Which ferromagnetic material has the lowest Curie temperature?

It is hard to search for materials by their properties in general and I am trying to find a material with a very low Curie temperature. At the moment I am browsing different sites but can only find a number of 'normal' ferromagnetic materials but few with a small Curie temperature.

So which material has the lowest Curie of all ferromagnets?

(By material I mean pure substance, e.g. NdFeB, but not a material under pressure, or nanocrystals or other special conditions which should all allow the suppression of ferromagnetism).

• If you take a magnetic-nonmagnetic alloy system (maybe iron-aluminum for example), the Curie temperature presumably changes continuously from above-room-temperature to absolute zero, as you change the composition from pure iron towards pure aluminum. Apart from the problem of inhomogeneities (and superconductivity), one imagines you can get even 0.0001K curie temperature. [Getting an arbitrary metal alloy may not be so easy though.] – Steve Byrnes Apr 10 '12 at 15:11
• @SteveB: this does not work so easily in practice. Often you get another ground state, e.g. a spin glass or a superparamagnet. By writing 'pure substance' I tried to exclude such dilutions. – Alexander Apr 10 '12 at 20:06
• @Alexander - what is the purpose of this question? Just curiosity? – Jen Apr 12 '12 at 12:19
• @Jen: Curiosity and some theoretical predictions that a 'normal' 2nd order phase transition in a ferromagnet is prevented at lowest temperatures (arxiv.org/abs/1203.3826). – Alexander Apr 12 '12 at 15:57

$\text{Y}\text{Ni}_3$ has a Curie temperature of 30 to 35 Kelvin. (Ref 1, Ref 2)
Here for example (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.57.1161), they use NMR to show nuclear ferromagnetism of adsorbed $^3$He with a Curie temperature in the order of $\sim$1 mK.