Structural stability of HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ magnetic nanoclusters*

The gas-aggregation-type cluster deposition has emerged as an attractive method to create uniaxially aligned nanoparticle building-blocks of metastable and new permanent-magnet materials such as HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ with appreciable coercivities ($H_{c} \approx $ 5.0 kOe), magnetocrystalline anisotropies ($K_{1} \approx $ 10 Mergs/cm$^{3})$, and magnetic polarization ($J_{s} \approx $ 10 kG) at 300 K. In comparison, bulk HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ alloys form only at ideal stoichiometries and high temperatures above 1000 $^{\circ}$C at thermal equiliburium conditions. We have investigated the structural stability of HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ phases on varying their stoichiometries from ideal values in HfCo$_{7\pm \delta }$ and Zr$_{2}$Co$_{11\pm \delta}$ nanoclusters (0$\le \delta \le $1), respectively and compared these results with the corresponding bulk phase diagrams. This study provides new insights to understand the underlying crystal structure and magnetic properties of the nanoclusters and to explore them for significant applications.