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 = vertical bar theta(CW)vertical bar/T-c. This leads to (longitudinal) ferrimagnetic ordering at much higher temperature, T-c approximate to 100 K and suppression of (transverse) antiferromagnetic ordering in MNPs (cf. T-c similar or equal to 65 K and T-s similar or equal to 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.