Magnetic films of negative Poisson's ratio in rotating magnetic fields

Mechanical properties of a thin magnetic film subjected to an external, rotating magnetic field have been studied by computer simulations. The film is modeled by a two-dimensional bead-spring model of a nonmagnetic polymer matrix with inclusions of magnetic nanoparticles (nanograins) of uniaxial anisotropy, The isotropic polymer matrix is represented as a hexagonal structure with sites decorated with small hexagonal particles containing magnetic nanograins. The beads correspond to polymer segments and the springs between them imitate chemical bonds, approximated by harmonic interactions. The beads also interact by the Lennalcl-Jones potential whose role is to mimic the core of non-bonded polymer segments. The Poisson ratio of the constructed polymer matrix is negative, i.e. the system expands laterally under longitudinal stretching force. It is shown that the magneto-elastic coupling can be used to control the expansion and contraction of the polymer matrix. Depending on the applied magnetic field, the Poisson ratio of the magnetic film may assume positive or negative Values. (c) 2008 Elsevier B.V. All rights reserved.