Study of Growth Kinetics of Fe₃Se₄ Nanocrystallites and the Influence of Size and Shape Tunability on their Magnetic Properties

Abstract

Crystalline iron selenide (Fe 3 Se 4 ) nanoparticles (NPs) present a useful model system for a fundamental understanding of magnetism apart from having potential applications in permanent-magnet-related technologies. Despite significant advancements in understanding of the nucleation and growth processes, control over size and shape tunability is nontrivial, especially for the transition metal chalcogenides (TMC). The Wulff theorem states that if a crystal can grow purely under thermodynamic control of parameters then it will adopt the shape determined by the surface-energy minimization, i.e., the surface energy minimization will drive the growth of each crystalline facet. However, the known or unknown, controllable or uncontrollable parameters of a typical reaction which makes kinetic growth more art than science, depending upon whether we understand them or not. There is hardly any study based on existing models and theories to explain the TMC morphology evolution. In this work, there is a conscious effort to know, understand, and control the individual role of various reaction pathways on the shape and size of the Fe 3 Se 4 nanocrystals. A qualitative growth mechanism is proposed based on the diffusion and reaction processes. Furthermore, the influence of shape/size on the magnetic parameters such as coercivity (H C ), magnetization (M 60kOe ), effective magnetic anisotropy constant (K eff ), energy product (BH max ), and average blocking temperature (T B ) is investigated for Fe 3 Se 4 NPs. After diving deeper into the science of crystal growth, our insight provides valuable guidance on experimental conditions for the synthesis of Fe 3 Se 4 NPs with tunable sizes/shapes which for the first time can be extended to most TMCs.

Remarkable Effect of Fe and Se Composition on Magnetic Properties─Comparative Study of the Fe–Se System at the Nanoscale

“…Fe 3 Se 4 (Fe 2+ Fe 2 3+ Se 4 2– ) has a monoclinic NiAs-type crystal structure with the unit cell lattice parameters a = 6.167 Å, b = 3.537 Å, and c = 11.17 Å having the I 2/ m space group (Figure a). , In Figure b,c, the magnetization ZFC and FC of Fe 3 Se 4 show bifurcation below 330 K and the Curie transition temperature is nearly 322 K, below which it goes into a ferrimagnetic phase. These results are also consistent with the earlier reports of T C ∼ 320 K for Fe 3 Se 4 . ,, Figure S6 shows the zoomed-in view of the M–T curves in the ZFC mode, illustrating an average blocking temperature of ∼313 K with a peak in the ZFC curve. Consequently, ferrimagnetism exists in the Fe 3 Se 4 nanorods with a transition temperature of 322 K. Furthermore, this material reveals semihard magnetic properties at the nanoscale. ,, In earlier reports, for the Fe 3 Se 4 nanoplatelets, the observed coercivity was ∼4 kOe at 300 K, which increases to ∼40 kOe at 10 K for 90 kOe of the maximum external field .…”

“…These results are also consistent with the earlier reports of T C ∼ 320 K for Fe 3 Se 4 . ,, Figure S6 shows the zoomed-in view of the M–T curves in the ZFC mode, illustrating an average blocking temperature of ∼313 K with a peak in the ZFC curve. Consequently, ferrimagnetism exists in the Fe 3 Se 4 nanorods with a transition temperature of 322 K. Furthermore, this material reveals semihard magnetic properties at the nanoscale. ,, In earlier reports, for the Fe 3 Se 4 nanoplatelets, the observed coercivity was ∼4 kOe at 300 K, which increases to ∼40 kOe at 10 K for 90 kOe of the maximum external field . The coercivity values for the Fe 3 Se 4 nanorods at 300 K were reported as ∼3.4 kOe and 2.74 kOe .…”

“…The as-synthesized Fe 3 Se 4 nanorods reported in this article also exhibited semihard magnetic properties, and their hysteresis loops recorded at 300 K (red curve) and 10 K (blue curve) for a field sweep up to ±60 kOe are shown in Figure d,e, respectively. These hysteresis loops showed the nonlinear behavior typical of the ferrimagnetic nature of Fe 3 Se 4 NPs, as reported in the literature. ,, The H C and M R values of Fe 3 Se 4 were determined to be 1.6 kOe and 1.2 emu/g, respectively, which are relatively low compared to the reported data due to the reduced size of the NPs. ,, All the phases were synthesized at the minimal possible temperature with an appropriate time. Thus, the pure Fe 3 Se 4 phase reaction started from this stage and growth was terminated by stopping the reaction that resulted in small NPs.…”

“…The selenides of iron have attracted much interest in recent years because of their exciting and unique thermal, electronic, optical, and magnetic properties. − The richness of physical properties also comes from the richness in the stoichiometric phases these families of compounds may exhibit. The Fe–Se system shows various stable polymorphs under ambient conditions, including FeSe 2 , Fe 3 Se 4 , Fe 7 Se 8 , and FeSe. − Hagg and Kindstrom’s original article referred to the iron selenides as a α-phase (FeSe x in a PbO-type structure with x up to 8/7) and β-phase (FeSe x in a NiAs-type structure with x ∼ 8/7–4/3) .…”

“…of these compounds have not been explored in depth so far. However, recently, the potential of Fe 3 Se 4 as a magnetic energy storage device was investigated, and it was found that this compound shows a colossal increase in the energy product while cooling from 300 to 10 K with the potential to replace expensive rare-earth hard magnets at lower temperatures. , …”