange.

Thin films made of nano-columns have been manufactured and described as extremely useful for their high porosity [19,32-34]. The thin films described in this present paper, formed by oblique nanosheets, also present a high porosity. This porous character can also be used for different applications. In particular, the aforementioned possibility of inserting different elements in the free space between the nano-sheets [29,30], molecules or compounds gives these thin films new and extensive possibilities of use for new properties and their control.

In conclusion, the use in sensor applications of new composite magnetic materials with controlled oblique nano-sheets morphology which confer them anisotropic physical properties is open to explore.