Complex Three-Dimensional Magnetic Ordering in Segmented Nanowire Arrays

Abstract

A comprehensive three-dimensional picture of magnetic ordering in high-density arrays of segmented FeGa/Cu nanowires is experimentally realized through the application of polarized small-angle neutron scattering. The competing energetics of dipolar interactions, shape anisotropy, and Zeeman energy in concert stabilize a highly tunable spin structure that depends heavily on the applied field and sample geometry. Consequently, we observe ferromagnetic and antiferromagnetic interactions both among wires and between segments within individual wires. The resulting magnetic structure for our nanowire sample in a low field is a fan with magnetization perpendicular to the wire axis that aligns nearly antiparallel from one segment to the next along the wire axis. Additionally, while the low-field interwire coupling is ferromagnetic, application of a field tips the moments toward the nanowire axis, resulting in highly frustrated antiferromagnetic stripe patterns in the hexagonal nanowire lattice. Theoretical calculations confirm these observations, providing insight into the competing interactions and resulting stability windows for a variety of ordered magnetic structures. These results provide a roadmap for designing high-density magnetic nanowire arrays for spintronic device applications.

Magnetic small-angle neutron scattering

“…With typical dimensions of a few tens of nm in cross section, such structures still fit in the framework of nanomaterials (i.e., with two dimensions smaller than ∼ 100 nm). Magnetization reversal in aligned nanowire arrays has been studied using both polarized and unpolarized SANS, and spin misalignment is commonly observed (Grigoryeva et al, 2007;Grutter et al, 2017;Günther et al, 2014;Maurer et al, 2014). However, with a typical nanowire length approaching the micrometer range and usually oriented parallel to the neutron beam, arrays of magnetic nanowires act as a grating, and strong multiple scattering has to be taken into account depending on the nanowire length; this effect is discussed in depth in Grigoriev et al, 2010.…”

Magnetic Nanowires and Nanotubes

“…This is valid for either two types of magnetic materials (Ivanov et al 2016a; Fig. 13c), or one magnetic and the other one nonmagnetic (Grutter et al 2017, Bran et al 2018. In case the segments display a transverse shape or magnetocristalline anisotropy, each may display a type of domain on its own, e.g.…”

“…Arrays of magnetic nanowires have been investigated by polarized small angle neutron scattering (PSANS) (Maurer et al 2014, Günther et al 2014, Grutter et al 2017. The power of this method is that fine information may be extracted from diffraction peaks of the lattice in case an ordered array is considered, the same way structural crystallography provides fine information about lattice symmetry and unit cell.…”

Galfenol Thin Films and Nanowires

“…Newer collaborations include those at National Institute of Standards and Technology (NIST), Gaithersburg, where Grutter et al used polarization-analyzed small angle neutron scattering (PASANS) to observe complex three-dimensional ordering between Galfenol segments in FeGa/Cu nanowires [34]. Combined dipolar interwire and intersegment interactions led to ordering that was ferromagnetic between nanowires and antiferromagnetic between segments parallel to the nanowires.…”