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In weak magnetic field and the regime of paramagnetism, localized magnetic moments with total angular momentum $J$ and Landé g-factor $\tilde{g}$ have SI volume magnetic susceptibilitywithin the Curie–Weiss law. Here $n_m$ counts magnetic ions per volume, ${\mu }_0$ is the vacuum permeability, ${\mu }_B$ is the Bohr magneton, and $\Theta$ is the Weiss temperature. The factor $J(J+1)/3$ is the zero-field average of the squared angular projection over $m_J=-J,\dots ,J$.

For one magnetic ion per formula unit, a molar volume $V_m$ contains $N_A$ magnetic ions, where $N_A$ is the Avogadro constant. Consequently $n_m{V}_m=N_A$, and the molar magnetic susceptibility isDefining the dimensionless effective paramagnetic moment $m_{\mathrm{eff}}$ by ${\mu }_{\mathrm{eff}}=m_{\mathrm{eff}}{\mu }_B$, numerical substitution givesThe normalization is per mole of formula units, which here also means per mole of magnetic ions.

Write the fitted inverse molar magnetic susceptibility asThe vertical scale is in $10^6\mathrm{mol}{\mathrm{m}}^{-3}$, not unscaled inverse SI magnetic susceptibility. The source line is approximately $8.8\times 10^6\mathrm{mol}{\mathrm{m}}^{-3}$ at $100\mathrm{K}$ and $19\times 10^6\mathrm{mol}{\mathrm{m}}^{-3}$ at $300\mathrm{K}$. These readings giveTherefore the effective paramagnetic moment and Weiss temperature areEquivalently, ${\mu }_{\mathrm{eff}}\simeq (3.5\text{–}3.6){\mu }_B$. An uncertainty of order $10\mathrm{K}$ in the graphical Weiss temperature is reasonable; the plotted fit does not justify exact integer parameters. Its positive intercept at $T=0$ requires a negative extrapolated temperature-axis intercept. Thus the dominant interactions have the antiferromagnetic sign in the interpretation of part (b).