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

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 (gamma-Mn(2)O(3) or Mn(3)O(4)) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly gamma-Mn(2)O(3) shell, larger ones have increasing amounts of Mn(3)O(4). A considerable enhancement of the Néel 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.

Morphology influence on nanoscale magnetism of Co nanoparticles: Experimental and theoretical aspects of exchange bias

“…The different contrast obtained by TEM between the darker core and the lighter shell of the nanoparticles after room temperature oxidation suggests their uncomplete oxidation. X-ray diffraction of these samples 20 Similar examples, showing that the phase of the 3d-metal oxides depends on the size of the nanoparticles, have also been recently reported for Fe 21 and Mn, 22 most probably indicating that cation vacancies are formed as a result of the metal oxide passivation.…”

Antiferromagnetic order in MnO spherical nanoparticles

“…Both samples exhibit a suppressed magnetic moment compared to bulk MnO. This behavior is also observed in confinement geometry and core-shell MnO particles 11,13,14 and is believed to be the result of disordered spins on the surface of the nanoparticles. 11,17 However, whether the actual moment for the Mn site is the same as in the bulk has not been directly determined yet.…”

“…9 In contrast, an increase has been observed in the magnetic ordering temperature and magnetic moment in several nanoscale materials. [10][11][12][13][14][15] For example, in some nanoscale 3d metals, the moment is enhanced as a result of band narrowing at the surface resulting from the reduced coordination number. This behavior has been observed experimentally in iron, cobalt, and nickel clusters 10 and has been supported by theoretical calculations.…”

“…16 On the other hand, in ionic oxide compounds, due to the relatively localized electronic distribution, the moment is less affected by the surface and surface disorder results in a reduced average moment. 17 A reduced moment and enhanced transition temperature have been observed in MnO particles [11][12][13][14][15] and in the spinel ferrites Ni(Mn)Fe 2 O 4 . 18 In either case, the origin of the moment and ordering temperature enhancement or reduction is unclear and, consequently, further study of magnetic nanoparticles is important.…”

Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles

“…Hence, a great deal about the defect structure and particle morphology can be inferred from conventional X-ray diffractograms simply by determining the crystal domain size along different crystallographic directions. Although the analysis is not trivial for very small particles due to peak broadening, in the case of core/shell nanoparticles from the volume ratio of the components it is possible to determine indirectly the morphology of the crystallites if a secondary technique is available, e.g., TEM [38,90].…”