K. Juntunen and J. Tuoriniemi
The research efforts at the YKI-cryostat are shared between two main projects: the search for superfluidity in 3He-4He mixtures and the studies of nuclear magnetism in pure metals, most recently in lithium.
During the year 2003 the cryostat was occupied for the measurements on lithium metal (K. Juntunen and J. Tuoriniemi). We cooled down two different types of samples, both hermetically sealed by copper capsules. The first was meant for the search of superconductivity. For this purpose the sample could be bulky but had to be extremely well protected from any magnetic stray fields (at nT level). The second was meant for the NMR measurements on highly polarized nuclei. For this purpose the sample had to be thin (tens of _m) and had to be placed into the bore of our second superconducting magnet in the cryostat (to be polarized in several T). Both types of samples produced interesting and to some extent unexpected results. No superconductivity of lithium was detected down to about 100 _K, but, instead, rather peculiar magnetic behavior was observed below fields of about 1 _T and temperatures below about 500 _K. Data on only one sample of this type could be collected due to problems with the SQUID sensors, and so, obviously, these investigations have to be carried on further in order to draw any conclusions about the origin of the magnetic signals. A wealth of NMR data was collected during two cool downs, about four months each. The behavior at high nuclear polarizations and in low magnetic fields was characterized by extraordinarily strong response at very low frequencies, well below the usual Larmor band in the local magnetic field. On the basis of such alterations of the NMR line shapes, measurements of hysteretic losses, and of the determination of the entropy-field-temperature relationships, we can sketch a phase diagram with at least four distinct regions in the temperature-magnetic field plane. The paramagnetic and a ferromagnetic phase are most reliably identifiable but the nature of the other two spin states is far less clear. The analysis of the results is still on its way and will eventually establish the thesis work of K. Juntunen.