|
|
Rebecca
Scatena
,
Alberto
Hernández-Melián
,
Benjamin M.
Huddart
,
Sam
Curley
,
Robert C.
Williams
,
Pascal
Manuel
,
Stephen J.
Blundell
,
Zurab
Guguchia
,
Zachary E.
Manson
,
Jamie L.
Manson
,
G. Timothy
Noe
,
John
Singleton
,
Tom
Lancaster
,
Paul A.
Goddard
,
Roger D.
Johnson
Open Access
Abstract: We present single-crystal neutron diffraction, powder muon spin rotation, and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain [Cu(pym)(H2O)4]SiF6·H2O (pym = pyrimidine), which displays a fourfold-periodic rotation of the local environment around the Cu(ii) 𝑆=1/2 ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures ≥20 mK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3 T. From the polarization of magnetic moments observed with magnetic fields applied in the [−1,2,0] direction, we can identify the static magnetic structure that best accounts for the data. The proposed model is supported microscopically by the presence of an alternating component of the 𝑔 tensor, which produces an internal twofold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.
|
Aug 2025
|
|
I10-Beamline for Advanced Dichroism - scattering
|
M.
Gomilšek
,
T. j.
Hicken
,
M. n.
Wilson
,
K. J. A.
Franke
,
B. M.
Huddart
,
A.
Štefančič
,
S. J. R.
Holt
,
G.
Balakrishnan
,
D. A.
Mayoh
,
M. t.
Birch
,
S. H.
Moody
,
H.
Luetkens
,
Z.
Guguchia
,
M. t. F.
Telling
,
P. J.
Baker
,
S. J.
Clark
,
T.
Lancaster
Open Access
Abstract: Skyrmions are particlelike vortices of magnetization with nontrivial topology, which are usually stabilized by Dzyaloshinskii-Moriya interactions (DMI) in noncentrosymmetric bulk materials. Exceptions are centrosymmetric Gd- and Eu-based skyrmion-lattice (SL) hosts with zero DMI, where both the SL stabilization mechanisms and magnetic ground states remain controversial. We address these here by investigating both the static and dynamical spin properties of the centrosymmetric SL host Gd2PdSi3 using muon spectroscopy. We find that spin fluctuations in the noncoplanar SL phase are highly anisotropic, implying that spin anisotropy plays a prominent role in stabilizing this phase. We also observe strongly anisotropic spin dynamics in the ground-state (IC-1) incommensurate magnetic phase of the material, indicating that it hosts a meronlike multi-𝑞 structure. In contrast, the higher-field, coplanar IC-2 phase is found to be single 𝑞 with nearly isotropic spin dynamics.
|
Jan 2025
|
|
I11-High Resolution Powder Diffraction
|
S.
Vaidya
,
A.
Hernández-Melián
,
J. P.
Tidey
,
S. P. M.
Curley
,
S.
Sharma
,
P.
Manuel
,
C.
Wang
,
G. L.
Hannaford
,
S. J.
Blundell
,
Z. E.
Manson
,
J. L.
Manson
,
J.
Singleton
,
T.
Lancaster
,
R. D.
Johnson
,
P. A.
Goddard
Open Access
Abstract: We investigate the magnetic properties of 𝑆=1 antiferromagnetic diamond-lattice, Ni𝑋2(pyrimidine)2 (𝑋=Cl, Br), hosting a single-ion anisotropy (SIA) orientation which alternates between neighboring sites. Through neutron diffraction measurements of the 𝑋=Cl compound, the ordered state spins are found to align collinearly along a pseduo-easy axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseduo-easy axis is also present for 𝑋=Br. We estimate the Hamiltonian parameters by combining analytical calculations and Monte Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseduo-easy axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseduo-easy axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields.
|
Nov 2024
|
|
I16-Materials and Magnetism
|
M. C.
Rahn
,
M. N.
Wilson
,
T. J.
Hicken
,
F. L.
Pratt
,
C.
Wang
,
F.
Orlandi
,
D. D.
Khalyavin
,
P.
Manuel
,
L. S. I.
Veiga
,
A.
Bombardi
,
S.
Francoual
,
P.
Bereciartua
,
A. S.
Sukhanov
,
J. D.
Thompson
,
S. M.
Thomas
,
P. F. S.
Rosa
,
T.
Lancaster
,
F.
Ronning
,
M.
Janoschek
Open Access
Abstract: Eu
5
In
2
Sb
6
is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of
Eu
5
In
2
Sb
6
single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic x-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of
Eu
5
In
2
Sb
6
results in coplanar noncollinear magnetic structures with an Ising-like net magnetization along the
a
axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-
q
structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.
|
May 2024
|
|
I10-Beamline for Advanced Dichroism - scattering
|
T. J.
Hicken
,
M. N.
Wilson
,
Z.
Salman
,
S. L.
Zhang
,
S. J. R.
Holt
,
T.
Prokscha
,
A.
Suter
,
F. L.
Pratt
,
G.
Van Der Laan
,
T.
Hesjedal
,
T.
Lancaster
Diamond Proposal Number(s):
[18898]
Open Access
Abstract: Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-
μ
+
SR
) measurements on a stack comprising
[
Ta
/CoFeB/
MgO
]
16
/Ta on a Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like structure that changes from Néel-like near the surface to Bloch-like deeper within the sample. LE-
μ
+
SR
reveals that the magnetic field distribution in the trilayers near the surface of the stack is distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support a quantitative analysis of the
μ
+
SR
results. By increasing the applied magnetic field, we find a reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a region dominated by skyrmions above approximately 180 mT.
|
Apr 2024
|
|
I11-High Resolution Powder Diffraction
|
Jamie L.
Manson
,
Zachary E.
Manson
,
Ashley
Sargent
,
Danielle Y.
Villa
,
Nicole L.
Etten
,
William J. A.
Blackmore
,
Samuel P. M.
Curley
,
Robert C.
Williams
,
Jamie
Brambleby
,
Paul A.
Goddard
,
Andrew
Ozarowski
,
Murray N.
Wilson
,
Benjamin M.
Huddart
,
Tom
Lancaster
,
Roger D.
Johnson
,
Stephen J.
Blundell
,
Jesper
Bendix
,
Kraig A.
Wheeler
,
Saul H.
Lapidus
,
Fan
Xiao
,
Serena
Birnbaum
,
John
Singleton
Diamond Proposal Number(s):
[18786]
Open Access
Abstract: We examine the crystal structures and magnetic properties of several S = 1 Ni(II) coordination compounds, molecules and polymers, that include the bridging ligands HF2-, AF62- (A = Ti, Zr) and pyrazine or non-bridging ligands F-, SiF62-, glycine, H2O, 1-vinylimidazole, 4-methylpyrazole and 3-hydroxypyridine. Pseudo-octahedral NiN4F2, NiN4O2 or NiN4OF cores consist of equatorial Ni-N bonds that are equal to or slightly longer than the axial Ni-Lax bonds. By design, the zero-field splitting (D) is large in these systems and, in the presence of substantial exchange interactions (J), can be difficult to discriminate from magnetometry measurements on powder samples. Thus, we relied on pulsed-field magnetization in those cases and employed electron-spin resonance (ESR) to confirm D when J << D. The anisotropy of each compound was found to be easy-plane (D > 0) and range from ≈ 8-25 K. This work reveals a linear correlation between the ratio d(Ni-Lax)/d(Ni-Neq) and D although the ligand spectrochemical properties may also be important. We assert that this relationship allows us to predict the type of magnetocrystalline anisotropy in tailored Ni(II) quantum magnets.
|
Jan 2020
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Open Access
Abstract: We present long-wavelength neutron diffraction data measured on both single crystal and polycrystalline samples of the skyrmion host material Cu2OSeO3. We observe magnetic satellites around the
(0
1
⎯
⎯
1)
(01¯1)
diffraction peak not accessible to other techniques, and distinguish helical from conical spin textures in reciprocal space. Our measurements show that not only the field-polarised phase but also the helical ground state are made up of ferrimagnetic clusters instead of individual spins. These clusters are distorted Cu tetrahedra, where the spin on one Cu ion is anti-aligned with the spin on the three other Cu ions.
|
Dec 2019
|
|
|
|
M. T.
Birch
,
R.
Takagi
,
S.
Seki
,
M. N.
Wilson
,
F.
Kagawa
,
A.
Štefančič
,
G.
Balakrishnan
,
R.
Fan
,
P.
Steadman
,
C. J.
Ottley
,
M.
Crisanti
,
R.
Cubitt
,
T.
Lancaster
,
Y.
Tokura
,
P. D.
Hatton
Abstract: Previous observations of metastable magnetic skyrmions have shown that close to the equilibrium pocket the metastable state has a short lifetime, and therefore, rapid cooling is required to generate a significant skyrmion population at low temperatures. Here, we report that the lifetime of metastable skyrmions in crystals of
Cu
2
OSeO
3
is extended by a factor of 50 with the introduction of only 2.5% zinc doping, allowing over 50% of the population to survive when field cooling at a rate of just 1 K/min. Our systematic study suggests that the lifetime enhancement is due to the increase in the pinning site density, rather than an alteration to the energy barrier of the decay process. We expect that doping can be exploited to control the lifetime of the metastable SkL state in other chiral magnets, offering a method of engineering skyrmion materials towards application in future devices.
|
Jul 2019
|
|
I10-Beamline for Advanced Dichroism - scattering
I16-Materials and Magnetism
|
Diamond Proposal Number(s):
[15119]
Abstract: We present the results of x-ray scattering and muon-spin relaxation (μ+SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120 periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. μ+SR measurements indicate a magnetically ordered phase throughout the bulk of the material below T N=105(5) K. There are signs of fluctuating magnetism in a narrow range of temperatures above T N involving low-energy excitations, while at temperatures well below T N behaviour characteristic of freezing of dynamics is observed, likely reflecting the effect of disorder in our polycrystalline sample. Using density functional theory we propose a distinct muon stopping site in this compound and assess the degree of distortion induced by the implanted muon.
|
Apr 2019
|
|
I11-High Resolution Powder Diffraction
I19-Small Molecule Single Crystal Diffraction
|
A.
Stefancic
,
S. H.
Moody
,
T. J.
Hicken
,
M. T.
Birch
,
G.
Balakrishnan
,
S. A.
Barnett
,
M.
Crisanti
,
J. S. O.
Evans
,
S. J. R.
Holt
,
K. J. A.
Franke
,
P. D.
Hatton
,
B. M.
Huddart
,
M. R.
Lees
,
F. L.
Pratt
,
C. C.
Tang
,
M. N.
Wilson
,
F.
Xiao
,
T.
Lancaster
Abstract: We present an investigation into the structural and magnetic properties of Zn-substituted Cu2OSeO3, a system in which the skyrmion lattice (SkL) phase in the magnetic field–temperature phase diagram was previously seen to split as a function of increasing Zn concentration. We find that splitting of the SkL is only observed in polycrystalline samples and reflects the occurrence of several coexisting phases with different Zn content, each distinguished by different magnetic behavior. No such multiphase behavior is observed in single-crystal samples.
|
Nov 2018
|
|
