Quantum phase transitions in NbFe₂and 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 low-temperature 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. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim