Phase formation and magnetic properties of M-type lanthanum substituted strontium ferrites

Rare earth ion substitution is one of the most important methods for adjusting the magnetic properties of M-type hexagonal ferrites; however, the regularity of these phase formations has rarely been studied. In this work, La substituted Sr hexaferrite LaxSr1-xO·nFe2O3 (La-SrM, 4.9 ≤ n ≤ 6.0, 0 ≤ x ≤ 0.6) was prepared using the traditional ceramic method. The effects of the Fe/Sr molar ratio (n), calcining temperature, and La3+ substitution (x) on SrM phase formation, the crystalline structure, and magnetic properties were investigated. With an increase of x up to a maximum value of 0.5–0.6, a higher calcining temperature is required to form the single M-phase of La-SrM samples. However, the optimal n values of single-phase La-SrM samples differ as the La substitution varies: when x = 0.1, n = 5.5–6.0; x = 0.2, n = 5.5–5.9; x = 0.3, 0.4 and 0.5, n = 5.7–5.8. The magnetic measurements show that La0.2Sr0.8O·5.8Fe2O3 has the highest specific saturation magnetization (σs), which is 2.2% higher than that of unsubstituted SrM (SrO·6Fe2O3), while the anisotropic field (HA), the anisotropic constant (K1), and Neel point (TN) of La3+ substituted SrM decreased. Detailed structure analyses were conducted to explain the changes in magnetic properties. Fe3+ in the spin-up 2a sublattice of LaxSr1-xO·5.8Fe2O3 decreased by approximately 5% from 98.5% (x = 0) to 93.85% (x = 0.4) with an increase in x. Additionally, a small amount of Fe3+ was reduced to Fe2+ in the spin-down 4f2 sublattice with the maximum reduction amount of 4.13% reached at x = 0.2, thereby improving σs. The decrease in the bond angle of (4f1) Fe3–O2–Fe5 (12k), (2a) Fe1–O4–Fe3 (4f1), and (4f1) Fe3–O4–Fe5 (12k) lead to the weakening of Fe–O–Fe superexchange of La-SrM so that HA, K1, and Tn decreased with increasing values of x. This work lays a solid foundation for the study of process regulation and ion substitution of permanent magnet ferrite.