Past exam of the condensed matter physics course of the University of Oxford / 2022 / 4 / c / Solution

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).