Enhancement of anisotropy energy of SmCo5 by ceasing the coupling at 2c sites in the crystal lattice with Cu substitution

SmCo5 and SmCo5−xCux magnetic particles were produced by co-precipitation followed by reduction diffusion. HRTEM confirmed the Cu substitution in the SmCo5 lattice. Non-magnetic Cu was substituted at “2c” site in the SmCo5 crystal lattice and effectively stopped the coupling in its surroundings. This decoupling effect decreased magnetic moment from SmCo5 (12.86 μB) to SmCo4Cu (10.58 μB) and SmCo3Cu2 (7.79 μB) and enhanced anisotropy energy from SmCo5 (10.87 Mega erg/cm3) to SmCo4Cu (14.05 Mega erg/cm3) and SmCo3Cu2 (14.78 Mega erg/cm3). Enhancement of the anisotropy energy increased the coercivity as its values for SmCo5, SmCo4Cu and SmCo3Cu2 were recorded as 4.5, 5.97 and 6.99 kOe respectively. Being six times cheaper as compared to Co, substituted Cu reduced the price of SmCo3Cu2 up to 2%. Extra 15% Co was added which not only enhanced the Mr value but also reduced the 5% of the total cost because of additional weight added to the SmCo3Cu2. Method reported in this work is most energy efficient method on the synthesis of Sm–Co–Cu ternary alloys until now.

. It was theoretically predicted that by substitution of Cu with Co, coercivity of SmCo 5 can be increased on the expense of M r and M s values 12 . Decrease in the M r is the serious disadvantage that reduces the energy density/ BH max despite of enhancement of the coercivity.
Second disadvantage associated with SmCo 5 magnets is their high cost. Sm is the most expensive element among its group and Co is also more expensive than Fe and these make the SmCo 5 most expensive class of the permanent magnets. Biggest reason behind high cost of SmCo 5 is the low quantity of the Sm and Co in the earth crust as shown in Fig. S-1-a. Cu is almost twice abundant (0.0068%) as compared to the Co (0.0003%) in the earth crust and this is one of the reasons that it is six times cheaper as compared to the Co (Fig. S-1-b). Another reason for the low price of Cu is the acquisition of Cu as the byproduct, during the extraction of Co from ores by pyro-metallurgy, hydro-metallurgy, and vapor-metallurgical methods 13 . Hence some ores those are primarily source of the Co (e.g. Carrollite) are also an important source of Cu.
Cu substitution for the Co can reduce the cost of most expensive magnet, SmCo 5 . But the price of SmCo 5 is critically determined by the Sm due to the high price of Sm (750 USD/kg) as compared to the Co (28 USD/kg). Although Co is six times more expensive as compared to the Cu but Co substitution with Cu (in SmCo 3 Cu 2 ) decreases the price of the magnet produced, only up to 2%. Another important factor that may reduce the price of the SmCo 3 Cu 2 is use of extra fused Co phase. This extra Co not only enhances the magnetic properties but also increases the overall weight of the produced SmCo 3 Cu 2 . SmCo 5 consists of almost 33% of Sm by weight, hence 15% addition of Co saved the 5% of the Sm. By considering both the factors (substitution of Co with Cu and addition of extra Co), overall price of SmCo 3 Cu 2 could be reduced up to 6%. Reduced price of the SmCo 5  18 . Annealing at such high temperature also leads to the loss of Sm which is evaporated and compensated with the addition of extra Sm. In this work reduction diffusion method is used for the synthesis of the magnetic particles which energy and time efficient as compared to the regular physical methods [19][20][21][22] .
This study suggests an eco-friendly chemical method for the synthesis of the Cu substituted SmCo 5 particles. In this energy efficient process, precursors were annealed at 900 °C for only 2 h. Cu substitution enhanced the coercivity, while reduction in the Mr value caused by Cu substitution is compensated by addition of extra Co. Cost of the SmCo 5 is reduced by substitution of Cu with Co. Extra added Co also increased the weight of SmCo 5−x Cu x magnet which further reduced its cost.

Experimental section
Materials. Samarium chloride (SmCl 3 . 6H 2 O, 99% purity), cobalt chloride (CoCl 2 . 62H 2 O), copper chloride (CuCl 2 . 2H 2 O, 99% purity), and potassium chloride (KCl, 99% purity) were obtained from Sigma Aldrich. 16 mesh granular Ca (99.5% purity) was obtained from alfa Aesar. These reagents were used without further purification. Milli-Q IQ 7000 water purifying system was used to obtain deionized water.  13, so that all of the metal chlorides could be changed to metal hydroxide. The reaction mixture was stirred continuously during the addition of NaOH solution. While maintaining the pH at 13, the solution was stirred for 1 h. Then, products were washed twice with DI water and ethanol, and dried at 80 °C for 1 h. XRD, TEM, and TEM-EDS analysis of SmCoCu hydroxides are provided as Fig S-2, S-3 and S-4 and S-5 in supporting information. The second step was the reduction-diffusion reaction. Hydroxides obtained from the first step were mixed with Ca and KCl in a glove box and then pressed into pellet form. Weight ratio of Ca, hydroxides and KCl was kept as 6:1:1. The pellet was reduced and diffused in a tube furnace by heating at 900 °C for 2 h, while Ar was flowing in the furnace. CaO produced during R-D can reduce the magnetic properties 23,24 . Hence, the product was washed with water again and again to remove CaO completely and washed twice with acetone before being stored in inert conditions. All steps of the experimental procedures are shown in Fig

Results and discussion
SmCo 5 and SmCo 5−x Cu x particles were produced by the experimental process described in the experimental section. XRD patterns for all SmCo 5 and SmCo 5−x Cu x particles produced are similar (Fig. 1a) because the crystal structure of these products is quite similar. Co and Cu have nearly similar atomic radii (125 pm and 128 pm respectively) but these variations are detectable. Hence, there is slight enhancement in the d-spacing in the crystal of SmCo 5−x Cu x because of substitution of Co with Cu. This enhanced d spacing also shifts the peak to the smaller value of theta (θ) in XRD pattern (Fig. 1b). Cobalt peak is also observed around 45° (Fig. 1a) in SmCo 5−x Cu x. SEM images in Fig. 1 reveal that the particles of all three products are in irregular shape and the size varies from 0.1 to 10 μm. Apparently it seems that Cu substitution enhances the particle size but exact mechanism behind this increased size is yet unknown. Maybe this due to faster reduction-diffusion in SmCo 5−x Cu x as compared to the SmCo 5. Hence SmCo 5−x Cu x particles are formed earlier (as compared to the SmCo 5 ) and stay for  Figure 2e is EDS line analysis of an isolated SmCo 4 Cu line. Line mapping confirms the homogeneous distribution of Cu throughout the particle. This also suggests the presence of Cu in the SmCo 4 Cu crystal, as elemental distribution is very regular. However, density of Sm, Co, and Cu is much higher in the center of the particle. This is because of higher thickness of the particle in the center. Small cobalt particles are attached on SmCo 3 Cu 2, can be detected in TEM-EDS images, Fig. 2f-i. Schematic illustration of SmCo 3 Cu 2 /Co interface is explained in Fig. 2j 25 .
There is possibility that Co particles detected in TEM-EDS might be left after the reduction diffusion process and now exist as the free detached, Co particles in SmCo 4 Cu, and SmCo 3 Cu 2 . Other possibility is that these Co particles were fused with SmCo 4 Cu, and SmCo 3 Cu 2 at high temperature during the reduction diffusion process, which was further proved by HRTEM analysis. Fused Co is identified at [100] facet in Fig. 3a. A portion of Fig. 3a, shown in red dotted box is zoomed in and described as Fig. 3b. Rhombus shown in Fig. 3d connects the four Sm atoms in the [001] zone axis. There are three Co atoms between these four Sm atoms but not visible in the TEM image because of their small size. Figure 3c is the modeling of SmCo 5 structure which is taken along "a/b" axis showing four Sm atoms connected by rhombus 25 . Figure 3d is the hexagonal cubic pack structure of the SmCo 5−x Cu x /SmCo 5 . These four Sm atoms are arranged in the "a/b" axis of the crystal lattice. HRTEM analysis confirms the presence of Co and SmCo 3 Cu 2 phases and their fusion in a single particle.   26 . Hence Cu reduces and diffuses much faster than Co and all Cu becomes the part of the SmCo 5−x Cu x crystal lattice (Fig. 3e). Left over Co is found as fused Co phase, detected in XRD, HRTEM and EDS mapping. Sm and Co are distributed in the two layers of hcp crystal SmCo 5 , with the similar atomic distribution found in CaCu 5 type crystals 27 . These layers are "3g" and "2c". Co always occupies 3g layer. "2c" layer is shared among Co and Sm, where Sm are on the edges of the SmCo 5 crystal and Co is located between the Sm atoms. Cu substitutes the Co in "2c" layer ( Fig. 3e) and enhances the magneto-crystalline anisotropy 12 . Instability of Cu substitution at 3g sites is also confirmed by previous studies [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] . Cu substitution has been clearly explained (Fig. 4) with top and side view of arrangement of the atoms in SmCo 5 and SmCo 5−x Cu x . Enhancement in the crystal parameters "a, b, c" and crystal volume was recorded after substitution. SmCo 5 and SmCo 5−x Cu x have hexagonal close pack crystal structure. Crystal parameters in the hcp were calculated with the help of the following equation as reported by the Sinha et al. 29 "d" and "hkl" values were determined from the XRD patterns. Peak with [200] facet was selected and "d" spacing value for the [200] facet was determined from the XRD patterns. By putting the value of "d" in the equation above, value of the "a" parameter was calculated.  facet was determined and were put in the equation above. With the already known value of "a" parameter, value of "c" parameter was determined. In the hcp crystal "a = b", hence no calculations for the "b" were performed. Substitution increased the unit cell volume by 3.9% (SmCo 4 Cu) and 5% (SmCo 3 Cu 2 ) as shown in the Fig. 4. Collectively, all the crystal parameters were increased after the substitution.
With electronic configuration of [Xe] 4f 6 6s 2 , Sm has 6 unpaired electrons in 4f orbital. Before coupling with Co, electrons in the "f " orbital of Sm hybridize and move to the 5d orbital. Hence actual electronic configuration of the Sm in SmCo 5 is [Xe] 5d 6 6s 2 and it has four unpaired electrons in the valance shell. However, after the hybridization, 5d electrons are in the spin down state 30 . Valance unpaired electrons in Co are in spin up configuration, hence Sm couples antiferro-magnetically with the Co (Fig. 5a). Co with electronic configuration of [Ar] 3d 7 4s 2 has three unpaired electrons in 3d orbital. Co has all the unpaired 3d electrons in the spin up configuration, which confirms that Co-Co exchange coupling is ferromagnetic (Fig. 5a).
Cu has electronic configuration of 3d 10 4s 1 therefore it does not have any unpaired electron in the valance shell. Hence Cu being non-magnetic, does not couple with Co or Sm either (Fig. 5b).
When one Co, Cu or Sm atom come contact with the other Co, Cu or Sm atoms, their valence electrons make a valence band. This valence band can be explained with rigid band model (Fig. 5c). Density or abundance of the electrons is taken on X-axis while energy of electrons is taken on the Y-axis. E f (fermi level) of the rigid band of the Cu is located in the "s" orbital which indicates that "3d" orbital is completely filled. It further tells that there is no inter-orbital movement of electrons in between 3d and 4s orbitals. All electrons in 3d orbitals are retained there unpaired therefore, being non-magnetic there is no chance for the Cu to couple with Sm or Co either. In case of Sm or Co, energy level of 3d electrons is similar to the 4s electrons, hence movement of electrons between the 4s and 3d orbital is possible. Fermi level of the Sm and Co is almost in the center of 3d band therefore, 3d band is partially filled (Fig. 4). Conclusively these band in Sm and Co are magnetic and can couple with each other.
Cu reduced magnetic moment more effectively when it substitutes Co at "2c" position. Substitution at "3g" site can eliminate the anisotropy energy, in consequence magnetic properties may vanish 12 . SmCo 4 Cu or SmCo 3 Cu 2 are anisotropic and easily magnetized from the a/b dimension of the crystal lattice (Fig. 6a). "2c" site substitution can decouple the ferromagnetic interaction between cobalt atoms of "2c" and "3g" layers and magnetic moment between these layers reduces drastically. Hence magnetization in the a/b direction (easy direction for magnetization) of the crystal becomes harder, that leads to the enhancement of coercivity and reduction of Mr value. It is concluded that in this work substitution occurred at the "2c" site as shown in the Fig. 6a.
Being non-magnetic after substitution, Cu strongly affects the magnetic properties of SmCo 5. Measured magnetic hysteresis curves for the SmCo 5−x Cu x and SmCo 5−x Cu x are shown in Fig. 6b. The magnetic moment of SmCo 5 , SmCo 4 Cu and SmCo 3 Cu 2 are to be 12.86, 10.58 and 7.79 μ B , respectively. In our work magnetic moment for SmCo 5 and SmCo 5−x Cu x was calculated from the M s values in the hysteresis loop. These values are lower than theoretically calculated values, because the theoretical conditions are ideal conditions (e.g. no thermal energy in the system and no oxidation of the product). The measured hysteresis curves for the SmCo 5 and SmCo 5−x Cu x are shown in Fig. 6b.
Reduction in magnetic moment critically affected on the anisotropy energy and coercivity. Coercivity (Hc) values of SmCo 5 , SmCo 4 Cu and SmCo 3 Cu 2 were recorded as 4.5, 5.97 and 6.99 kOe, respectively. The increasing  Energy density or energy product is intrinsic property and is the amount of energy stored in the lattice because of arrangement of the atoms in the crystal. It was found that Cu substitution enhanced the energy density. Energy densities for SmCo 5 , SmCo 4 Cu and SmCo 3 Cu 2 were recorded as 10.87, 14.05, and 14.78 M erg/cm 3 respectively. Furthermore, it is also evident from the hysteresis loop that squareness ratio was also increased after the Cu substitution. Complete hysteresis loop is given in the supplementary information as Fig. S-8. Possible effect of Cu substitution on the domain wall of SmCo 5−x Cu x is also explained in the supplementary information.
Role of the extra Co phase present in the SmCo 5−x Cu x is very important. Cu enhances the coercivity at the cost of magnetic moment. Formation of SmCo 3 Cu 2 after the substitution of Cu in SmCo 5 can reduce the M r value up to 35% 12 and 32% 31 . Reduced M r value will reduce the energy density drastically despite of enhancement of the coercivity. Purpose of keeping the extra Co phase was to maintain the M r value as well as the cost reduction of the product. Proposed mechanism of reduction of M r value by coupling between Co and SmCo 3 Cu 2 is explained in Fig. S-9.
Coercivity of the SmCo 5 particles produced by chemical method is quite low as compared to the SmCo 5 magnets. But the coercivity reported in our work is still relatively lower as compared to the coercivity reported in other recent chemical methods. One reason behind the lower value of the coercivity is the presence of small Sm 2 Co 17 phase (Fig. 1). Second main reason for the lower coercivity value is the irregular size and morphology of the magnetic particles. It is common observation, that when particle size of magnetic powder approaches the single domain size, magnetic properties are excellent. Similar conclusion was concluded by the Ma et al. 5 and they provided the magnetic properties of the mono-dispersed particles. Chuev et al. and Chen et al. explained the size and morphology dependence of magnetic properties (especially coercivity in detail) 32,33 . In our work, particles with the similar size (average size 300 nm) were also separated and they exhibited much better magnetic

Conclusion
SmCo5 and SmCo 5−x Cu x magnetic particles were synthesized by energy efficient, chemical method. Microstructure confirmed the Cu substitution in the SmCo 5 lattice and presence of Co phase fused together with SmCo 5−x Cu x . After substitution at "2c" site in the SmCo 5 crystal lattice, Cu almost blocked the coupling in the surrounding. The resulted decoupling in the crystal lattice affected the magnetic moment, anisotropy and coercivity. Magnetic moment was reduced as the result of Cu substitution, but coercivity and anisotropy energy were enhanced. The substitution of Co with Cu and extra Co phase decreased the price of SmCo 5−x Cu x , up to 5%.