Development of a novel one-pot synthetic method for the preparation of (Mn<inf>0.2</inf>Ni<inf>0.4</inf>Zn<inf>0.4</inf>Fe<inf>2</inf>O<inf>4</inf>)<inf>x</inf>-(BaFe<inf>12</inf>O<inf>19</inf>)<inf>1-x</inf> nanocomposites and the study of their microwave absorption and magnetic properties