5 Spintronics and Synchrotron Radiation
149
anisotropy in Pt/Co/MgO [38]. This magnetic anisotropy modification with voltage
can originate from charge accumulation or oxygen electromigration.
C. Bi et al. have performed Hall resistivity and XAS measurements at the Co
L 3 edge on a Pt/Co/GdOx sample [39]. A clear correlation between the evolution
of the magnetic anisotropy and the Co oxidation was demonstrated [Fig. 5.12(a,b)].
In Fig. 5.12b, one can see that the shape of the Co XAS spectra are modified with
voltage. Whereas a negative voltage induces some fine structures in the XAS spectra
indicating a Co oxidation, a positive voltage reduces the Co layer, and therefore Co
spectra become similar to that of metallic Co. Thus the Co magnetic anisotropy in
this Pt/Co/GdOx structures can be reversibly controlled by voltage via Co oxidation
and reduction. This result confirms the earlier experiments performed by F. Bonell
et al. [40] on Au/CoFe/MgO samples. Hence, in these examples, the modification of
magnetic anisotropy is rather due to oxygen electromigration than charge accumulation.
In V/Fe/MgO devices, S. Miwa et al. [41] have indeed observed a different
behaviour. A slight change of coercive field with bias voltage has been measured
[see Fig. 5.12c] but without any modification of the Fe L 2,3 XAS and X-ray magnetic circular dichroism (XMCD) spectra with the applied voltage, suggesting that
Fe is not oxidized [Fig. 5.12d]. This is quite surprising since the electric field applied
is similar in Pt/Co/GdOx and V/Fe/MgO experiments. This result shows that modifications of the coercive field is not induced by electromigration but is rather due to
charge accumulation.
To conclude, this careful investigation of the electronic properties of the FM/oxide
interface has allowed to unveil the origin of voltage control of magnetic anisotropy
in each NM/FM/oxide structures and thus to discriminate between electromigration
and charge effects.
5.2.1.2 Effect of Strain
Strain has been also used to tune magnetic anisotropy and can be very efficient if
magnetostrictive materials are used. By combining piezoelectric and magnetostrictive materials, it is possible to control the magnetic anisotropy by voltage. Here we
show an example where photoelectron emission microscopy (PEEM) measurements
have been performed to highlight the correlation between strain and magnetism [42].
A magnetostrictive CoFe 2 O 4 layer has been deposited on a piezoelectric BaTiO 3 substrate. BaTiO 3 can have domains with different unit cell parameters, and therefore
can induce different domains with different strains in the CoFe 2 O 4 layer. X-ray linear dichroism (XLD) being sensitive to local coordination, different XLD spectra
will be obtained. In Fig. 5.13d, XLD-PEEM
4 image recorded at the Fe L 3 edge are
4 XAS spectra can be also recorded on the different domains allowing to measure the local electronic
properties.
149
anisotropy in Pt/Co/MgO [38]. This magnetic anisotropy modification with voltage
can originate from charge accumulation or oxygen electromigration.
C. Bi et al. have performed Hall resistivity and XAS measurements at the Co
L 3 edge on a Pt/Co/GdOx sample [39]. A clear correlation between the evolution
of the magnetic anisotropy and the Co oxidation was demonstrated [Fig. 5.12(a,b)].
In Fig. 5.12b, one can see that the shape of the Co XAS spectra are modified with
voltage. Whereas a negative voltage induces some fine structures in the XAS spectra
indicating a Co oxidation, a positive voltage reduces the Co layer, and therefore Co
spectra become similar to that of metallic Co. Thus the Co magnetic anisotropy in
this Pt/Co/GdOx structures can be reversibly controlled by voltage via Co oxidation
and reduction. This result confirms the earlier experiments performed by F. Bonell
et al. [40] on Au/CoFe/MgO samples. Hence, in these examples, the modification of
magnetic anisotropy is rather due to oxygen electromigration than charge accumulation.
In V/Fe/MgO devices, S. Miwa et al. [41] have indeed observed a different
behaviour. A slight change of coercive field with bias voltage has been measured
[see Fig. 5.12c] but without any modification of the Fe L 2,3 XAS and X-ray magnetic circular dichroism (XMCD) spectra with the applied voltage, suggesting that
Fe is not oxidized [Fig. 5.12d]. This is quite surprising since the electric field applied
is similar in Pt/Co/GdOx and V/Fe/MgO experiments. This result shows that modifications of the coercive field is not induced by electromigration but is rather due to
charge accumulation.
To conclude, this careful investigation of the electronic properties of the FM/oxide
interface has allowed to unveil the origin of voltage control of magnetic anisotropy
in each NM/FM/oxide structures and thus to discriminate between electromigration
and charge effects.
5.2.1.2 Effect of Strain
Strain has been also used to tune magnetic anisotropy and can be very efficient if
magnetostrictive materials are used. By combining piezoelectric and magnetostrictive materials, it is possible to control the magnetic anisotropy by voltage. Here we
show an example where photoelectron emission microscopy (PEEM) measurements
have been performed to highlight the correlation between strain and magnetism [42].
A magnetostrictive CoFe 2 O 4 layer has been deposited on a piezoelectric BaTiO 3 substrate. BaTiO 3 can have domains with different unit cell parameters, and therefore
can induce different domains with different strains in the CoFe 2 O 4 layer. X-ray linear dichroism (XLD) being sensitive to local coordination, different XLD spectra
will be obtained. In Fig. 5.13d, XLD-PEEM
4 image recorded at the Fe L 3 edge are
4 XAS spectra can be also recorded on the different domains allowing to measure the local electronic
properties.