Size-Dependent Passivation Shell and Magnetic Properties in Antiferromagnetic/Ferrimagnetic Core/Shell MnO Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/ja1021798

J. Am. Chem. Soc., 2010, 132 (27), pp 9398–9407
Alberto López-Ortega, Dina Tobia, Elin Winkler, Igor V. Golosovsky, German Salazar-Alvarez, Sònia Estradé, Marta Estrader, Jordi Sort, Miguel Angel González, Santiago Suriñach, Jordi Arbiol, Francesca Peiró, Roberto D. Zysler, Maria Dolors Baró, Josep Nogués

DOI: 10.1021/ja1021798

Abstract:

The magnetic properties of bimagnetic core/shell nanoparticles consisting of an antiferromagnetic MnO core and a ferrimagnetic passivation shell have been investigated. It is found that the phase of the passivation shell (γ-Mn₂O₃ or Mn₃O₄) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly γ-Mn₂O₃ shell, larger ones have increasing amounts of Mn₃O₄. A considerable enhancement of the Nel temperature, T_N, and the magnetic anisotropy of the MnO core for decreasing core sizes has been observed. The size reduction also leads to other phenomena such as persistent magnetic moment in MnO up to high temperatures and an unusual temperature behavior of the magnetic domains.