On the propagation of perturbations in confined magnetic materials

RUBIO PUZZO, M L; EZEQUIEL V ALBANO

Magnetic Materials Research.

Nova

Lugar: New York, USA; Año: 2008; p. 1 - 100

The properties of confined magnetic materials are relevant for thedevelopment of new devices in material science, micro- andnanotechnology, and related fields. Furthermore, these properties are often quite different from those observed  in the bulk of the samples, so that their understanding posses  a formidable theoretical challenge.Within this context, this  article provides an extensive review onrecent progress achieved  in the study of the properties of interfaces and the propagation of perturbations in confined magnets. A magnetic Ising film placed ina semi-infinite slab, such that the  confining walls exertshort-range competing magnetic fields (h) on the confinedmaterial, undergoes a wetting transition at a  well-definedcritical temperature T_w(h) such that T_w(h) < T_{cb}, whereT_{cb} is the critical temperature of the bulk. In fact, thecompeting fields cause the occurrence of an interface betweenmagnetic domains of different orientation. For T < T_w(h) (T > T_w(h)) such interface is bound (unbound) to the walls, while right  at T_w(h) the interface is essentially located at thecenter of the film. A similar phenomenon is also observed byconfining the magnet in a corner geometry with competing surfacefields. The unbinding of the interface is then the analogouscounterpart of the wetting of a surface by a fluid, polymer,alloy, etc., and consequently the main conclusions of the studycan be generalized  to a large number of physical systems thatplay a key role in many technological applications, e.g., adhesion,coating, lubrication, etc. On the other hand, for each value of the surface field itis observed that, above certain  critical temperature T_D(h),the propagation of perturbations proceeds forever, leading toan active phase. However, below T_D(h) the propagation  stopsirreversibly leading to a  frozen phase. The critical points for the onset of perturbation spreading can be evaluated, showing that the wetting transition effectively shifts the location of these critical points, which are now placed within the nonwet phase. It is found that in theseconfined geometries, the spatiotemporal spreading ofperturbations becomes considerably enhanced by the  presence ofinterfaces between magnetic domains of different orientation,which act as a ´´catalyst´´ for the propagation, causing anenhancement  of the total affected area. As expected, thepropagation of perturbations  along the direction parallel to aninterface is faster than in the  perpendicular direction. Also,the propagation into the bulk is even slower.