Tailoring the Exchange Bias in FeNi/FeMn Bilayers by Heat Treatment and FeMn Surface Oxidation

The FeNi(30 nm)/FeMn(20 nm) bilayers were deposited by magnetron sputtering at room temperature onto glass substrates in a magnetic field. Afterward, they were subjected to heat treatments at temperatures from 50 °C to 400 °C in vacuum or in a gas mixture containing oxygen. The as-deposited FeNi/FeMn bilayers presented an exchange bias field H_ex = 30 Oe, a coercivity H_c ≈ 3 Oe, and a blocking temperature T_b ≈ 140 °C. An increase of the annealing temperature results in a monotonous decrease of Hex for temperatures up to 200 °C. For higher annealing temperatures, Hex remains almost constant. Hc sharply increases in the range from 200 °C to 300 °C, and finally decreases dramatically upon annealing>300 °C. Preferential surface oxidation of Mn during heat treatment in a gas mixture results in the formation of the ferromagnetic α-Fe(Mn) phase and leads to the change of the magnetic moment of the samples. The adjustment of temperature and annealing time in a gas mixture allows us to increase the switching field value of exchange biased FeNi/FeMn bilayers.