Magnetism of directly ordered Sm-Co clusters

Abstract

Sm-Co bulk alloys have shown superior permanent-magnet properties, but research on Sm-Co nanoparticles is challenging because of the need to control particle size, size-distribution, crystalline ordering, and phase purity. In the present study, a cluster-deposition method was used to produce Sm-Co nanoparticles having desired crystal structures without the requirement of subsequent high-temperature thermal annealing. Poorly crystallized SmCo5 nanoparticles exhibit a low room-temperature coercivity of only 100 Oe, whereas crystalline SmCo5 and Sm2Co17 nanoparticles show room-temperature coercivities of 2000 and 750 Oe, respectively. The direct synthesis of Sm-Co nanoparticles having sizes of less than 10 nm and a high degree of atomic ordering is an important step toward creating nanoparticle building blocks for permanent-magnets and other significant applications.

Prospects for nanoparticle-based permanent magnets

“…Compared with chemical methods, the desired crystalline ordering in the case of rare-earth alloy [18,27] and FePt nanoparticles [30] can be obtained directly without subsequent heat treatment using high sputtering powers (≤ 120 W) or modifying the plasma conditions as revealed in the high-resolution TEM (HRTEM) image of the directly ordered YCo 5 nanoparticles (bottom inset of Figure 2). Assembly of cluster-deposited nanoparticles, for example FePt, has been achieved by depositing them onto Si substrates, which are pre-coated with multilayers of amphiphilic phospholipid molecules [33,34].…”

“…Cluster-deposition methods performed under high-vacuum conditions usually prevent the surface oxidation of rare-earth elements. These particles can exhibit appreciable H c values (2-8 kOe) at room temperature and a high J s (~10 kG), comparable with their bulk values, as shown in the magnetization (M) vs applied field (H) loops (Figure 3a) [18,27,41]. Similarly, SmCo 5 and Nd 2 Fe 14 B nanoparticles of diameter ~10 nm produced by surfactantassisted ball milling exhibit high H c at room temperature in the range ~2.0-18.6 kOe and 1.2-4.0 kOe, respectively [37,38].…”

“…For example, the sputtering of atoms from the solid surface of a desired composition (such as YCo 5 or FePt) in a cooled inert-gas atmosphere (mixture of Ar and He)>10 −2 torr leads to successive collisions of sputtered atoms with the inert gas ions, and thus results in the formation of nanoparticles in the gas phase prior to deposition on substrates kept at room temperature [18,27,30,32]. This method produces monodisperse nanoparticles of size d ≤ 10 nm with an rms standard deviation σ/d ≈ 0.15, as shown in the TEM image of YCo 5 nanoparticles ( Figure 2) and corresponding particle size histogram (top inset of Figure 2) [18].…”

“…The cluster-deposition method using physical vapor deposition processes such as sputtering, thermal evaporation or laser ablation is based on the inert-gas-condensation principle and has been successful in producing both FePt and rare-earth alloy nanoparticles of smaller sizes (2-15 nm) with uniform size distribution and a high degree of atomic ordering [18,[26][27][28][29][30][31][32]. For example, the sputtering of atoms from the solid surface of a desired composition (such as YCo 5 or FePt) in a cooled inert-gas atmosphere (mixture of Ar and He)>10 −2 torr leads to successive collisions of sputtered atoms with the inert gas ions, and thus results in the formation of nanoparticles in the gas phase prior to deposition on substrates kept at room temperature [18,27,30,32].…”

Assembly of uniaxially aligned rare-earth-free nanomagnets

“…11 Second, HfCo 7 nanoparticles exhibit H c of 4.6 kOe at 10 K and 2.9 kOe at 300 K and J s of about 10.9 kG, which are comparable with those of reported rare-earth alloy nanoparticles. 7,[25][26][27][28][29] In addition, the room-temperature coercivity of HfCo 7 nanoparticles (2.9 kOe) is higher than that of bulk HfCo 7 alloys (1.8 kOe). In contrast, Co nanoparticles and bulk metal exhibit H c less than 50 Oe and a low K 1 of about 5 Mergs/cm 3 (not shown here).…”