Magnetic behaviour of assemblies of interacting cobalt-carbide nanoparticles

Abstract

Recent work [1] demonstrated high coercivity and magnetic moment in cobalt carbide nanoparticle assemblies and explained the high coercivity from first principles in terms of the high magnetocrystalline anisotropy of the cobalt carbide nanoparticles. In this work, we comprehensively model the interaction between the nanoparticles comprising the assembly and systematically understand the effect of particle size, distribution of the orientations of the nanoparticles' magnetocrystalline anisotropy axis with respect to the applied magnetic field, and dipole coupling between nanoparticles on the temperature dependent magnetic behavior of the nanoparticle assembly. We show that magnetocrystalline anisotropy alone is not enough to explain the large hysteresis over the 50K-400K temperature range and suggest that defects and inhomogeneties that pin the magnetization could also play a significant role on this temperature dependent magnetic behavior.Permanent magnets are used in an increasing number of applications and are typically alloys that have 4f elements (rare earth materials). These 4f elements lead to both high magnetic moment and high magnetocrystalline anisotropy that lead to properties desirable in permanent magnets: high remanence and high coercivity [2]. However, the mining process for rare earth materials is detrimental to the environment, their yield is low and their extraction cost is high [3]. This motivates research on rare earth free permanent magnets with high coercivity and high magnetic moment.

Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications

“…These Co x C nanorods showed an M s value of 38 emu/g and an H c value of 2.4 kOe. The nanowire dimensions could be better controlled by reductive decomposition of Co­(NO 3 ) 2 in a tetra­(ethylene glycol) solution of NaOH and poly­(vinylpyrrolidone) to further improve their magnetic properties. , Iron carbides are intermetallic compounds with carbon atoms occupying the interstitial sites surrounded by Fe atoms, offering some interesting magnetic properties for various applications. − However, the easy formation of interstitial C in the Fe structure also makes it challenging to prepare iron carbide NPs with structure, size, and composition controls. , Nickel carbides show interesting magnetic properties, but these properties are dependent on the formation of carbon defects in Ni-rich phases, which makes the synthesis even more challenging. , …”

“…The nanowire dimensions could be better controlled by reductive decomposition of Co(NO 3 ) 2 in a tetra(ethylene glycol) solution of NaOH and poly(vinylpyrrolidone) to further improve their magnetic properties. 146,149 intermetallic compounds with carbon atoms occupying the interstitial sites surrounded by Fe atoms, offering some interesting magnetic properties for various applications. 150−152 However, the easy formation of interstitial C in the Fe structure also makes it challenging to prepare iron carbide NPs with structure, size, and composition controls.…”