This study examined university engineering and physical science students' misconceptions of the nature of magnetic field. It is assumed that a significant knowledge of the sources of magnetic field is a basic prerequisite when students have to think about electromagnetic phenomena. To analyze students' conceptions, we have taken into account the fact that individuals build mental representations to help them understand the functioning of a physical system. These mental representations include different explanatory categories of reality in one same individual, depending on the context and the contents concerned. Therefore, we have designed an interview and an open-question questionnaire with an emphasis on explanations, so as to analyze the students' reasoning. We found that most of the students failed to identify the source of the magnetic field and they confuse magnetic force and magnetic field. It is concluded that although the questionnaire and interviews involved a wide range of phenomena, the misconceptions identified fall into four main categories of explanations which can inform curriculum development by identifying the strengths and weaknesses of the students' conceptions.