Tuning the exchange bias in NiFe/Fe-oxide bilayers by way of different Fe-oxide based mixtures made with an ion-beam deposition technique

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 At gas ratio of 0% during deposition had a bottom layer that consisted of pure b.c.c.. Fe (a = 2.87 angstrom) whereas films prepared with 19%O-2/Ar and 35%O-2/Ar had either Fe3O4(a = 8.47 angstrom) or alpha-Fe2O3 (a = 5.04 angstrom, c = 13.86 angstrom) 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 similar to 25 Oe) for a film prepared with 19%O-2/Ar indicates the existence of a magnetically hard ferrimagnetic Fe3O4 phase that is enhancing the plain NiFe (H-c similar to 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 similar to 135 Oe at 10 K) is with alpha-Fe2O3 (35%O-2/Ar) as the bottom film layer. This indicates that the pure antiferromagnetic 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.