151 research outputs found
Induced ferromagnetism in Mn3N2 phase embedded in Mn/Si3N4 multilayers
Room temperature ferromagnetism has been obtained for different sets of Mn/Si3N4 multilayers
prepared by sputtering. In order to find the most suitable conditions to stabilize the ferromagnetic
ordering in this system, the evolution of the magnetic properties has been studied for films in which
the Si3N4 layer thickness was maintained constant while that of the Mn layer was varied,
Mn tm/Si3N4 3.4 nm n, and conversely, in Mn 0.7 nm/Si3N4 tsn 43 samples, in which the Mn
layer thickness was kept constant while varying the Si3N4 layer thickness. Structural, compositional,
electronic and magnetic characterizations have been performed by means of x-ray reflectometry,
Rutherford backscattering spectrometry, x-ray photoemission spectroscopy, x-ray absorption, and
superconducting quantum interference device for further knowledge of the magnetic-structural
relationship in this system. Our results show that the peculiar magnetic behavior of these films is
mainly related to the stabilization of a slightly distorted Mn3N2 phase that is induced by the Si3N4
at the interfaces. For samples with larger Mn layer thickness, metallic Mn and Mn3N2 phases
coexist, which leads to a reduction of the total magnetization per Mn atom due to the presence of
metallic Mn. For small Mn layer thickness tm 0.86 nm, where noncontinuous Mn3N2 layers are
formed, the magnetization decreases noticeably due to the superparamagnetic size limit. It has been
found that the best conditions for the stabilization of the ferromagnetism in this system occur when
both, the manganese-rich and the silicon nitride layers, are continuous and with similar thickness,
close to 3.5 nm.Ministerio de Educación y Ciencia de España-MAT2006-01004, MAT2008-06542-C04-01, MAT2008-06765-C02-02, S-0505/MAT/0194, Consolider 2010_26400 y Nanoselect CSD2007-0004
Spontaneous Formation of Core@shell Co@Cr Nanoparticles by Gas Phase Synthesis
This work presents the gas phase synthesis of CoCr nanoparticles using a magnetron-based gas aggregation source. The effect of the particle size and Co/Cr ratio on the properties of the nanoparticles is investigated. In particular, we report the synthesis of nanoparticles from two alloy targets, Co90Cr10 and Co80Cr20. In the first case, we observe a size threshold for the spontaneous formation of a segregated core@shell structure, related to the surface to volume ratio. When this ratio is above one, a shell cannot be properly formed, whereas when this ratio decreases below unity the proportion of Cr atoms is high enough to allow the formation of a shell. In the latter case, the segregation of the Cr atoms towards the surface gives rise to the formation of a shell surrounding the Co core. When the proportion of Cr is increased in the target (Co80Cr20), a thicker shell is spontaneously formed for a similar nanoparticle size. The magnetic response was evaluated, and the influence of the structure and composition of the nanoparticles is discussed. An enhancement of the global magnetic anisotropy caused by exchange bias and dipolar interactions, which enables the thermal stability of the studied small particles up to relatively large temperatures, is reported
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