Spontaneous magnetization in Ni-Al and Ni-Fe layered double hydroxides.

Layered double hydroxides containing paramagnetic Ni-II and diamagnetic/paramagnetic Al-III/Fe-III ions have been prepared and characterized. Ni2Al(OH)(6)(NO3) center dot nH(2)O (1), Ni2Fe(OH)(6)(NO3) center dot nH(2)O (2), Ni2Fe(OH)(6)(C6H8O4)(0.5) center dot nH(2)O (3), and Ni2Fe(OH)(6)(C10H16O4)(0.5) center dot nH(2)O (4) were prepared by coprecipitation at controlled pH as polycrystalline materials with the typical brucite-like structure, with alternating layers of hydroxide and the corresponding anions, which determine the interlayer separation. Magnetic studies show the appearance of spontaneous magnetization between 2 and 15 K for these compounds. Interestingly, the onset temperature for spontaneous magnetization follows a direct relationship with interlayer separation, since this is the only magnetic difference between compounds 2, 3, and 4. Magnetic and calorimetric data indicate that long-range magnetic ordering is not occurring in any of these materials, but rather a freezing of the magnetic system in 3D due to the magnetic disorder and competing intra- and interlayer interactions. Thus, these hydrotalcite-like magnetic materials can be regarded as spin glasses.