A study of crossover from 3D ferrimagnetic bulk NiCr₂O₄ compound into 2D spin-glass-like nanophase

In this report, the magnetic behaviour of bulk and nanoparticles of NiCr₂O₄ under different applied magnetic fields has been investigated extensively. Nanoparticles of NiCr₂O₄ (12 nm) were obtained by mechanical milling of polycrystalline powder prepared by polyol method. FC–ZFC measurement of bulk at different applied magnetic fields has revealed the existence of a ferrimagnetic transition around 66 K followed by an antiferromagnetic transition close to 30 K. However, its nano counterpart has shown remarkable change in magnetic properties—a suppression of ferrimagnetic transition accompanied by strengthening low-temperature magnetic phase and observation of a new transition at 90 K (T _P), which is weakly magnetic in nature. The frequency-dependent ac susceptibility data of nanoparticles have been fitted to the well-known de Almeida–Thouless equation and a H^2/3 dependence of the low-temperature peak is observed with a resulting zero-field freezing temperature (T _f ⁰ ) equal to 10.1 K. Further, the dynamical behaviour near freezing temperature has been analysed in terms of critical behaviour and the obtained fitted parameter values were found to be τ ₀ (relaxation time constant) = 3.6 × 10⁻⁶ s, T _f ⁰  = 8.7 K and zν = 11.1. Moreover, Vogel–Fulcher law has been used to understand the nature of freezing transition, and the parameters after fitting are obtained as E _a/k _B = 58.9 K, τ ₀ = 5.22 × 10⁻⁸ s and T ₀ = 8.03 K. Finally, the spin-glass phase is concluded. Moreover, in contrast to bulk, the H^2/3 dependence of freezing temperature of nanoparticles sample (75 h) does support the 2D surface-like spin-glass nature.