  We studied the exchange coupling and decoupling occurring in a nanocomposite
spin system based on a 3D Heisenberg model by means of Monte Carlo numerical
computation simulation. Different from conventional micromagnetism approach
which usually adopts finite elements method to compute, in a top-down way, the
magnetic property of micromagnetic ensemble in micron even nanometer scale, our
approach in this paper is peculiar to the structure of a complex spin lattice,
i.e. two species of spins building up from single spin to cluster spins in a
bottom-up way. The simulation revealed the influence of exchange coupling
constant Jab, the size of cluster spins d and system reduced temperature t upon
the exchange coupling and decoupling between component spin phases of a
nanocomposite magnets, respectively. Smaller value of Jab, larger d and lower
temperature t usually lead to the decoupling of originally exchange-coupled
component phases and the occurrence of a characteristic two-stage shoulder with
an inflexion on the demagnetization curve. The results reported in this paper
are of, to some extent, universality and applicable to other dual-phase
magnetic systems since our simulation simply focus on a pure duplex spin system
rather than a specific material and all physical variables were treated in a
reduced form.
