The discovery of superconductivity at high pressure (albeit over a restricted range) in the ferromagnetic material UGe2 raised the possibility that bulk superconductivity might be found in other ferromagnets. The exact symmetry of the paired state and the dominant mechanism responsible for the pairing, however, remain unidentified. Meanwhile, the conjecture that superconductivity could occur more generally in ferromagnets has been fuelled by the recent observation of a low-temperature transition that suggests an onset of superconductivity in high-quality crystals of the itinerant-ferromagnet ZrZn2 (ref. 2), although the thermodynamic signature of this transition could not be detected. Here we show that the ferromagnet URhGe is superconducting at ambient pressure. In this case, we find the thermodynamic signature of the transition-its form is consistent with a superconducting pairing of a spin-triplet type, although further testing with cleaner samples is needed to confirm this. The combination of superconductivity and ferromagnetism may thus be more common and consequently more important than hitherto realized.

We have investigated the magnetic relaxation of clusters of eight iron ions characterized by a spin ground state of ten and an Ising anisotropy. Below 400 mK the relaxation rate is temperature independent suggesting that tunneling of the magnetic moment across its anisotropy energy barrier occurs. Using the anisotropy constants derived from EPR data, we can calculate both the crossover temperature T c and the expected tunneling frequency 1͞t. The field dependence of the relaxation shows evidence of resonant tunneling. [S0031-9007(97)03302-4]

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Below 360 mK, Fe8 magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted "square-root time" relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P (ξH) of magnetic energy bias ξH acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby "digging a hole" in P (ξH) (around the resonant condition ξH = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.PACS numbers: 75.45.+j, 75.60Ej Strong evidence now exists for thermally-activated quantum tunneling of the magnetization (QTM) in magnetic molecules such as Mn 12 ac and Fe 8 [1][2][3][4][5]. Crystals of these materials can be thought of as ensembles of identical, iso-oriented nanomagnets of net spin S = 10 for both Mn 12 ac and Fe 8 , and with a strong Ising-like anisotropy. The energy barrier between the two lowest lying spin states with S z = ±10 is about 60 K for Mn 12 ac and 25 K for Fe 8 [6,7]. Theoretical discussion of thermallyactivated QTM assumes that thermal processes (principally phonons) promote the molecules up to high levels, not far below the top of the energy barrier, and the molecules then tunnel inelastically to the other side. The transitions are therefore almost entirely accomplished via thermal excitations.At temperatures below 360 mK, Fe 8 molecular clusters display a clear crossover from thermally activated relaxation to a temperature independent quantum regime, with a pronounced resonance structure of the relaxation time as a function of the external field [5]. This can be seen for example by hysteresis loop measurements (Fig. 1). In this regime only the two lowest levels of each molecule are occupied, and only "pure" quantum tunneling through the anisotropy barrier can cause direct transitions between these two states. It was surprising however that the observed relaxation of the magnetization in the quantum regime was found to be non-exponential and the resonance width orders of magnitude too large [5,8]. The key to understanding this seemingly anomalous behavior now appears to involve the ubiquitous hyperfine fields as well as the (inevitable) evolving distribution of the weak dipole fields of the nanomagnets themselves [9].In this letter, we focus on the low temperature and low field limits, where phonon-mediated relaxation is astronomically long and can be neglected. In this limit, the S z = ±10 spin states are coupled by a tunneling matrix element ∆ tunnel which is estimated to be about 10 −8 K [9]. In order to tunnel between these states, the magnetic energy bias ξ H = gµ B SH due to the local magnetic field H on a molecule must be smaller than ∆ tunnel implying a local field smaller than 10 −9 T for Fe 8 clusters. Since the typical intermolecular dipole fields are of the order of 0.05 T, it seems at first that almost all...