Abstract
We have investigated the structural and magnetic properties of ion-beam deposited polycrystalline NiFe (25 nm)/Fe-oxide (35 nm) bilayers. A film prepared with an assist beam O 2 to Ar gas ratio of 0% during deposition had a bottom layer that consisted of pure b.c.c. Fe (a = 2.87 Å) whereas films prepared with 19%O 2 /Ar and 35%O 2 /Ar had either Fe 3 O 4 (a = 8.47 Å) or α-Fe 2 O 3 (a = 5.04 Å, c = 13.86 Å) bottom layers, respectively. Cross-sectional transmission electron microscopy revealed a smooth interface between the top nano-columnar NiFe and bottom nano-columnar Fe-oxide layer for all films. At room temperature, the observed coercivity (H c ∼ 25 Oe) for a film prepared with 19%O 2 /Ar indicates the existence of a magnetically hard ferrimagnetic Fe 3 O 4 phase that is enhancing the plain NiFe (H c ∼ 2 Oe) by way of exchange coupling. A significant amount of exchange bias is observed below 50 K, and at 10 K the size of exchange bias hysteresis loops shift increases with increasing oxygen in the films. Furthermore, the strongest exchange coupling (H ex ∼ 135 Oe at 10 K) is with ş-Fe 2 O 3 (35%O 2 /Ar) as the bottom film layer. This indicates that the pure antiferro-magnetic phases work better than ferrimagnetic phases when in contact with ferromagnetic NiFe. H ex (T) is well described by an effective AF domain wall energy that creates an exchange field with a (1 - T/T crit ) temperature dependence. H c (T) exhibits three distinct regimes of constant temperature that may indicate the existence of different AF spin populations that couple to the FM layer at different temperatures. Copyright © 2007 American Scientific Publishers All rights reserved.