NanoSQUID Magnetometry on Individual As-grown and Annealed Co Nanowires at Variable Temperature.

Performing magnetization studies on individual nanoparticles is a highly demanding task, especially when measurements need to be carried out under large sweeping magnetic fields or variable temperature. Yet, characterization under varying ambient conditions is paramount in order to fully understand the magnetic behavior of these objects, e.g., the formation of nonuniform states or the mechanisms leading to magnetization reversal and thermal stability. This, in turn, is necessary for the integration of magnetic nanoparticles and nanowires into useful devices, e.g., spin-valves, racetrack memories, or magnetic tip probes. Here, we show that nanosuperconducting quantum interference devices based on high critical temperature superconductors are particularly well suited for this task. We have successfully characterized a number of individual Co nanowires grown through focused electron beam induced deposition and subsequently annealed at different temperatures. Magnetization measurements performed under sweeping magnetic fields (up to ∼100 mT) and variable temperature (1.4-80 K) underscore the intrinsic structural and chemical differences between these nanowires. These point to significant changes in the crystalline structure and the resulting effective magnetic anisotropy of the nanowires, and to the nucleation and subsequent vanishing of antiferromagnetic species within the nanowires annealed at different temperatures.

Transmission XMCD-PEEM imaging of an engineered vertical FEBID cobalt nanowire with a domain wall.

Using focused electron-beam-induced deposition, we fabricate a vertical, platinum-coated cobalt nanowire with a controlled three-dimensional structure. The latter is engineered to feature bends along the height: these are used as pinning sites for domain walls, which are obtained at remanence after saturation of the nanostructure in a horizontally applied magnetic field. The presence of domain walls is investigated using x-ray magnetic circular dichroism (XMCD) coupled to photoemission electron microscopy (PEEM). The vertical geometry of our sample combined with the low incidence of the x-ray beam produce an extended wire shadow which we use to recover the wire's magnetic configuration. In this transmission configuration, the whole sample volume is probed, thus circumventing the limitation of PEEM to surfaces. This article reports on the first study of magnetic nanostructures standing perpendicular to the substrate with XMCD-PEEM. The use of this technique in shadow mode enabled us to confirm the presence of a domain wall without direct imaging of the nanowire.