Local structures and magnetic properties of Fullerene[sbnd]Co systems studied by XAFS and XMCD analyses

Abstract

In this work we have measured Co K- and L2,3-edge X-ray absorption near edge structure (XANES) and Co L2,3-edge X-ray magnetic circular dichroism (XMCD) spectra, and also carried out their calculations for C60Cox compounds. The observed XANES and XMCD are sensitive to the Co concentration. In the low density region (x=1.0−1.5), one Co is surrounded by three C60 balls. The Co L2,3-edge XMCD analyses gives the spin magnetic moment on Co in the range 0.5–0.9μB.In the high density region (x=4.0−4.3) calculated XANES for one of the proposed model, 4C604Co model, by Nakajima and Kaya give rather good agreement with the observed XANES, where a Co4 cluster is surrounded by four C60 balls. Fourier transformed extended X-ray absorption fine structure (EXAFS) is well explained by this model. In contrary to these results, the calculated XMCD spectrum for that high-symmetric model shows rather poor agreement with the observed spectrum. Lowering the symmetry, we can improve the agreement. The cluster density functional calculations for the 4C604Co model show that spin quintet (4μB) is in the lowest energy. Each Co has 1μB in ferromagnetic like order. The results also indicate that the band gap for the minority spin is smaller than that for the majority spin. This result provides us with fundamental information to understand the tunnel magnetoresistance (TMR) mechanism in those systems.