On the behavior of Ni Magnetic Nanowires as studied by FMR and the effect of “blocking”

C. A. RAMOS; E. VASSALLO BRIGNETI; E. DE BIASI; M. VÁSQUEZ

Nanowires

InTech - Open Access Publisher

Lugar: Viena; Año: 2011; p. 333 - 356

In this chapter we have reviewed some of the characteristics of Ni ferromagnetic nanowires grown by electrodeposition into AAO membranes with very good uniformity. A uniaxial anisotropy model was detailed and some results of FMR were show to be consistent with small variations in the filling factors indicating dipolar interactions compete with the uniaxial character of the magnetic anisotropy.  Furthermore, the measured filling factors indicate small variations in the magnetic anisotropy between samples could be associated with these subtle structural changes in the AAO matrix.  We showed that a contribution to the FMR line-widths can be originated in fluctuations in the porosity or filling factors of the sample, particularly in the condition H//NW. On the other hand, local distortions from the cylindrical shape would affect more strongly the line-width in the perpendicular condition.The most important result of this work is that there is no blocking in FMR in the sense in which is normally used in AC susceptibility, essentially because FMR is not performed at H = 0. In many cases the applied field is higher than the irreversibility field and this condition defines a single energy minimum, and therefore the system will be performing small oscillations around this minimum because the magnetization tends to relax extremely fast to the equilibrium position [Kittel, 2005].  What many researchers have believed  to be caused by “blocking” can be explained as being caused by the effective anisotropy field as a function of temperature growing larger than the characteristic exciting frequency, w/g, and thus not being able to excite resonantly the material at any applied field. In this sense the Ni NW are an excellent system to study the response far above (Q-Band), above and near (X-Band), and below  (L-Band) the anisotropy field gap.