We investigate the ground states of antiferromagnetic Mn nanochains on Ni(110) by spin-polarized scanning tunneling microscopy in combination with theory. While the ferrimagnetic linear trimer experimentally shows the predicted collinear classical ground state, no magnetic contrast was observed for dimers and tetramers where noncollinear structures were expected based on ab initio theory. This striking observation can be explained by zero-point energy motion for even-numbered chains derived within a classical equation of motion leading to nonclassical ground states. Thus, depending on the parity of the chain length, the system shows a classical or a quantum behavior.

• Received 20 October 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.157206