An Investigation of the Magnetic and Electrical Properties of YNi3
Weak itinerant electron ferromagnets such as ZrZn2, Ni3Al and YNi3 are of interest to solidstate research because they demonstrate non-Fermi-Liquid behaviour and allow us to probe the quantum nature of the solid state in magnetic materials. Spin fluctuation theory is currentlythe most successful tool we have for trying to describe critical behaviour on the border of magnetism, and these materials are particularly suited to such investigations. In this thesis the magnetic and electrical properties of YNi3 have been investigated both in magnetic field and under hydrostatic pressure. Some properties are in agreement with the literature, while others, such as the critical exponent at low temperature, differ from previous work. Some of the experiments performed are the first such studies of this material. The key new findings in our high purity samples with residual resistivity ratios (RRR =R300K/R4K) of at least 130, can be summarized as follows: • The electrical resistivity in the low temperature regime below about 10K varies approximately as T3/2 at low fields and as T2 at high fields beyond 6T; this is in contrast to the T2 variation previously reported down to low fields, with no evidence for a T3/2 regime, in low purity samples (RRR 4). • In contrast to previous findings in low purity samples we show that Arrott plots, M2 vs. H/M, where M is the magnetization and H the applied field, are markedly non-linear.. • More strikingly, we find that the magnetic transition in low fields appears weakly 1st order rather than 2nd order as had been previously assumed, thus identifying YNi3 as an example of a system on the border of a quantum tri-critical point. • By comparing the temperature dependence of the heat capacity at different fields, it has been possible to isolate the subtle magnetic heat capacity anomaly at the Curie temperature which had previously evaded detection and thus determine the elusive entropy change associated with the magnetic transition in a weak itinerant electron ferromagnet. • By studying the pressure dependence of the Curie temperature, we find that the magnetic transition collapses linearly with pressure and tends to vanish at approximately 55 kbar. This identifies the quantum critical or quantum tri-critical regime where an unconventional form of superconductivity is expected to arise in samples of sufficiently high purity and at sufficiently low temperatures in the low millikelvin temperature range.
Magnetism, Quantum Criticality, YNi3, Solid State, Itinerant Electron Ferromagnet, High Pressure
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