The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO(3)) and the wide-gap insulator lanthanum aluminate (LaAlO(3)) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO(3) as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO(3) layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

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 ...

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.R ecently great efforts have been undertaken to use the spin degree of freedom in electronic devices. These activities are fuelled by the potential prospects of spin-electronic (or 'spintronic') devices for example in terms of increased processing speed and integration, non-volatility, reduced power consumption, multifunctionality and their suitability for quantum computing 1 . The most common method for using the spin in devices is based on the alignment of the electron spin ('up' or 'down') relative to either a reference magnetic field or the magnetization orientation of a ferromagnetic layer. Device operation normally proceeds with measuring a quantity such as the electrical current that depends on how the degree of spin alignment is transferred across the device. The so-called 'spin valve' is a prominent example of such a spin-enabled device that has already revolutionized hard-drive read heads and magnetic memory 1 . The efficient transfer of spin polarization in real device structures remains one of the most difficult challenges in spintronics, because it is dependent on more than just the properties of the individual materials that comprise the device.Recently 2,3 , the use of organic materials in spintronics has become of significant interest, primarily owing to their ease and small cost of processing and electronic and structural flexibility. Furthermore, the extremely long spin coherence times found in organic materials offer considerable advantages over other materials 3 . This favourable property is related to two factors, first the weak spin-orbit coupling of light elements such as carbon and second to the small nuclear hyperfine interaction 4,5 . The latter arises because the electron transport in π-conjugated molecules is normally confined to molecular states, delocalized to the carbon rings, the predominant isotope of which, 12 C, has zero nuclear spin 4 .A common way to measure spin diffusion is based on time-resolved optical techniques, where spin-polarized charge carrie...

We report the first direct observation of the oxygen-isotope ((16)O/(18)O) effect on the in-plane penetration depth lambda(ab) in a nearly optimally doped YBa(2)Cu(3)O(7-delta) film using the novel low-energy muon-spin rotation technique. Spin-polarized low-energy muons are implanted in the film at a known depth z beneath the surface and process in the local magnetic field B(z). This feature allows us to measure directly the profile B(z) of the magnetic field inside the superconducting film in the Meissner state and to make a straightforward determination of lambda(ab). A substantial isotope shift Delta lambda(ab)/lambda(ab)=2.8(1.0)% at 4 K is observed, implying that the in-plane effective supercarrier mass m*(ab) is oxygen-isotope dependent with Delta m*(ab)/m*(ab)=5.5(2.0)%. These results are in good agreement with magnetization measurements on powder samples.

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves 1 to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts 2 . Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.The development and understanding of new hybrid organic/inorganic interfaces will enable considerable progress in organic spintronics for technological purposes, including processing elements, sensors, memories and conceptually different future applications. In addition to the 'standard' spintronic applications, newly developed interfaces could bring spintronic effects to the field of organic light-emitting diodes (OLEDs), as well as in the fast progressing field of organic field-effect transistors. For example, the injection of carriers with a controlled spin state could enable the amplification of either singlet or triplet exciton states 2 leading to a significant increase in the efficiency of the electroluminescence in OLEDs. Although these considerations are conceptually straightforward, no efficiency amplification has yet been reported in the literature, despite several attempts 3 . The failure of those approaches was caused by the simple reason that light emission can be detected starting from an applied voltage of a few volts, whereas state-of-the-art spin injection in organic materials persists to a maximum of around 1 V (refs 4-6). As yet, this is unexplained. Further complications arise from the fact that various reports on working devices show a wide spread of performances for apparently similar structures, highlighting the issue of reproducibility [7][8][9] . The poor reproducibility is mainly due to the unknown interplay between processing and spin transfer performance and there is little deterministic control of the interface properties. However, it has recently been demonstrated that the insertion of a barrier

No abstract

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.