中央研究院 原子與分子科學研究所

Spin structures of an exchange-coupled-bilayer system of expanded-face-centered-tetragonal (e-fct) Mn(001) ultrathin films grown on Co/Cu(001) were resolved by means of spin-polarized scanning-tunneling microscopy. With an in-plane spin-sensitive probe, a layered antiferromagnetic-spin ordering of Mn overlayers was evidenced directly. In addition, the spin frustration across the same Mn layer creating a narrow domain wall down to nanometer scale was also observed along the buried step of Co underlayers. According to the micromagnetic simulation, the step-induced domain-wall width is in agreement with the experimental results. Such in-plane layered antiferromagnetic-spin structures of e-fct Mn(001) provide uncompensated spins at the interface with Co underlayers and elucidate the mechanism of the corresponding exchange-bias field observed in the previous studies.

Using x-ray photoemission electron microscopy and the magneto-optical Kerr effect, we have demonstrated a perpendicular magnetic anisotropy that could be due to exchange coupling between the ferromagnetic and antiferromagnetic layers. The results of magnetic imaging and hysteresis loops show that the magnetization of Fe and permalloy (Py) films orients from the in-plane to perpendicular direction, as an Mn underlayer is above a threshold value that depends on the Fe or Py layer thickness. Their thickness-dependent behaviors can be quantitatively described by a phenomenological model that takes into account the finite-size effect of the antiferromagnet on exchange coupling. The anisotropy energy extracted from the model and the thermal stability of perpendicular magnetization enhanced with the increase of the Mn underlayer further demonstrate the exchange coupling nature.FIG 1. (a)-(c) Magnetic domain images of a 6 ML Fe/wedged-Mn bilayer measured with different relative orientations between the polarization of incoming x-ray and samples at 220 K. The magnetization direction of the Fe domains switches from in-plane to perpendicular direction while the thickness of Mn layer is increased.FIG 2.Magnetic easy axis phase diagram for (a) Fe/Mn biayers and (b) Py/Mn bilayes, measured by MOKE at 220 K. The solid lines are the SRT boundaries fitted with theoretical model







 

Electrons may experience inelastic coupling with the organic spacer layer during tunneling between two ferromagnetic electrodes.
To probe the transport behavior of spin-polarized electrons in organic materials, organic spin valves were fabricated utilizing
a relatively thin organic barrier of 3,4,9,10-perylene-teracarboxylicdianhydride (PTCDA) dusted with alumina at the organic/ferromagnetic interfaces.
These structures, with an organic barrier layer, exhibited magnetoresistance up to 12% at room temperature.
In studies of the inelastic tunneling spectrum, the observed characteristic peak of the organic layer provides direct evidence of the
interplay between the spin-polarized electrons and the organic molecules.
Combining the inelastic tunneling results with a simple molecular vibration calculation yields further information on the configuration
of the molecular thin film and the possible tunneling states of the spin-polarized electrons.
Such interplay indicates a true transport of spin-polarized electrons through organic material rather than through
defects or interdiffusion compounds formed at the interfaces within the organic spin valve.