Magnetic ordering in rare-earth fluorides withKY3F10structure and axial moments

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The rare-earth fluorides with the cubic $\mathrm{K}{\mathrm{Y}}_{3}{\mathrm{F}}_{10}$ structure contain rare-earth sites having tetragonal symmetry, with one-third of the sites having symmetry axes aligned along each of the cubic axes. We present the magnetic properties for compounds in which ${\mathrm{Er}}^{3+}$, ${\mathrm{Yb}}^{3+}$, ${\mathrm{Tm}}^{3+}$, and ${\mathrm{Tb}}^{3+}$ substitute for ${\mathrm{Y}}^{3+}$. Similar to $\mathrm{K}{\mathrm{Dy}}_{3}{\mathrm{F}}_{10}$, reported earlier, these materials display nearly complete (Ising) anisotropy of the magnetic moments along the local symmetry axis, yielding three orthogonal Ising lattices. Estimates of magnetic exchange are antiferromagnetic in all compounds, and the dipole-dipole interaction favors antiferromagnetism in the absence of exchange in each case. At low temperature, we find that only the Tm compound clearly displays antiferromagnetic order, while in the Er and Yb compounds we observe a transition to a ferromagnetic state, very similar to that seen in $\mathrm{K}{\mathrm{Dy}}_{3}{\mathrm{F}}_{10}$. We suggest a reason for this by comparing ground state energies as a function of single-ion anisotropy and the magnitudes of the two interactions.