Morphology, structure and magnetic coupling relationship in hard-soft SrFe12O19-CoFe2O4 nanostructures

Abstract

In recent years, magnetic nanocomposites (NCs) have obtained an ever-increasing attention, due to the possibility to finely control and modify their features at the nanoscale, which is fundamental for extending their applicability as permanent magnets in a multitude of energy-related technological areas [1,2]. In this regard, ferrites have shown to be a promising class of materials, in particular M-type SrFe12O19 (SFO) and spinel CoFe2O4 (CFO) nanoparticles (NPs): their respective hard and soft magnetic properties make them interesting candidates for hard-soft exchange-coupled nanostructures. Effectively, the combination of two prototypical phases with different extrinsic properties expands the potentiality and hence the suitability of such nanosystems [3]. Our main focus here is on the investigation of the synthesis strategy in controlling the magnetic coupling, through the optimization of the interface between the hard-soft magnetic phases, thus achieving efficiently strongly coupled nanostructures.This work addresses a systematic investigation of the magnetic interaction in SFO-CFO hard-soft NCs. Several samples were synthesized through a self-combustion sol-gel approach, with compositions ranging from 50/50 to 90/10 w/w %, in step of 10%. In order to elucidate the synthesis method, a thermogravimetric/differential thermal analysis (TG/DTA) was performed, showing a first evidence that the reaction proceeds in a symbiotic way, where the formation of CFO after-combustion assists the evolution of SFO precursors, when the CFO % is increased [4]. The subsequent investigation on the inner structure of the NCs, by transmission electron microscopy (TEM) technique, revealed that SFO NPs are in the form of platelets, with CFO nanocrystals confined between the SFO ones. This clearly suggests a strong interaction between the two structures [5]. The evidence of a symbiotic growth was furtherly confirmed by X-ray powder diffraction (XRPD): the crystallite size extracted by Rietveld refinement shows an evident dependence of SFO crystallite size on the amount of inserted soft phase. When CFO increases, the dc (size extracted along the c-axis) and dab (size extracted along the ab-plane) for SFO drop from 104 to 51 nm from 132 to 77 nm, respectively. Hence, we illustrate here for the first time that the CFO grains act as nucleation sites for the SFO phase, resulting in an effect of confinement, which restricts the growth of one phase by the other. To clarify the relationship between the morphological-structural features and magnetic coupling, the static magnetic properties of NCs were investigated at 300 K using a superconducting quantum interference device (SQUID) magnetometer. The analysis of field-dependent magnetization loops shows that the two magnetic phases are homogeneously dispersed and strongly coupled (as reported in Figure 1). Furthermore, the switching field distributions clearly exhibit a single reversal process of magnetization, confirming that the SFO and CFO phases are strongly coupled in the composite (as shown in Figure 2). As expected, the variation of coercivity (HC) is consistent with the increasing amount of soft CFO phase, as it decreases from ~463 to 178 kA/m, within the limit of exchange-coupling, whereas the saturation moment (MS) increases up to ~74 Am2/kg. Our study shows that it is possible to control and limit the decrease of the coercivity of hard-soft bi-magnetic NCs to our desire by controlling the size and distribution of the hard-soft regions, and their interface.We thank the Swedish Energy Agency and Swedish Research Council (VR) for financially supporting this work. **