Structural, Magnetic and Mechanical Properties of Nd16 (Fe76−xCox)B8 0 ≤ x ≤ 25 Alloys

In this work, the structural, magnetic and mechanical properties of Nd16Fe76−xCoxB8 alloys with a varying Co content of x = 0, 10, 20 and 25 were experimentally investigated by X-ray diffraction (XRD), Mössbauer spectrometry (MS) and vibrating sample magnetometry (VSM) at room temperature (RT), and microhardness tests were performed. The system presented hard Nd2Fe14B and the Nd1.1Fe4B4 phases for samples with x = 0; when the concentration increased to x = 20 and 25, the CoO phase appeared. All MS data showed ferromagnetic behavior (eight sextets: sites 16k1, 16k2, 8j1, 8j2, 4c, 4e, sb) associated with the hard and soft magnetic phases, and one paramagnetic component (doublet: site d) associated with the minority Nd1.1Fe4B4 phase, which was not identified by XRD. All samples were magnetically hard and presented hard magnetic behavior. The increase of Co content in these samples did not improve the hard magnetic properties but increased the critical temperature of the system and decreased the crystallite size of the hard phase. There was a general tendency towards increased microhardness with cobalt content that was attributable to cobalt doping, which reduces the lattice parameters and porosities within the sample, improving its hardness.


Introduction
NdFeB permanent magnets have been investigated since 1983 [1]- [3] and continue to be investigated nowadays [4], [5] due to their high energy density (~450 kJm -3 ) [6] that makes them useful for different applications: acoustic transducers, air conditioning, electric bikes, wind turbines, hybrid and electric cars, hard disk drives and others [7] which are the main reason of efforts to obtain an improvement in their magnetic and physics properties. The nanocomposed permanent magnets are composed of soft (α-Fe or Fe3B) and hard (Nd2Fe14B) nanocrystalline magnetic phases and they present much interest because they exhibit unusual properties such as: remanence relation (Mr/Ms) larger than 0.5 (the Stoner-Wohlfarth limitation) due the exchange coupling between these two phases [8]. Among earlier works on this nanocomposites magnet was carried out by Coehoorn, R et. al [9] that investigated Fe3B/Nd2Fe14B through of melt-spinning, showed values of 0.3 T and 93 kJm -3 for coercive and magnetic energy, respectively. After these works, different theoretical models [10]- [12] were proposed in order to improve our understanding of the exchange-coupled nanocomposite magnet. Many works that using it model have been carry out by thin film; for example, Cui, W et. al [13] investigated Nd2Fe14B/FeCo anisotropic nanocomposite films, and they have obtained values of maximum energy product between 400 and 500 kJm -3 through exchange-coupled. On the other hand, Yang. F et. al [4] recently investigated the bonded magnets of NdFeB with SrFe12O19 ferrite by additive manufacturing (3D printing); for the sample with 20 wt.% of ferrite they obtained a relatively low surface roughness of ~6 μm and a tensile strength of 12 MPa. However, they obtained decrease in magnetic properties saturation magnetization (Ms), remanent magnetization (Mr), coercive field (Hc) and maximum energy product (BH)max with the increased content wt.% of ferrite. Other recent work in the system NdFeB using the same principle is reported in the reference [5], where the phase and hyperfine structure of melt-spun nanocrystalline (Ce1-xNdx)16Fe78B6 alloys are studied, they found that alloys are composed specifically by the phase (NdCe)2Fe14B, however phases like (CeNd)Fe2 and (CeNd)Fe4B4 was also identified. The Mössbauer fitted was carry out to used six sextets and two doublet component; the average magnetic hyperfine field by Mössbauer and the magnetic properties Ms, Hc and (BH)max increased for the composition from x=0 to x=0.7. These properties are sensible to the type of elements of the hard [14]- [19] and soft phases [14], [20], to the grain size of the hard phase [15], [16], [21]- [24], and to the different techniques used to prepare them [25]- [36].
Considering that Co is a ferromagnetic transition metal and its atomic ratio is 1.25 Å that is comparison whit that of the Fe (1.26 Å ), moreover that the Co have a tetragonal crystalline structure with uniaxial anisotropy. In this direction we are interested in study the effect of Co in the properties of Nd16Fe76-xCoxB8 alloys (x = 0, 10, 20, and 25) obtained through arc-furnace, and characterized by xray diffraction (XRD), Mossbauer spectrometry (MS), vibrating sample magnetometry (VSM), and microhardness studies.

Materials and Methods
Alloys of the Nd16Fe76-xCoxB8 (x=0, 10, 20, and 25) system were prepared by mixing and compacting high purity fine powders (>99.9%) of Fe, Nd, B and Co. These samples were prepared by arc-melting method under Ar atmosphere. Afterwards, the samples were encapsulated in evacuated quartz tubes and finally heat treated at 1073 K for 30 minutes and quenched in ice water. All the prepared samples were characterized by MS, XRD, VSM, and Microhardness studies. Mössbauer measurements were performed in a constant acceleration spectrometer at room temperature with transmission geometry using a 57 Co(Rh) source of 25 mCi. All spectra were fitted with the MOSFIT program [37] and the isomeric deviation values were referred to α-Fe. The XRD measurements were obtained by using the Cu-Kα radiation and the patterns were refined by using the GSAS program [38], which is based in the Rietvelt method combined with Fourier analysis to describe the broadening of the lines. This refinement yields the average values of the lattice parameters and of the crystallite sizes, and quantify the obtained structural phases. The behavior of the magnetization as a function of the field was studied by using the VSM of the Physics Properties Measurements System (PPMS) of the Excellence Centre for New Materials (CENM) of Universidad del Valle, Colombia. Vickers hardness tests were performed on heat-treated samples, which have been polished to a mirror finish with alumina 0.05 µ m particle size. Microhardness maps were performed on a Leco AMH43 microhardness automated, on different areas of the samples, applying a load of 200 g and a holding time of 15 s. Figure 1 shows XRD patterns obtained for some samples with different Co concentrations. These patterns reveal that all samples present the tetragonal Nd2Fe14B hard phase (space group P42/mmm) and the Nd1.1Fe4B4 phase (tetragonal structure and orthorhombic space group Pccn) [39]- [42]. Also, the CoO phase (Wurzita structure and space group P63mc) for high concentrations of Co (x = 20 and 25) was detected. In Fig. 1 it can be noted how the intensity of the peaks corresponding to the Nd2Fe14B phase is slightly reduced and those of the Nd1.1Fe4B4 increase when Co content increases.

XRD
Rietveld refinement allowed us to determine the type of structures having these alloys. The parameter values for samples with stoichiometry Nd16Fe76-xCoxB8 with x = 0, 10, 20 and 25 are reported in Table 1. These data indicate that the crystallite size for Nd1.1Fe4B4 and Nd2Fe14B phases are in the nanometer range, from 5 to 90 nm. Crystallite size parallel to the Nd1.1Fe4B4 and Nd2Fe14B phases increases with increased concentration of cobalt. It can be noted that systematically, the two phases parallel crystallite sizes are greater than the perpendicular ones, indicating that the crystallite shape is not spherical but elongated in that direction. The refinement of the mean crystallite size of the hard phase is required in order to obtain higher remanence values [43], [44] and to increase the ferromagnetic exchange coupling between the soft and hard grains [10]. The obtained values of the crystallite size indicate that these melted alloys are nanostructured.   In Table 1 the quantitative evolution with the Co content of the weight fraction of the different obtained phases. It can be noted that the Co also decreases the stability of the hard phase and the decomposition of this phase conduces to the increment of the Nd1.1Fe4B4 phase.

Mossbauer Results
The Mössbauer spectra of Nd16Fe76-xCoxB8 (x = 0, 10, 20 and 25) samples, collected at RT, are shown in Fig. 3. To fit these spectra, it was necessary several subspectra, which were associated with the Nd2Fe14B, α-Fe and Nd1.1Fe4B4 phases. The last one was not identified by XRD. Seven sextets were used to fit the magnetic part of the spectra, six of them corresponding to those reported for the hard phase (16k1, 16k2, 8j1, 8j2, 4c and 4e) and the other corresponds to the α-Fe phase (s b ) reported by Hernandez et al. [47]- [49]. As can be observed, the Mössbauer spectrum of the sample with x = 0 is the typical one of the Nd2Fe14B phase [8]. It was necessary to add a small doublet (site d) in order to obtain the best fit, and this was attributed to the paramagnetic Nd1.1Fe4B4 phase [47]. The Mössbauer parameters, like a hyperfine magnetic field (Hhf), isomer shift (IS), quadrupole splitting (QS), and area of each subspectra, are listed in Table 2   Fig. 4a shows the variation of the hyperfine field, Hhf, of the different inequivalent Fe sites in the Nd2Fe14B hard magnetic phase as a function of Co content. It can be noted that the hyperfine field of the different sites has a tendency to decrease. This decrease is mainly due to the cobalt has a magnetic moment (1.71μB/atom) less than the magnetic moment of iron (2.22μB/atom), and this result suggests that cobalt atoms are entering within the lattice of Nd2Fe14B. But for site 4c the higher decreasing of hyperfine field with the cobalt concentration occurs; this result indicates that the cobalt atoms have a preference for substituting iron atoms at those sites. These results agree with Liao et al. [49].
In Fig. 4b, the results are also consistent, which shows a high decrease in the relative area 4c, confirming the preference of cobalt atoms replacing iron atoms at these sites, comparing 8j1, 8j2, 16k1, 16k2 and 4e sites, which have a tendency to increase their relative area; then the cobalt does not replace iron atoms of these sites and ferromagnetism of these sites increases.  . It can be observed that there is a general tendency of the mean hyperfine field (MHF) to decrease as cobalt content increases. Fig. 5b shows the behavior of the relative area of the site d (Nd1.1Fe4B4) and site s b (-Fe) relative to Co content. A behavior to diminish the relative area of the site d (Nd1.1Fe4B4), suggests that substituting iron atoms by cobalt atoms destabilizes Nd1.1Fe4B4 phase. Besides, the Fig.5b shows that the relative area of the -Fe phase decreases with the cobalt content. These results indicate that the cobalt atom diffuse into the structure of Nd2Fe14B substituting and expels iron atoms, producing Fe segregated.  Figure 6 shows the hysteresis loops for the alloys with x = 0, 10, 20 and 25. In all samples predominates the Nd2Fe14B hard magnetic phase. But the addition of cobalt reduces the size and width of the hysteresis loop, especially for x = 25. These samples reach the saturation magnetization when the fields are higher than 2.5 T. On the other hand, the values (see Table 4) of the coercive field Hc decrease with the increase of Co content, as shown in Fig. 7a. Despite such decrease in coercivity, all samples are still displaying a hard-magnetic character.

Microhardness
We carried out hardness evolution of the system Nd16Fe76-xCoxB8 with x = 0, 10, 20 and 25 as a function of the atomic percentage of cobalt. The microhardness values of the samples with respect to the cobalt content. Fig. 8a shows the data obtained in relation to the cobalt concentration. We can see a general tendency to increase the microhardness with the cobalt content, due to cobalt doping which reduces the lattice parameters and the porosities in the sample improving its hardness as can be appreciated in the microhardness images presented in Fig. 8b.

Conclusions
From the results of present work, it can be concluded that the substitution of Fe by Co atoms in the Nd16Fe76-xCoxB8 nanocomposite alloy decreases the magnetic hard character of the system. The increase of the Co content decreases the stability of the hard phase. The hard phase decreases due the decrease of the 4c site when Co content increases. The magnetic softening of the samples is principally due to the low ferromagnetic coupling between the soft and hard phases. There is a general trend of microhardness increase with cobalt content, which is attributable to the role of cobalt doping in reducing the lattice parameters and porosities in the sample In summary, the coupling between NdFeB and Co generates nanostructured systems that favor the formation of hard and soft magnetic phases and improves the ductility conditions which improves the magnetic and mechanical properties of exchange coupling magnets.
Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used "conceptualization, Juan Trujillo,