Strain-stabilized surface defects produce room temperature ferromagnetism in Mn₄Si₇nanostructures

Bulk Mn₄Si₇is a metallic ferromagnet with a weak magnetic momentµ= 0.012µ_B/Mn below its Curie temperatureT_C∼40 K. Here we report results from magnetic and electron spin resonance (ESR) studies in two samples of nanoparticles (NPs) of Mn₄Si₇prepared by high-energy ball milling for 40 h (B40) and 50 h (B50). Williamson-Hall analysis of the linewidths of various x-ray diffraction (XRD) lines of the two samples yielded average particle diameter <D>= 123 nm with strainη= 1.45 × 10^-2for B40 and <D>= 43 nm withη= 1.22 × 10^-2for B50. Using transmission electron microscopy, a near symmetrical log-normal distribution of particle sizes was found for B40 with <D>= 95 nm whereas for B50, <D>= 10.7 nm was found with the distribution highly skewed towards larger sizes. The larger 〈D〉 from XRD is explained on the basis of this size distribution. Both samples show hysteresis loops in the magnetizationMvsHdata at 340 K as well as at 3 K with coercivityH_C∼ 1 kOe (0.5 kOe) for B50 (B40) at 3 K. TheMvsTvariations for the field-cooled (FC) and zero-field-cooled (ZFC) cases withH= 500 Oe along with that of ΔM=M(FC)-M(ZFC) shows blocking temperatureT_B∼ 30 K with ΔMbeing positive at 300 K for both samples signifyingT_C> 300 K. ESR spectroscopy at 300 K yielded spectra characteristic of Mn²⁺ions with zero field splitting and effective spinS= 5/2 andg∼ 2.0. It is argued that the observed room temperature ferromagnetism is due to these strain-stabilized Mn²⁺ions present on the surface of the NPs in the reduced coordination environment of atoms on the surface of NP. The enhanced magnetization and ESR in the smaller B50 NP vis-a-vis larger B40 NP supports this conclusion.