2017 - 2018

Organizers: José Miguel Mancheño y Esther Rebollar

On May 30th we will have a seminar by Rodolfo Miranda, from IMDEA Nanociencia and Departamento de Física de la Materia Condensada, Instituto Nicolás Cabrera and IFIMAC, Universidad Autónoma de Madrid.

The search for long range magnetic order in 2D systems has a long history, spanning from single monolayers of magnetic metals epitaxially grown on non-magnetic metallic substrates to recent reports of ferromagnetic order for single monolayers of van der Waals crystals, such as Cr2Ge2Te6 or CrI3. In all these cases, however, the collective magnetic properties are associated with metal atoms with d or f electrons that provide the individual magnetic moments.

A fully spin-polarized ground state based on π electrons was rigorously predicted long ago in half-filled flat-band materials, but it has remained a challenge to realize it experimentally. Recently we showed by STM and DFT that a self-assembled monolayer of organic molecules (tetracyano-p-quinodimethane, i.e. TCNQ) deposited on graphene epitaxially grown on Ru(0001) acquire charge from the n-doped graphene substrate and develop a magnetic moment of 0.18 µB per molecule. The TCNQ monolayer develops spatially extended spin-split electronic bands, and a magnetically ordered ground-state spin alignment visualized using spin-polarized Scanning Tunneling Spectroscopy (STS). The ferromagnetic order is attributed to the direct Heisenberg exchange interaction mediated by overlapping frontier orbitals of the molecules. The magnetic anisotropy required for a 2D system to display long range magnetic order provides an easy axis in the surface plane, along the intermolecular bands where the electrons can be delocalized.

When an epitaxial ferromagnetic Co layer is sandwiched between a Pt(111) substrate and a graphene layer, the giant Dzyaloshinskii–Moriya interaction (DMI) that appears at the gr/Co interface stabilizes a chiral spin texture. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy (PMA) up to 4 nm thick Co films, and of the existence of chiral left-handed Néel-type Domain Walls stabilized by the effective Dzyaloshinskii–Moriya interaction (DMI) in the stack. In the case of 1 nm thick Co layer underneath graphene, we found large perpendicular orbital momentum mL=0.23 µB/atom, PMA of 1.3 MJ/m3 and an effective DMI of 0.7 mJ/m2. In addition, the presence of graphene results in: i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect fcc film, pseudomorphic with Pt and ii) an efficient protection from oxidation allowing the existence of chiral spin textures at ambient conditions.

SEMINAR’S DATE, TIME AND PLACE: 30 de mayo de 2018, 12:00h, Salón de Actos IQFR-CSIC, C/ Serrano 119, Madrid

SPEAKER: Rodolfo Miranda