Extraordinary Hall Effect for plasma and natural oxidation monitoring

 

Principle

We propose here a new technique for monitoring the oxidation of thin Al-Ox barriers used for example in the fabrication of magnetic tunnel junctions. This technique is based on the variation of the perpendicular magnetic anisotropy as a function of oxidation in Pt/Co/AlOx trilayers. It is very sensitive, reproducible and does not require any microfabrication process.

The magnetic anisotropy is measured by Extraordinary Hall Effect (the Hall voltage proportional to the z component of the magnetisation). It allows to measure magnetic layers as thin as 0.5 nm. The technique is based on the difference of magnetic anisotropy for Co/Al and Co/Al2O3 interfaces : While the former exhibits an in-plane anisotorpy, the latter leads to an out-of plane anisotropy (perpendicular magnetization).

Plasma oxidation of Pt 3nm/Co 0.6nm/Al 1.5nm trilayers

Figure 1 shows the evolution of the Hall signal as a function of oxidation time. The magnetisation is in-plane for short oxidation times (1’), goes perpendicular for about 2’ and back in-plane for longer oxidation times.

This very strong variation of the initial EHE slope is linked to the progress of the oxidation front through the Al layer. The initial Co/Al interface transforms into Co/AlOx and then into Co/CoOx/AlOx (cf. figure 2).

The optimum oxidation time thus corresponds to the maximum of the EHE slope.

Natural oxidation of very thin Al barriers

Samples of the form Pt 3nm/Co 0.6nm/Al xnm (x between 0.3 and 1.1nm) are oxidised for 30 minutes under oxygen pressure in the sputtering chamber.

The Hall effect measurements allow to determine the optimal thickness of the Al layer (full oxidation of the Al layer without oxidation of the Co layer).

In this case also (figure 3), one observes a very sharp maximum of the EHE slope (FWHM= 0.04 nm) for an Al thickness of 0.8 nm.

In the meantime, the magnetic component of the Hall amplitude (figure 4) progressively decreases to zero when the Al thickness decreases from 0.8 to 0.4 nm. This corresponds to the progressive oxidation of the Co layer.

Team

-  Bernard Rodmacq
-  Stéphane Auffret
-  Gérard Casali
-  Bernard Dieny

Publications

S. Monso et al., Crossover from in-plane to perpendicular anisotropy in Pt/CoFe/AlOx sandwiches, Applied Physics Letters, 80, 4157 (2002)


Fig. 1

Fig. 2

Fig. 3

Fig. 4