Multi-scale magnetic study on Ni(111) and graphene on Ni(111)

TL;DR

This study combines multiple magnetic imaging techniques and ab initio calculations to analyze the magnetic properties of Ni(111) surfaces and graphene-covered Ni(111), revealing domain structures, spin polarization, and effects of graphene coverage across various length scales.

Contribution

It provides a comprehensive multi-scale analysis of Ni(111) and graphene-covered Ni(111), combining experimental imaging and theoretical calculations to uncover domain behaviors and spin polarization effects.

Findings

Domain walls are 60-90 nm wide and movable by small magnetic fields.

Graphene coverage induces spin polarization detectable in vacuum.

Surface domain patterns originate from bulk domain structures.

Abstract

We have investigated the magnetism of the bare and graphene-covered (111) surface of a Ni single crystal employing three different magnetic imaging techniques and ab initio calculations, covering length scales from the nanometer regime up to several millimeters. With low temperature spinpolarized scanning tunneling microscopy (SP-STM) we find domain walls with widths of 60 - 90 nm, which can be moved by small perpendicular magnetic fields. Spin contrast is also achieved on the graphene-covered surface, which means that the electron density in the vacuum above graphene is substantially spin-polarized. In accordance with our ab initio calculations we find an enhanced atomic corrugation with respect to the bare surface, due to the presence of the carbon pz orbitals and as a result of the quenching of Ni surface states. The latter also leads to an inversion of spinpolarization with respect to the pristine surface. Room temperature Kerr microscopy shows a stripe like domain pattern with stripe widths of 3 - 6 {\mu}m. Applying in-plane-fields, domain walls start to move at about 13 mT and a single domain state is achieved at 140 mT. Via scanning electron microscopy with polarization analysis (SEMPA) a second type of modulation within the stripes is found and identified as 330 nm wide V-lines. Qualitatively, the observed surface domain pattern originates from bulk domains and their quasi-domain branching is driven by stray field reduction.