Electron Microscopy of Annealed (Ni,Zn,Co)Fe2O4

( Ni,Zn)Fe 2 O 4 samples containing small amounts of Co were characterized in a transmission electron microscope to ascertain the micro structural changes accompanying low‐temperature oxidation of the samples. Although no new features resulting from oxidation were observed, prominent surjace reduction occurred in the thin foil specimens. Formation and growth of Ni particles on the ferrite surface are explained using the heats of formation of the oxides. 2