I16-Materials and Magnetism
|
Diamond Proposal Number(s):
[27353]
Open Access
Abstract: Magnetic topological insulators and semimetals are a class of crystalline solids whose properties are strongly influenced by the coupling between non-trivial electronic topology and magnetic spin configurations. Such materials can host exotic electromagnetic responses. Among these are topological insulators with certain types of antiferromagnetic order which are predicted to realize axion electrodynamics. Here we investigate the highly unusual helimagnetic phases recently reported in EuIn2As2, which has been identified as a candidate for an axion insulator. Using resonant elastic x-ray scattering we show that the two types of magnetic order observed in EuIn2As2 are spatially uniform phases with commensurate chiral magnetic structures, ruling out a possible phase-separation scenario, and we propose that entropy associated with low energy spin fluctuations plays a significant role in driving the phase transition between them. Our results establish that the magnetic order in EuIn2As2 satisfies the symmetry requirements for an axion insulator.
|
Jun 2023
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Haonan
Jin
,
Wancong
Tan
,
Yizhou
Liu
,
Kejing
Ran
,
Raymond
Fan
,
Yanyan
Shangguan
,
Yao
Guang
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Jinsheng
Wen
,
Guoqiang
Yu
,
Shilei
Zhang
Diamond Proposal Number(s):
[27692, 28933, 30749]
Open Access
Abstract: Topological defects are fundamental concepts in physics, but little is known about the transition between distinct types across different dimensionalities. In topological magnetism, as in field theory, the transition between 1D strings and 0D monopoles is a key process whose observation has remained elusive. Here, we introduce a novel mechanism that allows for the controlled stabilization of emergent monopoles and show that magnetic skyrmion strings can be folded into monopoles. Conversely, they act as seeds out of which the entire string structure can unfold, containing its complete information. In chiral magnets, this process can be observed by resonant elastic X-ray scattering when the objects are in proximity to a polarized ferromagnet, whereby a pure monopole lattice is emerging on the surface. Our experimental proof of the reversible evolution from monopole to string sheds new light on topological defects and establishes the emergent monopole lattice as a new 3D topological phase.
|
Jun 2023
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Diamond Proposal Number(s):
[30613]
Open Access
Abstract: Magnetic chiral soliton lattices (CSLs) emerge from the helical phase in chiral magnets when magnetic fields are applied perpendicular to the helical propagation vector, and they show great promise for next-generation magnetic memory applications. These one-dimensional structures are previously observed at low temperatures in samples with uniaxial symmetry. Here, it is found that in-plane fields are the key to stabilizing the CSL in cubic Co8Zn10Mn2 over the entire temperature range from 15 K to below the Curie temperature (365 K). Using small-angle resonant elastic X-ray scattering, it is observed that the CSL is stabilized with an arbitrary in-plane propagation vector, while its thin plate geometry plays a deciding role in the soliton wavelength as a function of applied field. This work paves the way for high temperature, real world applications of soliton physics in future magnetic memory devices.
|
Apr 2023
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Xiaodong
Xie
,
Kejing
Ran
,
Yizhou
Liu
,
Raymond
Fan
,
Wancong
Tan
,
Haonan
Jin
,
Manuel
Valvidares
,
Nicolas
Jaouen
,
Haifeng
Du
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Shilei
Zhang
Diamond Proposal Number(s):
[20437, 26148, 22629]
Abstract: We identify a three-dimensional skyrmion side-face state in chiral magnets that consists of a thin layer of modulated surface spirals and an array of phase-locked skyrmion screws. Such chiral spin structures lead to a characteristic X-shaped magnetic diffraction pattern in resonant elastic x-ray scattering, reminiscent of Photo 51 of the DNA double-helix diffraction. By measuring both thin plates and bulk
Cu
2
OSeO
3
crystals in the field-in-plane geometry, we unambiguously identify the modulated skyrmion strings by retrieving their chirality and helix angle. The breaking of the translational symmetry along the side faces suppresses the bulk-favored conical state, providing a stabilization mechanism for the skyrmion lattice phase that has been overlooked so far.
|
Feb 2023
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Margaret R.
Mccarter
,
Kook Tae
Kim
,
Vladimir A.
Stoica
,
Sujit
Das
,
Christoph
Klewe
,
Elizabeth P.
Donoway
,
David M.
Burn
,
Padraic
Shafer
,
Fanny
Rodolakis
,
Mauro A. p.
Gonçalves
,
Fernando
Gómez-Ortiz
,
Jorge
Íñiguez
,
Pablo
García-Fernández
,
Javier
Junquera
,
Stephen W.
Lovesey
,
Gerrit
Van Der Laan
,
Se Young
Park
,
John W.
Freeland
,
Lane W.
Martin
,
Dong Ryeol
Lee
,
Ramamoorthy
Ramesh
Diamond Proposal Number(s):
[24797]
Abstract: An escalating challenge in condensed-matter research is the characterization of emergent order-parameter nanostructures such as ferroelectric and ferromagnetic skyrmions. Their small length scales coupled with complex, three-dimensional polarization or spin structures makes them demanding to trace out fully. Resonant elastic x-ray scattering (REXS) has emerged as a technique to study chirality in spin textures such as skyrmions and domain walls. It has, however, been used to a considerably lesser extent to study analogous features in ferroelectrics. Here, we present a framework for modeling REXS from an arbitrary arrangement of charge quadrupole moments, which can be applied to nanostructures in materials such as ferroelectrics. With this, we demonstrate how extended reciprocal space scans using REXS with circularly polarized x rays can probe the three-dimensional structure and chirality of polar skyrmions. Measurements, bolstered by quantitative scattering calculations, show that polar skyrmions of mixed chirality coexist, and that REXS allows valuation of relative fractions of right- and left-handed skyrmions. Our quantitative analysis of the structure and chirality of polar skyrmions highlights the capability of REXS for establishing complex topological structures toward future application exploits.
|
Dec 2022
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Diamond Proposal Number(s):
[23784]
Open Access
Abstract: Magnetic skyrmions (skyrmions hereafter) are magnetization configurations, whose topological robustness and nanoscale size have led to speculation that they could find use as a next-generation information carrier. Skyrmions have been observed in magnetic multilayer materials that are thin compared to the radius of a skyrmion, and chiral cubic single crystals that can be far larger than any characteristic skyrmion scale. In these single crystals, one would expect that skyrmions could exhibit interesting three-dimensional (3D) characteristics. Here, the symmetry of the micromagnetic free energy is investigated. This symmetry permits a complex 3D modulation of a skyrmion string, which we show to be a requirement of a skyrmion coexisting with the conical state. We discuss the implications of this modulation with respect to Thiele's equation and interskyrmion interactions. Further to this internal modulation, we study theoretically and show experimentally that the strings themselves must contort towards the surfaces of their confining crystals.
|
Dec 2022
|
|
I16-Materials and Magnetism
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Abstract: The burgeoning field of quantum materials concerns systems that do not adhere to the traditional theories of condensed matter physics. A key feature of these materials is a strong coupling between structural, electronic and magnetic degrees of freedom, which is especially prominent in 4d and 5d transition- metal oxides. The consequences of this coupling are wide, stabilising a range of emergent phases that are sensitive to perturbation. In this thesis, I develop novel techniques based on neutron and x-ray scattering to characterise and control electron-lattice coupling in 4d and 5d quantum materials.
I begin with Ca3Ru2O7, a 4d polar metal that hosts a spin-reorientation transition. Using neutron and resonant x-ray scattering, I reveal a new cy- cloidal magnetic phase, arising from spin-orbit coupling, that rapidly evolves with temperature to mediate the transition. I further show that the cycloid- mediated spin-reorientation can be driven by anisotropic strain, demonstrating the control enabled by coupling to the lattice.
I then turn to resonant inelastic x-ray scattering (RIXS), which has re- cently received interest as a new probe of electron-phonon coupling (EPC). Using graphite as a model system, I demonstrate the power of RIXS to probe the momentum-dependent EPC for a range of excited electronic states. Our RIXS data reveal some key deficiencies of current theoretical models of phonon excitations in RIXS, and prompt the development of a new Green’s-function– based model by our collaborators to address these issues.
Finally, I present a study of the 5d material Sr2IrO4, a famous jeff = 1/2 spin-orbit Mott insulator. I characterise the phonon spectrum with non- resonant inelastic x-ray scattering, before using RIXS to explore the phonon and magnon excitations. I find a strong EPC similar to that seen in the cuprates, and offer a new interpretation of the magnon dispersion involving coupling to spin-orbit excitons.
|
Jun 2022
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Kook Tae
Kim
,
Margaret R.
Mccarter
,
Vladimir A.
Stoica
,
Sujit
Das
,
Christoph
Klewe
,
Elizabeth P.
Donoway
,
David M.
Burn
,
Padraic
Shafer
,
Fanny
Rodolakis
,
Mauro A. P.
Gonçalves
,
Fernando
Gómez-Ortiz
,
Jorge
Íñiguez
,
Pablo
García-Fernández
,
Javier
Junquera
,
Sandhya
Susarla
,
Stephen W.
Lovesey
,
Gerrit
Van Der Laan
,
Se Young
Park
,
Lane W.
Martin
,
John W.
Freeland
,
Ramamoorthy
Ramesh
,
Dong Ryeol
Lee
Diamond Proposal Number(s):
[24797]
Open Access
Abstract: Resonant elastic X-ray scattering (REXS) offers a unique tool to investigate solid-state systems providing spatial knowledge from diffraction combined with electronic information through the enhanced absorption process, allowing the probing of magnetic, charge, spin, and orbital degrees of spatial order together with electronic structure. A new promising application of REXS is to elucidate the chiral structure of electrical polarization emergent in a ferroelectric oxide superlattice in which the polarization vectors in the REXS amplitude are implicitly described through an anisotropic tensor corresponding to the quadrupole moment. Here, we present a detailed theoretical framework and analysis to quantitatively analyze the experimental results of Ti L-edge REXS of a polar vortex array formed in a PbTiO3/SrTiO3 superlattice. Based on this theoretical framework, REXS for polar chiral structures can become a useful tool similar to x-ray resonant magnetic scattering (XRMS), enabling a comprehensive study of both electric and magnetic REXS on the chiral structures.
|
Apr 2022
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Yao
Guang
,
Kejing
Ran
,
Junwei
Zhang
,
Yizhou
Liu
,
Senfu
Zhang
,
Xuepeng
Qiu
,
Yong
Peng
,
Xixiang
Zhang
,
Markus
Weigand
,
Joachim
Gräfe
,
Gisela
Schütz
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Shilei
Zhang
,
Guoqiang
Yu
,
Xiufeng
Han
Diamond Proposal Number(s):
[23785]
Abstract: A three-dimensional singular point that consists of two oppositely aligned emergent monopoles is identified in continuous CoTb thin films, as confirmed by complementary techniques of resonant elastic x-ray scattering, Lorentz transmission electron microscopy, and scanning transmission x-ray microscopy. This new type of topological defect can be regarded as a superposition of an emergent magnetic monopole and an antimonopole, around which the source and drain of the magnetic flux overlap in space. We experimentally prove that the observed spin twist seen in Lorentz transmission electron microscopy reveals the cross section of the superimposed three-dimensional structure, providing a straightforward strategy for the observation of magnetic singularities. Such a quasiparticle provides an excellent platform for studying the rich physics of emergent electromagnetism.
|
Nov 2021
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Diamond Proposal Number(s):
[17402, 21868]
Open Access
Abstract: Magnetic skyrmions are vortex-like spin textures, which are usually treated as two-dimensional objects. In their lattice state, they form well-ordered, hexagonal structures, which have been studied in great detail. To obtain a three-dimensional (3D) skyrmion crystal, these planes can be envisaged to be stacked up forming skyrmion strings in the third dimension. Here, we report the observation of a 3D skyrmion phase in Cu2OSeO3 by carrying out reciprocal space mapping in resonant elastic x-ray scattering. We observe regions in the magnetic field-cooling phase diagram in which the skyrmion phase apparently coexists with the conical phase. However, such a coexistence is forbidden due to symmetry arguments. Instead, the skyrmion strings themselves are periodically modulated along their axes, as confirmed by micromagnetic simulations. The periodic modulation is in fact a necessary consequence of the evolution of the skyrmion phase out of the conical state and should therefore be a universal property of skyrmion strings in chiral helimagnets.
|
Aug 2021
|
|
