Discussion
The high temperature magnetic susceptibility of solids having ions with magnetic moments can be described by \(\chi \propto \frac{1}{T + \theta}\) with \(T\) as absolute temperature and \(\theta\) as constant. The three behaviors i.e. paramagnetic, ferromagnetic and anti-ferromagnetic are described, respectively, by
Answer: C
The three behaviours correspond to \(\theta = 0, \theta \lt 0, \theta \gt 0\).
With \(\chi \propto 1/(T + \theta)\):
- A paramagnet obeys Curie’s law, \(\chi \propto 1/T\), so \(\theta = 0\).
- A ferromagnet above its Curie temperature obeys the Curie-Weiss law \(\chi \propto 1/(T - T_{C})\), which diverges at \(T_{C}\); this is \(\theta = -T_{C} \lt 0\).
- An antiferromagnet above its Néel temperature follows \(\chi \propto 1/(T + \theta)\) with \(\theta \gt 0\), so the susceptibility stays finite and the extrapolated intercept is at negative temperature.
The options with \(\theta \gt 0\) for the ferromagnet reverse the signs of the two ordered cases, and any option giving the paramagnet a non-zero \(\theta\) contradicts Curie’s law.
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