Mrs. Yarden Nathan Yadgar - M.Sc. Candidate
14/06/2026
David Wang Auditorium, 3rd Floor, Dalia Maydan Bldg.
13:30
Fe-Co alloys exhibit a unique combination of magnetic and mechanical properties, yet their behavior at the scale of individual nanoparticles is largely unknown. In this work we fabricated FeCo nanoparticles using top-down approach and uncovered the physics that governs their strength at the nanoscale.
The nanoparticles are produced by solid-state dewetting of 30 nm Fe-Co bilayer thin films deposited on sapphire substrates. After annealing at 900°C, the films break up into well-faceted, single-crystalline particles of a homogeneous body centered cubic (BCC) phase with a (110) top facet — ideal specimens for nanomechanical testing. A central goal was to investigate how chemical ordering — the A2 (disordered BCC) to B2 (ordered) phase transformation — affects mechanical properties. Achieving the ordered phase, however, proved to be a story in itself: despite annealing at 500°C, the particles remained disordered. Kinetic experiments revealed atomic intermixing already at 380°C, with superlattice diffraction peaks appearing only after annealing at the temperature of 600°C. This behavior is explained by the high vacancy formation energy of 1.45 eV in FeCo, which suppresses the vacancy-mediated diffusion required for long-range ordering. Partially ordered FeCo nanoparticles were obtained by prolonged dewetting treatment of the bilayers at the temperature of 600°C.
Mechanical properties were measured by employing in-situ microcomression tests inside the SEM using a flat circular diamond indenter. Both ordered and disordered particles demonstrated the load-displacement curves typical for dislocation nucleation-controlled plasticity: elastic loading segment followed by abrupt displacement burst. The results reveal a striking size effect on the particle strength: the normalized critical resolved shear stress scales as τ_”CRSS” /G∝(D/b)^(-1.15), where G, D, and b are the shear modulus, top facet diameter, and Burgers vector, respectively. Especially noteworthy was a giant strength of the smallest studied B2 nanoparticles. These findings point to dislocation starvation and surface-controlled plasticity as the dominant deformation mechanisms in these nanoscale BCC and B2 nanoparticles.