The origin of magnetism in anatase Co-doped TiO2 magnetic semiconductors


Lee, Yunjae (2010) The origin of magnetism in anatase Co-doped TiO2 magnetic semiconductors. thesis.

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Abstract:Dilute magnetic semiconductors (DMS) can be tailored by doping a small amount of
elements containing a magnetic moment into host semiconductors, which leads to a new class of semiconductors with the functionality of tunable magnetic properties. Recently, oxide semiconductors have attained interests for the possibility of developing room temperature dilute magnetic semiconductors. However, a large number of conflicting results have pointed towards both an intrinsic and extrinsic origin for ferromagnetism. In earlier studies of oxides in relation to DMS, the controversy originated mainly from imprudent conclusions relying on measurements of macroscopic magnetic properties without careful structural studies of secondary phases in the materials under investigation. This thesis describes the results of experiments with a broad set of complementary tools to investigate the origin of ferromagnetism in anatase Co:TiO2, which shows room temperature magnetism and an anomalous Hall effect (AHE). In previous work carried out in the NanoElectronics group, ferromagnetism and the AHE at room temperature were observed in anatase Co:TiO2 thin films grown under oxygen poor conditions, while the remanence and coercivity disappear for films grown under oxygen rich conditions. We employed synchrotron based techniques, namely x-ray absorption spectroscopy
(XAS) and x-ray magnetic circular dichroism (XMCD), which rely on the excitation of
core-level electrons by soft x-rays. One of appealing features of these techniques for
DMS studies is that they can provide element specific information on the electronic and
magnetic properties of materials. Here, we focused on Co-derived electronic states.
We observed a suppressed XAS multiplet structure due to Co2+ ions at Co L-edges in
oxygen poor samples, as compared to oxygen rich samples. This suppressed fine structure
can be explained by a larger featureless Co metal contribution superimposed on the
spectral features related to Co2+ ions. A Co metal contribution is confirmed by XMCD
measurements, which show a featureless dichroism spectrum characteristic of metallic Co.
Both oxygen poor and oxygen rich Co:TiO2 showed a similar featureless XMCD
spectrum, with the circular dichroism effect being significantly weaker for the oxygen
rich case. These findings show that measurements of XAS L-edges alone may confirm
the presence of ionic species, but cannot rule out secondary phases that may not yield a
discernable fingerprint.
It can thus be concluded from the XAS/XMCD measurements that the Co distribution is
heterogeneous for all Co:TiO2 films, grown under oxygen rich as well as oxygen poor
conditions. Co-heterogeniety, in the form of metallic clusters, is also confirmed by
energy filtered transmission electron microscopy (EF-TEM). This means that metallic Co
clusters are present in all the investigated films, and that they contribute to the
ferromagnetism (as shown by XMCD) and AHE (as discussed below). We conclude that
XMCD and EF-TEM are especially useful tools to check for heterogeneity in DMS
Using XAS, we also observed clear charge transfer (CT) satellite peaks at Co L edges for
the thickest (160 nm) oxygen poor films. The CT peaks can be explained by the
interaction of Co (3d) electrons with donor defect states in the band gap. It is known that
Co segregates to defects sites, where it may form clusters. The observation that the CT
peaks become stronger for thicker films, which exhibit a higher defect density resulting from the relief of strain, may be related to defect states that are more energetically close
to and significantly hybridized with the Co(3d) states. These findings also confirm that
the electronic structure of Co ions is heterogeneous, related to defect states in Co:TiO2
thin films.
The introduction of a TiO2 buffer layer at the substrate/film interface, intended to reduce
strain in Co:TiO2, leads to a dramatic change of magnetic and structural properties in
Co:TiO2. First, a reduction of the density of Co clusters and a more homogeneous Co
distribution in the thin films are confirmed by XRD, AFM and EF-TEM measurements.
Second, the AHE is suppressed. These results can be explained by a cluster-induced AHE,
also taking into account the results from EF-TEM analysis which showed that the
location at which Co clusters form is markedly different for films with/without buffer
layer. For Co:TiO2 films without buffer layer, Co clusters formed preferentially at the
substrate/film interface give rise to an anomalous transverse Hall resistivity by polarizing
nearby electrons. On the other hand, for Co:TiO2 films without buffer layer, Co clusters
where mainly observed at the surface, such that they are placed outside the current path
and do not contribute to the associated transverse scattering. The very small value of the
ratio of the anomalous Hall resistivity to the longitudinal resistivity (10-4) also confirms
that the AHE originates from Co clusters, since Co clusters polarize only nearby electrons.
Therefore, the presence of AHE cannot be considered as a definite test for carrier
mediated magnetism in dilute magnetic semiconductors.
We also observed that metallic impurity band conduction and ferromagnetism occur
simultaneously in both types of Co:TiO2 films, with and without a buffer layer. Since the
metallic impurity band conduction is found to co-occur with ferromagnetism due to Co
clusters, we can conclude that the phenomena of impurity band conduction and
ferromagnetism are not related.
Last, the spin polarization of the charge carriers in ultrathin anatase Co:TiO2 layers is
investigated by studying spin-polarized tunneling in a magnetic tunnel junction (MTJ)
configuration. It is found that the tunnel magnetoresistance (TMR) and junction
resistance of epitaxial LSMO/STO/Co magnetic tunnel junctions changes significantly
upon the insertion of ultrathin layers of Co:TiO2 at the STO/Co interface. The magnitude
of the TMR decreases but remains negative, while the junction resistance increases
strongly. This is consistent with an effectively insulating and paramagnetic Co:TiO2
adding to the tunnel barrier, with the tunneling electrons originating mostly from the
Co:TiO2/Co interface, and experiencing spin-flip scattering by paramagnetic Co in the
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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