Influence of Ionic Order on the Magnetocrystalline Anisotropy and Crystalline Field Parameters in Lithium Ferrite Monocrystals

Measurements of the first order anisotropy constant ${\mathrm{(K}}_1)$ . utilizing the torque method, are reported for various degrees of long range order (LRO) in lithium ferrite. The LRO parameters were determined by x-ray diffraction analysis. It is found that the degree of cation “order” in the octahedral sites strongly influences $K_1$ . For example, measurements at 77°K yield the values ${\text{−127×10}}^3\hspace{0.17em}\mathrm{ergs}/{\mathrm{cm}}^3$ and ${\text{−162×10}}^3\hspace{0.17em}\mathrm{ergs}/{\mathrm{cm}}^3$ for $K_1$ in the “ordered” and “disordered” states, respectively. The experimental results were interpreted on the basis of the ferric ion spin Hamiltonian. Certain parameters ($a_{\mathrm{B\prime }}$ for octahedral sites and $a_{\mathrm{A\prime }}$ for tetrahedral sites) which include the contributions of the cubic term and the quartic axial term to cubic anisotropy were deduced from measurements of the temperature dependences of $K_1$ and saturation magnetization. It is found that in “disordered” lithium ferrite, $a_{\mathrm{B\prime }}\text{\hspace{0.17em}=\hspace{0.17em}+0.0242\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ and $a_{\mathrm{A\prime }}\text{\hspace{0.17em}=\hspace{0.17em}−0.0118\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ in excellent agreement with the corresponding parameters determined by Folen and Rado in magnesium ferrite which is also a “disordered” ferrite. In the “ordered” lithium ferrite, it is found that $a_{\mathrm{B\prime }}\text{\hspace{0.17em}=\hspace{0.17em}+0.0104\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ and $a_{\mathrm{A\prime }}\text{\hspace{0.17em}=\hspace{0.17em}+0.0034\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . The experimental temperature dependences of $K_1$ in both the “ordered” and “disordered” states are in quantitative agreement with that obtained from the Yosida-Tachiki theory. The changes in the magnitudes and signs of the observed $\mathrm{a\prime }$ -values resulting from the “order-disorder” transition are consistent with alterations of the quartic axial term.