The effects of interfaces on the magnetic properties of ultra-thin ferromagnetic films

In this dissertation, the manipulation of the magnetic properties of ultra-thin
ferromagnetic (FM) films by varying the interfacial coupling on both sides of the
film, e.g. with the capping layer and with the substrate, has been studied by means
of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism
(XMCD).
Firstly, XAS and XMCD measurements were carried out at the Fe L2;3 absorption
edges of an ultra-thin iron wedge grown on a ferroelectric [Pb(Mg1/3Nb2/3)O3]0.68-
[PbTiO3]0.32 (PMN-PT) (001) substrate using the total electron yield mode. Upon
switching the ferroelectric polarization of the substrate from Pup to Pdown, we
observe a strong change of approximately 20% of the Fe total magnetic moment
for a film thickness of 1:5 nm. However, when the dimensionality of the system
increases, i.e. for a film thickness of 3 nm, the same FE polarization only weakly
affect the Fe magnetic moments. Taking the sampling depth of the measurement
method into account, this difference is compatible with a magnetic anisotropy
change taking place only in the first interfacial layer in contact with the ferroelectric
substrate. We attribute this interfacial coupling to a charge accumulation or
depletion at the interface. Similar measurements were carried out for different
systems such as Pd/Fe/PMN-PT system in which the 3 nm Cr capping was replaced
by the 3 nm Pd capping. In this heterostructure system, we did not observe a strong
anisotropy for Pup as the remanent moments are observed along both the in-plane and out-of-plane directions. Upon switching the FE polarization of the PMN-PT to
down in Pd/Fe/PMN-PT system, the remanence magnetic moments increase along
the in-plane direction while they decrease along the out-of-plane direction. Further,
we report an increase in the saturation moments for the Pd/Fe/PMN-PT system
compared to the bcc Fe.
In order to gain further insight into the magnetic properties at the interface
between oxides and metals, we investigated ultra-thin films of 1:2 nm thickness
Co40Fe40B20 (CoFeB), using the XMCD technique, by probing at the Fe and Co L2;3
absorption edges. Systems with different oxide interfaces such as Ta/CoFeB/MgO,
Ta/CoFeB/BaTiO3 (BTO) and Ta/CoFeB/Ba0.1Sr0.9TiO3 (BSTO) samples have been
investigated. For the CoFeB/MgO system, the XMCD measurement revealed that
magnetization along in-plane and out-of-plane direction are similar, i.e., the system
is very isotropic. However, when the same thickness of CoFeB is grown on BSTO
and BTO substrates, the easy axis of the magnetization aligns along the in-plane
direction. The magnetic moments for Fe and Co shown by all the three systems are
less than the bulk bcc Fe and bulk hcp Co, which is attributed to a dead layer at the
CoFeB/Ta interface. We also report an enhancement in the orbital moments for Fe
and Co atoms in comparison to bulk bcc Fe and bulk hcp Co due to the interface
with different oxides.
Lastly, the effect of capping layers of Tantalum (Ta), Platinum (Pt) and Ruthenium
(Ru) on the CoFeB/MgO system has been studied. The Ta/CoFeB/MgO system
shows an isotropic behavior, while the Pt/Ru capped/CoFeB/MgO systems show
an in-plane magnetization. The Fe and Co magnetic moments were calculated
using the XMCD sum rules, in which we clearly observed the change in moments
among the three systems. Pt/CoFeB/MgO system shows an increase in the magnetic
moments of Fe and Co atoms as compared to the Ta/CoFeB/MgO system. On
the other hand, Ru/CoFeB/MgO system shows a decrease in the Fe moments as compared to Ta/CoFeB/MgO. The Pt/CoFeB/MgO system shows Co moments
similar to the bulk hcp Co. XAS spectra at the Fe and Co L2;3 edges, show a shift
in the energy of the XAS peak when capped with Pt and Ru in comparison to Ta
capped films. In addition the Ru/CoFeB/MgO systems shows a shoulder at the Fe
L3-edge due to the interaction of Ru and Fe orbitals. Whereas, the Pt/CoFeB/MgO
system shows a shift of the Co L3-edge to lower energy in comparison to the
Ta/CoFeB/MgO system which can be the result of interaction between Pt and Co
orbitals.