Abstract
Transition-metal oxide nanoparticles are relevant for many applications in different areas where their superparamagnetic behavior and low blocking temperature are required. However, they have low magnetic moments, which does not favor their being turned into active actuators. Here, we report a systematical study, within the framework of the density functional theory, of the possibility of promoting a high-spin state in small late-transition-metal oxide nanoparticles through alloying. We investigated all possible nanoalloys ABO (A, B = Fe, Co, Ni; n = 2, 3, 4; ) with different oxidation rates, m, up to saturation. We found that the higher the concentration of Fe, the higher the absolute stability of the oxidized nanoalloy, while the higher the Ni content, the less prone to oxidation. We demonstrate that combining the stronger tendency of Co and Ni toward parallel couplings with the larger spin polarization of Fe is particularly beneficial for certain nanoalloys in order to achieve a high total magnetic moment, and its robustness against oxidation. In particular, at high oxidation rates we found that certain FeCo oxidized nanoalloys outperform both their pure counterparts, and that alloying even promotes the reentrance of magnetism in certain cases at a critical oxygen rate, close to saturation, at which the pure oxidized counterparts exhibit quenched magnetic moments.
Highlights
We have performed an extensive DFT study of the possibility of inducing ferromagnetic-like order to promote a high-spin state in small transition-metal oxide (TMO)-NP (TM = Fe, Co, Ni) by their doping with a different transition metal (TM) elements of the list
We investigated all nanoalloys resulting from the different stoichiometries and with different oxidation rates up to saturation
From the energetic point of view, the binding energies indicate that the higher the concentration of Fe in the TMO-NP, the higher their absolute stability
Summary
Transition-metal oxide (TMO) nanoparticles (NP) have been the matter of intense research due to their relevance in a large variety of technological applications, such as those in medicine [1,2,3,4,5,6,7,8,9], new generation batteries [10,11,12], bactericidal agents [13,14,15], and catalytic processes [16,17,18,19,20,21]. There is one thing in common for the efficiency of any magnetic NP, independently of the application in which it will be used: it should be as strongly magnetic as possible for it to be turned into active actuators This is precisely the weak point of TMO-NP in general, since TM–O interactions induce antiparallel (AP) magnetic couplings which render them to be in a low-spin state, and to have a small total magnetic moment. Attempts have been made to increase the total moment of TMO-NP by their doping with another element [23,24] In this context, Szczerba et al have experimentally investigated the doping of iron oxide nanoparticles with Zn with the goal of avoiding the spin missalignment, thereby enhancing the total moment [24].
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