Magnetic dead layers (MDLs) can be formed at the interface of ferromagnetic (FM) and non-magnetic thin films as a result of interfacial diffusion typically during the fabrication process 1, causing degradation of magnetic layers’ properties, such as the magnetic moment and exchange coupling. For patterned films it also poses thermal stability problems due to a decrease in the magnetic anisotropy energy, leading to a reduction in thermal activation energies 3.

X-Ray reflectometry (XRR) has been used in conjunction with polarised neutron reflectometry (PNR) to investigate the formation of magnetic dead layers in Co70Fe30 thin films with Ta interfaces in addition to sample degradation over time. The rate at which the Ta diffuses into the Co70Fe30 layer differs between the top and bottom interfaces resulting in MDLs of different thicknesses, 3.04Å and 1.87Å respectively as indicated in figure 1(b). It is clear that the formation of magnetic dead layers has resulted in a loss of magnetisation in the thin Co70Fe30 layer.

This is of particular technological importance for spintronic devices in which the magnetic properties at the interfaces are crucial. Ta has been known to provide very good adhesion between the substrate and the soft FM layer. It is also widely used as a passivation layer to prevent oxidisation in air 4. Our results show that this is not the case, with Ta unsuitable as a seed due to the formation of MDLs and not viable as a capping layer due to oxygen passivation over time as shown by XRR in figure 2.
References:
1 T. L. Monchesky and J. Unguris, Phys. Rev. B 74, 24130 (2006).

2 K. Oguz et al., J. Appl. Phys. 103, 07B526 (2008).

3 M. Tokaç et al., AIP Advances 7, 115022 (2017).

4 H. S. Jung et al., J. Appl. Phys., Vol. 93, No. 10, (2003).

Fig. 1. (a) PNR measurements, raw data shown as points while the fitted simulated data is indicated by a solid line. (b) Depth profiles of nuclear and magnetic scattering length density (SLD) for the sample structure in the schematic above. MDLs have been introduced in the interfaces between the top and bottom interfaces of the CoFe layer.

Fig. 2. XRR curves for the sample structure denoted in the inset above. Sample A was measured as soon as deposited whilst sample B was measured 15 months later which has clearly degraded over time as seen on the fringes and as well as the differences in thickness of the sample.