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 µm. 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.
We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance by nuclear resonant scattering of synchrotron radiation at the 14.4 keV resonance of 57 Fe. The precession of the magnetization leads to an apparent reduction of the magnetic hyperfine field acting at the 57 Fe nuclei. The spin dynamics is described in a stochastic relaxation model adapted to the ferromagnetic resonance theory by Smit and Beljers to model the decay of the excited nuclear state. From the fits of the measured data the shape of the precession cone of the spins is determined. Our results open a new perspective to determine magnetization dynamics in layered structures with very high depth resolution by employing ultrathin isotopic probe layers.
Based on micromagnetic simulations, we report on a novel magnetic helix in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers with different anisotropies. We show that such a confined helix stays stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal magnetic field created by the exchange interaction. We show that this internal field stores a magnetic energy density of a few kJ/m3. We also find that it dramatically modifies ferromagnetic resonances, such that the helix can be used as a ferromagnetic resonance filter and a fast acting attenuator.
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