Shell pressure on the core of MnO/Mn3O4core/shell nanoparticles

Abstract

Here we show that spontaneous oxidation of MnO nanoparticles into MnO/Mn 3 O 4 core/shell nanoparticles has the effect of a local pressure, decreasing the MnO cell parameter and increasing strain, resulting in the increase of the MnO antiferromagnetic/paramagnetic transition temperature T N . These effects are more severe in smaller nanoparticles.

Probing the meta-stability of oxide core/shell nanoparticle systems at atomic resolution

“…The local processes underlying the reduction transformation can be monitored via atomic resolution imaging. High-resolution STEM-high angle annular dark field (STEM-HAADF) images recorded along the spinel [1][2][3][4][5][6][7][8][9][10] zone axis allow tracking insitu electron beam-induced selective reduction of Mn3O4 in both Fe3O4/Mn3O4 and Mn3O4/Fe3O4 core/shell systems. As summarized in Fig.…”

“…In this case, the smaller relative changes in the lattice parameters of the shell reflect its large volume fraction, which dominates over the core. In general, such comparisons to the bulk values show that both the Mn3O4 and Fe3O4 contract and/or expand to keep the system energy low by generating a coherent interface [2,60,61] in which the configuration, i.e., Mn3O4 in the core or in the shell, and the volume fractions of each phase are very important to understand which phase undergoes the largest deformation. In fact, for Fe3O4/Mn3O4, with an incomplete shell, only the shell suffers strains to accommodate the coherent interface (see Fig.…”

“…Hybrid nanoparticles [1] are very attractive as model systems for fundamental studies [2][3][4][5][6] and also for a very wide range of applications. The combination of reduced dimensionality effects with the interplay between the properties of the constituents can give rise to multiple, and sometimes new, functionalities in systems of interest for catalytical [7], optical [8,9] magnetic [10,11] or biomedical applications [12][13][14][15].…”

Magnetism of monomer MnO and heterodimer FePt@MnO nanoparticles

“…The exchange coupling between the "spin glass like" or "FMlike" shell and the AF core could cause the EB effect in pure AF NPs 11 . However, the EB effect observed here is more likely to be caused by an oxidation of the NP surface from MnO to ferrimagnetic Mn 3 O 4 or Mn 2 O 3 [43][44][45][46] . Due to the exchange interaction between the AF MnO core and the ferrimagnetic shell, an EB effect can occur 11,43,44 .…”

Continuous local model for two-dimensional spin ice

“…These ground states obey the so called ice rule, the spins arranged in such a way that, on each vertex of the lattice, two of them point outward and the other two point inward the vertex, resulting in zero magnetic flux. It was predicted that excitations on the ground state of the spin ice, corresponding to the violation of the ice rule by spin inversion, lead to the emergence of quasiparticles with magnetic properties that justifies their denomination as magnetic monopoles [1,15]. There are some characteristics of the spin ice systems that are essential for the emergence of magnetic monopoles in both three-dimensional (3D) real spin ice compounds, such as Dy 2 T i 2 O 7 or Ho 2 T i 2 O 7 , or in artificial two-dimensional (2D) spin ice, as 2D square lattices of permalloy nano-islands [16,21,8] -the ice rule (divergenceless of the ground state), periodicity (of the pyrochlore and artificial square lattices) and anisotropy (the magnetic moments are constrained to point along fix axes).…”