It is accepted that only three elements are ferromagnetic at room temperature, the transition metals iron, cobalt and nickel. The Stoner criterion explains why, for example, iron is ferromagnetic but manganese is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: the product of the density of states with the exchange integral must be greater than unity for spontaneous ordering to emerge.1,2 Here, we demonstrate that it is possible to alter the electronic states of nonferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, in 2 order to drive them ferromagnetic at room temperature. This remarkable effect is achieved via interfaces between metallic thin films and C 60 molecular layers. The emergent ferromagnetic state can exist over several layers of the metal before being quenched at large sample thicknesses by the material's bulk properties. While the induced magnetisation is easily measurable by magnetometry, low energy muon spin spectroscopy 3 provides insight into its magnetic distribution by studying the depolarisation process of low energy muons implanted in the sample. This technique indicates localized spin-ordered states at and close to the metallo-molecular interface.Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms due to electron transfer. 4,5 This opens a path for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic elements such as organic semiconductors. Charge transfer at molecular interfaces can then be used to control spin polarisation or magnetisation, with far reaching consequences in the design of devices for electronic, power or computing applications. 6,7 Multifunctional materials with the spin degree of freedom such as multiferroics, magnetic semiconductors and molecular magnets have all aroused huge interest as potentially transformative components in quantum technologies. [8][9][10][11][12] Strategies used to bring magnetic ordering to these materials typically rely on the inclusion of magnetic transition metals, heavy elements with a large atomic moment or rare earths. In thin film structures, proximity effects and coupling at interfaces play an essential role. 13,14 This is especially the case for molecular spintronics, 15,16 where organic thin films grown on copper have demonstrated spin filtering. 17The organic magnetic coupling can propagate for long distances in systems such as nanoscale vortex-like configurations or nanoskyrmion lattices. 183We choose C 60 as a model molecule due to its structural simplicity and robustness as well as its high electron affinity. C 60 /transition metal complexes exhibit strong interfacial coupling between metal 3d z electrons and molecular π-bonded p electrons. The potential created by the mismatch of molecular and metal work functions leads to a partial filling of the interface states. [19][20][21] Other molecules with close electron affinity and the potential for 3d z /p coupling ...

In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux-the diamagnetic Meissner effect-from the interior of superconducting lead. This discovery was crucial in formulating the BardeenCooper-Schrieffer (BCS) theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a superconductor-magnet hybrid system where magnetic ordering breaks time-reversal symmetry of the superconducting condensate and results in the stabilization of an odd-frequency superconducting state. It has been predicted that under appropriate conditions, odd-frequency superconductivity should manifest in the Meissner state as fluctuations in the sign of the magnetic susceptibility, meaning that the superconductivity can either repel (diamagnetic) or attract (paramagnetic) external magnetic flux. Here, we report local probe measurements of faint magnetic fields in a Au=Ho=Nb trilayer system using low-energy muons, where antiferromagnetic Ho (4.5 nm) breaks time-reversal symmetry of the proximity-induced pair correlations in Au. From depth-resolved measurements below the superconducting transition of Nb, we observe a local enhancement of the magnetic field in Au that exceeds the externally applied field, thus proving the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.

Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour. Here we create a thermally active artificial kagome spin ice that is made up of a large array of dipolar interacting nanomagnets and undergoes phase transitions predicted by microscopic theory. We use low energy muon spectroscopy to probe the dynamic behaviour of the interacting nanomagnets and observe peaks in the muon relaxation rate that can be identified with the critical temperatures of the predicted phase transitions. This provides experimental evidence that a frustrated magnetic metamaterial can be engineered to admit thermodynamic phases.

We study the influence of a voltage-driven nonequilibrium of quasiparticles on the properties of short mesoscopic superconducting wires. We employ a numerical calculation based upon the Usadel equation. Going beyond linear response, we find a nonthermal energy distribution of the quasiparticles caused by the applied bias voltage. It is demonstrated that this nonequilibrium drives the system from the superconducting state to the normal state, at a current density far below the critical depairing current density. DOI: 10.1103/PhysRevLett.96.147002 PACS numbers: 74.78.Na, 74.20.Fg, 74.25.Bt, 74.25.Sv The energy distribution function of quasiparticles in a normal metal is under equilibrium conditions given by the Fermi-Dirac distribution f 0 . In recent years it has been demonstrated that in a voltage (V)-biased mesoscopic wire (length L) a two-step nonequilibrium distribution develops [1] with additional rounding by quasiparticle scattering due to spin-flip and/or Coulomb interactions [2]. Figure 1(a) shows the distribution, which resembles two shifted FermiDirac functions:with " the quasiparticle energy and x the coordinate along the wire. For strong enough relaxation (L L , with L the phase coherence length) and/or high temperatures (k B T eV) the distribution returns to a Fermi-Dirac distribution with a local effective temperature.The questions we address here are how the distribution function is modified when the normal wire is replaced by a superconducting wire [for a typical result see Fig. 1(b)] and how this affects observable properties such as the currentvoltage characteristics of the system and the breakdown of the superconducting state. The static nonequilibrium distribution leads to the occurrence of a resistance of the superconductor. Another source of voltage might potentially develop due to phase-slip events, either thermally activated or as quantum phase slips [3,4]. The problem that we study focuses on wires which are wide enough to ignore the contribution of quantum phase slips-but still more narrow than the superconducting phase coherence length 0 -to the resistance and are also far enough below the critical temperature T c to ignore the thermally assisted contribution. Within these constraints we relate the distribution function to observable quantities. To do this, it is convenient to separate the part of f which is symmetric in particle-hole space, f L (energy mode), from the asymmetric part, f T (charge mode), since they each have a different spatial and spectral form [ Figs. 1(c) and 1(d)]. In particular, we will show that the breakdown is characterized by a voltage rather than by a current; in other words, the system cannot be trivially treated as two resistors modelling the normal current to supercurrent conversion, with a superconducting element characterized by its depairing current in between.The transport and spectral properties of dirty superconducting systems (' e 0 , with ' e the elastic mean free path) are described by the quasiclassical Green functions obeying the Usadel equat...

Superconducting spintronics has emerged in the past decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom 1,2 . Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by proximity of a conventional superconductor to a ferromagnetic material with inhomogeneous macroscopic magnetization 3 . Using low-energy muon spin-rotation experiments we find an unanticipated e ect, in contradiction with the existing theoretical models of superconductivity and ferromagnetism: the appearance of a magnetization in a thin layer of a non-magnetic metal (gold), separated from a ferromagnetic double layer by a 50-nm-thick superconducting layer of Nb. The e ect can be controlled either by temperature or by using a magnetic field to control the state of the remote ferromagnetic elements, and may act as a basic building block for a new generation of quantum interference devices based on the spin of a Cooper pair.The ability to manipulate the spin degree of freedom of charge carriers is key to realizing future spin-based electronics. Integrating superconductors into spintronic devices can greatly enhance performance 1 and allows the transport of spin over long distances without the dissipation of heat 2 . To achieve the alignment of electron spins, ferromagnetic materials are used. Superconductivity and ferromagnetism are, however, antagonistic states of matter, and the interplay between these two states results in the conversion of conventional spin-singlet into spin-triplet pair correlations 3 . Whereas spin-singlet pairs have spin angular momentum S = 0, spin-triplet pairs have S = 1, with three possible spin projections s z = −1, 0, +1. The realization of such spin-triplet pairs in mesoscopic systems containing interfaces between superconducting (S) and ferromagnetic (F) layers has attracted much interest from both the theoretical and experimental communities. Interaction of spin-singlet superconductivity with collinear ferromagnetism leads to oscillations and suppression of the pair correlation at a short distance ξ f due to the exchange magnetic field in the ferromagnet, which tends to align the spins of electrons parallel 4-7 . However, to create longer-range penetration of spin-triplet superconductivity into the ferromagnet, interaction with a non-collinear magnetism is required [8][9][10] , motivating the discovery of superconducting currents through ferromagnetic metals over distances far longer than the singlet penetration length ξ f (refs 11-13). These long-range triplet components (LRTC) have parallel spin projections (s z = ±1), and are not suppressed by the exchange field. Theory predicts that the conversion into spin-triplet pairs should also give rise to an induced magnetic moment in the superconductor, decaying away from the interface [14][15][16] , often called the inverse or magnetic proximity effect. For diffusive systems this induced m...