Herein,　the　crystal　structure,　surface　morphology,　and　magnetic　and　thermal　properties　of　the　BaFe11.9In0.1O19-substituted　M-type　hexaferrite　are　investigated.　This　solid　solution　is　fabricated　using　＂two-step＂　topotactic　reactions.　It　is　determined　through　the　X-ray　diffraction　method　that　the　crystal　structure　of　this　solid　solution　under　normal　conditions　has　hexagonal　symmetry　with　P6(3)/mmc　space　group　(No.　194)　with　the　unit　cell　parameters　a　=　5.8992(1)　angstrom　and　c　=　23.2275(7)　angstrom.　It　is　determined　by　the　scanning　electron　microscope　(SEM)　researches　that　the　average　size　of　the　grains　is　d　over　bar　=　6　mu　m.　It　is　determined　by　the　vibration　magnetometry　that　the　ferrimagnet-paramagnet　phase　transition　happens　at　around　T　(C)　=　420　degrees　C.　Using　differential　scanning　calorimetry　(DSC),　it　is　found　that　air　and　water　molecules　leave　the　sample　surface　up　to　420　degrees　C　and　the　resulting　oxygen　anions　appearing　as　a　result　of　a　solvothermal　reaction　fill　the　oxygen　vacancies　in　the　crystal　structure　at　high　temperatures　up　to　800　degrees　C.　The　thermal　transitions　repeated　in　the　high　temperatures　are　established.　The　oxidation　behavior　of　this　compound　is　determined　from　thermogravimetric　analysis　(TGA)　in　a　wide　temperature　range.