May Wheeler

Beating the Stoner criterion using molecular interfaces.

Only three elements are ferromagnetic at room temperature: the transition metals iron, cobalt and nickel. The Stoner criterion explains why iron is ferromagnetic but manganese, for example, is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: according to this criterion, the product of the density of states and the exchange integral must be greater than unity for spontaneous spin ordering to emerge. Here we demonstrate that it is possible to alter the electronic states of non-ferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, to overcome the Stoner criterion and make them ferromagnetic at room temperature. This effect is achieved via interfaces between metallic thin films and C60 molecular layers. The emergent ferromagnetic state exists over several layers of the metal before being quenched at large sample thicknesses by the material’s bulk properties. Although the induced magnetization is easily measurable by magnetometry, low-energy muon spin spectroscopy provides insight into its distribution by studying the depolarization process of low-energy muons implanted in the sample. This technique indicates localized spin-ordered states at, and close to, the metal–molecule interface. Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer. This mechanism might allow for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors. Charge transfer at molecular interfaces may thus be used to control spin polarization or magnetization, with consequences for the design of devices for electronic, power or computing applications (see, for example, refs 6 and 7).

Synthetic ferrimagnet nanowires with very low critical current density for coupled domain wall motion

Domain walls in ferromagnetic nanowires are potential building-blocks of future technologies such as racetrack memories, in which data encoded in the domain walls are transported using spin-polarised currents. However, the development of energy-efficient devices has been hampered by the high current densities needed to initiate domain wall motion. We show here that a remarkable reduction in the critical current density can be achieved for in-plane magnetised coupled domain walls in CoFe/Ru/CoFe synthetic ferrimagnet tracks. The antiferromagnetic exchange coupling between the layers leads to simple Néel wall structures, imaged using photoemission electron and Lorentz transmission electron microscopy, with a width of only ~100 nm. The measured critical current density to set these walls in motion, detected using magnetotransport measurements, is 1.0 × 1011 Am−2, almost an order of magnitude lower than in a ferromagnetically coupled control sample. Theoretical modelling indicates that this is due to nonadiabatic driving of anisotropically coupled walls, a mechanism that can be used to design efficient domain-wall devices.

Optical conversion of pure spin currents in hybrid molecular devices

Carbon-based molecules offer unparalleled potential for THz and optical devices controlled by pure spin currents: a low-dissipation flow of electronic spins with no net charge displacement. However, the research so far has been focused on the electrical conversion of the spin imbalance, where molecular materials are used to mimic their crystalline counterparts. Here, we use spin currents to access the molecular dynamics and optical properties of a fullerene layer. The spin mixing conductance across Py/C60 interfaces is increased by 10% (5 × 1018 m−2) under optical irradiation. Measurements show up to a 30% higher light absorbance and a factor of 2 larger photoemission during spin pumping. We also observe a 0.15 THz slowdown and a narrowing of the vibrational peaks. The effects are attributed to changes in the non-radiative damping and energy transfer. This opens new research paths in hybrid magneto-molecular optoelectronics, and the optical detection of spin physics in these materials.Carbon-based molecules could prove useful in terahertz and optical devices controlled by pure spin currents. Here, conversely, the authors use spin currents to probe molecular dynamics and enhance the optical response of a fullerene layer, enabling hybrid magneto-molecular optoelectronic devices.

Effects of spin doping and spin injection in the luminescence and vibrational spectrum of C60

We have studied the Raman spectrum and photoemission of hybrid magneto-fullerene devices. For C60 layers on cobalt, the spin polarized electron transfer shifts the photoemission energy, reducing the zero phonon contribution. The total luminescence of hybrid devices can be controlled via spin injection from magnetic electrodes, with changes of the order of 10%–20% at room temperature. Spin polarised currents alter as well the Raman spectrum of the molecules, enhancing some modes by a factor 5 while shifting others by several wavenumbers due to a spin-dependent hopping time and/or enhanced intermolecular interactions. These results can be used to measure spin polarisation in molecules or to fabricate magneto-optic and magneto-vibrational devices.

Enhanced Exchange Bias of Spin Valves Fabricated on Fullerene-Based Seed Layers

The magnetic properties of exchange biased systems deposited on top of fullerene-based materials have been studied. An enhanced exchange bias field has been observed for Si/SiO2/C60/Co/Cu/Co/IrMn/Ta and Si/SiO2 /FeC60 /Co/Cu/Co/IrMn/Ta in comparison to a typical structure of Si/SiO2/Ta/Co/Cu/Co/IrMn/Ta. Here the magnetic properties of the devices are changed significantly with an organic underlayer or seed layer. The magnetic ordering is well maintained, and C60 seeding results in an exchange bias of (19.5 ±0.5) mT, some 7 mT higher than that of Ta seeded spin valves (12.5 ±0.5 mT). A combination of MOKE (magneto-optical Kerr effect), X-ray diffraction and Raman spectroscopy have been used to characterize the materials and further understand the interactions between the Co and fullerene-based materials.

Direct Measurement of Spin Polarization in Ferromagnetic-C 60 Interfaces Using Point-Contact Andreev Reflection

An important step has been developed combining the potential of spintronics with organic electronics to reveal the promising field of molecular spintronics, which can offer more flexibility, higher recyclability, and low-production costs compared with inorganic devices. Room temperature magnetoresistance (MR) of 5% has been obtained from C60 spin valves with the structure Co(20 nm)/Al2 O3(1.2 nm)/C60 (5-60 nm)/Py(20 nm)/Al(1.5 nm). We observe an asymmetric dependence at low temperatures of the MR with voltage, and surprisingly with the magnetic field as well. This behavior has been attributed to the organic interface formed at the junction, which results in a change of the ferromagnets spin polarization. The spin polarization at the organic-ferromagnetic interface is extracted by measuring the bias dependence of the conductance of a metallic-superconducting point contact and analyzed the spectra with the modified Blonder-Tinkham-Klapwijk theory. Point-contact Andreev reflection measurements reveal that the Co-C60 possess spin polarization of 30% ± 1%, compared with 40% ± 1% for our Co films without C60. This could account for the asymmetry of the MR in spin valves incorporating C60 and the need for an alumina spacer to maximize the MR.

Developing Hollow-Channel Gold Nanoflowers as Trimodal Intracellular Nanoprobes

Gold nanoparticles-enabled intracellular surface-enhanced Raman spectroscopy (SERS) provides a sensitive and promising technique for single cell analysis. Compared with spherical gold nanoparticles, gold nanoflowers, i.e., flower-shaped gold nanostructures, can produce a stronger SERS signal. Current exploration of gold nanoflowers for intracellular SERS has been considerably limited by the difficulties in preparation, as well as background signal and cytotoxicity arising from the surfactant capping layer. Recently, we have developed a facile and surfactant-free method for fabricating hollow-channel gold nanoflowers (HAuNFs) with great single-particle SERS activity. In this paper, we investigate the cellular uptake and cytotoxicity of our HAuNFs using a RAW 264.7 macrophage cell line, and have observed effective cellular internalization and low cytotoxicity. We have further engineered our HAuNFs into SERS-active tags, and demonstrated the functionality of the obtained tags as trimodal nanoprobes for dark-field and fluorescence microscopy imaging, together with intracellular SERS.