Quantum phase transitions in NbFe₂ and Ca₃Ru₂O₇

We examine the low temperature states of two transition metal compounds: (i) NbFe₂ is poised on the threshold of ferromagnetism and can be pushed into a spin-aligned state at low temperature by modifying the composition slightly. Stoichiometric NbFe₂ has been reported as a rare example of lowtemperature spin density wave order in a d-metal system. We have used pressure, field and composition tuning to examine the phase diagram of NbFe₂. Near the quantum critical point, we find distinct non-Fermi liquid forms of the resistivity and heat capacity, whereas we observe strong, hysteretic magnetoresistance effects deep in the ordered phase. (ii) Ca₃Ru₂O₇ undergoes first a magnetic transition (T_N = 56 K) and then a structural transition (T_S = 48 K) on cooling. Most of the Fermi surface is gapped out at low temperature, leading to a very low carrier density and small Fermi surface pockets. Pressure suppresses both T_N and T_S and, for p>3.5 GPa, induces a third low temperature state, which is robust up to at least 7.5 GPa.