Magnetoresistance measurement of tailored Permalloy nanocontacts

Abstract

We study the evolution of the magnetoresistance ͑MR͒ in Permalloy nanocontacts prepared by controlled low-temperature UHV electromigration in nanoring segment structures with constrictions. The ring geometry allows for the controlled and reproducible positioning of a domain wall in the nanocontacts. We observe three different resistance levels, corresponding to distinct domain-wall positions. A change in the sign of the MR difference, between a domain wall at the constriction and a domain wall next to the constriction, occurs with decreasing constriction width. This is in line with our micromagnetic simulations, where the MR is calculated based on the anisotropic MR ͑AMR͒ effect.

Scaling of intrinsic domain wall magnetoresistance with confinement in electromigrated nanocontacts

“…For the initial state of the contact -i.e. the state with a resistance of 436 Ω, the plot shows two main resistance levels as a function of the angle, which can be understood as follows: Depending on the angle of the saturating field, a domain wall may or may not be nucleated in the structure and as a result the two resistance states can be attributed to the presence and absence of a domain wall between the probes [22]. When the field is initially applied along angles in the range 325 • -035 • , or equivalently 145 • -215 • , the remnant configuration is expected to be a quasiuniform state with no domain wall in the half-ring.…”

“…[17,18]. In general, larger effects are predicted and observed as the confinement of a DW is increased on reducing the size of a wire or for domain walls at constrictions [19][20][21][22].…”

“…Furthermore the influence of contamination or local oxidation can often dominate the signals [33][34][35][36], making an understanding of the fundamental intrinsic properties of a system difficult and requiring careful experimental design in order to exclude or account for such parasitic contributions. As a result, and due to the large range of approaches to determine MR in confined magnetic systems, there is a large spread in the reported values [11,18,22,[37][38][39][40][41][42][43][44][45][46][47] and hence it can be difficult to understand the regimes of applicability of different theories. One proven approach to a robust determination of different contributions to the MR involves the formation of magnetic Py nanocontacts in ultra-high vacuum (UHV), with different regimes of behaviour observed depending on the contact size.…”

“…One proven approach to a robust determination of different contributions to the MR involves the formation of magnetic Py nanocontacts in ultra-high vacuum (UHV), with different regimes of behaviour observed depending on the contact size. For larger contacts no significant intrinsic DWMR is observed with only relatively small effects, which are of less interest for devices, and which can be accounted for due to the AMR effect from the components of the magnetization within the wall which are no-longer collinear with the current [22,38,48]. Alternatively for ultra-narrow contacts much larger intrinsic effects have been reported [49], yet these effects are very sensitive to the exact atomic-coordination at the domain wall position and change in both magnitude and sign based on small atomic rearrangements [50], hence they are difficult to employ in a device context.…”

“…In this work we employ our previously established approach of tailoring the size of magnetic nanocontacts in clean UHV conditions [22,49,58] to study the evolution of intrinsic DWMR for nanocontacts in a regime where the contacts are expected to be a few nm in size.…”