We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T C 3 K and a small ordered moment m 0 0:03 B . Superconductivity is observed with a resistive transition temperature T s 0:8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T s , and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations. DOI: 10.1103/PhysRevLett.99.067006 PACS numbers: 74.70.Tx, 74.20.Mn, 75.30.Kz In the standard theory for superconductivity (SC) due to Bardeen, Schrieffer, and Cooper ferromagnetic (FM) order impedes the pairing of electrons in singlet states [1]. It has been argued, however, that on the border line of ferromagnetism, critical magnetic fluctuations could mediate SC by pairing the electrons in triplet states [2]. The discovery several years ago of SC in the metallic ferromagnets UGe 2 (at high pressure) [3], URhGe [4], and possibly UIr (at high pressure) [5], has put this idea on firm footing. However, later work provided evidence for a more intricate scenario in which SC in UGe 2 and URhGe is driven by a magnetic transition between two polarized phases [6 -8] rather than by critical fluctuations associated with the zero temperature transition from a paramagnetic to a FM phase. Here we report a novel ambient-pressure FM superconductor UCoGe. Since SC occurs right on the border line of FM order, UCoGe may present the first example of SC stimulated by critical fluctuations associated with a FM quantum critical point (QCP).UCoGe belongs to the family of intermetallic UTX compounds, with T a transition metal and X is Si or Ge, that was first manufactured by Troć and Tran [9]. UCoGe crystallizes in the orthorhombic TiNiSi structure (space group P nma ) [10,11], just like URhGe. From magnetization, resistivity (T 4:2 K) [9,10] and specific-heat measurements (T 1:2 K) [12] it was concluded that UCoGe has a paramagnetic ground state. This provided the motivation to alloy URhGe (Curie temperature T C 9:5 K) with Co in a search for a FM QCP in the series URh 1ÿx Co x Ge (x 0:9) [13]. Magnetization data showed that T C upon doping first increases, has a broad maximum near x 0:6 (T max C 20 K) and then rapidly drops to 8 K for x 0:9 [13]. This hinted at a FM QCP for x & 1:0. In this Letter we show that the end (x 1:0) compound UCoGe is in fact a weak itinerant ferromagnet. Moreover, metallic ferromagnetism coexists with SC below 0.8 K at ambient pressure.Polycrystalline UCoGe samples were prepared with nominal compositions U 1:02 CoGe (sample 2) and U 1:02 Co 1:02 Ge (sample 3) by arc melting the constituents (natural U 99.9%, Co 99.9%, and Ge 99.999%) under a high-purity argon atmosphere in a water-cooled copper crucible. The as-cast samples were annealed for 10 days at 850 C. Sampl...

Whereas resistivity and ac susceptibility measurements on the magnetic (T M ∼ 130 K) superconductor (T c,onset ∼ 50 K) RuSr 2 GdCu 2 O 8 (Ru-1212Gd) reported by different research groups reveal a universal behavior in its superconducting state, this is not the case with the superconducting quantum interference device (SQUID) magnetization measurements. The reported SQUID measurements in the superconducting state of Ru-1212Gd reveal a variety of behaviors, leaving the question of bulk superconductivity for this compound open. We review several of the reported behaviors by presenting measurements done on our samples belonging to the same batch. Based on the observed sensitivity of Ru-1212Gd to magnetic field inhomogeneities when it is moved in the superconducting magnet of the SQUID magnetometer during the measurements, we suggest that the reported different behaviors can be the result of different field profiles in the superconducting magnet and not of different superconducting properties. In order to avoid the artifacts arising from moving the sample in an inhomogeneous field, we did measurements on a stationary Ru-1212Gd sample employing a home made SQUID magnetometer. The measured curves showed none of the suspicious "symptoms" present in the curves measured with a magnetometer employing sample movement (e.g. no reversal of the features observed in the superconducting state of Ru-1212Gd by a field reversal) and if verified by measurements on stationary samples by other groups a universal behavior in the superconducting state of Ru-1212Gd can be revealed by the SQUID measurements too. Our considerations support the existence of bulk superconductivity for Ru-1212Gd.

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