Spin resonance, magnetic measurements, and structural analysis are reported for metal-semiconductor separated SWCNTs after filling with ferrocene. Raman scattering performed after a heat treatment confirms partial transformation to double-walled CNTs but results from spin resonance (FMR), X-ray diffraction, and TEM evidence in addition the growth of ferromagnetic nanoparticles. For the metallic tubes the particles are identified as magnetite (Fe3O4) with full chemical stoichiometry. From the temperature dependence of the FMR response and from measurements of the magnetization by dc and ac SQUID the magnetite crystals are shown to undergo a Verwey transition. The transition temperature from the SQUID experiment is around 125 K as expected but considerably higher than observed from the FMR analysis. Results for semiconducting tubes are similar but magnetic particles are an order of magnitude smaller and exhibit different structures in addition to magnetite.