= Solution
In the SI linear-response convention $\mathbf M=\chi_H\mathbf H$, a diamagnetic contribution has negative susceptibility: the induced moment opposes the applied field. Orbital motion supplies a familiar microscopic mechanism, and closed-shell atoms can be diamagnetic even without permanent moments. <Paramagnetism> has positive susceptibility, with induced <magnetization> along the field; thermal alignment of permanent localized moments is one mechanism, while conduction-electron <spin> response is another.
Neither sign alone is a definition of <ferromagnetic ordering>. An ordinary paramagnetic phase has no spontaneous zero-field <magnetization>. A diamagnetic background can coexist with a larger paramagnetic contribution in the same specimen. Curie's inverse-temperature law describes an appropriate localized-moment model, not every possible paramagnet.
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