I10-Beamline for Advanced Dichroism
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Maciej
Dabrowski
,
Andreas
Frisk
,
David M.
Burn
,
David G.
Newman
,
Christoph
Klewe
,
Alpha T.
N’diaye
,
Padraic
Shafer
,
Elke
Arenholz
,
Graham J.
Bowden
,
Thorsten
Hesjedal
,
Gerrit
Van Der Laan
,
Gino
Hrkac
,
Robert J.
Hicken
Diamond Proposal Number(s):
[17745, 19116, 20760]
Abstract: Microwave and heat-assisted magnetic recordings are two competing technologies that have greatly increased the capacity of hard disk drives. The efficiency of the magnetic recording process can be further improved by employing non-collinear spin structures that combine perpendicular and in-plane magnetic anisotropy. Here, we investigate both microwave and optically excited magnetization dynamics in [Co/Pt]/NiFe exchange spring samples. The resulting canted magnetization within the nanoscale [Co/Pt]/NiFe interfacial region allows for optically stimulated magnetization precession to be observed for an extended magnetic field and frequency range. The results can be explained by formation of an imprinted domain structure, which locks the magnetization orientation and makes the structures more robust against external perturbations. Tuning the canted interfacial domain structure may provide greater control of optically excited magnetization reversal and optically generated spin currents, which are of paramount importance for future ultrafast magnetic recording and spintronic applications.
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Nov 2020
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I06-Nanoscience
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Diamond Proposal Number(s):
[838]
Abstract: Biogenic nanoscale vanadium magnetite is produced by converting V(V)- bearing ferrihydrites through reductive transformation using the metal- reducing bacterium Geobacter sulfurreducens. With increasing vanadium in the ferrihydrite, the amount of V-doped magnetite produced decreased due to V-toxicity which interrupted the reduction pathway ferrihydrite – magnetite, resulting in siderite or goethite formation. Fe L2,3 and V L2,3 X-ray absorption spectra and data from X-ray magnetic circular dichroism analysis revealed the magnetite to contain the V in the Fe(III) Oh site, predominately as V(III) but always with a component of V(VI) present a consistent V(IV)/V(III) ratio in the range 0.28 to 0.33. The bacteriogenic production of V-doped magnetite nanoparticles from V-doped ferrihydrite is confirmed and the work reveals that microbial reduction of contaminant V(V) to V(III)/V(IV) in the environment will occur below the Fe-redox boundary where it will be immobilised in biomagnetite nanoparticles.
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Jul 2020
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I06-Nanoscience
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Yujia
Wang
,
Qing
He
,
Wenmei
Ming
,
Mao-Hua
Du
,
Nianpeng
Lu
,
Clodomiro
Cafolla
,
Jun
Fujioka
,
Qinghua
Zhang
,
Ding
Zhang
,
Shengchun
Shen
,
Yingjie
Lyu
,
Alpha T.
N’diaye
,
Elke
Arenholz
,
Lin
Gu
,
Cewen
Nan
,
Yoshinori
Tokura
,
Satoshi
Okamoto
,
Pu
Yu
Open Access
Abstract: Epitaxial strain provides important pathways to control the magnetic and electronic states in transition-metal oxides. However, the large strain is usually accompanied by a strong reduction of the oxygen-vacancy formation energy, which hinders the direct manipulation of their intrinsic properties. Here, using a postdeposition ozone annealing method, we obtain a series of oxygen stoichiometric
SrCoO
3
thin films with the tensile strain up to 3.0%. We observe a robust ferromagnetic ground state in all strained thin films, while interestingly the tensile strain triggers a distinct metal-to-insulator transition along with the increase of the tensile strain. The persistent ferromagnetic state across the electrical transition therefore suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which then calls for further investigation of the intrinsic mechanism of this magnetic compound beyond the double-exchange mechanism.
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May 2020
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I10-Beamline for Advanced Dichroism
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Maciej
Dabrowski
,
Takafumi
Nakano
,
David
Burn
,
Andreas
Frisk
,
David G.
Newman
,
Christoph
Klewe
,
Qian
Li
,
Mengmeng
Yang
,
Padraic
Shafer
,
Elke
Arenholz
,
Thorsten
Hesjedal
,
Gerrit
Van Der Laan
,
Zi Q.
Qiu
,
Robert J.
Hicken
Diamond Proposal Number(s):
[17745, 19116, 20760]
Abstract: Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.
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May 2020
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I10-Beamline for Advanced Dichroism
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Christoph
Klewe
,
Qian
Li
,
Mengmeng
Yang
,
Alpha T.
N’diaye
,
David M.
Burn
,
Thorsten
Hesjedal
,
Adriana
Figueroa
,
Chanyong
Hwang
,
Jia
Li
,
Robert J.
Hicken
,
Padraic
Shafer
,
Elke
Arenholz
,
Gerrit
Van Der Laan
,
Ziqiang
Qiu
Diamond Proposal Number(s):
[18542, 19116, 20483, 20493, 21616]
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Apr 2020
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Abstract: Previous work has documented time‐ and temperature‐dependent variations in the Curie temperature (Tc) of natural titanomagnetites, independent of any changes in sample composition. To better understand the atomic‐scale processes responsible for these variations, we have generated a set of synthetic titanomagnetites with a range of Ti, Mg, and Al substitution; a subset of samples was additionally oxidized at low temperature (150°C). Samples were annealed at temperatures between 325‐400 °C for up to 1000 h and characterized in terms of magnetic properties; Fe valence and site occupancy were constrained by X‐ray magnetic circular dichroism (XMCD) and Mössbauer spectroscopy. Annealing results in large (up to ~100 °C) changes in Tc, but Mössbauer, XMCD, and saturation magnetization data all demonstrate that intersite reordering of Fe2+/Fe3+ does not play a role in the observed Tc changes. Rather, the data are consistent with vacancy‐enhanced nanoscale chemical clustering within the octahedral sublattice. This clustering may be a precursor to chemical unmixing at temperatures below the titanomagnetite binary solvus. Additionally, the data strongly support a model where cation vacancies are predominantly situated on octahedral sites; Mg‐substitution is largely accommodated on octahedral sites; and Al‐substitution is split between the two sites.
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Apr 2019
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I09-Surface and Interface Structural Analysis
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Paul C.
Rogge
,
Ravini U.
Chandrasena
,
Antonio
Cammarata
,
Robert J.
Green
,
Padraic
Shafer
,
Benjamin M.
Lefler
,
Amanda
Huon
,
Arian
Arab
,
Elke
Arenholz
,
Ho Nyung
Lee
,
Tien-Lin
Lee
,
Slavomir
Nemsak
,
James M.
Rondinelli
,
Alexander
Gray
,
Steven J.
May
Diamond Proposal Number(s):
[17824]
Abstract: We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe sites undergo no observable spectroscopic change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ∼5–10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.
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Jan 2018
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I06-Nanoscience
I10-Beamline for Advanced Dichroism
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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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James
Byrne
,
Victoria
Coker
,
Eva
Cespedes
,
Paul L.
Wincott
,
David J.
Vaughan
,
Richard
Pattrick
,
Gerrit
Van Der Laan
,
Elke
Arenholz
,
Floriana
Tuna
,
Martin
Bencsik
,
Jonathan R.
Lloyd
,
Neil
Telling
Abstract: The magnetic moments of magnetite nanoparticles are dramatically enhanced through the addition of zinc in a microbiologically driven synthesis procedure. The particles are produced through the reduction of Fe(III)-compounds containing Zn(II) by the iron reducing bacterium Geobacter sulfurreducens .
Results indicate a signifi cant increase in the saturation magnetization by over 50% compared to magnetite at both room and low temperatures for relatively minor quantities of zinc substitution. A maximum saturation magnetization of nearly 100 emu g −1 of sample is measured at room temperature. Analysis of the cation site ordering reveals a complex dependence on the Zn content, with the combined effect of Zn substitution of Fe 3+ ions on tetrahedral sites, together with Fe 2+ cation oxidation, leading to the observed magnetization enhancement for low Zn doping levels. The improved magnetic properties give superior performance in MRI applications with an MRI contrast enhancement among the largest values reported, being more than 5 times larger than a commercial contrast agent (Feridex) measured under identical conditions.The synthesis technique applied here involves an environmentally benign route and offers the potential to tune the magnetic properties of magnetic
nanoparticles, with increased overall magnetization desirable for many different commercial applications.
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May 2014
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