Peningkatan sifat magnetik magnet permanen nanokomposit (Ba1-xLax) Fe12O19/Co0, 6Zn0, 4Fe2O4 melalui sintesa pemaduan mekanik dan destruksi ultrasonik = Magnetic properties enhancement in nanocomposite (Ba1-xLax) Fe12O19/Co0, 6Zn0, 4Fe2O4 permanent magnets by mechanical alloying and ultrasonic irradiation