Strain-stabilized surface defects produce room temperature ferromagnetism in Mn 4 Si 7 nanostructures

Abstract Bulk Mn 4 Si 7 is a metallic ferromagnet with a weak magnetic moment µ = 0.012 µ B /Mn below its Curie temperature T C ∼40 K. Here we report results from magnetic and electron spin resonance (ESR) studies in two samples of nanoparticles (NPs) of Mn 4 Si 7 prepared 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 strain η = 1.45 × 10 −2 for B40 and < D> = 43 nm with η = 1.22 × 10 −2 for 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 magnetization M vs H data at 340 K as well as at 3 K with coercivity H C ∼ 1 kOe (0.5 kOe) for B50 (B40) at 3 K. The M vs T variations for the field-cooled (FC) and zero-field-cooled (ZFC) cases with H = 500 Oe along with that of Δ M = M (FC)– M (ZFC) shows blocking temperature T B ∼ 30 K with Δ M being positive at 300 K for both samples signifying T C > 300 K. ESR spectroscopy at 300 K yielded spectra characteristic of Mn 2+ ions with zero field splitting and effective spin S = 5/2 and g ∼ 2.0. It is argued that the observed room temperature ferromagnetism is due to these strain-stabilized Mn 2+ 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.