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Design of Face Centered Cubic Co81.8Si9.1B9.1 with High Magnetocrystalline Anisotropy

Despite the composition close to glassy forming alloys, face centered cubic (FCC) Co81.8Si9.1B9.1, designed based on Co9B atomic cluster (polyhedral), are synthesized as singlephase ribbons successfully. These ribbons, with grain sizes of ca. 92 nm, show supreme ductility and strong orientation along (111), which couples with shape anisotropy leading to high magnetocrystalline anisotropy comparable to Co rich Co-Pt nanoscale thin films, with a coercivity of 430 Oe and squareness of 0.82 at room temperature. The stability and magnetic behaviors of the phase are discussed based on experimental electronic structure. This work not only develops low cost Co-based materials for hard magnetic applications, but also extends the atomic cluster model developed for amorphous alloys into the design of new crystalline materials.

Metallic cobalt (Co) has been widely studied as an important magnetic material and its magnetic properties can vary significantly depending on its structures: [1] the face centered cubic (FCC, stable above 450 °C) phase shows good soft magnetic behaviours, and amorphous CoSiB materials based on the structure (with a general composition at ca. Co80(Si,B)20) are one of best soft magnetic materials, used in many high-performance electric devices; [2] the hexagonal close-packed (HCP, stable below 450 °C) Co shows hard magnetic behavior because of the structural anisotropy, and its compounds with rare earth (RE) elements (e.g. SmCo5) or noble metals (e.g. CoPt) do show significantly enhanced hard magnetic behaviours due to the high anisotropy of RE elements or strong 3d-5d hybridizations. [3,4] The high cost of these materials inhibits their applications as permanent magnets in many cases. On another hand, many of these compounds show unsatisfactory mechanical performance. The fabrication of nanomaterials especially nanowires does improve the hard magnetic behaviours significantly, [5,6] but the complex process and oxidization are main drawbacks.
In metallic glasses, it is generally agreed that there are one or two kinds of atomic clusters (short-range orders) dominating the structure and thus controlling the physical properties. [7] It is reported that based on different dominant clusters, CoSiB metallic glasses with close compositions but significantly different structure and magnetic behaviors (from ferromagnetism to anitferromagnetism) can be achieved. [8,9] In infinite solid state solutions, it is also found that their atomic structures are not completely disordered. Instead, dominant atomic clusters determining the magnetic properties also exist in these compounds, and can vary depending on the compositions. [5,6,10] Thus, it is possible to design this kind of magnetic compounds based on the dominant atomic clusters by using the atomic cluster model previously developed for amorphous alloys. [8] CoB/CoSiB metallic glasses are widely studied in both structure and magnetic properties since 1970s. The atomic clusters favouring glassy formation have to be avoided, because the absence of anisotropy in glassy states will deteriorate the hard magnetism. According to Co-B phase diagram, [1] there is a Co rich FCC phase, Co23B6, stable at high temperature. Thus the cluster favouring the FCC structure can be extracted from this compound, [11] as Co9B polyhedral (see the inset structure in Figure 1a). In order to reduce the cost and increase the magnetic anisotropy, it is better to include Si and B as much as possible while remaining the FCC or HCP Co-type structure. The maximum solubility of Si is 10 at. % (Co:Si=9:1) in HCP Co and 18 at. % in FCC Co. [1] Si concentration more than the maximum solubility may favour the amorphous state instead of Co structure. Thus the composition is designed as Co9BSi, normalized as Co81.8Si9.1B9.1. The synthesized structure is expected to be FCC Co type, because the dominant cluster is from FCC Co23B6. As a result, in our work, we obtained the The morphologies of these ribbons indicate the good ductility of these samples, as shown in the inset of Figure 1(a). The lattice parameter estimated from the (111) reflection is a=3.514 Å, smaller than the value 3.537-3.558 Å of FCC Co at room temperature. [12] It is in agreement with the small atomic size of the substituting elements B and Si. The (111) reflection peak is broadened obviously, suggestive of the nanocrystalline nature.
The apparent crystallite size was estimated as ca. 92 nm using the Scherrer's formula. [ Magnetic hysteresis loops were measured under applied fields along longitude direction and perpendicular to ribbons at room temperature. As shown in Figure 2, Co81.8Si9.1B9.1 exhibits high magnetocrystalline anisotropy with (111) easy direction. Such strong anisotropy was mainly observed in nanoscale thin film or nanomaterials of anisotropic shapes. [6,14] The magnetization is almost saturated at 2000 Oe along the easy direction close to the field to saturate soft CoB/CoSiB amorphous alloys, and at 10000Oe along the perpendicular direction close to the field for bulk and nanoscale Co in both FCC and HCP structures.
However, the saturation magnetization (Ms) is only 31.7 emu/g, corresponding to 28 emu per Co gram, much smaller than the reported Ms in bulk Co and most nanoscale Co. [15,16] Obviously the reduced Ms is not due to the dilution effect of the substituted elements. It was reported that Co nanoparticles of 2 nm in size show a smaller Ms, very likely owing to the surface oxidation. [17] Apparently, it is not the case here. Though antiferromagnetic CoSiB amorphous alloys are also reported, the small filed for saturation along (111) rules out the possibility. Thus the reduced Ms should result from the partial delocalization of Co 3d electrons due to the strong interaction with B or Si.
The coercivity (Hc) is ca. 430 Oe, in both easy and hard directions, as evident in the inset of Figure 3. The coercivity is much larger than bulk Co and most of reported nanoscale HCP and FCC Cobalt (10-300 Oe), [15,17] and close to some Co-Pt thin films. [14] Despite the reduced Co magnetic moment, the saturation remanence (Mr) is still up to 26 emu/g with a high squareness (Mr/Ms) of 0.82. With proper tuning, the materials will be promising hard magnetic materials.
To reveal the origin of the improvement in hard magnetization, magnetic domains in the samples were observed by magnetic force microscaopy (MFM). The samples show (see Despite the fact that the composition of the FCC Co81.8Si9.1B9.1 is very close to compositions of many CoSiB amorphous alloys, neither amorphous nor crystalline impurities were observed in XRD patterns and TEM observations. It is already reported that two CoSiB amorphous alloys having very close compositions show distinctive differences in their magnetisms, due to different short-range orders or clusters. [8,9] To understand the origin of the special structure and magnetism of the present Co81.8Si9.1B9.1 phase, electronic structures around fermi level ware measured on the present Co81.8Si9.1B9.1 phase, HCP Co and a soft magnetic CoSiB alloy Co63.1B27Si9.9. Because the major phase after crystallization of soft magnetic CoSiB amorphous alloy is generally the FCC Co structure, the electronic structure of soft magnetic CoSiB amorphous alloy should be similar to that of FCC Co. [8,19] Compared with the electronic structures of HCP Co (see Figure 4), many soft magnetic CoSiB alloys show similar feature around the fermi level: peak electronic density of state (DOS, N(E)) at fermi level followed by a shoulder at 1.5 eV (black arrows in Figure 4). [8,20] Both HCP Co and CoSiB amorphous alloy have large DOS close to the peak value at their fermi level, while the FCC Co81.8Si9.1B9.1 exhibits a small DOS at the fermi level. It is evident that the FCC Co81.8Si9.1B9.1 is more stable than both hcp Co and amorphous CoSiB alloys. As the electronic structure around the Fermi level determines the magnetic behaviours, the Co atoms in both HCP Co and amorphous CoSiB show similar magnetic moment, in agreement with literatures. The shoulder at 1.5 eV corresponds to the electrons in the spin-down states, while the peak at Fermi level is from spin-up electrons. [20] For the FCC Co81.8Si9.1B9.1, the shift of peak towards high binding energy and the absence of the shoulder indicate that the strong interactions between Co and B/Si leads to the delocalisation of Co 3d electrons and thus low Co moments. It is also supported by the fact that the hallmark peak of localized B 2p-Co 3d hybridisation at ∼ 8 eV in soft magnetic CoSiB is not observed in the present FCC Co81.8Si9.1B9.1 phase. [21] Such delocalization yielding significantly reduced magnetic moments was also observed in FeBY glassy alloys. [22] Obviously, the compositions of FCC CoSiB phase are tuneable to achieve a balance between better magnetic performance and structural stability. show that the hard magnetic behaviours can be further improved without destruction of the structure, although a balance between structural stability and magnetic performance has to be taken into account.

Experimental Section
Synthesis of FCC Co81.8Si9.1B9.1: the mixtures (10 g) of pure constituent elements Co chunks      . Electronic structures of the Co81.8Si9.1B9.1 phase(black), bulk HCP Co (red) and a typical soft magnetic CoSiB glassy alloy (Co63.1B27Si9.9). The black arrows mark a shoulder next to the peak electronic DOS, and the blue arrow indicates the shoulder induced by B-Co interactions.