Observation of Oxidation and Mechanical Strain in Cr Nanoparticles Produced by Ball-Milling

Abstract

Nanoparticles of Cr, produced by ball-milling a commercial chromium powder for up to 70 h in Ar-filled glove box, are characterized by room temperature X-ray diffraction (XRD) and magnetization (M) studies as a function of temperature from 5 to 300 K. Analysis of the XRD linewidths in terms of particle size (L) and mechanical strain (η) shows that with increase in milling-time, L decreases and η increases. However, for milling times ≥40 h, saturation in L (at 20 nm) and η is reached and presence of Cr2O3 becomes evident in the XRD patterns. In the magnetic studies, M increases as L decreases, also reaching saturation values for milling time ≥40 h. The variations of M versus T are indicative of nanoparticles effects such as blocking. However, it is evident that the effect of Cr2O3 is present even for samples for which XRD patterns do not show the presence of Cr2O3 (samples milled for less than 40 h). The question of the observed large magnitudes of M for Cr nanoparticles vis-à-vis bulk Cr in terms of undetected surface oxides and uncompensated surface moments is discussed.