Magnetic Moment of Lanthanum Magnetoplumbite Ferrite

The composition ${\mathrm{La}}^{3+}$ ${\mathrm{Fe}}^{2+}$ ${\mathrm{Fe}}_{11}^{3+}$ ${\mathrm{O}}_{19}$ was prepared by sintering the oxides, and its magnetic moment was measured from liquid air temperature to the Curie point. At the lowest temperature a value of 17.5 Bohr magnetons per formula weight was obtained, as compared with 19 Bohr magnetons of Ba${\mathrm{Fe}}_{12}^{3+}$ ${\mathrm{O}}_{19}$. The difference is qualitatively understood if the ${\mathrm{Fe}}^{2+}$ ion is a nearest neighbor to the ${\mathrm{La}}^{3+}$ in the crystal, i.e., in the [$12K$] site, thus decreasing the total magnetic moment. The Curie temperature is 695°K, compared to 725°K of Ba${\mathrm{Fe}}_{12}^{3+}$ ${\mathrm{O}}_{19}$. The lower Curie point can again be qualitatively explained by less exchange interaction with the divalent iron of La${\mathrm{Fe}}^{2+}$ ${\mathrm{Fe}}_{11}^{3+}$ ${\mathrm{O}}_{19}$, or possibly by the change in unit cell dimension which modifies the short-range exchange interaction between the iron ions.