Strong dipolar interactions in high magnetization FeCo nanoparticles

In this study, FeCo nanoparticles are synthesized using the chemical solution approach. XRD pattern shows cubic crystal structure with the average crystallite size of 19 nm. The nanoparticles exhibit a high saturation magnetization of 230 emu/g with a coercivity of 268 Oe at room temperature. The sharp increase in the ZFC magnetization with temperature till ∼50 K of random nanoparticles indicates the relaxation of randomly oriented surface and superspins. However, the sharp increase in the ZFC magnetization of aligned nanoparticles is not observed in this temperature range, which implies that the superspins were already aligned and did not get relaxed due to the existence of strong dipolar interactions. This observation is interpreted as the dominance of dipolar interactions over the superspin relaxation. The flat FC curve for randomly oriented nanoparticles is an evidence for the existence of strong dipolar interactions, which further get increased with the alignment of nanoparticles. The sharp decrease in the FC magnetization of the aligned sample with the decreasing temperature is an interesting feature that may be due to the existence of enhanced interparticle magnetic dipolar interactions in the aligned nanoparticles. Furthermore, the sharp increase in coercivity below 50 K is also attributed to the anisotropy induced by the surface and superspin dipolar interactions. More importantly, the coercivity of the randomly oriented nanoparticles at 5 K is observed to be 416 Oe which got increased by ∼2 times (780 Oe) for the aligned sample, further confirming the presence of anisotropy induced by the strong magnetic dipolar interactions. The emergence of significant anisotropy from the aligned nanoparticles via dipolar interactions makes it an interesting system for further investigations.