Building Composite Iron–Manganese Oxide Flowerlike Nanostructures: A Detailed Magnetic Study

Abstract

Here we show that it is possible to produce different magnetic core−multiple shell heterostructures from monodisperse Fe3O4 spherical magnetic seeds by finely controlling the amount of a manganese precursor and using, in a smart and simple way, a cation-exchange synthetic approach. In particular, by increasing the amount of precursor, we were able to produce nanostructures ranging from Fe3O4/ manganese ferrite core−single-shell nanospheres to larger, flowerlike Fe3O4/manganese ferrite/Mn3O4 core−double-shell nanoparticles. We first demonstrate how formation of the initial thin manganese ferrite shell determines a dramatic reduction of the superficial disorder in the starting Fe3O4, bringing nanomagnets with lower hardness. Then, the growth of the second and most external manganese oxide shell causes magnetic hardening of the heterostructures, while its magnetic exchange coupling with the rest of the heterostructure can be either antiferromagentic or ferromagnetic, depending on the strength of the applied external magnetic field. This response is similar to that of an iron oxide−manganese oxide core−shell system but differs from what is observed in multiple-shell heterostructures. Finally, we report that the most external shell becomes magnetically irrelevant above the ferrimagnetic−paramagnetic transition of the manganese oxide, and the resulting magnetic behavior of the flowerlike structures is studied in depth

Surface Compositional Change of Iron Oxide Porous Nanorods: A Route for Tuning their Magnetic Properties

“…As indicated by their corresponding H E values, S3 and S3_Mn at 4 K are both affected by SEB, which is in fact maximized in S3_Mn. This is in accordance with our previous studies on spherical NPs of different size, where the introduction of a new cation by a CE approach did lower the SEB at first by forming a mixed shell and reducing the initial superficial disorder, but also led to a further increase once the new cation formed a second shell on top of the mixed shell [18,19]. Here, the curing effect of Mn atoms on the superficial disorder of S3, which is limited to the sole external surface, is likely masked by the additional disorder introduced by the newly formed external Mn 3 O 4 shell.…”

“…The increased size of the S3_Mn NRs following the introduction of Mn can be explained in the light of the compositional analysis by EDS as the effect of the formation of a Mn-rich shell on top of the S3 NRs. This result is in good accordance with previous studies conducted by our group [10,18] and once again confirms that the superficial disorder is the key parameter driving the initial penetration of Mn ions in the NRs. However, the magnetite structure is not prone to further continue with the CE once the superficial vacancies have been occupied [19].…”

“…Furthermore, the vertical shift (VS), evidenced by the non-null values of ∆M R , and the occurrence of a training effect for repeated hysteresis loops (in both Figures 5 and 6, R0 and R1 are the first and second hysteresis loops) confirm that the superficial disorder of the inner and external surfaces work as makeshift shells from a magnetic point of view, thus suggesting that the superficial magnetic disorder gives rise to spinglass-like magnetic responses [30]. VS is always lower in the FC hysteresis loops of S3_Mn, which proves that the introduction of Mn did also actually have a curing effect on the disorder of the external surface of the magnetite NRs, in agreement with what we recently observed on similar systems [18].…”

“…In fact, while M S becomes almost eight times lower than that measured for the S3 sample, the coercivity measured at 5 K for S3 is roughly five times higher than what was previously observed for similar-sized NRs [25,26] and is even higher than those of much bigger NRs [17]. In this case too, a further comparison can be drawn between the NRs of S3_Mn and spherical NPs of similar volume, but given the multilayered nature of our samples, flower-like isotropic core/shell NPs of magnetite and manganese oxide represent the most fitting counterpart [18]. Both systems are characterized by an AFM coupling between core and shell, but once again the NRs have higher coercivity due to their anisotropic shape, while their saturation magnetization is lowered by the partially hollow magnetite core.…”

Magnetic Study of CuFe2O4-SiO2 Aerogel and Xerogel Nanocomposites

“…This finding is in good accordance with the presence of spontaneous exchange bias (HE) in the hysteresis loops of the 10 wt% and 15 wt% aerogel samples, whose presence usually indicates structural disorder that depletes the number of available magnetic moments and affects negatively the net magnetization of the samples. [33][34][35][36] On the other hand, the low coercivity, higher magnetization and null exchange bias (HE) observed for the 5 wt% sample can be explained by considering the 3 nm size of the crystallites, which puts them in the lower portion of the single domain regime. Such small-sized crystallites correspond to small single magnetic domains, which are easily aligned to applied magnetic fields and undergo superparamagnetic relaxation at very low temperatures, as shown by the low TB and TIRR values observed in the ZFC-FC curves in the present case.…”