Electron transfer and spin injection in C60-ferromagnetic composites

The magnetic properties of spin doped fullerenes are investigated in hybrid organic/inorganic structures with the aim of establishing the extent to which magnetic states can be induced and controlled in these materials. Volume magnetometry is used to measure a reduction of net magnetization and an increase in coercivity in cobalt which can be understood in terms of a transfer of majority spin electrons from the transition metal d-band into spin polarized hybrid interface states. This is supported by PNR and XAS studies of Co/C60 which reveal AF coupling between Co metal films and a hybrid interfacial region where magnetic ground states are induced in fullerenes through charge transfer. Investigations of hybridization between C60 and the RE-TM alloy CoGd show that the compensation temperature of the ferrimagnet is altered by the presence of C60. PNR measurements of CoGd/C60 MLs reveal interfacial coupling which creates an AF region 1.5 $\pm$ 0.1 nm thick. Magnetometry of Gd/C60 bilayers indicates that hybridization between the metal conduction bands and the C60 LUMO modifies magnetic ordering in Gd. This is supported by the observation of novel features in the temperature dependence of magnetization and resistivity in the composite. XAS of Gd/C60 bilayers shows a large peak in the carbon K-edge at 282 eV which is attributed to interfacial hybridization. It is shown that PL quenching in C60 is greater over Co than Au which is attributed to the greater electron transfer between Co and C60. PL quenching is proposed as an effective way to measure magnetic coupling and electron transfer in interfaces. Raman spectra are recorded in C60 junctions during spin polarised transport. The Ag(2) peak splitting is shown to depend on the polarisation of injected current acting as an effective probe of triplet formation in C60. Finally, XAS at the carbon K-edge is recorded during spin transport. A suppression of the LUMO to zero and increase in the intensity of the 282 eV peak occurs after removal of external bias and is shown to be reversible and repeatable under cycles of grounding and charge injection. A proposed mechanism involving the redistribution of charge following the removal of bias which causes electrons to become trapped in interfacial states is suggested

A Hybrid Magneto-Optic Capacitive Memory with Picosecond Writing Time

The long-term future of information storage requires the use of sustainable nanomaterials in architectures operating at high frequencies. Interfaces can play a key role in this pursuit via emergent functionalities that break out from conventional operation methods. Here, spin-filtering effects and photocurrents are combined at metal-molecular-oxide junctions in a hybrid magneto-capacitive memory. Light exposure of metal-fullerene-metal oxide devices results in spin-polarized charge trapping and the formation of a magnetic interface. Because the magnetism is generated by a photocurrent, the writing time is determined by exciton formation and splitting, electron hopping, and spin-dependent trapping. Transient absorption spectroscopy measurements show changes in the electronic states as a function of the magnetic history of the device within picoseconds of the optical pumping. The stored information is read using time-resolved scanning magneto optic Kerr effect measurements during microwave irradiation. The emergence of a magnetic interface in the picosecond timescale opens new paths of research to design hybrid magneto-optic structures operating at high frequencies for sensing, computing, and information storage

Emergent magnetism at transition-metal–nanocarbon interfaces

Charge transfer at metallo–molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc–C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo–carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2−π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz–π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices

Enhanced Spin-Orbit Coupling in Heavy Metals via Molecular Coupling

5d metals are used in electronics because of their high spin–orbit coupling (SOC) leading to efficient spin-electric conversion. When C60 is grown on a metal, the electronic structure is altered due to hybridization and charge transfer. In this work, we measure the spin Hall magnetoresistance for Pt/C60 and Ta/C60, finding that they are up to a factor of 6 higher than those for pristine metals, indicating a 20–60% increase in the spin Hall angle. At low fields of 1–30 mT, the presence of C60 increased the anisotropic magnetoresistance by up to 700%. Our measurements are supported by noncollinear density functional theory calculations, which predict a significant SOC enhancement by C60 that penetrates through the Pt layer, concomitant with trends in the magnetic moment of transport electrons acquired via SOC and symmetry breaking. The charge transfer and hybridization between the metal and C60 can be controlled by gating, so our results indicate the possibility of dynamically modifying the SOC of thin metals using molecular layers. This could be exploited in spin-transfer torque memories and pure spin current circuits

Enhanced spin-orbit coupling in a heavy metal via molecular coupling

Heavy metals are key to spintronics because of their high spin-orbit coupling (SOC) leading to efficient spin conversion and strong magnetic interactions. When C60 is deposited on Pt, the molecular interface is metallised and the spin Hall angle in YIG/Pt increased, leading to an enhancement of up to 600% in the spin Hall magnetoresistance and 700% for the anisotropic magnetoresistance. This correlates with Density Functional Theory simulations showing changes of 0.46 eV/C60 in the SOC of Pt. This effect opens the possibility of gating the molecular hybridisation and SOC of metals

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