I06-Nanoscience (XPEEM)
I10-Beamline for Advanced Dichroism - scattering
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C. J.
Durrant
,
L. R.
Shelford
,
R. A. J.
Valkass
,
R. J.
Hicken
,
A. I.
Figueroa
,
A. A.
Baker
,
G.
Van Der Laan
,
L. B.
Duffy
,
P.
Shafer
,
C.
Klewe
,
E.
Arenholz
,
S. A.
Cavill
,
J. R.
Childress
,
J. A.
Katine
Diamond Proposal Number(s):
[8782, 11585]
Abstract: Spin pumping has been studied within Ta / Ag / Ni81Fe19 (0–5 nm) / Ag (6 nm) / Co2MnGe (5 nm) / Ag / Ta large-area spin-valve structures, and the transverse spin current absorption of Ni81Fe19 sink layers of different thicknesses has been explored. In some circumstances, the spin current absorption can be inferred from the modification of the Co2MnGe source layer damping in vector network analyzer ferromagnetic resonance (VNAFMR)
experiments. However, the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thicknesses of less than 0.6 nm are found to be discontinuous and super-paramagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5-nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR line width that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.
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Oct 2017
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J.
Li
,
L. R.
Shelford
,
P.
Shafer
,
A.
Tan
,
J. X.
Deng
,
P. S.
Keatley
,
C.
Hwang
,
E.
Arenholz
,
G.
Van Der Laan
,
R. J.
Hicken
,
Z. q.
Qiu
Abstract: Despite recent progress in spin-current research, the detection of spin current has mostly remained indirect. By synchronizing a microwave waveform with synchrotron x-ray pulses, we use the ferromagnetic resonance of the Py (Ni 81 Fe 19 ) layer in a Py/Cu/Cu 75 Mn 25 /Cu/Co multilayer to pump a pure ac spin current into the Cu 75 Mn 25 and Co layers, and then directly probe the spin current within the Cu 75 Mn 25 layer and the spin dynamics of the Co layer by x-ray magnetic circular dichroism. This element-resolved pump-probe measurement unambiguously identifies the ac spin current in the Cu 75 Mn 25 layer.
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Aug 2016
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I06-Nanoscience (XPEEM)
I10-Beamline for Advanced Dichroism - scattering
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Robert Alexander James
Valkass
,
Leigh R.
Shelford
,
Chris J.
Durrant
,
Adriana
Figueroa
,
Alex A.
Baker
,
Padraic
Shafer
,
Elke
Arenholz
,
Jeffrey R.
Childress
,
Jordan A.
Katine
,
Gerrit
Van Der Laan
,
Robert
Hicken
Diamond Proposal Number(s):
[8782, 11585, 13063]
Open Access
Abstract: In spin valve structures the damping of a ferromagnetic layer driven at resonance can be modified by the transfer of spin angular momentum into a ‘sink’ ferromagnetic layer. This effect, known as spin pumping, is interface dominated and expected to increase with increasing sink layer thickness up to a saturation absorption depth, previously reported to be 1.2 nm regardless of the sink layer’s composition [1]. Using vector network analyser ferromagnetic resonance (VNA-FMR), we have studied the variation in damping as a function of sink layer thickness in a series of CoMnGe (5 nm) / Ag (6 nm) / NiFe (x nm) spin valves. These measurements show only small variations in the CoMnGe Gilbert damping parameter for x ≤ 1.8 nm, although damping is observed to increase at x = 0.3 and 0.6 nm. Element-resolved x-ray detected ferromagnetic resonance (XFMR) [2] measurements confirm spin transfer torque due to spin pumping as the origin of the damping for x = 1.5 and 1.8 nm, with both thicknesses having the same effective spin mixing conductance, supporting the findings of Ghosh et al [1]. For thicker sink layers the source and sink FMR fields are seen to coincide, hampering the identification of spin pumping. [1] A Ghosh, et al. Physical Review Letters 109, 127202 (2012) [2] M Marcham, et al. Physical Review B 87, 180403 (2013)
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Apr 2016
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I06-Nanoscience (XPEEM)
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R.
Valkass
,
W.
Yu
,
L.
Shelford
,
P. S.
Keatley
,
T. H. J.
Loughran
,
R.
Hicken
,
S.
Cavill
,
G.
Van Der Laan
,
S.
Dhesi
,
M. A.
Bashir
,
M. A.
Gubbins
,
P. J.
Czoschke
,
R.
Lopusnik
Diamond Proposal Number(s):
[9287]
Abstract: Four different designs of partially built hard disk write heads with a yoke comprising four repeats of NiFe (1nm)/CoFe (50nm) were studied by both x-ray photoemission electron microscopy (XPEEM) and time-resolved scanning Kerr microscopy (TRSKM). These techniques were used to investigate the static equilibrium domain configuration and the magnetodynamic response across the entire structure, respectively. Simulations and previous TRSKM studies have made proposals for the equilibrium domain configuration of similar structures, but no direct observation of the equilibrium state of the writers has yet been made. In this study, static XPEEM images of the equilibrium state of writer structures were acquired using x-ray magnetic circular dichroism as the contrast mechanism. These images suggest that the crystalline anisotropy dominates the equilibrium state domain configuration, but competition with shape anisotropy ultimately determines the stability of the equilibrium state. Dynamic TRSKM images were acquired from nominally identical devices. These images suggest that a longer confluence region may hinder flux conduction from the yoke into the pole tip: the shorter confluence region exhibits clear flux beaming along the symmetry axis, whereas the longer confluence region causes flux to conduct along one edge of the writer. The observed variations in dynamic response agree well with the differences in the equilibrium magnetization configuration visible in the XPEEM images, confirming that minor variations in the geometric design of the writer structure can have significant effects on the process of flux beaming.
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Jun 2015
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I06-Nanoscience (XPEEM)
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G.
Stenning
,
L. R.
Shelford
,
S. A.
Cavill
,
F.
Hoffmann
,
M.
Haertinger
,
T.
Hesjedal
,
G.
Woltersdorf
,
G.
Bowden
,
S. A.
Gregory
,
C. H.
Back
,
P. A. J.
De Groot
,
G.
Van Der Laan
Diamond Proposal Number(s):
[7745]
Open Access
Abstract: Exchange-coupled hard and soft magnetic layers find extensive use in data storage applications, for which their dynamical response has great importance. With bulk techniques, such as ferromagnetic resonance (FMR), it is difficult to access the behaviour and precise influence of each individual layer. By contrast, the synchrotron radiation-based technique of x-ray detected ferromagnetic resonance (XFMR) allows element-specific and phase-resolved FMR measurements in the frequency range 0.511 GHz. Here, we report the study of the magnetization dynamics of an exchange-coupled Ni0.81Fe0.19 (43.5 nm)/Co0.5Fe0.5 (30 nm) bilayer system using magnetometry and vector network analyser FMR, combined with XFMR at the Ni and Co L2 x-ray absorption edges. The epitaxially grown bilayer exhibits two principal resonances denoted as the acoustic and optical modes. FMR experiments show that the Kittel curves of the two layers cannot be taken in isolation, but that their modelling needs to account for an interlayer exchange coupling. The angular dependence of FMR indicates a collective effect for the modes of the magnetically hard CoFe and soft NiFe layer. The XFMR precessional scans show that the acoustic mode is dominated by the Ni signal with the Co and Ni magnetization precessing in phase, whereas the optical mode is dominated by the Co signal with the Co and Ni magnetization precessing in anti-phase. The response of the Co signal at the Ni resonance, and vice versa, show induced changes in both amplitude and phase, which can be ascribed to the interface exchange coupling. An interesting aspect of phase-resolved XFMR is the ability to distinguish between static and dynamic exchange coupling. The element-specific precessional scans of the NiFe/CoFe bilayer clearly have the signature of static exchange coupling, in which the effective field in one layer is aligned along the magnetization direction of the other layer.
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Jan 2015
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Theoretical Physics
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Abstract: Ferromagnetic resonance in thin films and heterostructures is studied by micromagnetic simulation. Using a Fourier transform, we obtain the power spectrum of modes of precession and demonstrate that they agree with experimental results. Additional resonance modes associated with demagnetization effects are observed and studied as a function of the sample dimensions. Sufficiently confined geometries lead to a reorientation of the resonance mode, and a dramatic change in its frequency. Finally, the simulations are extended to exchange-coupled bilayers where energy is transferred between magnetic materials.
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May 2014
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I10-Beamline for Advanced Dichroism - scattering
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Open Access
Abstract: For decades, a link between increased levels of iron and areas of Alzheimer's disease (AD) pathology has been recognized, including AD lesions comprised of the peptide β-amyloid (Aβ). Despite many observations of this association, the relationship between Aβ and iron is poorly understood. Using X-ray microspectroscopy, X-ray absorption spectroscopy, electron microscopy and spectrophotometric iron(II) quantification techniques, we examine the interaction between Aβ(1–42) and synthetic iron(III), reminiscent of ferric iron stores in the brain. We report Aβ to be capable of accumulating iron(III) within amyloid aggregates, with this process resulting in Aβ-mediated reduction of iron(III) to a redox-active iron(II) phase. Additionally, we show that the presence of aluminium increases the reductive capacity of Aβ, enabling the redox cycling of the iron. These results demonstrate the ability of Aβ to accumulate iron, offering an explanation for previously observed local increases in iron concentration associated with AD lesions. Furthermore, the ability of iron to form redox-active iron phases from ferric precursors provides an origin both for the redox-active iron previously witnessed in AD tissue, and the increased levels of oxidative stress characteristic of AD. These interactions between Aβ and iron deliver valuable insights into the process of AD progression, which may ultimately provide targets for disease therapies
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Mar 2014
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I06-Nanoscience (XPEEM)
I10-Beamline for Advanced Dichroism - scattering
|
M. D.
Watson
,
L. J.
Collins-Mcintyre
,
L. R.
Shelford
,
A. I.
Coldea
,
D.
Prabhakaran
,
S. C.
Speller
,
T.
Mousavi
,
C. R. M.
Grovenor
,
Z.
Salman
,
S. R.
Giblin
,
G.
Van Der Laan
,
T.
Hesjedal
Diamond Proposal Number(s):
[7345]
Open Access
Abstract: Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi2Te3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi2Te3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with TC varying between 9 and 13 K (bulk values) and a saturation moment that reaches 4.4(5) μB per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab-plane. The electrical transport data show an anomaly around TC that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n-doped with carrier concentrations of ~ 0.5–3.0 × 1020 cm−3. X-ray magnetic circular dichroism (XMCD) at the Mn L2,3 edge at 1.8 K reveals a large spin magnetic moment of 4.3(3) μB/Mn, and a small orbital magnetic moment of 0.18(2) μB/Mn. The results also indicate a ground state of mixed d4–d5–d6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga1−xMnxAs. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c-axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.
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Oct 2013
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I06-Nanoscience (XPEEM)
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M.
Marcham
,
W.
Yu
,
P.
Keatley
,
L.
Shelford
,
P.
Shafer
,
S.
Cavill
,
H.
Qing
,
A.
Neudert
,
J. R.
Childress
,
J. A.
Katine
,
E.
Arenholz
,
N. D.
Telling
,
G.
Van Der Laan
,
R.
Hicken
Abstract: Precessional dynamics of a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1)/Ru(3) (thicknesses in nm) thin film have been explored by low temperature time-resolved magneto-optical Kerr effect and phase-resolved x-ray ferromagnetic resonance measurements. As the temperature was decreased from 300 to 140?K, the magnetic damping was found to increase rapidly while the resonance field was strongly reduced. Static x-ray magnetic circular dichroism measurements revealed increasing ferromagnetic order of the Dy moment antiparallel to that of Co50Fe50/Ni90Fe10. Increased coupling of the Dy orbital moment to the precessing spin magnetization leads to significantly increased damping and gyromagnetic ratio of the film while leaving its magnetic anisotropy effectively unchanged.
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Feb 2013
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I10-Beamline for Advanced Dichroism - scattering
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G. B. G.
Stenning
,
G. J.
Bowden
,
S. A.
Gregory
,
J.-M. L.
Beaujour
,
P. A. J.
De Groot
,
L. R.
Shelford
,
P.
Bencok
,
P.
Steadman
,
A. N.
Dobrynin
,
T.
Hesjedal
,
G.
Van Der Laan
Diamond Proposal Number(s):
[7162]
Abstract: Magnetic reversal in a (110)-oriented [DyFe2(60Å)/YFe2(240Å)]×15
[
DyF
e
2
(
60
Å
)
/
YF
e
2
(
240
Å
)
]
×
15
multilayer film is investigated using magnetometry, micro-magnetic modeling, and element-specific soft x-ray magnetic circular dichroism. At temperatures between ∼60 and 120 K, the magnetic reversal involves a two-step process. It is shown that the reversal mechanism can be described as switching from an in-plane [001] reversed anti-ferromagnetic state, to an out-of-plane [100] transverse spring exchange state, and finally to an in-plane [001⎯⎯]
[
00
1
¯
]
magnetic exchange spring state.
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Aug 2012
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