MFM Study of Magnetic Charge Order in Fibonacci-Distorted, Honeycomb Artificial Spin Ice*

We study the ordering of magnetic charges in honeycomb artificial spin ice (ASI) having distortions created by applying an aperiodic Fibonacci sequence of binary digits mapped onto short (d₁) and long (d₂) 2D lattice parameters. Patterned Permalloy (Ni_0.80Fe_0.20) thin films were deposited on Si₃N₄ using standard electron beam lithography (EBL) to create samples with a range of distortion ratios d₂/d₁ = 1.00, 1.15, 1.30, 1.45 and 1.62.

The lengths of disconnected segments and their location around three-fold honeycomb vertices were modified by these distortions. Two pattern types were studied: 1) Varied segment length; 2) Varied segment distance to vertex. In both cases, the values of d₁ and d₂ uniquely determine the positions of the pattern vertices.

The magnetic charge distribution of each pattern was imaged using standard magnetic force microscopy (MFM) techniques before and after the system is annealed into a low-energy ASI state. The magnetic charge distributions are then analyzed to determine the effect of the distortions on frustration of the magnetic textures, and relation to the coercive fields measured by SQUID magnetometry.