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Q & A: paramagnetic Gadolinium

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Most recent answer: 08/15/2016
Is Gadolinium ferromagnetic up to its Curie point (68 F or so) and then paramagnetic after that?I think the answer is "yes" but I get differing answers as to what the real effect is.Take a small piece of Gd below 68 F and it will be attracted to a magnet. Warm the Gd to 100 F (for example using warm tap water) and what happens?I've seen a video when the Gd is no longer attracted to a magnet but my on experimenting with a small piece of Gd is that it is still just as attracted to a magnet.The explanation given to me by some is that it is still paramagnet (Gd) and that if the Gd were magnetized while below the Curie point it would stop being a magnet past the Curie point.That doesn't make sense to me as paramagnetism generally is a weak effect usually demonstrated by moving a piece of paramagnetic that is on a piece of cork floating in a bowl of water (to greatly reduce friction).Why ultimate question is why is my small piece of Gd just as attracted by a magnet above its Curie point as it is below it?
- Gray (age 61)

Your observation doesn't sound too strange to me. Even in the ferromagnetic state, the domains in Gd will only partly align with the external field. At temperatures far above the Curie point, the paramagnetic alignment will become weak. You're talking about a temperature of 100°F or 311K, about 18K above the Curie temperature of 293K, only about 6% on an absolute scale. The magnetic susceptibility should follow a Curie-Weiss law, proportional to 1/(T-TC) rather than simple paramagnetism, which goes as 1/T. So the susceptibility is enhanced by a factor of around 16 by the ferromagnetic interactions, even though it's not actually ferromagnetic at that point. Also, those comparison paramagnets may have a fairly low concentration of spins, compared to Gd.

Mike W.

(published on 08/15/2016)

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