Hong-Ji Lin

Electronic structure and magnetism of the diluted magnetic semiconductor Fe-doped ZnO nanoparticles

We have studied the electronic structure of Fe-doped ZnO nanoparticles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy, resonant photoemission spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe3+ ionic state with mixture of a small amount of Fe2+ and that Fe3+ ions are dominant in the surface region of the nanoparticles. It is shown that the room temperature ferromagnetism in the Fe-doped ZnO nanoparticles primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼2–3u2002nm.

Surface- and bulk-sensitive x-ray absorption study of the valence states of Mn and Co ions in Zn1-2xMnxCoxO nanoparticles

We have performed x-ray absorption spectroscopy (XAS) measurements on Zn1−2xMnxCoxO nanoparticles. From the XAS results, it seems that the Mn and Co ions are in a mixed-valence (2+, 3+, and 4+) state and the relative concentrations of the high-valence (3+ and 4+) Mn and Co ions are higher in the surface region than in the deep core region. We suggest that this is a distinct trend of nanoparticle diluted magnetic semiconductor (DMS) unlike the case of DMS in film and bulk forms, where the transition-metal ions are expected to be 2+.

X-ray magnetic circular dichroism characterization of GaN/Ga1- xMnxN digital ferromagnetic heterostructure

The authors have investigated the magnetic properties of a GaN∕Ga1−xMnxN (x=0.1) digital ferromagnetic heterostructure (DFH) showing ferromagnetic behavior using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The Mn L2,3-edge XAS spectra were similar to those of Ga1−xMnxN random alloy thin films, indicating a substitutional doping of high concentration Mn into GaN. From the XMCD measurements, it was revealed that paramagnetic and ferromagnetic Mn atoms coexisted in the Ga1−xMnxN digital layers. Subtle differences were also found from the XMCD spectra between the electronic states of the ferromagnetic and paramagnetic Mn2+ ions. The ferromagnetic moment per Mn atom estimated from XMCD agreed well with that estimated from superconducting quantum interference device measurements, indicating that the ferromagnetic behavior of the GaN∕Ga1−xMnxN DFH sample arises only from substitutional Mn2+ ions in the Ga1−xMnxN digital layers and not from ferromagnetic precipitates.

X-ray Magnetic Circular Dichroism Investigations of the Origin of Room Temperature Ferromagnetism in Fe-Doped ZnO Nanoparticles

We have studied the electronic structure of Zn0.9Fe0.1O nanoparticles, which have been reported to be ferromagnetic at room temperature, by using soft X-ray absorption (XAS) and magnetic circular dichroism (XMCD). The XAS results indicate that Fe ions are predominantly in the Fe3+ state with mixture of a small amount of Fe2+. It is shown that the room temperature ferromagnetism in the Zn0.9Fe0.1O nanoparticles is primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ~2–3 nm.