Magnetic nanoparticles and/or nanowires are being encapsulated in carbon nanotubes (CNTs) as an alternative to introduce a magnetic response to CNTs. This work proposes a novel route to control the magnetic response of CNTs filled with iron-containing nanomaterial through the synthesis process and heat treatment. CNTs were synthesized through chemical vapor deposition from ferrocene at 750 degrees C. They were characterized by Raman spectroscopy and microscopy. In addition, the CNTs were heat treated after the synthesis; CNTs were heated at 1000 degrees C for 10 min in an inert atmosphere. The magnetic properties of CNTs as grown and after heat treatment were evaluated using the hysteresis loop and Mossbauer spectroscopy. Results show that there is a correlation between the manufacturing process - considering synthesis parameters and heat treatment - and the composition of iron-containing phases in the CNTs. By varying the temperature and dwell time of synthesis, the composition of the phases encapsulated by CNTs will be different. Heat treatment performed on the CNTs after the synthesis, also resulted in significant modification of the final phases trapped by the nanotubes. These methods enabled the control of which phases remained encapsulated by the nanotubes. This study shows it is possible to control the final magnetic properties, e.g. coercivity, of CNTs filled with magnetic material so as to synthesize the most appropriate magnetic nanomaterial for a specific application. (C) 2013 Elsevier Ltd. All rights reserved.