In situ crystallization and magnetic measurement of hexaferrite glass-ceramics

Glass-ceramic (GC) materials have the formability advantages of glasses and the opportunity for controlled crystallization of functional ceramic phases. Here, we obtain GCs containing the hard magnetic phase Sr-hexaferrite (SrFe12O19) from a borate glass. Ten compositions in the B2O3-Fe2O3-SrO system were explored, varying B2O3 and Sr/Fe ratio. Compositions forming glass on quenching, according to X-ray diffraction (XRD), were subsequently heat treated to promote crystallization. Three selected compositions were investigated with vibrating sample magnetometry (VSM), thermal analysis, and electron probe microanalysis (EPMA). Phases identified by XRD after air heat treatment included α-Fe2O3, Fe3O4, SrB2O4, and SrFe12O19. Glasses were also crystallized in situ in a VSM in an argon environment, measuring magnetic properties during heating to 650°C after in situ heat treatment at 800°C. In samples with SrFe12O19, wasp-waisted loops were observed. First order reversal curve (FORC) measurements confirmed low (magnetite) and high (Sr-hexaferrite) coercivity phases. Room temperature VSM measurements of argon in situ treated samples were compared with two ex situ air heat treated protocols. The microstructures of the three investigated air heat treated GC materials were completely different, and compositional position on the phase diagram appeared to influence crystallization progress. These results suggest that careful control of composition as well as heat treatment protocol including atmosphere is necessary for crystallization of desired magnetic phases. Also, VSM was confirmed to be sensitive to magnetic phases at low concentrations not visible to XRD.