Session X36: Dimensional Effects in Magnetic Ordering

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The electronic band structure and the magnetic properties of two-dimensional magnets are decisively controlled by the Coulomb interaction between carriers within the layered material. At the same time we can externally modify this interaction by means of dielectric substrates or dielectric coating. This allows for an external manipulation of internal static and dynamic magnetic properties.

Here we use a combination of the Magnetic-Force theorem and state-of-the-art ab-initio down-folding techniques to theoretically study how dielectric substrates affect the microscopic magnetic exchange interactions in monolayer CrI₃. We find that short-range exchange interactions can be drastically tuned by the environmental material, which strongly affects the magnetic transition temperature as well as the magnon excitation spectra. Based on these findings we suggest novel magnetic devices from externally spatially controlled magnetic properties.   

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In magnetoelectrics, an electric field induces a magnetization and a magnetic field induces a polarization, while the system remains in thermal equilibrium. We present a comprehensive theory [1] for magnetoelectricity in magnetically ordered quasi-2D systems. Considering ferromagnetic (FM) zincblende and antiferromagnetic (AFM) diamond structures, we obtain quantitative expressions for the magnetoelectric responses due to electric and magnetic fields that reveal explicitly the inherent duality of these responses required by thermodynamics. For this, AFM order plays a central role. We define a Néel operator t that describes AFM order, in the same way a magnetization m reflects FM order. While m is even under space inversion and odd under time reversal, t describes a toroidal moment that is odd under both symmetries. Thus m and t quantify complementary aspects of magnetic order. In quasi-2D systems, FM order can be attributed to dipolar equilibrium currents that give rise to a magnetization. In the same way, AFM order arises from quadrupolar currents that generate the toroidal moment.
[1] Phys. Rev. Research 2, 043060 (2020)   

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Increasing the ferromagnetic transition temperature in 2D ferromagnetic insulators is an important challenge for technological applications. Recently, we have reported an enhanced magnetism in Cr₂Ge₂Te₆/NiO heterostructures. The enhanced magnetism manifested in significantly increased Curie temperature and coercive field, suggesting a strongly enhanced perpendicular magnetic anisotropy. At the time, the microscopic mechanism of this enhanced magnetism remained elusive [1]. Here, we further study the possible cause of the enhancement. We performed magneto optic Kerr effect and Raman microscopy measurements on thin Cr₂Ge₂Te₆ (CGT) flakes covered by NiO. We found a correlation between the enhanced magnetism with the presence of wrinkles in the heterostructure and the Raman spectrum of the heterostructure showed CGT’s characteristic peaks shifting when probed on and around the wrinkles. Moreover, the Raman peak shift and the enhanced magnetism are more pronounced for positions closer to the wrinkles, which suggests that wrinkle-induced-strain in CGT is a possible origin of the strongly enhanced magnetism.

[1] H. Idzuchi, A.E. Llacsahuanga, X.C. Pan, K. Tanigaki, and Y.P. Chen, Appl. Phys. Lett. 115, 232403 (2019)   

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We provide a complete and systematic first-principles study on the thermodynamic stability, structural parameters, and magnetic properties of the T1 non-collinear antiferromagnetic L1₂-IrMn₃ surface and exchange-biased L1₂-IrMn₃/Fe heterostructure. We find two stable surface and heterostructure terminations, which are in good agreement with experimental HR-TEM data. Using a comparative approach, we analyzed the exchange-bias properties in the heterostructure, discovering that the number of Mn-Fe interactions at the interface is related to exchange bias intensity. This finding could lead to novel exchange-bias tailoring by controlling the terminating layers. Finally, we describe the interface magnetic coupling atom-by-atom, finding a relationship between antiferromagnetic order near the interface, and heterostructure stability. Our analysis uncovers a possible mechanism for the appearance of exchange bias in non-collinear/collinear heterostructures. These results are in good agreement with hysteresis measurements of the IrMn₃/Fe bilayer.   

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A recent experiment has shown that the V₅S₈ thin films exhibit an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition with reducing thickness. Here, for the first time, using density functional theory calculations, we report the antiferromagnetic order of bulk V₅S₈, which is consistent with the previous experiments. We find that the origin of the magnetic ordering is from super-exchange interaction. We found that there is an antiferromagnetic to ferromagnetic phase transition when V₅S₈ is thinned down to 2.2 nm. The main magnetic moments of the antiferromagnetic and ferromagnetic states of the thin films are located on the interlayered vanadium atoms, which is the same as that in the bulk. Meanwhile, the strain also influences the AFM–FM phase transition. Our results not only reveal the magnetic order and origin in bulk V₅S₈ and thin films, but also provide a set of parameters which can be used in future calculations. [Chin. Phys. B . 2020, 29(7): 077504]   

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We use neutron scattering to directly witness entanglement between quantum spins in the 1D Heisenberg antiferromagnet KCuF₃. We apply three entanglement witnesses---one-tangle, two-tangle, and quantum fisher information---to its inelastic neutron spectrum, and compare with simulated DMRG spectra. We find that each of these provides direct access to entanglement, but quantum fisher information is the most robust experimentally. This 1D system serves as a proof-of-principle that solid state quantum spin entanglement can be directly measured. We outline how these results can be applied to higher dimensional and higher spin materials to witness and quantify entanglement in quantum materials.   

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Quasiparticles are constructs for simplifying the complicated many-body description of solids. Their dynamics and mutual effective interactions provide rich information about real world material properties. We probe dynamics of magnon quasiparticles in a ferromagnetic spin-1 Heisenberg chain with easy-axis anisotropy and analyze the effect of magnon-magnon interactions on its thermal properties. This model is relevant for recent terahertz (THz) optics experiments on NiNb2O6 by Chauhan et al. [Phys. Rev. Lett. 124, 037203 (2020)], where a tunable, field direction sensitive, temperature-dependent shift of the dynamical magnetic susceptibility was reported. We perform numerical and visual analyses of the corresponding Kubo formula by utilizing matrix elements between various n and n+1 magnon states, identifying the most active transitions at a given temperature. Using a combination of exact diagonalization and density matrix renormalization group calculations, our study reveals that single-ion uniaxial anisotropy results in field dependent magnon attraction or repulsion. Attraction stabilizes composite bound states of magnons, and we propose ways of detecting them in future experiments.   

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Structured functional liquids combine mechanical versatility of fluids with solid-state properties, such as ferromagnetism, and offer a route to synthesize and control magnetic liquids for adaptive liquid robotics. Studies on these intriguing materials are only in their nascent state [1,2], and a profound understanding of the physical state is still lacking. We use hydrodynamics experiments to probe how the magnetization of ferromagnetic liquid droplets, governed by the assembly and jamming of magnetic nanoparticles at liquid-liquid interfaces, and their response to external stimuli can be tuned by chemical, structural and magnetic means. These results are augmented by structural analysis of short-range ordered nanoparticle assemblies, and numerical modeling of the forced rotational motion. Our results highlight the leading role of structural short-range order and nanoparticle jamming on magnetic properties, which provide a path toward nano patterning structured liquids.

[1] X. Liu, et al., Science 365, 264 (2019).
[2] R. Streubel, X. Liu, X. Wu, T.P. Russell, Materials 13, 2712 (2020).   

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The metallic delafossites, which are among the highest conductivity materials, have recently emerged as an interesting system for materials research. In particular, PdCoO₂ has an extremely long electron mean free path, and its layered crystal structure results in a large anisotropy between the in-plane and out-of-plane direction, making it an outstanding candidate for studying electronic behaviors. However, none of the delafossites are ferromagnetic, mainly as a result of the underlying triangular-lattice. However, we find that it is possible to make PdCoO₂ films strongly ferromagnetic, via hydrogenation. In this process, the crystal transforms into a structure consisting of layers of intermixed Pd and Co, and results in a ferromagnet with strong out-of-plane anisotropy and a high Curie temperature of ~650 K. Furthermore, these films exhibit anomalous hall effect whose sign can be switched by controlling the hydrogenation parameters. This work shows that it is possible to introduce changes to the magnetic and electronic properties of delafossites via hydrogenation.   

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Semiconductor nanowires with epitaxially grown superconductor are a promising potential host for Majorana bound states. The addition of a magnetic insulator to the system has been shown to induce Zeeman splitting in the nanowire via proximity effect, eliminating the need for an external magnetic field to produce a topologically nontrivial state. We use scanning SQUID microscopy (SSM) to study 2D heterostructures of thin film EuS, a magnetic insulator, on InAs and InSb substrates. Our measurements are a local probe of magnetism as a function of temperature, allowing us to characterize the homogeneity and domain structure of possible proximity-induced magnetism in this magnetic insulator/semiconductor structure. We report the local Curie temperature as a function of the EuS layer thickness and substrate material. These results contribute to a more thorough understanding of the magnetic coupling between EuS and InAs/InSb, which can inform design choices for future devices.   

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Time-integrated four wave mixing measurements were performed on Monolayer WSe₂. Magnetic fields up to 7.5 T were applied parallel and perpendicular to the monolayer plane and the dephasing time was measured. Furthermore, the effects of valley spin selection on the dephasing time and the brightening of spin forbidden dark excitons were explored by applying external magnetic fields and by varying the polarizations.   

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Curvilinear magnetism is in focus of intensive study, as it allows to tailor chiral and anisotropic responses and stabilize topologically non-trivial magnetic textures at the nanoscale [1]. In curved magnets all energy functionals, that contain spatial derivatives are reshaping in a way of appearance additional curvature-induced chiral and anisotropy terms. Here, we will discuss theoretical and experimental aspects behind the appearance of curvature-induced chiral effects in parabolic nanostripes [2]. We show that a pinning of domain wall at the parabolic apex is originated due to the presence of local curvature-induced chiral term, that could be quantified from the depinning field. The interplay between the intrinsic and extrinsic chiral terms paves the way to a mesoscale one, whose symmetry and strength depend both on the geometrical and material parameters [3]. These concept allows us to propose a novel approach towards artificial ME materials, whose magnetic states could be controled through small geometrical changes of curvature and torsion [4].
[1] R. Streubel et. al., J. Phys. D: Appl. Phys. 49,363001 (2016).
[2] O. Volkov et al.. Phys. Rev. Lett. 123, 077201 (2019).
[3] O. Volkov et al., Sci. Rep. 8, 866 (2018).
[4 ]O. Volkov et al., J. Phys. D: Appl. Phys. 52, 345001 (2019).