Resistance behavior and magnetization reversal analysis of individualConanowires

Abstract

Since the direct observation of magnetization reversal processes in nanostructured systems is hard to achieve, we use magnetoresistance measurements as an indirect sensing tool. To avoid dipolar interactions present in nanowire gratings individual Co nanowires are prepared by electron beam lithography on Si substrates. Using a special two step process nonmagnetic gold contacts are attached to the Co nanowires to exclude an influence of the contact structures on the magnetization reversal process. To support the magnetoresistance analysis also the structure and morphology as well as the magnetic properties of the Co nanowires have been characterized using various microscopy and measurement techniques. Depending on the direction of the applied field three different magnetoresistance measurements can be distinguished. Based on the anisotropic magnetoresistance effect the transversal and the polar magnetoresistance reflect a coherent rotation of the magnetization independent of the wire width. In contrast, the longitudinal magnetoresistance shows a wire widths dependence of the magnetization reversal processes. In small wires nucleation processes occur whereas in wide wires the formation of complex domain structures has been observed. Nanowires with a width in between these groups show "transition"-like reversal processes.

Reversal processes and domain wall pinning in polycrystalline Co-nanowires

“…It exhibits two resistance minima at a coercive field of B c Ϸ 29 mT as expected for this wire width. 4 In contrast to the discussion of the two narrower nanowires above, the resistance at B = 0 T is much lower as compared to its saturation value. This is due to the fact that the remanent state is not monodomainlike, which is typical for wider wires with low aspect ratio.…”

“…Upon application of a small magnetic field in the reverse direction, a small amount of magnetization components is rotated from a direction along the long wire axis to a transverse direction while simultaneously vortextype domain configurations form at the wire ends. 4 This leads to a resistance decrease due to AMR. At the coercive field B c Ϸ 105 mT the resistance sharply increases and returns to its original value.…”

“…Magnetoresistance measurements were carried out via a four-terminal ac technique in a 4 He bath cryostat at a temperature of T = 4.2 K. The sample geometry of a nanowire with width w, thickness t, and length l, and the labeling of its axes, is shown in Fig. 1.…”

“…3 Particularly, detailed investigations have revealed that the magnetization reversal in wider Co nanowires is accomplished by the generation of a multidomain state of the magnetization in the whole wire, while in narrower wires domain walls nucleate at the wire ends and traverse the wire during reversal. 4 The coercive field in both cases strongly depends on the wire width w and increases as 1 w with decreasing wire width. 5,6 In the present paper we will show by magnetoresistance measurements in combination with Monte Carlo simulations and magnetic force microscopy in which way the switching process in ferromagnetic nanowires is affected by the sample geometry.…”

Domain wall propagation in Permalloy nanowires with a thickness gradient

“…only in nanowires one encounters head-to-head transverse (TDWs) or vortex domain walls (VDWs). The occurrence of which depends on the nanowire width, w, and thickness, t [3][4][5][6][7][8][9][10][11][12]. A w-t-phase diagram for Permalloy (Ni 81 Fe 19 ) nanowires has been explored both experimentally [9] and theoretically [5,11] showing the regions of existence of either TDWs or VDWs.…”