Core Size and Interface Impact on the Exchange Bias of Cobalt/Cobalt Oxide Nanostructures

Abstract

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.

Modeling of the Influence of Oxidation on the Energy of Interfacial Exchange Interaction in Co/CoO Films

“…This observation of the bias field H E dependence on the thickness d AF was noted in core/shell nanoparticle systems [6][7][8][9][10]. Note that the well-known relationship (see, for example, [3]) between the energy of the interphase exchange interaction and the field E in = H eb M F M V F M , where M F M and V F M are saturation magnetization and ferromagnetic volume, allows experimental energy estimate E in .…”

“…The thickness of the antiferromagnetic layer may be adjusted by controlling the duration of post-synthetic annealing or the partial pressure of oxygen during spraying. Through oxidation, it is feasible to establish the value of the H E field and, therefore, the interphase exchange interaction energy [9,10].…”

Structure, magnetic, and electrodynamic properties of SrInxFe12‐xO4/Ni0.5Zn0.5Fe2O4 composites at low temperature

“…[23][24][25] Isolators, microwave absorption materials, filters, hyperthermia materials, and biosensors are only a few of the applications of exchange-coupled hard-soft bi-magnetic ferrites. 23,[26][27][28][29][30][31] Hence, it is required to estimate the magnetic properties of hard-soft bi-magnetic materials in order to determine their proper use in an appropriate field. 32,33 Hard-soft ferrite materials usually comprise magnetism types such as ferromagnetic (FM), ferrimagnetic (FiM), and antiferromagnetic (AFM).…”

“…Thus, both scientists and industry can design hard magnets with improved energy properties. [23][24][25] Isolators, microwave absorption materials, filters, hyperthermia materials, and biosensors are only a few of the applications of exchange-coupled hard-soft bi-magnetic ferrites. 23,[26][27][28][29][30][31] Hence, it is required to estimate the magnetic properties of hard-soft bi-magnetic materials in order to determine their proper use in an appropriate field.…”

“…The combinations result in proximity effects, reversal of magnetization in two stages, and blocking temperatures that can be tuned in bi‐magnetic hard‐and‐soft ferrites. Thus, both scientists and industry can design hard magnets with improved energy properties 23–25 . Isolators, microwave absorption materials, filters, hyperthermia materials, and biosensors are only a few of the applications of exchange‐coupled hard–soft bi‐magnetic ferrites 23,26–31 .…”