Effect of Co and Ni additions as doping materials on the micro-structures and the magnetic properties of barium hexa-ferrites oleh Masno Ginting, Perdamean Sebayang, Martha Rianna, Marhaposan Situmorang, Heryani Fujiati, Anggito P. Tetuko, Eko Arief Setiadi, Candra Kurniawan, Achmad Maulana Soehada Sebayang