Magnetism

Magnetic excitations and interactions

We are interested in the nature of magnetism in correlated materials such as complex oxides and how it relates to emergent phenomena such as high-temperature superconductivity. Our tool of choice for this is resonant inelastic x-ray scattering (RIXS), as illustrated in Fig. 1, in which a x-ray core hole resonance is used to couple x-rays to magnetism in order to measure collective magnetic excitations.

Figure 1: The resonant inelastic x-ray scattering (RIXS) process. The initial, intermediate, and final states are shown from left to right. Photons are represented as wavy red lines and electronic transitions are shown as green arrows. In this process spin excitations are created and their dispersion can be measured by determining the energy and momentum change of the photon.

References

2022

Emergence of Spinons in Layered Trimer Iridate Ba₄Ir₃O₁₀

Y. Shen, J. Sears, G. Fabbris, A. Weichselbaum, W. Yin, H. Zhao, D. G. Mazzone, H. Miao, M. H. Upton, D. Casa, R. Acevedo-Esteves, C. Nelson, A. M. Barbour, C. Mazzoli, G. Cao, and M. P. M. Dean

Phys. Rev. Lett. 129, 207201 (2022)

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Spinons are well known as the elementary excitations of one-dimensional antiferromagnetic chains, but means to realize spinons in higher dimensions is the subject of intense research. Here, we use resonant x-ray scattering to study the layered trimer iridate Ba₄Ir₃O₁₀, which shows no magnetic order down to 0.2 K. An emergent one-dimensional spinon continuum is observed that can be well described by XXZ spin-1/2 chains with a magnetic exchange of 55 meV and a small Ising-like anisotropy. With 2% isovalent Sr doping, magnetic order appears below TN=130 K along with sharper excitations in (Ba_1-xSr_x)₄Ir₃O₁₀ . Combining our data with exact diagonalization calculations, we find that the frustrated intratrimer interactions effectively reduce the system into decoupled spin chains, the subtle balance of which can be easily tipped by perturbations such as chemical doping. Our results put Ba₄Ir₃O₁₀ between the one-dimensional chain and two-dimensional quantum spin liquid scenarios, illustrating a new way to suppress magnetic order and realize fractional spinons.
Role of Oxygen States in the Low Valence Nickelate La₄Ni₃O₈

Y. Shen, J. Sears, G. Fabbris, J. Li, J. Pelliciari, I. Jarrige, Xi He, I. Bozovic, M. Mitrano, Junjie Zhang, J. F. Mitchell, A. S. Botana, V. Bisogni, M. R. Norman, S. Johnston, and M. P. M. Dean

Phys. Rev. X 12, 011055 (2022)

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[BNL Press Release]

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O K-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La4Ni3O8 and a prototypical cuprate La2−xSrxCuO4 (x=0.35). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer–Mott-Hubbard character with the Coulomb repulsion U of similar size to the charge-transfer energy Δ. Nevertheless, the transition-metal-oxygen hopping is larger in La4Ni3O8 than in La2−xSrxCuO4, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La4Ni3O8 despite its larger Δ. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

2021

Strong Superexchange in a d^9-δ Nickelate Revealed by Resonant Inelastic X-Ray Scattering

J. Q. Lin, P. Villar Arribi, G. Fabbris, A. S. Botana, D. Meyers, H. Miao, Y. Shen, D. G. Mazzone, J. Feng, S. G. Chiuzbăian, A. Nag, A. C. Walters, M. Garcı́a-Fernández, Ke-Jin Zhou, J. Pelliciari, I. Jarrige, J. W. Freeland, Junjie Zhang, J. F. Mitchell, V. Bisogni, X. Liu, M. R. Norman, and M. P. M. Dean

Phys. Rev. Lett. 126, 087001 (2021)

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The discovery of superconductivity in a d^9-δ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced d^9-1/3 trilayer nickelates R4Ni3O8 (where R=La, Pr) and associated theoretical modeling. A magnon energy scale of ∼80 meV resulting from a nearest-neighbor magnetic exchange of J=69(4) meV is observed, proving that d^9-δ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.

2019

Anomalous magnetoresistance due to longitudinal spin fluctuations in a Jeff = 1/2 Mott semiconductor

Lin Hao, Zhentao Wang, Junyi Yang, D. Meyers, Joshua Sanchez, Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, Daniel Haskel, Philip J. Ryan, Kipton Barros, Jiun-Haw Chu, M. P. M. Dean, Cristian D. Batista, and Jian Liu

Nature Communications 10, 5301 (2019)

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As a hallmark of electronic correlation, spin-charge interplay underlies many emergent phenomena in doped Mott insulators, such as high-temperature superconductivity, whereas the half-filled parent state is usually electronically frozen with an antiferromagnetic order that resists external control. We report on the observation of a positive magnetoresistance that probes the staggered susceptibility of a pseudospin-half square-lattice Mott insulator built as an artificial SrIrO3/SrTiO3 superlattice. Its size is particularly large in the high-temperature insulating paramagnetic phase near the Néel transition. This magnetoresistance originates from a collective charge response to the large longitudinal spin fluctuations under a linear coupling between the external magnetic field and the staggered magnetization enabled by strong spin-orbit interaction. Our results demonstrate a magnetic control of the binding energy of the fluctuating particle-hole pairs in the Slater-Mott crossover regime analogous to the Bardeen-Cooper-Schrieffer-to-Bose-Einstein condensation crossover of ultracold-superfluids.

2018

Giant magnetic response of a two-dimensional antiferromagnet

Lin Hao, D Meyers, Hidemaro Suwa, Junyi Yang, Clayton Frederick, Tamene R Dasa, Gilberto Fabbris, Lukas Horak, Dominik Kriegner, Yongseong Choi, Jong-Woo Kim, Daniel Haskel, Philip J. Ryan, Haixuan Xu, Cristian D. Batista, M. P. M. Dean, and Jian Liu

Nature Physics 14, 806 (2018)

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[BNL Press Release] [UTK Press Release] [ANL Highlight]

A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter1. Such coupling is possible via the Dzyaloshinskii–Moriya (DM) interaction2,3, but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy4. We solve this long-standing problem with a top-down approach that utilizes spin–orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square lattice artificially synthesized as a superlattice of SrIrO₃ and SrTiO₃. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. Our results demonstrate that symmetry can be exploited in spin–orbit-coupled magnets to develop functional AFM materials for fast and secured spintronic devices5,6,7,8,9.
Imaging antiferromagnetic antiphase domain boundaries using magnetic Bragg diffraction phase contrast

Min Gyu Kim, Hu Miao, Bin Gao, S-W Cheong, C Mazzoli, A Barbour, Wen Hu, SB Wilkins, Ian K Robinson, MPM Dean, and V. Kiryukhin

Nature communications 9, 5013 (2018)

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[BNL Press Release]

Manipulating magnetic domains is essential for many technological applications. Recent breakthroughs in Antiferromagnetic Spintronics brought up novel concepts for electronic device development. Imaging antiferromagnetic domains is of key importance to this field. Unfortunately, some of the basic domain types, such as antiphase domains, cannot be imaged by conventional techniques. Herein, we present a new domain projection imaging technique based on the localization of domain boundaries by resonant magnetic diffraction of coherent X rays. Contrast arises from reduction of the scattered intensity at the domain boundaries due to destructive interference effects. We demonstrate this approach by imaging antiphase domains in a collinear antiferromagnet Fe₂Mo₃O₈, and observe evidence of domain wall interaction with a structural defect. This technique does not involve any numerical algorithms. It is fast, sensitive, produces large-scale images in a single-exposure measurement, and is applicable to a variety of magnetic domain types.

2017

High-temperature charge density wave correlations in La_1.875Ba_0.125CuO₄ without spin–charge locking

H Miao, José Lorenzana, Götz Seibold, YY Peng, A Amorese, F Yakhou-Harris, K Kummer, NB Brookes, RM Konik, V Thampy, G. D. Gu, G. Ghiringhelli, L. Braicovich, and M. P. M. Dean

Proceedings of the National Academy of Sciences 114, 12430–12435 (2017)

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[BNL Press Release]

Although all superconducting cuprates display charge-ordering tendencies, their low-temperature properties are distinct, imped- ing efforts to understand the phenomena within a single con- ceptual framework. While some systems exhibit stripes of charge and spin, with a locked periodicity, others host charge den- sity waves (CDWs) without any obviously related spin order. Here we use resonant inelastic X-ray scattering to follow the evolution of charge correlations in the canonical stripe-ordered cuprate La_1.875Ba_0.125CuO₄ across its ordering transition. We find that high-temperature charge correlations are unlocked from the wavevector of the spin correlations, signaling analogies to CDW phases in various other cuprates. This indicates that stripe order at low temperatures is stabilized by the coupling of otherwise inde- pendent charge and spin density waves, with important implica- tions for the relation between charge and spin correlations in the cuprates. charge
Doping dependence of collective spin and orbital excitations in the Spin-1 quantum antiferromagnet La_2-xSr_xNiO₄ Observed by X rays

G Fabbris, D Meyers, L Xu, VM Katukuri, L Hozoi, X Liu, ZY Chen, J Okamoto, T Schmitt, AC Uldry, B. Delley, G. D. Gu, D. Prabhakaran, A. T. Boothroyd, J. Brink, D. J. Huang, and M. P. M. Dean

Phys. Rev. Lett. 118, (2017)

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We report the first empirical demonstration that resonant inelastic x-ray scattering (RIXS) is sensitive to collective magnetic excitations in S=1 systems by probing the Ni L₃ edge of La_2−xSr_xNiO₄ (x=0, 0.33, 0.45). The magnetic excitation peak is asymmetric, indicating the presence of single and multi-spin-flip excitations. As the hole doping level is increased, the zone boundary magnon energy is suppressed at a much larger rate than that in hole doped cuprates. Based on the analysis of the orbital and charge excitations observed by RIXS, we argue that this difference is related to the orbital character of the doped holes in these two families. This work establishes RIXS as a probe of fundamental magnetic interactions in nickelates opening the way towards studies of heterostructures and ultrafast pump-probe experiments.

2016

Ultrafast energy and momentum resolved dynamics of magnetic correlations in photo-doped Mott insulator Sr₂IrO₄

MPM Dean, Yue Cao, X Liu, S Wall, D Zhu, Roman Mankowsky, V Thampy, XM Chen, JG Vale, D Casa, Jungho Kim, A. H. Said, P. Juhas, R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada, T. Katayama, M. Yabashi, Yoshikazu Tanaka, T. Togashi, J. Liu, C. Rayan Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först, and J. P. Hill

Nature Materials 15, 601–605 (2016)

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Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and higherature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr₂IrO₄. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Neel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

2015

Insights into the high temperature superconducting cuprates from resonant inelastic X-ray scattering

MPM Dean

Journal of Magnetism and Magnetic Materials 376, 3–13 (2015)

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Recent improvements in instrumentation have established resonant inelastic X-ray scattering (RIXS) as a valuable new probe of the magnetic excitations in the cuprates. This paper introduces RIXS, focusing on the Cu L₃ resonance, and reviews recent experiments using this technique. These are discussed in light of other experimental probes such as inelastic neutron scattering and Raman scattering. The success of these studies has motivated the development of several new RIXS spectrometers at synchrotrons around the world that promise, among other improvements, 5–10 times better energy resolution. We finish by outlining several key areas which hold promise for further important discoveries in this emerging field.

2013

Ferromagnetic Exchange Anisotropy from Antiferromagnetic Superexchange in the Mixed 3 d- 5 d Transition-Metal Compound Sr₃CuIrO₆

Wei-Guo Yin, X Liu, AM Tsvelik, MPM Dean, MH Upton, Jungho Kim, D Casa, A Said, T Gog, TF Qi, G. Cao, and J. P. Hill

Phys. Rev. Lett. 111, 057202 (2013)

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We report a combined experimental and theoretical study of the unusual ferromagnetism in the one- dimensional copper-iridium oxide Sr₃CuIrO₆. Utilizing Ir L3 edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromag- netic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu²⁺O₄ plaquettes and Ir⁴⁺O₆ octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.
High-energy magnetic excitations in the cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ: towards a unified description of its electronic and magnetic degrees of freedom

MPM Dean, AJA James, RS Springell, X Liu, C Monney, KJ Zhou, RM Konik, JS Wen, ZJ Xu, GD Gu, V. N. Strocov, T. Schmitt, and J. P. Hill

Phys. Rev. Lett. 110, 147001 (2013)

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We investigate the high-energy magnetic excitation spectrum of the high-T(c) cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) using Cu L(3) edge resonant inelastic x-ray scattering. Broad, dispersive magnetic excitations are observed, with a zone boundary energy of ∼ 300 meV and a weak dependence on doping. These excitations are strikingly similar to the bosons proposed to explain the high-energy "kink" observed in photoemission. A phenomenological calculation of the spin response, based on a parametrization of the the angle-resolved photoemission spectroscopy derived electronic structure and Yang-Rice-Zhang quasiparticles, provides a reasonable prediction of the energy dispersion of the observed magnetic excitations. These results indicate a possible unified framework to reconcile the magnetic and electronic properties of the cuprates and we discuss the advantages and disadvantages of such an approach.
Persistence of magnetic excitations in La_2-xSr_xCuO₄ from the undoped insulator to the heavily overdoped non-superconducting metal

M. P. M. Dean, G. Dellea, R. S. Springell, F. Yakhou-Harris, K. Kummer, N. B. Brookes, X. Liu, Y.-J. Sun, J. Strle, T. Schmitt, and J. P. Hill

Nature Materials 12 12, 1019–1023 (2013)

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[BNL Press Release]

One of the most intensely studied scenarios of high- temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations1. Indeed, such excitations have been observed up to optimal doping in the cuprates2–7. In the heavily overdoped regime, neutron scattering mea- surements indicate that magnetic excitations have effectively disappeared8–10, and this has been argued to cause the demise of HTS with overdoping1,8,10. Here we use resonant inelastic X-ray scattering, which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La_2−xSr_xCuO₄ across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x=0.40). For x=0, well-defined magnon excitations are observed11. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations. Other factors, such as the redistribution of spectral weight, must be considered.

2012

Spin excitations in a single La₂CuO₄ layer

MPM Dean, RS Springell, C Monney, KJ Zhou, J Pereiro, I Božović, Bastien Dalla Piazza, HM Rønnow, E Morenzoni, J Van Den Brink, T. Schmitt, and J. P. Hill

Nature Materials 11, 850–854 (2012)

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Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates. In bulk cuprates such as La₂CuO₄, the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature8. Here, we measure the spin response of isolated layers of La₂CuO₄ that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La2CuO4, with no evidence for more complex correlations such as resonating valence bond correlations9,10,11. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories.