Unidirectional Magnetic Anisotropy in Molybdenum Dioxide–Hematite Mixed-Oxide Nanostructures

Abstract

MoO2-Fe2O3 nanoparticle systems were successfully synthesized by mechanochemical activation of MoO2 and α-Fe2O3 equimolar mixtures throughout 0–12 h of ball-milling. The role of the long-range ferromagnetism of MoO2 on a fraction of more defect hematite nanoparticles supporting a defect antiferromagnetic phase down to the lowest temperatures was investigated in this work. The structure and the size evolution of the nanoparticles were investigated by X-ray diffraction, whereas the magnetic properties were investigated by SQUID magnetometry. The local electronic structure and the specific phase evolution in the analyzed system versus the milling time were investigated by temperature-dependent Mӧssbauer spectroscopy. The substantially shifted magnetic hysteresis loops were interpreted in terms of the unidirectional anisotropy induced by pinning the long-range ferromagnetic order of the local net magnetic moments in the defect antiferromagnetic phase, as mediated by the diluted magnetic oxide phase of MoO2, to those less defect hematite nanoparticles supporting Morin transition. The specific evolutions of the exchange bias and of the coercive field versus temperature in the samples were interpreted in the frame of the specific phase evolution pointed out by Mӧssbauer spectroscopy. Depending on the milling time, a different fraction of defect hematite nanoparticles is formed. Less nanoparticles supporting the Morin transition are formed for samples exposed to a longer milling time, with a direct influence on the induced unidirectional anisotropy and related effects.

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“…Finally, the last class of reported heterogeneous nanostructured systems consists of interfaced ferromagnetic and antiferromagnetic nanophases presenting specific magneto-functionalities related to induced unidirectional magnetic anisotropy. Unidirectional magnetic anisotropy in molybdenum dioxide, MoO 2 , and hematite, α-Fe 2 O 3 , mixed oxide nanostructures was reported by Felicia Totea et al in [ 12 ]. The aim of the work was to extend the research on diluted magnetism by a comprehensive study on the possibility to control the room temperature ferromagnetism of MoO 2 by the interfacial magnetic interactions with the antiferromagnetic phase of hematite in equimolar mixtures of MoO 2 and α-Fe 2 O 3 nanoparticles ball-milled for different amounts of time.…”

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“…Many other research groups investigated the correlation of the electronic structure at an interface or within a single film with an electronic structure gradient. However, this topic is only scarcely discussed for nanostructures [72,73] and thus not further discussed here.…”