Noncollinear magnetism in manganese nanostructures

We present ab initio spin-density-functional calculations of the magnetic properties of Mn nanostructures with a geometry varying between a straight linear wire and a three-dimensional nanorod, including collinear and noncollinear, commensurate and incommensurate magnetic configurations. With decreasing tension along the axis of the nanostructure we find a series of transitions first from a straight to a zigzag wire, then to planar triangular or hexagonal stripes and further to a nanorod consisting of a periodic stacking of distorted octahedra. At local equilibrium all nanostructures are in a high-moment state, with absolute values of the local magnetic moments per atom varying between 3.79 mu(B) for a straight noncollinear antiferromagnetic Mn monowire, 3.54 mu(B) for a triangular collinear antiferromagnetic stripe, 3.40 mu(B) for a hexagonal collinear ferrimagnetic stripe, and 2.96 mu(B) for an octahedral noncollinear ferrimagnetic nanorod. For all low-dimensional nanostructures except the monowire we find collinear and noncollinear magnetic structures to be energetically nearly degenerate, if the geometric and magnetic degrees of freedom are relaxed simultaneously. The energetic consequences of a modest change in the interatomic distances are comparable to those of a large canting of the magnetic moments. Compression of the nanostructures leads to a decrease in the magnetic moments.