Enhancement of the Magnetic Properties in Si4+-Li+-Substituted M-Type Hexaferrites for Permanent Magnets

A series of charge-balanced Si4+-M1+,2+ (M = Mg2+, K+, Li+) substitution compositions with the chemical formulae of SrFe12−2xSixMgxO19 (x = 0, 0.05, 0.1, 0.2), Sr1−yFe12−ySiyKyO19 (y = 0.05, 0.1, 0.2), and SrFe12−z(Si0.6Li0.6)zO19 (z = 0.05, 0.1, 0.2, 0.4, 0.6) were prepared using conventional ceramic processes. While the sole doping of Si with x = 0.1 (SrFe12−xSixO19 (x = 0.1)) causes a noticeable Fe2O3 phase formation, the co-doping of Si-Mg and Si-Li allow a single M-type phase formation with x up to x = 0.1 and z = 0.4, respectively. Notably, a 2.6% increase in the saturation magnetization was obtained for Si-Li-substituted SrM with z = 0.05—that is, SrFe11.95Si0.03Li0.03O19. Enhancement of the magnet performance can also be achieved when anisotropic permanent magnets are fabricated based on the substitution composition SrFe11.95Si0.03Li0.03O19. The remnant magnetic flux density was improved by 2.5% compared to that of the unsubstituted SrM (from 4207 to 4314 G). The maximum energy product (BHmax) value also increased from 4.24 to 4.46 M·G·Oe. The enhancement in permanent magnet performance is attributed to the increase in the MS of SrM by the optimal Si-Li substitution. This is a promising result because enhanced permanent magnet performance is achieved with intrinsic magnetic property improvement.