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...

MnFeP 1−x Si x compounds with x = 0.10, 0.20, 0.24, 0.28, . . . , 0.80, 1 were prepared by high-energy ball milling and solid-state reaction. The structural, magnetic, and magnetocaloric properties are investigated as a function of temperature and magnetic field. X-ray diffraction studies show that the samples in the range from x = 0.28 to 0.64 adopt the hexagonal Fe 2 P-type structure with a small amount of second phase which increases with increasing Si content. The samples with lower Si content show the orthorhombic Co 2 P-type structure. Magnetic measurements show that the paramagnetic-ferromagnetic transition temperatures range from 214 to 377 K. Of much importance is the fact that these compounds do not contain any toxic components and exhibit excellent magnetocaloric properties.

We have performed specific-heat measurements on the heavy-fermion superconductor URu 2 Si 2 in magnetic fields up to 17.5 T. A sharp peak in the specific heat signals the antiferromagnetic transition at T N ϭ17.5 K, which shifts to lower temperatures in applied magnetic fields. In order to describe the specific heat below T N , we have used the characteristic features of the excitation spectrum measured by neutron scattering. The relative field dependence of the antiferromagnetic transition temperature T N and the energy gap ⌬ in the magnetic excitation spectrum can be described by a single scaling relation of the form ͓1Ϫ(B/B 0 ) 2 ͔. The scaling field of 48.5 T is close to the metamagnetic transition field B*ϭ40 T, where the heavy-fermion state is suppressed.

We have studied the magnetocaloric effect in MnFe(P,Si,Ge) compounds. The structural properties of the compounds were determined by x-ray diffraction. The homogeneity and the stoichiometry of the compounds were checked by electron probe microanalysis. The Curie temperature is found to be near room temperature. Specific-heat measurements made on these compounds show a first-order ferromagnetic—paramagnetic phase transition. The magnetocaloric effect derived from magnetization data shows that this effect in the MnFe(P,Si,Ge) compounds is as large as that in Gd-based compounds and MnFeP1−xAsx compounds. This means that we have succeeded in totally replacing As by (Ge,Si) in the latter compounds without losing the favorable magnetic properties. The upshot is that we have found relatively low cost and nontoxic materials for room-temperature cooling applications.

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