Stochastic nature of the domain wall depinning in permalloy magnetic nanowires

Johanna Akerman, Manuel Muñoz, Marco Maicas, and José L. Prieto

Phys. Rev. B 82, 064426 – Published 31 August 2010

Abstract

This study explores experimentally the stochastic nature of the domain wall depinning in permalloy nanowires using notches of various shapes and depths. The presence of the domain wall in the notch is detected through its anisotropic magnetoresistance (AMR), which is measured with high precision in order to detect even small changes in the domain wall profile. These measurements showed that variations in the depinning field are related with changes, sometimes very small, in the AMR profile, which indicates that small changes in the pinned domain wall profile can affect largely the depinning process. As these small changes are many times unpredictable and uncontrollable, the stochastic nature of the depinning could have negative consequences for practical applications based on permalloy nanowires.

Received 28 July 2010

DOI:https://doi.org/10.1103/PhysRevB.82.064426

Authors & Affiliations

Johanna Akerman¹, Manuel Muñoz², Marco Maicas¹, and José L. Prieto¹

¹Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), Universidad Politécnica de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain
²Instituto de Física Aplicada, CSIC, C/Serrano 144, 28006 Madrid, Spain

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Issue

Vol. 82, Iss. 6 — 1 August 2010

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Images

Figure 1
SEM picture of one of the wires used for this study (top). Within the picture there is a zoom of the double notch in the wire. On the bottom left, there is an schematic of the setup used for the electrical detection of the domain wall through its associated decrease in the AMR in the section B-C. On the bottom right there is a selected graph with a pinning and depinning event measured by AMR. The first fall edge is taken as the propagation field and the second rise edge is the depinning field.Reuse & Permissions
Figure 2
Propagation field (open squares) and depinning field (solid squares) for all the wires and the notches measured in this study. Each point represents the average value of at least 40 measurements. The top graph plots the values for wires with shallow notches and the bottom graph plots the values for deeper notches. On top of the figure there are SEM pictures for each of the notches studied: S—square, T—triangular, SL—square left, SR—square right, E—elliptical, and C—circular. The width of all the wires is 500 nm.Reuse & Permissions
Figure 3
Top graph represents the probability of pinning for each type of notch. Some data of the total number of measurements are given in the main text. Bottom graph represents the standard deviation for all the measurements for the different notches. Empty squares are measurements for shallow notches and solid triangles for deeper notches.Reuse & Permissions
Figure 4
Plots (a) and (b) show the two typical shapes found for the AMR profile in the pinning events measured for S-125 nm notch. The percentage of events for each type is indicated on top of the figures. For both (a) and (b) we show six events displaced for clarity. It is clear that if the profile is identical for different events (i.e., the bottom three curves), $H_{dep}$ is the same. On the other hand, if the shape changes even marginally (compare the bottom three curves with the top three in both (a) and (b) $H_{dep}$ can be considerably different. The dashed ellipses on the right plot indicate the only difference between the three top and three bottom curves.Reuse & Permissions
Figure 5
(Color online) Depinning sequences in different types of notches. The left sequence describes a clean depinning in a E-shallow notch where the structure of the DW is not altered in the process. In the middle sequence for a S-shallow notch, the vortex changes chilarity during the depinning. Finally, in the right sequence for a S-deep notch, a different vortex nucleates on the right of the notch before the incoming DW is extinguished on the left side of the notch.Reuse & Permissions

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