Purely dipolar versus dipolar-exchange modes in cylindrical nanorods

Abstract

Properties of purely dipolar and dipolar-exchange modes in finite circular rods are investigated numerically, with the sample static magnetization assumed to be uniform and parallel to the rod axis; we study magnetic excitations propagating along this direction. In particular, we demonstrate size-exchange effects induced by modifying the rod aspect ratio (the diameter to length ratio) and the strength of the exchange coupling. Untypical groups of purely dipolar modes are shown to occur in smooth rods: bulk-dead modes, characterized by a dead-amplitude region inside the rod, and comb modes, in which a nearly “dead” region is found at the sample borders, while numerous (“comb-like”) oscillations take place in the bulk. These two mode groups fade to disappear when the sample geometry evolves toward the thin-film one, or when the exchange becomes dominant. The existence of these untypical modes is related to local demagnetizing field profile nonhomogeneity, particularly to two edge wells found in the field profile. The bulk-dead modes are low-frequency modes confined to the well regions only (and therefore double-degenerated), while modes with higher frequency values (subjected to an interference within the barrier) become fast-oscillating comb modes. When the contribution of the exchange grows to prevail, the “dipolar” nonhomogeneity becomes negligible with respect to the total (exchange and dipolar) energy, and only the typically exchange-related bulk and surface modes are found to subsist.