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 O2 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%O2/Ar and 35%O2/Ar had either Fe3O4(a = 8.47 Å) or α-Fe2O3 (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 (Hc ∼ 25 Oe) for a film prepared with 19%O2/Ar indicates the existence of a magnetically hard ferrimagnetic Fe3O4 phase that is enhancing the plain NiFe (Hc ∼ 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 (Hex ∼ 135 Oe at 10 K) is with α-Fe2O3 (35%O2/Ar) as the bottom film layer. This indicates that the pure antiferromagnetic phases work better than ferrimagnetic phases when in contact with ferromagnetic NiFe. Hex(T) is well described by an effective AF domain wall energy that creates an exchange field with a (1 − T/Tcrit) temperature dependence. Hc(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.