Giant magnetic coercivity in Fe(3)C-filled carbon nanotubes

One of the major challenges in the synthesis of ferromagnetically filled carbon nanotubes is the achievement of high coercivities. Up to now the highest coercivity has been shown to be 2200 Oe at 2 K ranging down to 500 Oe at temperatures of 300 K. Here we show that the anomalously large coercivity of 3440 Oe is observed in comparable samples. By comparing our result to those reported in previous studies no correlation is found between coercivity and the shape anisotropy or the crystal-diameter. Instead we suggest that the main parameter which controls the coercivity of these structures is the interplay of the grain size and shape anisotropy. We attribute the anomalous coercivity to the grain size being below the calculated single magnetic domain limit.