Anomalous nano-magnetic effects in non-collinear spinel chromite NiCr2O4

We present the results of a detailed investigation of magnetism in spinel chromite NiCr2O4 magnetic nanoparticles (MNPs). Compared to the bulk NiCr2O4, the finite crystallite size of about 10 nm lowers the Jahn-Teller distortion and greatly enhances the collinearity of the spin structure in MNPs with considerably reduced “frustration index” = |θCW|/Tc. This leads to (longitudinal) ferrimagnetic ordering at much higher temperature, Tc≈100 K and suppression of (transverse) antiferromagnetic ordering in MNPs (cf. Tc ≃ 65 K and Ts ≃ 30 K in the bulk); a transition to the cluster spin glass state occurs at Tg = 19.0 K. Moreover, the M-H hysteresis loops show anomalous “hour-glass” behavior at fields near Hc in the vicinity of Tg; this non-monotonous Hc (T) variation can not be accounted from the celebrated Stoner-Wohlfarth model. The present study interprets the anomalous Hc (T) behavior in the framework of magnetically interacting core-shell structure with large surface anisotropy, and points out the importance of surface effects in nanochromites compared to their counterpart ferrites. © 2018 Elsevier B.V.