Nuclear magnetic resonance thermometry at ultra-low temperature

The electromagnetic signal of NMR in pure metallic samples emerges as the most reliable technique for measuring low temperatures from 1 K to below 1 mK (10⁻³ K). This fact stems from the self-consistency of the temperature measurement by NMR, which relies on the Curie law and the Korringa law separately. Samples of Pt, Al, and Cu have been studied with a careful evaluation of the effect of magnetic impurities. Finally the temperatures of the remarkable features of the ³He melting curve which may be regarded as thermometric fixed points have been measured.