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Paramagnetism of the Co sublattice in ferromagnetic Zn$_{1-x}$Co$_{x}$O films

A. Barla·G. Schmerber·E. Beaurepaire·A. Dinia·H. Bieber·S. Colis·F. Scheurer·J. -P. Kappler·P. Imperia·F. Nolting·F. Wilhelm·A. Rogalev·D. Muller·J. J. Grob
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Abstract

Using the spectroscopies based upon x-ray absorption, we have studied the structural and magnetic properties of Zn$_{1-x}$Co$_{x}$O films ($x$ = 0.1 and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature $T_{\mathrm{C}}$ above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the Co \textit{L}$_{2,3}$ edges reveals that the Co 3\textit{d} sublattice is paramagnetic at all temperatures down to 2 K, both at the surface and in the bulk of the films. The Co 3\textit{d} magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetisation measurements, suggesting that the Co 3\textit{d} electrons are not directly at the origin of the observed ferromagnetism.

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A. Barla, G. Schmerber, E. Beaurepaire, A. Dinia, H. Bieber, S. Colis, F. Scheurer, J. -P. Kappler, P. Imperia, F. Nolting, F. Wilhelm, A. Rogalev, D. Muller, J. J. Grob. 2006-12-07. Paramagnetism of the Co sublattice in ferromagnetic Zn$_{1-x}$Co$_{x}$O films. https://doi.org/10.1103/physrevb.76.125201

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