Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd(2)Ti(2)O(7), in its magnetically ordered phase below approximately 1 K, displays persistent spin dynamics down to temperatures as low as 20 mK. The characteristics of the induced muon relaxation can be accounted for by a scattering process involving two magnetic excitations, with a density of states characterized by an upturn at low energy and a small gap depending linearly on the temperature. We propose that such a density of states is a generic feature of geometrically frustrated magnetic materials.

We have investigated the thermal, transport and magnetic properties of URh$_{1-x}$Ru$_x$Ge alloys near the critical concentration $x_{cr} = 0.38$ for the suppression of ferromagnetic order. The Curie temperature vanishes linearly with $x$ and the ordered moment $m_0$ is suppressed in a continuous way. At $x_{cr}$ the specific heat varies as $c \sim TlnT$, the $\gamma$-value $c/T|_{0.5K}$ is maximum and the temperature exponent of the resistivity $\rho \sim T^n$ attains a minimum value $n=1.2$. These observations provide evidence for a ferromagnetic quantum phase transition. Interestingly, the coefficient of thermal expansion and the Gr\"uneisen parameter $\Gamma$ remain finite at $x_{cr}$ (down to $T = 1$ K), which is at odds with recent scaling results for a metallic quantum critical point.Comment: 5 pages; accepted for publication in Phys. Rev.

We have investigated the evolution of ferromagnetic order in the correlated metal URhGe (Curie temperature T C = 9.5 K) by chemical substitution of Ru, Co and Si. Polycrystalline samples URh 1−x Ru x Ge (x ≤ 0.6), URh 1−x Co x Ge (x ≤ 0.9) and URhGe 1−x Si x (x ≤ 0.2) have been prepared and the magnetic properties have been investigated by magnetization and transport experiments. In the case of Ru doping, T C initially increases, but then decreases linearly as a function of x and is completely suppressed for x cr ≈ 0.38. The Curie temperature in the URh 1−x Co x Ge series has a broad maximum T C = 20 K near x = 0.6 and then drops to 8 K for x = 0.9. In the case of Si doping T C stays roughly constant. We conclude that the alloy systems URh 1−x Ru x Ge and URh 1−x Co x Ge are interesting candidates to study the ferromagnetic instability.

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Results of a detailed investigation of the ferromagnetic superconductor UGe 2 using positive muon spin rotation and relaxation techniques are presented. The pressure and temperature dependences of the frequencies and related spin-spin relaxation rates show that the transition from the weakly to the strongly polarized magnetic ͑WP-SP͒ phases is still observable at T X Ӎ 3 K under a pressure of 1.33͑2͒ GPa. Thus this transition survives at higher pressures than previously believed. The temperature T X at 1.00͑2͒ GPa corresponds to a thermodynamic phase transition rather than a crossover. No such statement can be given reliably at lower pressure. A substantial shrinking of the component along the easy axis of the diagonal hyperfine tensor, at the muon site where it is large, is observed in the SP phase relative to the WP phase. This corresponds to an appreciable decrease in the electronic density at the Fermi level in the SP phase. The investigation of the paramagnetic-ferromagnetic critical spin dynamics at ambient pressure and at 0.95͑2͒ GPa shows that the simple one-band model is an oversimplification inconsistent with our critical spin-dynamics results. Data from specific heat, Fermi-surface studies, Hall effect, neutron form factor, and spectroscopic techniques supports this conclusion. Even at 0.95͑2͒ GPa the conduction electrons are characterized by a small magnetic moment, relative to the bulk magnetization per uranium atom.