On the origin of the magnetism of Mn-Zn-O systems: Structural, electronic, and magnetic study of exotic MnO 2-δ/ZnO thin films

The structural, electronic, and magnetic characterization of sputtered MnO 2-δ/ZnO films showing room temperature ferromagnetism has been carried out by a number of techniques. To elucidate the origin of the ferromagnetic order, a novel approach by studying XAS and XMCD signals at both Mn and Zn K edges in combination with the macroscopic magnetization measurements has been performed. The analysis of the XAS profile at the Mn edge indicates the coexistence of Mn 4+/Mn 3+ with a slightly distorted MnO 2-δ environment. The XMCD signal at the Zn K edge indicates the existence of an intrinsic magnetic moment in Zn. The Zn edge dichroic intensity is, however, 1 order of magnitude lower than in the case of Mn, being the magnetic moment of Mn, 2.1-2.4 μ B per Mn cation participating in the ferromagnetic phase, the main contribution to the total magnetization. The ferromagnetic phase has been assigned to high-density regions of Mn cations with a MnO 2-δ distorted environment, mainly allocated at the nonstoichiometric and highly defective ZnO grain boundaries, with a sufficient extension to overcome the thermal energy over room temperature. The ferromagnetic coupling has been related to modifications of the Mn-O-Mn bonds, distances, and angles, promoted by neighboring ZnO and forming stable magnetic configurations that lead to new forms of exchange interaction: ferromagnetic superexchange or a double-exchange mechanism between the Mn 4+ and Mn 3+. Highly defective regions and structural distortions, together with complex Mn oxidation states, appear to be key features in the magnetic properties of Mn-Zn-O. © 2011 American Chemical Society.We acknowledge financial support to Spanish MICINN through contract MAT2009-08786, MAT2008-06542-C04, MAT2010- 16022, MAT2010-09346, and CSD2009-00013 and to the Madrid regional government though contract S2009/MAT-1756. The Spanish MICINN is acknowledged by MAL-M (JdlC program), CG (FPI program), and RB, for the financial support.Peer Reviewe