The presence of small amounts of Mn3+ ions in both Y3MnxFe5−xO12 and rare‐earth substituted Y2.3Gd0.7Al0.4MnxFe4.6−x O12 will produce significant improvements in the hysteresis loop The samples studied were prepared by standard ceramic techniques and were found to be single phase with 10–15‐μ grain diameters. The sensitivity of the remanence ratio to external stress parallel to the magnetization1,2 is reduced monotonically with x and is eliminated in both compounds at x≃0.09. In addition, remanence ratios are increased because of an apparent reduction in the effects of internal stress, and all evidence suggests a decrease in coercive forces when densities are maximized. All hysteresis‐loop data, including stress sensitivity measurements, were obtained from a high‐field hysteresigraph.3 The electrical resistivity decreases with x, but remains well above 1010 Ω‐cm up to x=0.1 and the dielectric loss tangent is less than 2×10−4, which is highly satisfactory for microwave applications. From the results of resonance linewidth measurements compared with calculated contributions from porosity, it is concluded that anisotropy contributions to linewidths are reduced without significant increase in intrinsic linewidths. As a result, it appears that Mn3+ additions to these compounds also lower the magnetic anisotropy and, consequently, the lower limits of both linewidth and and coercive force.