Combined effect of 2D nature and structural glass behaviour and true magnetic contribution to the low-temperature heat capacity of layered double hydroxides

Abstract

Heat capacity of cobalt-aluminium and magnesium-aluminium layered double hydroxides (LDH) with the divalent-to-trivalent cations ratio (n) of 2 and 3, and intercalated with NO₃⁻, CO₃²⁻ and OH⁻, was measured as a function of temperature between 2 and 100 K. Magnetic contribution to the heat capacity (C_mag) of Co_nAl LDH was extracted from comparison of the C(T) dependences with the respective reduced dependences of their non-magnetic counterparts, Mg_nAl LDH. At the lowest measured temperatures, the magnetic contribution to the heat capacity (C_mag) of Co-containing LDH was found to be almost two orders of magnitude bigger than their lattice contribution. In a zero magnetic field, the low-temperature C_mag of Co₂Al LDH was measured to be proportional to T^2.6, while for Co₃Al LDH, C_mag ∼ T² was observed. An application of an external magnetic field was revealed to converge these dependences to C_mag ∼ T³ in compounds with both n = 2 and 3. However, the conversion in Co₂Al LDH occurred at lower values of external magnetic field (50 kOe) than that in Co₃Al LDH (90 kOe).

It was revealed from the C(T) behaviour of the non-magnetic LDH at the lowest measured temperatures that they possess the features typical for both a 2D solid and a structural glass. The reduced C(T)/T³ heat capacity of Mg_nAl LDH intercalated with nitrate was found to demonstrate a so-called boson peak, which is a manifestation of a glassy state. The observed low-temperature structural glassy state in LDH was associated with the disorder of the intercalated species.