I10-Beamline for Advanced Dichroism
|
Open Access
Abstract: Soft X-ray scattering is a powerful technique for measuring magnetic materials. By highlighting some examples using diffraction, small angle scattering and reflectivity the element sensitivity and strong dependence of the polarisation on both the size and direction of the magnetic moments in both single crystals and thin films will be demonstrated.
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Sep 2022
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I10-Beamline for Advanced Dichroism
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Open Access
Abstract: Resonant soft-x-ray reflectivity and vibrating sample magnetometry have been used to characterize field driven and spin orbit torque driven magnetization reversal in a CoFeTaB/Pt bilayer. Reversal of the magnetization occurs either along the applied field direction or perpendicular to the current flow direction. Magnetometry results show that field driven (current driven) coercivities are reduced by application of a current (field) highlighting the roles played by the two external parameters. In the current switching case, it is demonstrated with soft-x-ray hysteresis loops that only the layers near the interface with Pt switch, possibly highlighting the role of proximity effects of the magnetized Pt. We show how magnetization reversal perpendicular to the beam results in hysteresis behavior in the reflected intensity that is dependent on the magnetization but independent of the helicity of the circular polarization of the incident beam
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Sep 2022
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I10-Beamline for Advanced Dichroism
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Open Access
Abstract: Single-crystal exchange biased multilayer thin-films consisting of an Fe layer on single-crystal chemically ordered L12-IrMn3 and disordered γ-IrMn3 were investigated using circularly and linearly polarised soft X-ray reflectivity to de- termine the interfacial magnetic structure between Fe and IrMn3. In ordered L12-IrMn3, we found that the Mn uncompensated moments at the interface are strongly pinned and only Fe rotational moments are observed. In disor- dered γ-IrMn3 the uncompensated moments are partially pinned, where the Mn moments are rotatable but the rotations are restricted. These findings are a crucial development in exchange bias theory to understand the force that drives the magnetic reversal process in ordered and disordered IrMn3.
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Nov 2021
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[14915]
Abstract: Electrostatically defined perovskite oxide heterostructures, in which carriers are confined by the modulation of the A-site ion charge, offer new possibilities of tuning the magnetic properties of manganite oxides. We investigate the preferential orientation of ferromagnetic and antiferromagnetic moments in ultrathin
La
0.7
Sr
0.3
MnO
3
layers embedded in antiferromagnetic
Sr
MnO
3
as they undergo a metal-to-insulator transition with decreasing thickness. Our results evince the role of orbital occupation, metallicity, and competition of different magnetic phases, in absence of spurious effects occurring in thin films as a result of symmetry breaking at
La
0.7
Sr
0.3
MnO
3
interfaces and of incorporation of oxygen vacancies.
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Aug 2021
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[19996]
Abstract: We report a comprehensive study on the magnetic ground state of
La
1.5
Ca
0.5
Co
O
4
combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the
Co
L
2
,
3
edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at
T
N
≈
52
K
. Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic
A
-centered lattice but with
a
and
b
axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group
A
2
m
m
where two nonequivalent compressed and expanded
Co
O
6
octahedra are ordered showing a checkerboard pattern in the
a
b
plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as
(
1
/
4
,
1
/
4
,
0
)
t
,
(
1
/
4
,
1
/
4
,
1
)
t
, and
(
1
/
4
,
1
/
4
,
1
/
2
)
t
in the parent tetragonal cell were observed at low temperature at the
Co
L
2
,
3
-edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the
Co
L
2
edge, indicates that only
Co
2
+
-like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the
a
b
plane of the parent tetragonal cell with a slight tilt in the
c
axis. Despite the onset temperatures for the three resonant magnetic reflections being the same,
≈
55
K
, different thermal behavior is observed between
(
1
/
4
,
1
/
4
,
1
/
2
)
t
and
(
1
/
4
,
1
/
4
,
L
)
t
(
L
=
integer
) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the
(
1
/
4
,
1
/
4
,
1
/
2
)
t
magnetic reflection is at least ten times larger than that of the
(
1
/
4
,
1
/
4
,
L
)
t
(
L
=
integer
)
ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector
k
=
(
1
/
4
,
1
/
4
,
1
/
2
)
t
that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in
La
1.5
Ca
0.5
Co
O
4
corresponds then to this propagation vector
k
=
(
1
/
4
,
1
/
4
,
1
/
2
)
t
while
(
1
/
4
,
1
/
4
,
0
)
t
and
(
1
/
4
,
1
/
4
,
1
)
t
magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.
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May 2021
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I10-Beamline for Advanced Dichroism
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Abstract: Nanoparticles that combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core–shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide nanoparticles which hampers potential applications such as data storage or sensors. In this manuscript, we report on the design of nanoparticles with an onion-like structure which has been scarcely reported yet. These nanoparticles consist of a Fe3−δO4 core covered by a first shell of CoFe2O4 and a second shell of Fe3−δO4, e.g., a Fe3−δO4@CoFe2O4@Fe3−δO4 onion-like structure. They were synthesized through a multistep seed-mediated growth approach which consists consists in performing three successive thermal decomposition of metal complexes in a high-boiling-point solvent (about 300 °C). Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increasing. We investigated very precisely the structure of nanoparticles in performing high resolution (scanning) TEM imaging and geometrical phase analysis (GPA). The chemical composition and spatial distribution of atoms were studied by electron energy loss spectroscopy (EELS) mapping and spectroscopy. The chemical environment and oxidation state of cations were investigated by 57Fe Mössbauer spectrometry, soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The combination of these techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, whereas the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe3−δO4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core–shell nanoparticles, thus demonstrating the high potential of onion-like nanoparticles to accurately tune the magnetic properties of nanoparticles according to the desired applications.
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Mar 2021
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I10-Beamline for Advanced Dichroism
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Jamie R.
Massey
,
Rowan C.
Temple
,
Trevor P.
Almeida
,
Ray
Lamb
,
Nicolas A.
Peters
,
Richard P.
Campion
,
Raymond
Fan
,
Damien
Mcgrouther
,
Stephen
Mcvitie
,
Paul
Steadman
,
Christopher H.
Marrows
Diamond Proposal Number(s):
[17439, 18484]
Abstract: The phase coexistence present through a first-order phase transition means there will be finite regions between the two phases where the structure of the system will vary from one phase to the other, known as a phase boundary wall. This region is said to play an important but unknown role in the dynamics of the first-order phase transitions. Here, by using both x-ray photon correlation spectroscopy and magnetometry techniques to measure the temporal isothermal development at various points through the thermally activated first-order metamagnetic phase transition present in the near-equiatomic FeRh alloy, we are able to isolate the dynamic behavior of the domain walls in this system. These investigations reveal that relaxation behavior of the domain walls changes when phase coexistence is introduced into the system and that the domain-wall dynamics is different to the macroscale behavior. We attribute this to the effect of the exchange coupling between regions of either magnetic phase changing the dynamic properties of domain walls relative to bulk regions of either phase. We also believe this behavior comes from the influence of the phase boundary wall on other magnetic objects in the system.
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Oct 2020
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I10-Beamline for Advanced Dichroism
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M. T.
Birch
,
D.
Cortés-Ortuño
,
L. A.
Turnbull
,
M. N.
Wilson
,
F.
Groß
,
N.
Träger
,
A.
Laurenson
,
N.
Bukin
,
S. H.
Moody
,
M.
Weigand
,
G.
Schütz
,
H.
Popescu
,
R.
Fan
,
P.
Steadman
,
J. A. T.
Verezhak
,
G.
Balakrishnan
,
J. C.
Loudon
,
A. C.
Twitchett-Harrison
,
O.
Hovorka
,
H.
Fangohr
,
F. Y.
Ogrin
,
J.
Gräfe
,
P. D.
Hatton
Diamond Proposal Number(s):
[20866]
Open Access
Abstract: Magnetic skyrmions are topologically nontrivial particles with a potential application as information elements in future spintronic device architectures. While they are commonly portrayed as two dimensional objects, in reality magnetic skyrmions are thought to exist as elongated, tube-like objects extending through the thickness of the host material. The study of this skyrmion tube state (SkT) is vital for furthering the understanding of skyrmion formation and dynamics for future applications. However, direct experimental imaging of skyrmion tubes has yet to be reported. Here, we demonstrate the real-space observation of skyrmion tubes in a lamella of FeGe using resonant magnetic x-ray imaging and comparative micromagnetic simulations, confirming their extended structure. The formation of these structures at the edge of the sample highlights the importance of confinement and edge effects in the stabilisation of the SkT state, opening the door to further investigation into this unexplored dimension of the skyrmion spin texture.
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Apr 2020
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[19421]
Open Access
Abstract: We report small-angle x-ray scattering measurements of the skyrmion lattice in two 200-nm-thick
Cu
2
OSeO
3
lamellae aligned with the applied magnetic field parallel to the out of plane [110] or [100] crystallographic directions. Our measurements show that the equilibrium skyrmion phase in both samples is expanded significantly compared to bulk crystals, existing between approximately 30 and 50 K over a wide region of magnetic field. This skyrmion state is elliptically distorted at low fields for the [110] sample, and symmetric for the [100] sample, possibly due to crystalline anisotropy becoming more important at this sample thickness than it is in bulk samples. Furthermore, we find that a metastable skyrmion state can be observed at low temperature by field cooling through the equilibrium skyrmion pocket in both samples. In contrast to the behavior in bulk samples, the volume fraction of metastable skyrmions does not significantly depend on cooling rate. We show that a possible explanation for this is the change in the lowest temperature of the skyrmion state in this lamellae compared to bulk, without requiring different energetics of the skyrmion state.
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Jan 2020
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[14826, 17388]
Abstract: By employing resonant x-ray microdiffraction, we image the magnetization and magnetic polarity domains of the
Y
-type hexaferrite
Ba
0.5
Sr
1.5
Mg
2
Fe
12
O
22
. We show that the magnetic polarity domain structure can be controlled by both magnetic and electric fields and that full inversion of these domains can be achieved simply by reversal of an applied magnetic field in the absence of an electric field bias. Furthermore, we demonstrate that the diffraction intensity measured in different x-ray polarization channels cannot be reproduced by the accepted model for the polar magnetic structure, known as the 2-fan transverse conical (TC) model. We propose a modification to this model, which achieves good quantitative agreement with all of our data. We show that the deviations from the TC model are large and may be the result of an internal magnetic chirality, most likely inherited from the parent helical (nonpolar) phase.
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Sep 2019
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