Further Reading
A.S. Oja and O.V. Lounasmaa, Nuclear magnetic ordering in simple metals at positive and negative nanokelvin temperatures, Rev. Mod. Phys. 69, 1 (1997).
Adiabatic Demagnetization Cooling
The temperature of a given system is established by interactions between its constituents. Various subsystems may have different temperatures if they are effectively decoupled from each other. Fascinating examples of this are the assemblies of nuclear spins and conduction electrons in metals. Mutual interactions between the small nuclear moments are extremely weak and at low temperatures the nuclei are so well isolated that their temperature can be many orders of magnitude lower, or higher, than that of the lattice and conduction electrons.The temperature of an isolated spin system can be altered by an adiabatic change of the external magnetic field, which is the basis for nuclear demagnetization cooling. This technique has been developed to its extreme at the LTL by operating two nuclear stages in cascade. In these studies the nuclear spin system of the second stage is, in fact, also the system under experimental investigation.
Nuclear Magnetic Ordering
When the thermal energy of the spins is comparable to their mutual interactions, reordering of the system occurs. In the past, the nuclear spins of copper and silver have been cooled all the way to the magnetically ordered states, where SQUID-NMR and neutron diffraction measurements have given detailed information on the thermodynamics and magnetic structures of the nuclei. As a by-product, both experiments produced a low temperature record. During our present research on rhodium, the spin system has been cooled to 250 pK, the lowest temperature ever produced and measured, but no sign of magnetic ordering has been observed so far. High-resolution SQUID-NMR techniques are again used to investigate the nature of magnetic interactions in this metal in order to understand the observations and to make predictions about the possible ordering temperature.From Positive to Negative Temperatures
A rather unique property of nuclear magnets is the possibility of producing negative spin temperatures. This does not violate the laws of thermodynamics, i.e. inaccessibility of the absolute zero, because the negative side of the temperature scale is reached by a rapid magnetic field reversal. During this process 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.Future Goals
The cryogenic instrument itself, housing the present experiment on rhodium, is a fully upgraded state-of-the-art platform for ultralow temperature research of much wider scope. It is particularly designed for investigations of condensed matter below 100 mK. Interesting phenomena in this regime are expected, for example, in the dilute liquid mixtures of the helium isotopes 3He and 4He.
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