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Layered double hydroxides (LDH) belong to the family of anionic clays which are promising for such applications as water treatment, drug delivery and sensing. LDH are composed of the mixed metal layers where metal cations are surrounded by the edge-linked hydroxide octahedra [1]. The present study is focused on the low-temperature static and dynamic magnetic properties of the Ni$^{2+}_n$Fe$^{3+}$ LDH with the nickel-to-iron ratio ($n$) of $2$ and $3$ in magnetic fields up to $90$ kOe.
In all the Ni$_n$Fe LDH studied, the spontaneous magnetization appears below $17$ K which is accompanied by magnetization loops of a ferromagnetic type. Their peak of the temperature-dependent magnetic susceptibility in the vicinity of $17$ K correlates with the heat capacity anomaly in the same temperature range. The ac magnetic susceptibility peak is frequency-independent, which indicates that low-temperature magnetic behaviour of Ni$_n$Fe LDH may be associated with their long-range magnetic ordering. Upon cooling below about $5$ K, the magnetic behaviour of LDH with the different nickel-to-iron ratio is strongly different. All the Ni$_2$Fe LDH studied demonstrate a step like initial magnetization curve accompanied by the wasp-waisted magnetization hysteresis loop, while no such effect is seen for the LDH with $n = 3$. The effect found suggests that at least two magnetic phases coexist in Ni$_2$Fe LDH at low temperatures. The effect is most likely related to the cluster structure of layers for Ni$_2$Fe LDH which agrees with the experimentally found clustering in Ni-Fe LDH by using Mössbauer spectroscopy [2]. One possible scenario is as follows. Long range magnetic ordering below $17$ K occurs not in the entire layer, but in separate regions, clusters. Below about $5$ K temperature long range magnetic ordering occurs in the inter cluster space. Thus, at low temperatures two magnetic phases with different coercive field values are coexistent and negatively coupled. For a deeper exploration of this phenomenon, X-ray Absorption Spectroscopy (XAS) and Magnetic Circular Dichroism (XMCD) techniques have been applied. The respective studies are in progress.
