Controlled Synthesis of MnP Nanorods:  Effect of Shape Anisotropy on Magnetization

Manganese phosphide (MnP) nanorods have been prepared by high-temperature injection of Mn2(CO)10 dissolved in octadecene into a hot (350 °C) surfactant mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP). Uniform short rods (20.3 ± 3.6 nm × 5.2 ± 0.89 nm), grown along the b axis of orthorhombic MnP, were produced after 1 h of heating. The rods were ferromagnetic, with a coercive field of 4200 Oe at 10 K and a blocking temperature of 250.4 K at 500 Oe, and exhibited only weak interparticle interactions. Samples with longer rods having an aspect ratio of over 30 can be prepared by using longer heating times or multiple injections. The evolution of cubes and thicker rods was also observed with extended heating regimens. Surprisingly, the introduction of shape anisotropy into nanoparticles of MnP has little or no influence on the magnetic properties, suggesting that the inherent magnetocrystalline anisotropy is the dominating influence.