Exchange biasing and thermal stability of CoFe/PtPdMn films

Two sets of PtPdMn exchange biased films Ta 50 A/CoFe 100 A/PtPdMn (tAF) A/Ta 50 A, with PtPdMn thickness, tAF=350, 600 A, were deposited on Si substrates by dc magnetron sputtering techniques. After magnetic annealing, these two sets of films exhibited values of exchange bias field, Hex=229 and 254 Oe, respectively. The PtPdMn layer was then thinned to various thicknesses from 600 down to 50 A by ion beam etching. Hex does not retain its original value. It decreases with decreasing tAF and becomes zero at tAF∼75 A. In addition, we have observed that the training effect or the anomalous hysteresis loss becomes more pronounced with decreasing tAF. This confirms that not only face-centered-tetragonal phase but, more critically, tAF plays role in determining exchange biasing and its thermal stability. The blocking temperature, TB, appears unaffected by the thinning of the PtPdMn layer, and no apparent change occurs in the local blocking temperature distribution, as suggested by the finite size effect.