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” = |θ_CW|/T_c. This leads to (longitudinal) ferrimagnetic ordering at much higher temperature, T_c≈100 K and suppression of (transverse) antiferromagnetic ordering in MNPs (cf. T_c ≃ 65 K and T_s ≃ 30 K in the bulk); a transition to the cluster spin glass state occurs at T_g = 19.0 K. Moreover, the M-H hysteresis loops show anomalous “hour-glass” behavior at fields near H_c in the vicinity of T_g; this non-monotonous H_c (T) variation can not be accounted from the celebrated Stoner-Wohlfarth model. The present study interprets the anomalous H_c (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.