Magnets
Theoretical Physics
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L. J.
Collins-Mcintyre
,
L. B.
Duffy
,
A.
Singh
,
N-J
Steinke
,
C. J.
Kinane
,
T. R.
Charlton
,
A.
Pushp
,
A. J.
Kellock
,
S. S. P.
Parkin
,
S. N.
Holmes
,
C. H. W.
Barnes
,
G.
Van Der Laan
,
S.
Langridge
,
T.
Hesjedal
Open Access
Abstract: We report the structural, electronic, and magnetic study of Cr-doped Sb2Te3 thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of x = 0.42 (in CrxSb2−xTe3), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in the c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from 2 μB to almost 3 μB per Cr ion. The transition temperature (Tc) depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phase pure films of x = 0.42, a Tc of 125K was determined. Electric transport measurements find surface-dominated transport below 10K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.
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Sep 2016
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I05-ARPES
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L.
Bawden
,
S. P.
Cooil
,
F.
Mazzola
,
J. M.
Riley
,
L. J.
Collins-Mcintyre
,
V.
Sunko
,
K. W. B.
Hunvik
,
M.
Leandersson
,
C. M.
Polley
,
T.
Balasubramanian
,
T. K.
Kim
,
M.
Hoesch
,
J. W.
Wells
,
G.
Balakrishnan
,
M. S.
Bahramy
,
P. D. C.
King
Diamond Proposal Number(s):
[11383]
Open Access
Abstract: Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.
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May 2016
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I10-Beamline for Advanced Dichroism - scattering
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Diamond Proposal Number(s):
[10207]
Abstract: We investigate the magnetic properties of Ho-doped Bi2Te3 thin films grown by molecular beam epitaxy. Analysis of the polarized X-ray absorption spectra at the Ho M5 absorption edge gives an effective 4f magnetic moment which is ∼45% of the Hund's rule ground state value. X-ray magnetic circular dichroism (XMCD) shows no significant anisotropy, which suggests that the reduced spin moment is not due to the crystal field effects, but rather the presence of non-magnetic or antiferromagnetic Ho sites. Extrapolating the temperature dependence of the XMCD measured in total electron yield and fluorescence yield mode in a field of 7 T gives a Curie–Weiss temperature of ϑCW ≈ –30 K, which suggests antiferromagnetic ordering, in contrast to the paramagnetic behavior observed with SQUID magnetometry. From the anomaly of the XMCD signal at low temperatures, a Néel temperature TN between 10 K and 25 K is estimated.
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Apr 2016
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I10-Beamline for Advanced Dichroism - scattering
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Diamond Proposal Number(s):
[10207]
Abstract: We report a study on the transition temperature TC of Cr-doped topological insulator thin films, where an increase in the ferromagnetic onset can provide a pathway towards low-power spintronics in the future. Arrott plots, measured by surface-sensitive x-ray magnetic circular dichroism at the Cr L2,3 edge as a function of field at various low temperatures, give a TC ≈ 7 K for the pristine surface. This is comparable to the bulk value of the film, which means that there is no indication that the spontaneous magnetization is different near the surface. Evaporation of a thin layer of Co onto the pristine surface of the in-situ cleaved sample increases the ordering temperature near the surface to ∼19 K, while in the bulk it rises to ∼10 K. X-ray absorption spectroscopy shows that Cr enters the Bi2Se3 host matrix in a divalent state, and is unchanged by the Co deposition. These results demonstrate a straightforward procedure to increase the transition temperature of doped topological insulators.
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Sep 2015
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I10-Beamline for Advanced Dichroism - scattering
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Diamond Proposal Number(s):
[9210, 11500]
Abstract: In the field of spintronics ,the generation of a pure spin current (without macroscopic charge flow) through spin pumping of a ferromagnetic(FM) layer opens up the perspective of a new generation of dissipation-less devices. Microwave driven ferromagnetic resonance (FMR) can generate a pure spin current that enters adjacent layers, allowing for both magnetization reversal (through spin-transfer torque) and to probe spin coherence in non-magnetic materials. However, standard FMR is unable to probe multilayer dynamics directly, since the measurement averages over the contributions from the whole system. The synchrotron radiation-based technique of x-ray detected FMR (XFMR) offers an elegant solutiontothisdrawback,givingaccesstoelement-,site-,andlayer-specific dynamical measurements in heterostructures. In this work, we show how XFMR has provided unique information to understand spin pumping and spin transfer torque effects through a topological insulator (TI) layer in a pseudo-spin valve heterostructure. We demonstrate that TIs function as efficient spin sinks, while also allowing a limited dynamic coupling between ferromagnetic layers. These results shed new light on the spin dynamics of this novel class of materials, and suggest future directions for the development of room temperature TI-based spintronics.
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Jul 2015
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B18-Core EXAFS
I10-Beamline for Advanced Dichroism - scattering
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Diamond Proposal Number(s):
[9178, 9234]
Abstract: Ferromagnetically doped topological insulators with broken time-reversal symmetry are a prerequisite for observing the quantum anomalous Hall effect. Cr-doped (Bi,Sb)2(Se,Te)3 is the most successful materials system so far, as it combines ferromagnetic ordering with acceptable levels of additional bulk doping. Here, we report a study of the local electronic structure of Cr dopants in epitaxially grown Bi2Se3 thin films. Contrary to the established view that the Cr dopant is trivalent because it substitutionally replaces Bi3+, we find instead that Cr is divalent. This is evidenced by the energy positions of the Cr K and L2,3 absorption edges relative to reference samples. The extended x-ray absorption fine structure at the K edge shows that the Cr dopants substitute on octahedral sites with the surrounding Se ions contracted by delta d = 0.36 Angstrom , in agreement with recent band structure calculations. Comparison of the Cr L2,3 x-ray magnetic circular dichroism at T = 5 K with multiplet calculations gives a spin moment of 3.64 muB/Crbulk, which is close to the saturation moment for Cr2+ d4. The reduced Cr oxidation state in doped Bi2Se3 is ascribed to the formation of a covalent bond between Cr d(eg) and Se p orbitals, which is favored by the contraction of the Cr-Se distances.
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Oct 2014
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I10-Beamline for Advanced Dichroism - scattering
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Liam
Collins-Mcintyre
,
S. E.
Harrison
,
P.
Schönherr
,
N.-J.
Steinke
,
C. J.
Kinane
,
T. R.
Charlton
,
D.
Alba-Veneroa
,
A.
Pushp
,
A. J.
Kellock
,
S. S. P.
Parkin
,
J. S.
Harris
,
S.
Langridge
,
G.
Van Der Laan
,
T.
Hesjedal
Open Access
Abstract: We report the structural and magnetic study of Cr-doped Bi2Se3 thin films using x-ray diffraction (XRD), magnetometry and polarized neutron reflectometry (PNR). Epitaxial layers were grown on c-plane sapphire by molecular beam epitaxy in a two-step process. High-resolution XRD shows the exceptionally high crystalline quality of the doped films with no parasitic phases up to a Cr concentration of 12% (in % of the Bi sites occupied by substitutional Cr). The magnetic moment, measured by SQUID magnetometry, was found to be per Cr ion. The magnetic hysteresis curve shows an open loop with a coercive field of . The ferromagnetic transition temperature was determined to be analyzing the magnetization-temperature gradient. PNR shows the film to be homogeneously ferromagnetic with no enhanced magnetism near the surface or interface.
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Sep 2014
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[5771, 8321, 8608]
Open Access
Abstract: High-density growth of single-crystalline Bi2Se2Te nanowires was achieved via the vapour-liquid-solid process. The stoichiometry of samples grown at various substrate temperatures is precisely determined based on energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy on individual nanowires. We discuss the growth mechanism and present insights into the catalyst-precursor interaction.
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Jun 2014
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I06-Nanoscience (XPEEM)
I10-Beamline for Advanced Dichroism - scattering
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S. E.
Harrison
,
L.
Collins-Mcintyre
,
S.
Li
,
A.
Baker
,
L.
Shelford
,
Y.
Huo
,
A.
Pushp
,
S. S. P.
Parkin
,
J. S.
Harris
,
E.
Arenholz
,
G.
Van Der Laan
,
T.
Hesjedal
Abstract: Incorporation of magnetic dopants into topological insulators to break time-reversal symmetry is a prerequisite for observing the quantum anomalous Hall (QAHE) effect and other novel magnetoelectric phenomena. GdBiTe3 with a Gd:Bi ratio of 1:1 is a proposed QAHE system, however, the reported solubility limit for Gd doping into Bi2Te3 bulk crystals is between ?0.01 and 0.05. We present a magnetic study of molecular beam epitaxy grown (Gd x Bi1– x )2Te3 thin films with a high Gd concentration, up to x ? 0.3. Magnetometry reveals that the films are paramagnetic down to 1.5?K. X-ray magnetic circular dichroism at the Gd M 4,5 edge at 1.5?K reveals a saturation field of ?6?T, and a slow decay of the magnetic moment with temperature up to 200?K. The Gd3+ ions, which are substitutional on Bi sites in the Bi2Te3 lattice, exhibit a large atomic moment of ?7?? B , as determined by bulk-sensitive superconducting quantum interference device magnetometry. Surface oxidation and the formation of Gd2O3 lead to a reduced moment of ?4?? B as determined by surface-sensitive x-ray magnetic circular dichroism. Their large atomic moment makes these films suitable for incorporation into heterostructures, where interface polarization effects can lead to the formation of magnetic order within the topological insulators.
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Jan 2014
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I06-Nanoscience (XPEEM)
I10-Beamline for Advanced Dichroism - scattering
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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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