Session M64: Correlated Magnetism

Polaronic behavior in La1.2Sr1.8Mn2O7

La_1.2Sr_1.8Mn₂O₇ has been an intriguing candidate for probing electronic correlations due to the concomitant emergence of colossal magneto resistance with its paramagnetic-insulating to ferromagnetic-metallic transition at T_c = 120 K. Experimental evidence has demonstrated the relevance of polarons across the phase transition. In particular, early ARPES studies showed the abrupt formation of a small quasi-particle peak with strong mass renormalization in the metal phase which has been associated with a polaronic metal state. Yet, conflicting results revealing finite quasi particle spectral weight even above T_c and combined ARPES/STM studies showing sensitivity to the cleaved surface cast doubt on the polaronic metal scenario. Here we use resonant inelastic X-ray scattering (RIXS) which is a bulk-sensitive two-photon-probe. In contrast to ARPES which measures the single particle spectral function, RIXS probes collective excitations and is thus especially suitable for the investigation of charge-lattice coupled phenomena. In this presentation, we will discuss the RIXS signatures of polarons in La_1.2Sr_1.8Mn₂O₇.   

Visualizing ferro-rotational domains and domain walls using scanning nonlinear optical spectroscopy and microscopy

Ferroic orders have attracted growing interest over the past few decades. Among these include the ferro-rotational (FR) order, invariant under both time reversal and spatial inversion operations. Despite theoretical predictions of its wide presence and outstanding significance, the FR order is experimentally challenging to investigate due to its nontrivial conjugate coupling fields. Investigations of FR order domains and domain walls are critical not only to characterizing physical properties of the FR order but also in determining, for example, the multiferroic states in many type-II multiferroic candidates. Here, we report our efforts in using a series of second harmonic generation (SHG) techniques to investigate the FR domains and domain walls in NiTiO₃. We use a SHG rotational anisotropy (RA) technique to survey a NiTiO₃ sample and show the presence of two FR domain states that both and individually lack mirror symmetries but are related to each other by a vertical mirror operation. We then use scanning SHG microscopy to visualize the spatial distribution of the two FR domain states along with their corresponding domain walls. Finally, we take SHG RA measurements both across and along the domain walls and demonstrate the mirror symmetry is preserved at the domain walls, while broken within domains.   

Magnetic Dimers and Clusters in the Dilute (Sb1-xMnx)2Te3 Topological Insulator

Inelastic neutron scattering (INS) techniques reveal the predominant magnetic interactions in the dilute magnetic topological insulator (Sb_1-xMn_x)₂Te₃ (x = 0.03). The INS shows a series of local excitations that are associated with the formation of antiferromagnetically coupled Mn-Mn dimers, (confirming Mn²⁺ with s = 5/2).  Modeling the momentum-space dependence of these excitations confirms the formation of the dimer, and furthermore identifies them as interlayer next-nearest-neighbors (NNNs) Mn that substitute the Sb atoms in the adjacent A-B stacked bilayers.  We note that the dimers are connected through a Te with an Mn-Te-Mn bond angle of 180^o following the Goodenough-Kanamori-Anderson rule for p-d hybridized orbitals with dominant AFM. Low energy excitations (0.1 to 0.3 meV) in the (H,0,0) and (0,0,L) plane suggest in-plane FM nano-clusters coupled ferromagnetically across the van der Waals gap and antiferromagnetically in the A-B bilayer. The magnetic susceptibility of the dilute system is consistent with the INS results.  It is interesting that such low Mn concentrations (~3%) reveal interactions that are relevant to the intercalated MnSb_2nTe_3n+1 and with implications on the isostructural Bi-based compounds.   

Non-collinear chiral magnetic structure in rare earth nickelates

We resolved the debate on the non-collinear nature of the antiferromagnetic spin structure in nickelates. With the polarization resolved resonant X-ray scattering tuned to the Ni L₃-edge with azimuthal rotation, we revealed the unambiguous non-collinear chiral magnetic structure with a net chiral polarization on the domains in NdNiO₃. In combination with model calculations, our results allow accurate determine of the chiral magnetic structure and the population of the chiral domains that had not been observed in prior work on nickelates.   

Field Tunable Magnetic Transitions of CsCo2(MoO4)2(OH): A Triangular One-Dimensional Chain Structure with a Frustrated Geometry

Synthesis and characterization of complex competing magnetic interaction systems are a crucial part of the development of theoretical models that can predict the emergent physics in condensed matter systems. In particular, the oxyanion systems-based transition-metal (M) oxide sublattices that are magnetically isolated by closed-shell nonmagnetic oxyanions (SiO₄⁻⁴, PO₃⁻⁴, AsO₃⁻⁴) show great potential for exploring and characterizing new emergent phenomena. Recently we have synthesized single crystals of CsCo₂(MoO₄)₂(OH) and characterized the crystals structure. CsCo(MoO₄)₂(OH) belongs to a rare class of compounds, namely delta-chain or saw tooth type structure in which corner sharing isosceles triangles whose vertices consist of one Co(1) and two Co(2) atoms. The magnetic susceptibility reveals a long-range ordering around 5 K with a strong anisotropy. The isothermal magnetization data shown a stepwise magnetization when the magnetic field along the Co-O-Co (saw-tooth chain) direction. Two noticeable metamagnetic transitions were observed at 0.3 kOe and 40 kOe. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other.   

Application of machine learning to investigate the stabilization of skyrmions on a triangular Kondo lattice

Skyrmions are key contenders for future spintronic devices because of the topological protection caused by the whirling spin texture. The ideal candidates to realize skyrmions are materials with a band of conduction electrons coupled to the net moment of the localized f-electrons. Such materials are modelled via Kondo lattice model (KLM) which can be approximated to RKKY model in the weak-coupling regime. Away from this regime, significant four spin interactions that are non-analytic functions of the coupling constant emerge in the low-energy model rendering a perturbative treatment inapplicable. We  use supervised machine learning to obtain these effective interactions in both real space and momentum space. The resultant low-energy  effective Hamiltonian is used to study multiple-Q magnetic orderings on a triangular Kondo Lattice Model. We demonstrate the presence of the skyrmion phase even in absence of spin anisotropy, which is not possible within the pure RKKY model.   

Fractal Magnetic Domains in NdNiO3

Nanoscale probes often reveal complex, multiscale electronic structures at the surface of correlated quantum materials. Our resonant magnetic X-ray scattering nanoprobe measurements revealed a scale-free distribution of antiferromagnetic domains with fractal magnetic geometry [Li et al., Nat. Commun., 2019].  We use 4-state clock models to predict the expected shapes and statistics of these domains near the critical point.  Using cluster techniques, we compare the critical exponents from these models to the fractal magnetic textures observed with X-ray scattering in order to identify the critical point controlling the complex pattern formation.  We find that the critical exponents from the 2D Random Field 4-Clock Model are consistent with the critical exponents measured in NdNiO₃.    

Colossal Piezoresistance in Narrow-Gap Eu5In2Sb6

We report the discovery of a remarkably large piezoresistance in Eu₅In₂Sb₆ single crystals, wherein anisotropic metallic clusters naturally form within a semiconducting matrix due to electronic interactions. Eu₅In₂Sb₆ shows a highly anisotropic piezoresistance, and c-axis pressure of only 0.4 GPa leads to a resistivity drop of more than 99.95% that results in a record bulk gauge factor. Our result not only reveals the role of interactions and phase separation in the realization of colossal piezoresistance, but it also highlights a novel route to multi-functional devices with large responses to both pressure and magnetic field.   

Small moment antiferromagnetic ordering in single crystalline La2Ni7

Specific heat, electrical resistivity, and low field magnetization measurements indicate that there is a series of antiferromagnetic phase transitions at T₁ = 61.0 K, T₂ = 56.5 K and T₃ = 42.2 K in La₂Ni₇ single crystals at ambient pressure. They have very small entropy changes associated with them and the anisotropic M(H) data saturate at ~ 0.12 μ_B/Ni; both observations strongly suggest the AFM order is associated with very small, itinerant, moments. Analysis of M(T), M(H), ρ(T) and ρ(H) data allow for the construction of anisotropic H-T phase diagrams as well as the identification of the two lower temperature magnetically ordered states as antiferromagnetically ordered, with the moments aligned along the c-axis, and the higher temperature, T₂ < T < T₁, state as having a finite ferromagnetic component. Applying pressures up to 2.0 GPa in ρ(T) indicates that there is very little pressure dependence of T₁, T₂ and T₃ over this pressure range.   

Long-range magnetic order of La2Ni7

La₂Ni₇ has been recently shown to possess magnetic states associated with low-moment, itinerant Ni ordering.[1] Employing single-crystal neutron diffraction in zero magnetic field, we found one commensurate (C_c) and two incommensurate (IC_ab and IC_c) antiferromagnetic phases below 42.8(5), 58(1), and 62(1) K, respectively. The transition temperatures are consistent with the H-T phase diagrams shown in Ref. [1]. Although IC_ab and IC_c both have the same temperature dependent propagation vector, IC_c has magnetic moments aligned colinear to c whereas IC_ab has a significant magnetic moment component in the ab-plane. We found the phase transition to C_c to be first-order with a magnetic structure consistent with a triangular-wave modulated spin-density wave with magnetic moments colinear along c and propagation vector (0 0 1).

[1] Ribeiro, Raquel A., et al. arXiv:2110.06349 (2021).   

A DFT+U and many-body Quantum Monte Carlo study of monolayer MnO2 and VSe2

In the past decade, the search for two-dimensional (2D) ferromagnets has been at the forefront of the materials science community. Two promising 2D ferromagnets that have been synthesized and predicted to exceed the Curie temperature of CrI₃ are monolayer MnO₂ [1] and VSe₂ [2]. The theoretical methods used to predict the properties of these materials, such as density functional theory (DFT), have a demonstrated dependence on the approximations which can result in flawed predictions for the magnetic properties. For the case of VSe₂, DFT yields the semiconducting (H) or metallic (T) phases more favorable depending on which functional is used. For the case of MnO₂, the preferred magnetic coupling (ferromagnetic or antiferromagnetic) is also functional dependent. Due to these reasons, we employed many-body Diffusion Monte Carlo (DMC) to calculate the properties of monolayer MnO₂ and VSe₂. DMC has a much weaker dependence on the trial density functional and can accurately provide energetic and magnetic properties in 2D and bulk materials. Our results have achieved the statistical uncertainty required to resolve the energy differences between the magnetic and structural phases of 2D MnO₂ and VSe₂. Our work intends to serve as a terminal theoretical benchmark for MnO₂ and VSe₂​​​​​​​ monolayers, guide experimentalists, and aid in the future computational studies of other 2D magnetic materials using many-body methods.

[1] J. Phys. Chem. Lett., 4, 20, 3382–3386 (2013)

[2] Nature Nanotechnology, 13, 289–293 (2018)   

Electronic Structure and Magnetism of the Triple-layer Ruthenate Sr4Ru3O10

We report electronic band structure calculations for Sr₄Ru₃O₁₀ which displays both ferromagnetic and metamagnetic behavior. The density functional calculations find the ground state to be ferromagnetic in agreement with experiment, and inclusion of Coulomb Hubbard interaction U applied to the Ru 4d states has dramatic effects on the Fermi surface, which reveal the role of Coulomb interactions and correlated many-body physics.  The minority spin bands are mainly empty with Fermi surfaces in the outer areas of the Brillouin zone with bands that disperse steeply upward.   The majority-spin bands are full or nearly full and form narrow bands near the Fermi energy around the zone center. The results are in qualitative agreement with recent angle resolved photoemission spectroscopy (ARPES) experiments [1] and show the need for a combined theoretical study and ARPES investigation with better energy resolution to reveal the nature of the narrow bands close to the Fermi-level, which is critical for understanding the exotic magnetic properties observed in this material.

[1] P. Ngabonziza, et al., Surface and kink structures in Sr4Ru3O10 revealed by synchrotron-based ARPES, Sci. Rep. 10, 21062 (2020).   

Charge disorder induced magnetic order

In strongly correlated materials, clean non-magnetic samples, for example ruthenates[1-3], are sometimes found to develop long-range magnetic order via introduction of minute amount of non-magnetic disordered impurities. To explain such anti-intuitive behavior, we propose a scenario of inducing magnetic order via charge disorder in systems with coexisting local moment and itinerant degrees of freedom.  By disrupting the long-range quantum fluctuation originated from the itinerant carriers, the magnetic order preferred by the local moment can be re-established. We demonstrate this mechanism using a realistic spin-fermion model derived from the undoped FeSe as an example. We show that the magnetic order can indeed be recovered once the length scale of phase coherence of the itinerant carriers becomes shorter than that of the necessary quantum fluctuations as a result of enhanced disorder.  Our simple idea has a general applicability as it reflects the rich physics in quantum many-body systems through interplay between multiple components over different length scale.

[1] M. Braden et al., Phys. Rev. Lett.88, 197002 (2002).

[2] M. Minakata and Y. Maeno, Phys. Rev. B63, 180504 (2001).

[3] K. Ishida et al., Phys. Rev. B 67, 214412 (2003).   

Symmetry breaking Fermi liquid state in single-crystalline nanorod of Pr2Ir2O7

Pyrochlore Iridates display an interesting interplay of geometric frustration, spin-orbit coupling, and Coulomb correlation potentially leading to a wide variety of novel quantum phases[1].  Among the Pyrochlore Iridates, Pr₂Ir₂O₇ is known to be a chiral spin liquid with macroscopically broken time-reversal symmetry but no magnetic long-range ordering down to the lowest temperature. Moreover, unlike many other materials with Pyrochlore lattices, Pr₂Ir₂O₇ is a metal with relatively low resistivity and a positive temperature coefficient. At low temperatures, the resistivity shows a shallow minimum[2]. The local Pr³⁺ 4f² moments have non-Kramer’s J = 4 doublet ground state, separated from the excited Kramer’s doublet by a gap of 160 K. If we consider valence fluctuations to the excited Kramer’s doublet leading to a two-channel Kondo effect instead of the single-channel effect, it should lead to heavy Fermi liquid formation and an associated mean-field like phase transition similar to the famous ‘hastatic order’ transition in URu₂Si₂ [3]. Such ‘hastatic’ order breaks both single and double time-reversal symmetry due to the mixing of integer and half-integer spins[3]. We discuss the experimental evidence of emergent broken symmetry Fermi liquid state in a single-crystalline nanorod of Pr₂Ir₂O₇.

[1] W. Witczak-Krempa, Gang Chen, Yong Baek Kim, and Leon Balents, Annu. Rev. Condens. Matter Phys. 5, 57 (2014).

[2] S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. van Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan, Phys. Rev. Lett.96, 087204 (2006).

[3] P. Chandra, P. Coleman and R. Flint, Nature 493, 621 (2013).