1.1 Magnetic nanotechnology and magnetic nanostructures

Nowadays advanced technologies rely on research in materials performance and on developing new materials with novel superior properties. Nanotechnology is the engineering of materials of sizes less than microns and involves physics, chemistry, material science, biology and other fields. The interest in nanotechnology in our days is based on the promise of significant social implications, which include better understanding of nature, efficient manufacturing techniques for almost every human-made object and a new world of products beyond what has been possible with other technologies. Nanomaterials constitute an emerging sub discipline in materials sciences due to the use of devices with smaller dimensions for which a new physical behavior naturally appears. The fabrication of nanodevices is feasible since we have a close example: life itself. Some of the proposed paradigms can be found in nature. An example of molecular motors is kinesin and a form of nanoassembling is RNA replication. The prospects of nanotechnology are still not clear: whether it will generate a completely new technology. However, nanotechnology has been up to date a new form of understanding science.

Magnetism has been from its origins one of the more fertile fields of science. A historically important magnetic device such as the compass has stimulated the curiosity for centuries. Analogously, the nanomagnetism is between the most successful fields of nanotechnology. The special role of nanomagnetism is due to the fact that magnetic properties depend uniquely on both dimensionality and lengthscales. The reduced size of the nanostructures originates properties different from the bulk ones, as for example surface properties, due to the different surrounding of the magnetic atom. Generally speaking, new phenomena occur always when one of the dimensions of the nanostructure becomes comparable to some characteristic length, defining the magnetic behavior, such as the magnetostatic correlation length.

The magnetic nanostructured materials could be divided into two groups: (i) naturally occurring nanostructured materials such as policrystalline magnetic films and (ii) artificially prepared nanostructures. The control of the magnetic properties of policrystalline magnetic materials emerges via understanding of physical processes related to their granular structure and preparation techniques which can control the grain properties. The new materials for permanent magnets, SmCo and NiFeB, as well as the materials for magnetic recording, CoCrPt, are examples of nanostructured materials where the correct combination of different phases allows to achieve the desired performance.

From the point of view of fabrication of artificial nanostructured materials, the different methods can be grouped coarsely in:

• Top-down. Top-down approach has been the traditional approach to miniaturization via lithographic tools [Sheats 98]. One of other possible fabrication methods includes the extrusion as the example examined in Chapter 4.
• Bottom-up. The sample is assembled from small building units. A typical example is chemical synthesis from solution containing the precursors [Sellmyer 02]. The procedure allows to create 2D disposition of the nanoelements or 3D disposition, as for example Co particles in alumina matrix [Luis 02].

Top-down approach offers more external control of the geometry and properties of the nanostructures. On the other hand, bottom-up is less expensive than its counterpart and is preferred for applications where the cost is an important factor. Beside of these two methods to fabricate nanostructures, and complementing them, the use of simulations allows to predict and to design new materials in the virtual laboratory.

Fig. 1.1 shows some of the possible magnetic nanostructure geometries. These include chain of fine particles [He 07], arrays of striped nanowires [Shearwood 94], arrays of cylindrical nanowires [Zhan 02], nanojunctions [Rüster 03], arrays of nanodots [Novosad 02], etc.

The nanomagnets have a vast field of application and exhibit very interesting new properties. Nanomagnetism is already central to data storage, sensor and device technologies. The nanostructured magnetic materials have improved the performance in traditional applications as sensors, actuators, permanent magnets and transformer cores. Besides, the nanomagnetism is increasingly being used in the life sciences and medicine. For example, the magnetic nanoparticles can be used as a method for cell labeling, drug delivery and hyperthermia [Pankhurst 03]. The fundamental requirement is the biocompatibility of the nanoparticles in order to avoid toxicity. From more traditional technological point of view the exciting new phenomena include spin injection [Jedema 01], giant magnetoresistance (GMR) (the subject of the Nobel Price in Physics this year) [Baibich 88], tunnelling magnetoresistance (TMR) [Moodera 95], half metallic magnets [de Groot 83], etc. All this phenomena open a new field known as spintronics. The semiconductor devices could potentially be substituted by spin-aware devices as the spin transistor [Wolf 01]. The microelectronics has played a crucial and innovative role in driving the scientific and technological progress that has made a major contribution to social and economic growth worldwide for many decades. One can imagine the tremendous possibilities that can be offered by nanoelectronics, a natural extension of microelectronics in the present trend of miniaturization. However, this extension must be accompanied by a deeper understanding and control of the various aspects of nanoelements. Finally, spintronics and high density recording require fast magnetic switching processes. It is therefore of great interest to obtain a thorough understanding of the spin dynamics and magnetic relaxation on nano-second time scales.

Figure 1.1: Typical nanostructure geometries: (a) chain of fine particles, (b) striped nanowire, (c) cylindrical nanowire, (d) nanojunction, (e) vicinal surface step, (f) nanodots, (g) antidots and (h) particulate medium. From R. Skomski [Skomski 03a].

Besides of industrial applications, patterned magnetic structures are very attractive as model systems to study fundamental physical properties of small magnetic particles. The magnetization state of magnetic nanostructures is different from that of the bulk. In the bulk material the sample is divided into domains separated by domain walls. Nevertheless, if the size of the nanostructure is small compared to the domain wall or the geometry of the sample favors some disposition of the magnetization, this is not longer true. Examples of this are single domain particles [Puntes 01] or vortex state in circular nanodots [Novosad 02]. Magnetic thin films and multilayers, which can also be considered nanostructures, can exhibit a number of interesting properties, as for example thickness-dependent domain wall and coercive phenomena [Zhao 01]. The interlayer exchange interactions within magnetic multilayer structures is a very important property leading to different static, dynamic and thermal properties of magnetic multilayers and nanostructures (see Chapter 3 to see an example). Another example constitutes the phenomenon of exchange bias [Kodama 99]. The latter is widely used in current read head devices, for example.

To conclude, magnetic nanostructures exhibit various scientifically interesting and technologically important deviations from bulk magnets. The number of phenomena and the potential applications indicate a promising future for the nanomagnetism. The creation, understanding and exploitation of artificial nanostructures and the study of their magnetic properties remains a challenge for future research.