Magnetic phase diagram of helimagnetic Ba(Fe1−xScx)12O19 (0 ≤ x ≤ 0.2) hexagonal ferrite

•The magnetic phase diagram of Ba(Fe1−xScx)12O19 was constructed in the T-x plane.•From the phase diagram, it was determined that helimagnetism appeared at x ≳ 0.06.•The antiferromagnetic phase appeared at x ≳ 0.19 through the coexistence region with helimagnetism.•The turn angle of the helix as a function of x and T was clarified. [Display omitted] Hexagonal ferrite Ba(Fe1−xScx)12O19 is an important magnetic oxide material in both science and engineering because it exhibits helimagnetism around room temperature (300 K). In this study, the magnetic phase diagram of Ba(Fe1−xScx)12O19 consisting of ferri-, heli-, antiferro-, and paramagnetic phases has been completed through magnetization and neutron diffraction measurements. The magnetic phase transition temperature to paramagnetism decreases with the increase in x, and the temperature at which the magnetization reaches a maximum, which corresponds to the magnetic phase transition from heli- to ferrimagnetism, is determined for low x crystals. The temperatures at which helimagnetism appears are precisely determined by observing the magnetic satellite reflection peaks in neutron diffraction at various temperatures, which characterize helimagnetism. Based on these results, the magnetic phase diagram of the Ba(Fe1−xScx)12O19 system is constructed in the T-x plane. Helimagnetism appears at x ≳ 0.06, and magnetism with antiferromagnetic components appears as the extension phase of helimagnetism at x ≳ 0.19 through the coexistence region. The turn angle ϕ0 of the helix for each x crystal is calculated from the relationship, ϕ0 = 2πδ, where δ is the incommensurability. The turn angle ϕ0 decreases with the increase in temperature for the same x crystal, and increases with the increase in x at the same temperature. Furthermore, it is found that there are clear thresholds at which ϕ0 cannot take values between 0°