|
|
C. A. F.
Vaz
,
G.
Van Der Laan
,
S. A.
Cavill
,
H. A.
Dürr
,
A.
Fraile Rodríguez
,
F.
Kronast
,
W.
Kuch
,
P.
Sainctavit
,
G.
Schütz
,
E.
Weschke
,
F.
Wilhelm
,
H.
Wende
Abstract: X-ray magnetic circular dichroism (XMCD) is the difference in X-ray absorption between left and right circularly polarized light in magnetic materials. It is the X-ray counterpart of the magneto-optic effect for visible light but shows a magnetic contrast up to three orders of magnitude higher. The exploration of XMCD using high-flux, monochromatic and polarization-variable synchrotron sources has advanced the understanding of magnetism and magnetic materials, in particular, when combined with spectral analysis based on powerful sum rules that enable the quantification of spin and orbital moments with elemental, even chemical, selectivity and high sensitivity. As an essential cornerstone of techniques to probe magnetic nanostructures and spin textures as well as their dynamics, XMCD has become an indispensable tool for the study of magnetism at the nanoscale and atomic scale. This Primer provides an overview of the principles and physics underlying XMCD, the experimental techniques used to measure it and its application to the study and understanding of fundamental and technologically relevant magnetic phenomena.
|
May 2025
|
|
I06-Nanoscience (XPEEM)
|
Ryuji
Fujita
,
Gautam
Gurung
,
Mohamad‐assaad
Mawass
,
Alevtina
Smekhova
,
Florian
Kronast
,
Alexander Kang-Jun
Toh
,
Anjan
Soumyanarayanan
,
Pin
Ho
,
Angadjit
Singh
,
Emily
Heppell
,
Dirk
Backes
,
Francesco
Maccherozzi
,
Kenji
Watanabe
,
Takashi
Taniguchi
,
Daniel A.
Mayoh
,
Geetha
Balakrishnan
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
Diamond Proposal Number(s):
[31730]
Open Access
Abstract: The van der Waals interaction enables atomically thin layers of exfoliated 2D materials to be interfaced in heterostructures with relaxed epitaxy conditions, however, the ability to exfoliate and freely stack layers without any strain or structural modification is by no means ubiquitous. In this work, the piezoelectricity of the exfoliated van der Waals piezoelectric α-In2Se3 is utilized to modify the magnetic properties of exfoliated Fe3GeTe2, a van der Waals ferromagnet, resulting in increased domain wall density, reductions in the transition temperature ranging from 5 to 20 K, and an increase in the magnetic coercivity. Structural modifications at the atomic level are corroborated by a comparison to a graphite/α-In2Se3 heterostructure, for which a decrease in the Tuinstra-Koenig ratio is found. Magnetostrictive ferromagnetic domains are also observed, which may contribute to the enhanced magnetic coercivity. Density functional theory calculations and atomistic spin dynamic simulations show that the Fe3GeTe2 layer is compressively strained by 0.4%, reducing the exchange stiffness and magnetic anisotropy. The incorporation of α-In2Se3 may be a general strategy to electrostatically strain interfaces within the paradigm of hexagonal boron nitride-encapsulated heterostructures, for which the atomic flatness is both an intrinsic property and paramount requirement for 2D van der Waals heterojunctions.
|
Mar 2024
|
|
I10-Beamline for Advanced Dichroism - scattering
|
N.-J.
Steinke
,
S. L.
Zhang
,
P. J.
Baker
,
L. B.
Duffy
,
F.
Kronast
,
J.
Krieger
,
Z.
Salman
,
T.
Prokscha
,
A.
Suter
,
S.
Langridge
,
Gerrit
Van Der Laan
,
T.
Hesjedal
Diamond Proposal Number(s):
[11503]
Abstract: Chromium-doped
Sb
2
Te
3
is a magnetic topological insulator (MTI), which belongs to the
(
Sb
,
Bi
)
2
(
Se
,
Te
)
3
family. When doped with the transition metals V, Cr, and Mn this family displays long-range ferromagnetic order above liquid nitrogen temperature and is currently intensely explored for quantum device applications. Despite the large magnetic ordering temperature, the experimental observation of dissipationless electrical transport channels, i.e., the quantum anomalous Hall effect, is limited in these materials to temperatures below
≈
2
K. Inhomogeneities in the MTI have been identified as a major concern, affecting the coupling between the Dirac states and the magnetic dopants. Nevertheless, details on the local magnetic order in these materials are not well understood. Here, we report the study of the magnetic correlations in thin films using a combination of muon spin relaxation
(
μ
SR
)
, and magnetic soft x-ray spectroscopy and imaging.
μ
SR
provides two key quantities for understanding the microscopic magnetic behavior: The magnetic volume fraction, i.e., the percentage of the material that is ferromagnetically ordered, and the relaxation rate, which is sensitive to the magnetic static
(
≈
μ
s
)
and dynamic disorder. By choosing different implantation depths for the muons, one can further discriminate between near-surface and bulk properties. No evidence for a surface enhancement of the magnetic ordering is observed, but, instead, we find evidence of small magnetically ordered clusters in a paramagnetic background, which are coupled. The significant magnetic field shift that is present in all samples indicates a percolation transition that proceeds through the formation and growth of magnetically ordered spin clusters. We further find that fluctuations are present even at low temperatures, and that there appears to be a transition between superparamagnetism and superferromagnetism.
|
Dec 2022
|
|
|
|
Open Access
Abstract: Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.
|
Jul 2022
|
|
I06-Nanoscience (XPEEM)
|
C.
Schmitt
,
L.
Baldrati
,
L.
Sanchez-Tejerina
,
F.
Schreiber
,
A.
Ross
,
M.
Filianina
,
S.
Ding
,
F.
Fuhrmann
,
R.
Ramos
,
F.
Maccherozzi
,
D.
Backes
,
M.-A.
Mawass
,
F.
Kronast
,
S.
Valencia
,
E.
Saitoh
,
G.
Finocchio
,
M.
Klaui
Diamond Proposal Number(s):
[22448]
Abstract: Understanding the electrical manipulation of the antiferromagnetic order is a crucial aspect to enable the design of antiferromagnetic devices working at THz frequencies. Focusing on collinear insulating antiferromagnetic
Ni
O
/
Pt
thin films as a materials platform, we identify the crystallographic orientation of the domains that can be switched by currents and quantify the Néel-vector direction changes. We demonstrate electrical switching between different T domains by current pulses, finding that the Néel-vector orientation in these domains is along [
±
5
±
5
19], different compared to the bulk
⟨
112
⟩
directions. The final state of the in-plane component of the Néel vector
n
IP
after switching by current pulses
j
along the
[
1
±
1
0
]
directions is
n
IP
∥
j
. By comparing the observed Néel-vector orientation and the strain in the thin films, assuming that this variation arises solely from magnetoelastic effects, we quantify the order of magnitude of the magnetoelastic coupling coefficient as
b
0
+
2
b
1
=
3
×
10
7
J
/
m
3
. This information is key for the understanding of current-induced switching in antiferromagnets and for the design and use of such devices as active elements in spintronic devices.
|
Mar 2021
|
|
I06-Nanoscience (XPEEM)
|
L.
Baldrati
,
O.
Gomonay
,
A.
Ross
,
M.
Filianina
,
R.
Lebrun
,
R.
Ramos
,
C.
Leveille
,
F.
Fuhrmann
,
T. R.
Forrest
,
F.
Maccherozzi
,
S.
Valencia
,
F.
Kronast
,
E.
Saitoh
,
J.
Sinova
,
M.
Kläui
Diamond Proposal Number(s):
[18850, 20698]
Abstract: We probe the current-induced magnetic switching of insulating antiferromagnet–heavy-metal systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one-third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely, the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs.
|
Oct 2019
|
|
|
|
Abstract: The combination of topological insulators, i.e., bulk insulators with gapless, topologically protected surface states, with magnetic order is a love-hate relationship that can unlock new quantum states and exotic physical phenomena, such as the quantum anomalous Hall effect and axion electrodynamics. Moreover, the unusual coupling between topological insulators and ferromagnets can also result in the formation of topological spin textures in the ferromagnetic layer. Skyrmions are topologically-protected magnetization swirls that are promising candidates for spintronics memory carriers. Here, we report on the observation of skyrmionium in thin ferromagnetic films coupled to a magnetic topological insulator. The occurrence of skyrmionium, which appears as a soliton composed of two skyrmions with opposite winding numbers, is tied to the ferromagnetic state of the topological insulator. Our work presents a new combination of two important classes of topological materials and may open the door to new topologically inspired information-storage concepts in the future.
|
Jan 2018
|
|
|
|
Abstract: Palaeomagnetic measurements of meteorites1, 2, 3, 4, 5 suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields6. Convection on these bodies is currently thought to have been thermally driven7, 8, implying that magnetic activity would have been short-lived9. Here we report a time-series palaeomagnetic record derived from nanomagnetic imaging10 of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core11, 12, 13 is consistent with our measured magnetic field characteristics and cooling rates14. Solidification-driven convection was probably common among small body cores15, and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System.
|
Jan 2015
|
|
|
|
Open Access
Abstract: X-ray photoemission electron microscopy (XPEEM) enables natural remanent magnetisation to be imaged with
resolution across a field of view of 5–20 μm. The method is applied to structural features typical of the Widmanstätten microstructure (kamacite – tetrataenite rim – cloudy zone – plessite) in the Tazewell IIICD iron meteorite. Kamacite lamellae and the tetrataenite rim are multidomain, whereas plessite consists of laths of different phases displaying a range of stable magnetisation directions. The cloudy zone (CZ) displays a complex interlocking domain pattern resulting from nanoscale islands of tetrataenite with easy axes distributed along three possible crystallographic directions. Quantitative analysis of the coarse and intermediate CZ was achieved using a combination of image simulations and histogram profile matching. Remanence information was extracted from individual regions of interest
wide, demonstrating for the first time the capability of XPEEM to perform quantitative paleomagnetic analysis at sub-micron length scales. The three tetrataenite easy axis orientations occur with equal probability in the coarse and intermediate CZ, suggesting that spinodal decomposition in these regions was not strongly influenced by internal interaction fields, and that they are suitable candidates for future paleomagnetic studies. The fine CZ shows a strong dominance of one easy axis. This effect is attributed to island–island exchange interactions that render the fine CZ unsuitable for paleomagnetic study. Variations in the relative strength (proportion of dominant easy axis) and direction (direction of dominant easy axis) of a paleomagnetic field can be resolved from different regions of the CZ using XPEEM, raising the prospect of obtaining a time-resolved measurement of the active dynamo period in meteorites originating from the upper unmelted regions of differentiated asteroids (e.g. chondrites, pallasites, mesosiderites).
|
Jun 2014
|
|
|
|
Abstract: We analyze the origin of the electrical resistance arising in domain walls of perpendicularly magnetized materials by considering a superposition of anisotropic magnetoresistance and the resistance implied by the magnetization chirality. The domain wall profiles of L10-FePd and L10-FePt are determined by micromagnetic simulations based on which we perform first-principles calculations to quantify electron transport through the core and closure region of the walls. The wall resistance, being twice as high in L10-FePd than in L10-FePt, is found to be clearly dominated in both cases by a high gradient of magnetization rotation, which agrees well with experimental observations.
|
Feb 2012
|
|
