Shock compression response of ferromagnetic FePt nanoparticles

The shock-compression response of ferromagnetic FePt nanoparticles has been studied with the objective of making bulk nanocrystalline permanent magnets. Chemically synthesized FePt nanoparticles (10 nm size) with a partially ordered fct structure, produced after controlled thermal annealing at low temperature, were pressed to $\sim $45{\%} packing density and shock consolidated using a three-capsule plate-impact gas-gun fixture. The recovered disk-shaped magnets were densified to a density of $\sim $90{\%} higher than the initial packing density via intensive plastic deformation of the nano-sized particles, as revealed by high resolution transmission electron microscopy. Shock compression of the nanoparticles also induced an order-to-disorder phase transition from fct to fcc structure. The disordering transition resulted in decrease of magnetic properties; however, upon subsequent annealing, the ordered structure was completely recovered and the 10 nm grain size was still fully retained. The samples showed properties with characteristics of hard magnets - energy product (\textit{BH})$_{max }$up to 14 MGOe and coercivity up to 14.6 kOe, which are higher than those of un-shocked samples. This work has been supported by US DoD/DARPA through ARO under grant DAAD-19-01-1-0546.