Today, electronic and magnetic technologies allow us to store and process huge amounts of information. However, the growing demand for greater capacity, faster processing speeds, and improved energy-efficiency is driving the search for new technologies. Spintronics is one of the fields exploring alternative ways to store and process information by exploiting the magnetic properties of materials.

One of the most promising approaches involves nanodevices based on domain walls (DWs), which are tiny boundaries between regions with different magnetic orientations within a material. DWs can be thought of as signals that move rapidly along magnetic nanowires, with their position encoding information. An electrical current applied to the nanowire drives the walls along it. The smaller the current required, the less energy a potential device would consume. The challenge, therefore, is to achieve fast and reliable DW motion at very low current densities.

A new study led by researchers from ALBA Synchrotron and the Complutense University of Madrid (UCM), in collaboration with SPINTEC (Université Grenoble Alpes/CNRS/CEA), has demonstrated that DWs can be moved in a controlled and reproducible way using extremely low electrical currents —more than five times lower than those reported in previous studies— and has identified a factor that limits their movement at even lower currents.

The researchers studied cylindrical permalloy nanowires made of an iron-nickel alloy (Fe₍₂₀₎Ni₍₈₀₎), approximately 245 nm in diameter. By applying short electrical pulses, they were able to move the DWs along the wires and study their motion at different current densities.

They achieved controlled DW motion at current densities as low as 2.8 × 10¹⁰ A/m², and observed movement even at 4.5 × 10⁹ A/m². However, as they reduced the current further, some DWs unexpectedly stopped at specific locations along the nanowires. They had become pinned. Understanding why this happened was important: if these technologies are to be used for information storage and processing, DW motion needs to be reliable and predictable.

The team used magnetic X-ray microscopy at ALBA’s MISTRAL beamline to observe the magnetic state of the nanowires and track the movement of the DWs. This allowed them to identify precisely where the walls became trapped.

The researchers then examined these regions using electron microscopy, together with advanced spectroscopy and diffraction techniques. These complementary methods provided information about the morphology, chemical composition, and crystallographic structure of the nanowires. By combining this structural information with the magnetic images obtained at ALBA, the researchers could link the behaviour of the DWs to the microscopic properties of the material.

The answer was not a major change in chemical composition. Instead, the unexpected pinning sites were associated with small local variations in crystallinity —differences in how the atoms are arranged within the material. These microscopic variations can act as obstacles to DWs motion. When the current is strong enough, a wall can overcome them; as the current becomes weaker, the driving force eventually becomes too small, and the wall becomes trapped.

This finding points to an important consideration for the development of next-generation, energy-efficient spintronic devices: controlling the chemical composition of a nanowire is not enough. Its local crystal structure must also be carefully engineered to ensure reliable and reproducible DW motion at very low currents. Such control could help pave the way for more efficient technologies for storing and processing information, including new types of memory, logic devices, and brain-inspired computing.

XMCD-TXM (magnetic X-ray microscopy) images (top) and ADF-STEM (electron microscopy) images (bottom) of unintentional pinning regions (a) 3a and (b) 3b. (c) Fe and Ni EDS (energy-dispersive X-ray spectroscopy) elemental maps along with a composite Ni + Fe intensity map. (d) Ni/Fe line profile along the nanowire. (e) Lower angle ADF-STEM image containing both pinning sites. (f) Presents crystal orientation maps for regions 3a and 3b, showing both in- plane (bottom) and out-of-plane (top) orientations. The in-plane orientation is defined relative to the image’s vertical axis, which is perpendicular to the nanowire.