Finite size effects in shell nanoparticles

The finite size effects on magnetic properties of shell particles in the shape of truncated octahedron are studied using classical O(3) Heisenberg model. Both considered cases with nearest neighbor ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions show the existence of the well-defined ground state. The FM particles show FM order, while AFM particles show freezing in the non-collinear structure with very low magnetization due to the presence of frustrated triangular facets. The variation of magnetization, susceptibility and specific heat as a function of size shows considerable differences from that in the three-dimensional (3D) particles. We also observe effects of shell geometry, i.e. the fraction of sites in facets, edges and vertexes, which affects the apparent ordering temperature.