A study of crossover from 3D ferrimagnetic Bulk $NiC{r}_2{O}_4$ compound into 2D spin-glass like nanophase

Abstract

In this report, the magnetic behaviour of $NiC{r}_2{O}_4$ bulk and nanoparticle samples under different applied magnetic field has been investigated extensively. Nanoparticles of $NiC{r}_2{O}_4$ were obtained by mechanical milling of polycrystalline powder prepared by polyol method. FC-ZFC measurement of bulk at different applied magnetic field has revealed the existence of a ferrimagnetic transition around 66K followed by an antiferromagnetic transition close to 30K. 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 90K ($T_P$), which is weakly magnetic in nature. The frequency dependent ac susceptibility data of nanoparticle have been fitted to the well known de Almedia-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 10.1K. Further, the dynamical behaviour near freezing temperature has been analysed in terms of critical behaviour and the obtained fitted parameters values being as ${\tau }_0$(relaxation time constant) = $3.6X10^{-6}s$, $T_f^0=8.7$K and $z\nu =11.1$. Moreover, Vogel-Fulcher law has been used to understand the nature of freezing transition and the parameter after fitting are obtained as $E_a/k_B=58.9$K, ${\tau }_0=5.22\times 10^{-8}$ 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 nanoparticle sample (75h) does support the 2D surface like spin glass nature.

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