Evidence for intrinsic weak ferromagnetism in a C60 polymer by PIXE and MFM

In this study a C-60 polymer has been characterized for the first time with respect to impurity content and ferromagnetic properties by laterally resolved particle induced X-ray emission (PIXE) and magnetic force microscopy (MFM) in order to prove the existence of intrinsic ferromagnetism in this material. In the sample studied the main ferromagnetic impurity found was iron with an average concentration of 175 +/- 16 mug/g within the sample volume probed by the ion beam. However, the Fe distribution is very inhomogeneous and characterized by micrometer-large impurity grains of almost pure iron surrounded by an almost pure carbon matrix. With MFM, the ferromagnetic properties have been investigated both in pure and contaminated regions of the sample as determined by PIXE. We found that similar to30% of the area of pure regions (concentration of magnetic impurities <1 mug/g) is characterized by ferromagnetic domains, which are partly correlated to grains and grain boundaries. These magnetic domains are of intrinsic origin and not related to impurities. In contaminated regions of the sample the ferromagnetic properties are dominated by the impurities as expected from the high impurity concentrations. The combination of PIXE and MFM allowed us to separate between the intrinsic and extrinsic magnetic regions and to directly prove that intrinsic ferromagnetism exists in a C-60 polymer. (C) 2003 Elsevier B.V. All rights reserved.