Nitridation of Cr–urea complex into nanocrystalline CrN and its antiferromagnetic magnetostructural transition study

Abstract

An antiferromagnetic nanocrystalline CrN interstitial compound was prepared using Cr(NO₃)₃·9H₂O and urea as starting materials. Initially, a Cr–urea complex was obtained from the solid-state reactions between starting chemicals. Then, the resulting complex was thermally heated to partially decompose into a compound containing Cr–N/Cr–O–N bonds, followed by a nitridation process under the flow of NH₃ gas that replaced all the oxygen with nitrogen from the precursor and produced CrN nanoparticles. The powder X-ray diffraction (XRD) pattern of the nitridated product confirmed the phase-pure formation of cubic rock-salt CrN with the space group Fmm. Electron microscopy studies revealed that the CrN nanoparticles agglomerated and the average mean diameter of the nanoparticles was calculated to be 22.92 nm. The high-resolution X-ray photoelectron spectroscopy (XPS) spectra of Cr (2p) and N (1s) confirmed the existence of the (+III) oxidation state of Cr in the sample. The antiferromagnetic characteristics of the prepared agglomerated CrN nanoparticles are discussed in comparison to the literature data available of the bulk counterparts. The zero-field cooled (ZFC) and field cooled (FC) temperature-dependent magnetization studies showed that the CrN nanoparticles experience an antiferromagnetic transition at a Néel temperature of 265.44 K. At this temperature, the CrN nanocrystals undergo a magnetostructural transition, viz. a paramagnetic cubic lattice transforms into an antiferromagnetic orthorhombic lattice, which can be ascribed to magnetic stress arising during antiferromagnetic-ordering.