Epitaxial growth and exchange coupling of spinel ferrimagnet Ni0.3Zn0.7Fe2O4 on multiferroic BiFeO3

Thin films of the zinc nickel ferrite, Zn0.7Ni0.3Fe2O4 (ZNFO), were deposited by the RF magnetron sputtering on a number of substrates, including (001) oriented single crystals of LaAlO3 (LAO) and SrTiO3 (STO), polycrystalline Pt/Si, and epitaxial films of BiFeO3 (BFO) and LaNiO3 (LNO). Except for the films on Pt/Si, the ZNFO films grown on other substrates were epitaxial and their magnetic properties were affected by the heteroepitaxy induced strains. Typically, the coercivity (Hc) was increased with the strain, i.e. Hc varied from 31 Oe for the 150 nm thick polycrystalline films grown on Pt/Si, to 55 Oe and 155 Oe for the 20 nm thick epitaxial films grown on BFO and LAO, respectively. The saturation magnetization of the epitaxial films was reduced accordingly to about 470 emu/cm3 from 986 emu/cm3 in the polycrystalline films. The all-oxide architecture allowed field-annealing to perform at the temperature above the Neel temperature of BFO (~ 370 °C), after which clear exchange bias was observed.