Surface magnetism ultrathin films

Vollmer R 1998 Magnetization-induced second harmonic generation from surfaces and ultrathin films Nonlinear Optics in Metals e6 K H Bennemann (Oxford Clarendon) pp 42-131... 

J.C. Jacquet, T. Valet, in Magnetic Ultrathin Films, Multilayers and Surfaces, ed. by E. Marinero (Materials Research Society, Pittsburg, 1995). 

C. Wang, R. Osgood, R. White, B. Clemens, in Magnetic Ultrathin Films, Multilayers and Surfaces, ed. by E. Marinero, B. Heinrich, W. Egelhoff, A. Fert, H. Fujimori, G. Giintherodt, R. White (1995) (Mater. Res. Soc. Symp. Proc., San Francisco, 1995)... 

Different low dimensionality magnetic systems have been investigated, including surfaces, adsorbates, thin films, interfaces and multilayers. In the following we p resent a pplication o f s pin r esolved I PE t echnique t o s tudy I ayered magnetic nanosfructures, namely Fe/Cr/Fe(001) multilayers, Fe/ZnSe(001) ultrathin films, and LaSrMnO/SnTiO interfaces. 

An other important point is related to the surface hyperfine field which may be a priori smaller or larger than the bulk hyperfine field at low temperature in the case of ultrathin films, according to the substrate and the material coating the surface. Mossbauer studies of a-Fe nanoparticles have revealed that the hyperfine field values of inner Fe nuclei are similar to those of bulk crystalline a-Fe at low temperature (i.e. static blocked magnetic regime), but those characteristic of the superficial atomic layer are found either lower or higher [91-96]. Such conclusions are well supported by some examples of the literature with 2 nm a-Fe particles in organic liquids [93],... 

Experimental determinations of for fine particles show some inconsistencies. This is not surprising, as detailed studies on ultrathin films have shown that changes may depend on several parameters. Generally, is lowered with respect to the saturation magnetization value for the corresponding bulk material. The decrease should be mainly due to spin canting at the surface, that is, disorientations of spins directions, as evidenced by Mossbauer spectroscopy experiments. " A decrease of the magnetic moment per spin at the surface is also possible for metallic particles. However, for the present it is difficult to predict such effects because of the lack of systematic studies on the different types of particles. 

The effect of this anisotropy has been clearly evidenced for thin films where, for /C <0, perpendicular magnetization has been observed in many ultrathin film systems instead of parallel magnetization as expected from magnetostatic energy. ° However, for a surface of low symmetry, a second term for the surface anisotropy occurs, which is written by surface unit as follows ... 

Tremendous interest has arisen in recent years regarding die issues of chain mobility and glass transition at polymer surfaces (1-4). Keddie et al used ellipsometric measurements of thermal expansivity to study Tg of polystyrene on silicon as a function of film thickness (1). The observed Tg asymptotically approached the bulk value of 100 C as thickness was increased, but for thinner films lO s of nm), Tg was significantly depressed. Clearly, such effects are of critical importance in the application of ultrathin polymer films as lubricants and protective coatings in magnetic data storage devices and micromechanical systems (5). Until recently, however, direct mechanical measurements of polymer behavior at the nanoscopic scale (which is necessary for mechanical analysis of ultrathin films) and at elevated temperatures has not been possible. 

Magnetic Moment (and Magnetic Hyperfine Field) at Surfaces and in Ultrathin Films... 

Figure 7.1 Schematic representation of band narrowing and resulting increased magnetic moments at surfaces and in ultrathin films.

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