A new class of organometallic-inorganic magnetic material was engineered by a sonochemically assisted self-assembly process between magnetite nanoparticles (biogenic Fe3O4, hard constituent) functionalized with isonicotinic acid and a metamagnetic organometallic complex ([Ni(en)2]3[Fe(CN)6]2·3H2O, soft constituent). In such bottom-up methodology, hard and soft counterparts form well-organized microdimensional clusters that showed morphological fingerprints and magnetic behavior clearly distinct from those of the initial building units. In the engineered soft-hard material, the magnetite nanocrystals induced ferromagnetic ordering at room temperature of closer contact layers of [Ni(en)2]3[Fe(CN)6]2·3H2O, thus demonstrating the ability to sensibly modify the [Ni(en)2]3[Fe(CN)6]2·3H2O paramagnetic regime. The magnetic ordering of [Ni(en)2]3[Fe(CN)6]2·3H2O was triggered by the intrinsic local field of the hard magnetic nanocrystals, which resembled, to some extent, the effects promoted by large, external magnetic fields.