Size-driven enhancement of coercivity in tetragonal FeCo nanocrystals

We report large and tunable coercivity of body-centered tetragonal (bct) Fe 76 Co 24 nanocrystals grown on β-Fe 5.33 Co 1.67 Se 8 via topochemical deselenization. By controlling the reaction temperature and trioctylphosphine content, the mean grain size ( D ) and loading of the bct-Fe 76 Co 24 nanocrystals can be precisely regulated. This size control leads to dramatic variations in magnetic properties. At room temperature, the coercivity increases from 120 Oe to 600 Oe as D grows from 5.16 nm to 5.36 nm. The bct-Fe 76 Co 24 with D < ∼4.51 nm becomes superparamagnetic at 300 K. At 2 K, they exhibit a large coercivity, reaching 4230 Oe for 3.65 nm bct-Fe 76 Co 24 nanocrystals with the blocking temperature T B (≈ 160 K). In contrast, the nanocrystals with D = 5.36 nm shows a much lower coercivity of 760 Oe at 2 K, highlighting a strong size-dependent effect. Magnetic alignment of a bulk composed of 5.36 nm Fe 76 Co 24 at 300 K results in a coercivity of 360 Oe, which is slightly lower than that in randomly oriented state. Analysis of the initial magnetization curves reveals that magnetization reversal in the 3.65 nm nanocrystals is dominated by domain nucleation, whereas in the 5.36 nm particles, it involves a combination of domain nucleation and domain-wall pinning.