1 Magnetic nanoparticles and thin films

The rapid development of nanotechnology leads to a drastic reduction in the size of magnetic devices to dimensions below one micron and thus to an increase in the ratio between the surface and the volume of the system. It is clear that the surface effects in such low dimensional systems will become very important and will affect the overall magnetic behavior in multiple ways. Within the nanoscale magnetic systems, nanoparticles and ultra-thin films are examples of systems where surface effects play an important role in their magnetic properties.

Ultra-thin magnetic films are widely used for technological applications such as magnetic recording or micro-electromechanical applications. The use of magnetic multi-layers has offered multiple applications such as magnetic recording heads and sensors. The magnetic layers in magneto-resistive heads or spin valves for example, have thickness below than 10 nm. The ultra-thin films are normally grown on non-magnetic substrates and their coating is a widely used method to protect them from oxidation. Both under and top layers can change the structural and magnetic properties of a magnetically active thin film and allow the engineering of its properties.

Nanoparticles have various important technological applications such as in high-frequency electric circuits for mobile phones [1]; for magnetic refrigerators; data storage devices [2,3] or in biomedicine [4,5] (for drug delivery, imaging, sensing and hyperthermia for tumor therapy).

The application of magnetic nano-particles in data storage devices, has been a strong driven force for the development of new methods for growing well-define magnetic nanoparticles with controllable sizes ranging from a few nanometers up to tens of nanometers. The magnetic storage requires that each magnetic particle behaves as a mono-domain particle and that its magnetic state is thermally stable and switchable. This means that it does not easily lose its magnetization direction once the external magnetic field is removed and that the field necessary to reverse the magnetization of the particle does not exceed the field produced by the read head of the hard disk. Also it is necessary to consider the effect of the interactions between magnetic particles on the signal-noise-ratio (SNR).

The magnetic thermal stability and the blocking temperature are defined by the relevant magnetic energy barrier. The magnetic energy barrier is proportional to the nanoparticle diameter and the macroscopic magnetic anisotropy value. Additional possibility to control the energy barrier is provided by the surface modification, for example, the oxidation of nanoparticle may increase the energy barrier via the exchange-bias effect [6,7]. Magnetic nanoparticles embedded in non-magnetic matrices, such as Co in Au, Ag or Cu have also been reported to have a larger blocking temperature [8,9,10], as compared to the value given by pure Co. The combination of materials with different magnetic properties, as in the case of core-shell nanoparticles, allows to control the energy barrier almost independently from the coercive field [11,7,12].

In the case of biomedical applications we have to take into account diverse factors: we must not only evaluate the magnetic behavior of the system but also its bio-compatibility, specifically if we work with "in vivo" (inside the human body) applications . The nanomagnets also can be used in biomedical applications "in vitro" (out of the body) which main use is in diagnostic. Additionally, for biomedicine applications, nanoparticle's surface should be functionalized to act in a biological media or to deliver the drugs. This is known to alter the magnetic properties.

Successful application of magnetic nanoparticles in the areas listed above is strongly dependent on the stability of the particle under a range of different conditions. Additionally it is necessary that the nanoparticles have a narrow shape and size distributions, which implies sophisticated techniques of nanoparticle growth. Magnetic nanoparticles are also very often embedded in non-magnetic matrices to avoid oxidation.

In principle, we can divide the preparation of nanoparticles into two groups, depending on the growth strategy used:

Top-down

These methods start from the bulk material which is decomposed into increasingly smaller fragments. The method includes widely used deposition technique such as sputtering, laser ablation, etc.

Bottom-up

These methods grow nanoparticles via the nucleation of numerous atoms, obtaining particles with a diameter of 1 to 50 nm and narrow size distribution. The typical example of this kind of growth techniques is the chemical synthesis.

Depending on the preparation method and chemical environment the nanoparticles with different shapes such as spheres, octahedra, cubes, etc. are possible to prepare. The shape of the nanoparticle also affects its magnetic properties. In Fig. 1.1 we show different examples of magnetic nanoparticles: spherical and cubic nanoparticles of $\gamma -Fe_{2}O_{3}$ and octahedral nanoparticles of Co and Fe.

Figure 1.1: Transmission electron microscope (TEM) micrographs of (a) spherical and (b) cubic $\gamma -Fe_{2}O{3}$ nanoparticles. The insets show high resolution transmission electron microscope (HRTEM) images of the respective nanoparticles. (c) HRTEM image of a cobalt nanoparticle along a (110) (d) HRTEM observation of an iron nanoparticle along a (110) direction. (Images extracted from [13] (a-b) and from [14] (c-d))

The magnetic properties of nanoparticles could present some differences with respect to those of the bulk material principally by several key issues: the shape, the size and the surface effects [15,16].