Magnetism of ultra-short one dimensional atomic chains*

Driven by almost a century of theoretical work, the physical realization of one dimensional (1D) magnets has become essential to address pressing problems such as quantum criticality, many-body, spin transport, the emergence of new (topological) magnetic phases, and the extent and persistence of short- and long magnetic interactions as a function of length.

We report structural and magnetic properties of one-dimensional Fe chains as a function of length in the 10 to 200 atoms range. These Fe chains are grown using iron phthalocyanine (FePc) thin films and FePc/ metal-free-phthalocyanine (H₂Pc) superlattices (SLs). The length of 1D Fe chains is precisely controlled by the deposited thickness of the FePc layers.

Although structurally identical, 1D Fe chains formed in films and superlattices have different magnetic behavior. In films, the coercive field remains almost constant whereas in SLs increases with the length of the chain. This difference can be explained using a semi-classical model, which combines short range direct Exchange and Dzyaloshinskii-Moriya interactions. The increase of the coercive field in SLs is attributed to a magnetization reversal process which is governed by chiral symmetry breaking produced by a weak magnetic anisotropy. This anisotropy originates at the extreme of the Fe chains by proximity with the H₂Pc layers and is observable by element selective X-ray absorption spectroscopy (XAS).