In ferromagnetic bulk materials, the magnetostatic interaction and bulk magneto-crystalline anisotropy are the principal sources of the anisotropy, but when we are working with nano-scale systems, such as thin films, nanoparticles, wires, etc. strong surface effects are expected. As the particle size decreases, the surface and interface effects are enhanced due to the increase of the surface/volume ratio. The surface effects can yield different magnetic properties of a low-dimensional system with respect to the typical bulk ones.
The principal surface effects will be briefly discussed in below:
- Lattice relaxation: Normally the bond-breaking at the boundaries of the nanomagnet yields the structural relaxation of the system. The atomic positions on the surface of nanoparticle have been reported to correspond to lattice expansion or the contrary contraction . Also nanoparticles embedded in different matrices experience a mismatch of lattice parameters on the surface and the relaxation of the internal lattice structure. Additionally thin films grown on substrates can show large strain effects.
- Nanoparticles shape and surface reconstruction: Depending on the chemical environment, nanoparticles with different shapes such as spherical or more exotic cubes or neadles can be synthesized [13,19]. Nanoparticles are often reported in the octahedral and dodecahedral shape [20,14]. The existence of different surfaces and vertices obviously change local properties on the surface.
- Charge transfer: On the surface of nanoparticles we can find defects such as cations, which can promote charge transfer and change the magnetic character of the surface . The charge transfer in the case of magnetic nanoparticles coated with polymers is also known to occur , as well as in nanoparticles with organic molecules or embedded in different non-organic matrices.
- Oxidation: Metallic nanoparticles are chemically active and are easily oxidized in air, resulting generally in a reduction or even loss of the total magnetic moment. For instance, cobalt is a typical ferromagnetic (FM) material, nevertheless its oxide CoO has an anti-ferromagnetic (AFM) character , in a similar way as Ni and NiO . Additionally, when the surface of Co or Ni nanoparticles is oxidized, we find a system with two different magnetic phases which could lead to new magnetic phenomena. For example, this kind of a composite material (FM-AFM or vice-versa) could present the exchange bias effect [25,26,27].
- Variation of the magnetic moment: In ferromagnetic systems the magnetic moment may be enhanced by reducing the dimensionality . Magnetic moment enhancement with decreasing size has been observed experimentally and theoretically in metallic nano-clusters of Fe, Co, Ni, etc. [29,30,31,32]. Therefore in systems like metallic ferromagnet nanoparticles where a large fraction of the spins belong to the surface, one could expect an increment of the system's magnetization.
- Surface spin disorder: A reduction of the saturation magnetization, Ms, has been observed experimentally in nanoparticles. Initially, this reduction has been explained by models which postulated the existence of a "dead" magnetic layer at the surface , however there are other theories that relate the origin of that effect with the existence of a canted spin configuration at the surface , or the spin-glass-like spin state . But up to now, the origin of the canting of the spins in fine particles is an object of a continuing discussion .
- Surface anisotropy Another surface-driven effect is the enhancement of the magnetic anisotropy with decrease of the system size [36,32,37]. That increment is assumed to be originated by the anisotropy at surface and has been detected experimentally in nanoparticles of Co, Fe, etc. [###1###,8,10,38]. Also a systematic study with different coatings has revealed that they can influence to the effective anisotropy.
- Variation of the exchange interaction at surface: The variation of the exchange interaction energy at surface has been reported from a theoretical point of view in thin films [39,40] and also has been observed in magnetic nanoparticles. For example,
material has shown a drop of the Curie temperature of the system as the concentration of Cu is increased. Such a decrease of Tc is associated to the variation of the exchange interaction strength at the surface .
In practice it is impossible to separate these effects and consequently, all of them are normally embedded in the phenomenological concept of the "surface anisotropy". The detailed theoretical description of real experimental situation is almost impossible due to competition of many effects and large dispersion of individual nanoparticles properties.