We present magnetic quantum cellular automata (MQCA), fabricated by means of nanostencil lithography, i.e., using a resistless shadow masking technique in ultra-high vacuum. The nanostencil tool allows the fabrication and in situ investigation of structures using atomic force microscopy (AFM) and magnetic force microscopy (MFM). We analyze the error distribution within the structures to shed light on the performance and challenges of magnetic cellular logic devices. Simulations are performed to corroborate an improved concept for these devices which makes use of fourfold magnetic anisotropy. © 2010 IOP Publishing Ltd.