Asymmetric magnetic response induced by compositional gradients in Ni-Fe nanowires

Abstract

"The control over the domain wall motion in cylindrical nanostructures is a key factor for developing the next generation of spintronic and logic devices [1,2]. Among several strategies, introducing changes in the chemical structure of the wire (chemical barriers) has been proved to be an effective way to pin the domain walls [3], but determining the direction of the domain wall movement still remains as an open challenge. Following the same approach, in this work we introduced gradients of composition in Ni-Fe nanowires to pattern a ratchet profile of the domain wall energy along the nanowire axis, in order to create an asymmetry for the domain wall movement. Nanowires were synthesized using template assisted electrodeposition. Composition was gradually changed between Ni90Fe10 to Ni35Fe65 along the nanowire longitudinal axis in periods of a few micrometres (fig.1.a). Combining laterally resolved X-ray Absorption Spectro-microscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) we correlate the chemical structure of single nanowires with their 3D spin texture. By applying external magnetic fields along the nanowire axis, we studied the evolution of the magnetic state depending on the field direction. In addition, First Order Reversal Curves (FORC) were also measured in arrays of nanowires. The FORC diagrams of nanowires with homogeneous composition and nanowires with axial gradients of composition are shown in fig.1.b-c. While the diagram of homogeneous nanowires is highly symmetric with respect to the interaction field axis (HU)(b), an asymmetry arises in the diagram of nanowires with axial gradients (red square) (c), evidencing the emerging of asymmetrical magnetization processes in the nanowires [4]."