Cascade nuclear demagnetization cryostat for the neutron diffraction studies on spontaneously ordered nuclear spin systems in pure metals. A single crystal sample of isotopically pure silver-109 mounted on the apparatus orders antiferromagnetically at a nuclear spin temperature of about 600 pico-Kelvin.

Nuclear Magnetism in Pure Metals

We have investigated nuclear magnetism in pure copper, silver, rhodium, and lithium metals by the SQUID-NMR and/or neutron diffraction methods under the extreme low temperature conditions, where the spin entropy becomes low enough to require spontaneous ordering of the spin system. In some cases, also negative absolute spin temperatures can be produced and investigated.

Negative Absolute Temperatures

A rather unique property of nuclear magnets is the possibility of producing negative spin temperatures. This, of course, does not violate the laws of thermodynamics, i.e. the inaccessibility of the absolute zero: the negative side of the temperature scale is reached by a rapid magnetic field reversal, during which the spin temperature is, strictly speaking, ill defined, but can be thought of evolving via infinity. In a sense, negative absolute temperatures are not colder than zero but actually hotter than infinite temperature! This is so because the spin system then possesses more energy than at infinite temperature. Physically, the important consequence of this is that the ordered spin configurations may be entirely different depending on whether the absolute zero is approached from the positive or from the negative side. Silver, for example, has been observed to order antiferromagnetically at positive and ferromagnetically at negative temperatures.

Neutron Diffraction Studies

A wealth of information on the spin system and of its mutual interactions can be retrieved by the NMR methods, and the ordered state has always been first found by the susceptibility measurements. However, the microscopic nature of the ordered state cannot be conclusively identified by mere magnetic measurements, wherefore neutron diffraction experiments have been performed on copper and silver in collaboration with Risø and Hahn-Meitner laboratories in Denmark and Germany. Indisputable confirmation and many details of the ordered antiferromagnetic states were gained by these measurements.

Nuclear Magnetism and Superconductivity

Magnetism and superconductivity are controversial phenomena in that the superconducting state tends to be destroyed by any magnetic disturbance and the superconducting state excludes any bulk magnetic field due to the Meissner effect. Competition of the ordering tendencies may be encountered if a nuclear magnet is embedded in a weak superconductor with a low critical temperature and a low critical magnetic field. Rhodium is the weakest known superconductor with the transition temperature of about 0.5 mK, which was one of the main reasons for initiating our work on this metal. Mutual influence between the polarized nuclear spin system and the superconducting electron system was indeed observed, but, unfortunately, the spontaneously ordered nuclear magnetic state was never reached or properly identified in this metal. Lithium has now been observed to become a superconductor below 0.4 mK. This is thus the most promising candidate for further studying the interplay between nuclear magnetism and superconductivity. Lithium constitutes a fairly strong nuclear magnet which is supposed to severely influence the conditions for superconductivity.

Lithium Metal

We have refrigerated the electrons and the lattice of lithium samples down to 0.1 mK and observed the superconducting state of this light metal below 0.4 mK. This is the first uncompressed alkaline metal to undergo the superconducting transition, and Li has the lowest transition temperature of any element to date. The further cooled nuclear spin system in lithium was found to develop magnetic ordering below about 300 nK in zero magnetic field. The character of the ordered state differs in an essential manner from those of copper and silver investigated thus far.