## 1 Introduction  
High-magnetic-moment nanoparticles (MNPs), such as metallic Fe, Co, \(\alpha^{\prime\prime}\)-Fe\({}_{16}\)N\({}_{2}\), and FeCo(Ni) binary alloys, have been the subject of intense research activity, owing to their potential applications in many different fields [1; 2; 3; 4; 5; 6; 7; 8] exploiting their high saturation magnetization (M\({}_{8}\)) which exceed by a factor of two or more the typical values of oxide materials (e.g., Fe\({}_{3}\)O\({}_{4}\) and CoFe\({}_{2}\)O\({}_{4}\)) [5]. A high magnetic moment is strongly desired for theranostic applications, as it leads to (1) a more efficient manipulation of magnetic particles by an external magnetic field and (2) higher contrast in magnetic particle or magnetic resonance imaging and more efficient heat generation in hyperthermia treatments [9]. However, different from oxide materials, metallic nanoparticles can be toxic, owing to their strong reactivity with oxygen, and they must be covered with a biocompatible shell, such as carbon [9; 10; 11], SiO\({}_{2}\)[12], or Au [13]. On the other hand, in a non-oxidizing environment, the chemical activity of zero-valent metallic particles can be exploited to catalyze specific reactions. For example, the catalytic activity of Fe and Co is crucially important in hydrocarbon synthesis in the Fischer-Tropsch process [14], and the use of nanoparticles with a high surface-to-volume ratio enhances the rate of this reaction [4]. Additionally, in combination with magnetically hard materials, soft MNPs with a high magnetic moment can be used as building blocks of rare-earth-free nanostructured permanent magnets which match or surpass the performance of current permanent magnets containing critical rare-earth elements [15; 16]. Within this framework, [PERSON] et al. recently showed that it is possible to regulate the degree of reduction of cobalt ferrite MNPs to optimize the magnetic coupling with FeCo phases as a novel approach toward permanent magnets [17]. Among the family members, the Fe\({}_{100\times}\)Co\({}_{\rm x}\) intermetallic alloys show a distinctive combination of a high Curie temperature, low magneto-crystalline anisotropy, and high saturation magnetization, which reaches a maximum value of 240 Am\({}^{2}\)/kg in the Fe\({}_{65}\)Co\({}_{35}\) alloy [18; 19; 20]. Despite these interesting properties, FeCo particles are unstable in air and relatively difficult to synthesize with respect to magnetic oxide materials [21]. Several techniques have been proposed to synthesize pure FeCo alloy nanoparticles, including physical vapor nanoparticle deposition [13; 22], methane chemical vapor deposition [9; 10], and polyol methods [23; 24]. However, it is still challenging to find a large-scale, efficient, and low-cost method for their fabrication. A promising strategy, which was already applied to synthesize high-moment and air-stable \(\alpha\)-Fe MNPs [25], implies the use of oxide-based MNPs and their reduction in the H\({}_{2}\) atmosphere. Despite this method, referred to as hydrogen reduction, being widely used to produce bulk metals due to its industrial importance in metallurgy [26], its implementation in nanoparticle synthesis is more difficult, owing to the higher cost of the parent nanoparticle materials and the increase in particle size at high temperatures [4; 25]. In this regard, a bigger effort may be put forth to develop an easy and scalable way to synthesize oxide precursors with high-quality nanocrystallites to preserve the nanoscale features before undergoing reduction [27; 28]. Moreover, to make the process more efficient, different strategies have been proposed, such as the use of hydrogen at high pressure [29] or the application of a magnetic field during the reaction [30; 31]. The latter was demonstrated to be highly efficient in terms of reduction of the processing temperature. However, the effect of an external magnetic field on the topochemical reduction process is still under discussion, and further studies are necessary to better disclose and control the process.