Session F55: 2D Magnetism

Pressure Induced Unconventional Quantum Criticality in the Coupled Spin 1/2 Ladder Antiferromagnet with Frustrating Interactions

Quantum spin-1/2 ladders with two legs have been severing as a model low-dimensional spin system for discovering and understanding new phenomena. Observation of Higgs amplitude mode and magnetic field-induced spontaneous quasiparticle decay and renormalization of quasiparticle dispersion were reported on a coupled spin-1/2 ladder antiferromagnetic compound C₉H₁₈N₂CuBr₄ using neutron scattering technique [1-3]. In contrast to most spin ladders reported to date, C₉H₁₈N₂CuBr₄ is close to a quantum critical point at ambient pressure and zero applied magnetic field. Very recently, by carrying out neutron scattering and AC heat capacity studies in the presence of hydrostatic pressure, we discovered an unconventional quantum criticality with the Néel-ordered state breaking down at a critical pressure of P_C ~1.0 GPa through a phase transition into a quantum-disordered state [4]. Estimates of the critical exponents suggest that this transition may fall outside the traditional Landau paradigm. Moreover, broad magnetic excitation continua were observed near the phase transition. We propose that the frustrating interlayer couplings may contribute to drive the system into an exotic gapped quantum paramagnetic regime, such as a quantum spin liquid phase. Our work offers an interesting experimental platform to investigate the non-trivial interplay between frustrated magnetism and quantum critical phenomena.

References:

[1] T. Hong et al., Phys. Rev. B 89, 174432 (2014).

[2] T. Hong et al., Nat. Commun. 8, 15148 (2017).

[3] T. Hong et al., Nat. Phys. 13, 638 (2017).

[4] T. Hong et al., Nat. Commun. 13, 3073 (2022).   

Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe3-xGeTe2

Fe_3-xGeTe₂ is a 2D layered van der Waals (vdW) material that is a rare example of a ferromagnetic metal, with magnetism remaining robust in the bulk down to the monolayer. Despite much fundamental and applied interest, open questions exist even on basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. To address these questions we present results from neutron scattering experiments on a large single crystal with low Fe vacancy content (Fe_2.85GeTe₂). The measurements show antiferromagnetic spin fluctuations develop and coexist with ferromagnetism. To explain the underlying behavior we performed realistic dynamical mean-field theory calculations that revealed that the competing magnetic fluctuations are driven by an orbital selective Mott transition (OSMT). The detailed calculations allow the different orbital contributions to be disentangled and assigned to local and itinerant character, thereby explaining the apparently contradictory dual behavior. The results presented represent an advancement in understanding the coexistence of itinerant and local moments in a canonical quasi-2D vdW ferromagnetic material and consequences for spin and orbital dependent electronic functions within wider spintronic and topological transport research are discussed.   

Orbital Angular Momentum of Magnons

We study the orbital angular momentum (OAM) of magnons for collinear ferromagnet (FM) and antiferromagnetic (AF) systems with nontrivial networks of exchange interactions [1,2].   The OAM L_zn(k) of magnons at wavevector k and magnon band n for AF and FM zig-zag and honeycomb lattices becomes nonzero when the lattice contains two inequivalent sites and is largest at the avoided-crossing points or extremum of the frequency bands.  Hence, the arrangement of exchange interactions may play a more important role at producing the orbital angular momentum of magnons than the spin-orbit coupling energy and the resulting non-collinear arrangement of spins.  However, the spin-orbit induced Dzyalloshinskii-Moriya interactions between next-nearest neighbor sites dramatically change our results for the FM honeycomb lattice by producing a nonzero average value <L_zn(k)> for the OAM within each band n = 1 or 2.

 

References

 

[1.] R.S. Fishman, J.S. Gardner, and S. Okamoto, Phys. Rev. Lett. 129, 167202 (2022)

[2.] R.S. Fishman, L. Lindsay, and S. Okamoto, J. Phys: Cond. Mat. (in press).

    

Tunable Berezinskii-Kosterlitz-Thouless correlations in a quasi-2d Heisenberg magnet

We discuss the manifestation of field- and pressure-tuned Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly-coupled spin-1/2 Heisenberg layers of the material [Cu(pz)₂(2-HOpy)₂](PF₆)₂ (CuPOF). Due to the moderate intralayer exchange coupling of J/k_B = 6.8 K, laboratory magnetic fields induce a substantial XY anisotropy of the spin correlations. This provides a significant BKT regime, as the tiny interlayer exchange J′/k_B ≈ 1 mK only induces 3d correlations upon close approach to the BKT transition. We employed NMR and μ⁺SR measurements to probe the spin correlations that determine the critical temperatures of the long-range order and the BKT transition for various applied magnetic fields and hydrostatic pressures. Further, we performed stochastic series expansion QMC simulations based on the experimentally determined model parameters. Finite-size scaling of the spin stiffness yields an excellent agreement of the critical temperatures between theory and experiment.   

Spin-density waves and fractional magnetization plateau in triangular Cs2CoBr4

Despite being essentially two-dimensional (2D), spin insulators on distorted triangular lattice can retain exotic features of one-dimensional (1D) physics. Quantum fluctuations in applied magnetic field lift the classical ground-state degeneracy, leading to rich phase diagrams hosting emergent states with collective excitations. Examples are frustration-induced magnetization plateaux, a typical feature of 2D triangular geometries, and incommensurate spin-density waves (SDW), which in 1D result from the perfect nesting of the underlying fermionic Fermi surface. This talk focuses on the triangular antiferromagnetic insulator Cs₂CoBr₄ discovered in our group. Its existence in sizable single-crystals and suitable energy scales allow full experimental access and make it an ideal candidate to explore the above scenario. We observed several field-induced quantum states and completely characterized its magnetic phase diagram. For each phase, we report comprehensive measurements of the ground state and excitations in a combination of bulk thermodynamics, neutron diffraction, inelastic neutron scattering, and terahertz spectroscopy experiments. In line with the above picture, we found scattering evidence of multiple incommensurate SDWs. Their connection with a commensurate m = 1/3 magnetization plateaux with up-up-down structure is discussed.   

Cascade of dynamic and equilibrium magnetization steps in a Mn2+-based spin trimer in pulsed magnetic field

We investigated the magnetization process of Mn²⁺ -based spin trimer hybrid halide by measuring the magnetization and differential susceptibility in pulsed magnetic field up to 60 T. In equilibrium with the rate of change of magnetic field slow at temperatures below which the correlation between spins form, the magnetization rapidly approaches a 1/3 magnetization plateau at low fields with collinear up-down-up spin structure and five subsequent magnetization plateaus from energy level crossings at 7/15, 9/15, 11/15, 13/15 of before saturation magnetization were observed at higher fields. On the other hand, numerous additional magnetization plateaus below 1/3 plateau and two additional peaks between the plateaus at higher fields in the differential susceptibility appeared in pulsed magnetic field experiment where the rate of change of magnetic field exceeds 5000 T/s. More interestingly, the fraction of 1/3 plateau reduces to 3/11 with increasing the rate of magnetic field change at low temperature. In this presentation, we discuss diabatic transition and the role of phonons that drastically transform the magnetization process in pulsed magnetic field based on Landau-Zener effect.   

Dynamical electron correlation in two-dimensional FenGeTe2 (n=3, 4, 5) magnets

The Fe_nGeTe₂ (n=3, 4, 5) family of two-dimensional magnets has attracted a lot of attention recently as they exhibit high temperature ferromagnetism along with complex temperature dependent magnetization and structural reconstructions, skyrmionic features etc. The complexity is enriched due to the presence of electron correlation and intricate magnetic interactions, where systematic theoretical studies are inadequate to have a consensus. In this work, we have employed first-principles calculations to have a systematic study of this family using (i) standard density functional theory (DFT), (ii) static electron correlation (DFT+U) and (iii) dynamic electron correlation effect (DFT+DMFT) methods. Moreover, complex structural aspects in Fe₅GeTe₂ have been considered in connection to experimental observations. Our results show that DFT+DMFT is the most accurate method to correctly reproduce the magnetic interactions and experimentally observed transition temperatures. Moreover, Monte Carlo simulations reveal peculiar magnetic structures at low temperature. The inaccurate values of structural parameters, magnetic moments and exchange interactions obtained from DFT+U make this method inapplicable for the FGT family.    

Tipping the balance between ferromagnetism and charge density waves in monolayer VSe2

The field of two-dimensional ferromagnets has been reinvigorated by the discovery of monolayer VSe2, which is reported to be ferromagnetic with a Curie point higher than 330 K. While multiple studies have reported the observation of charge density waves in these systems, the ferromagnetic state of monolayer VSe2 remains highly debated. We hypothesize that ferromagnetism in monolayer VSe2 is driven by a Van Hove singularity at the Fermi level, while nested Fermi surface sheets drive the charge density waves. Based on this picture, we use first-principles calculations to investigate whether electron doping and pressure can tip the balance between these two competing states.   

Electrically-tunable tunneling anisotropy magnetoresistance in few-layer CrPS4

Electrical control of spin degree of freedom in magnetic materials lays the foundation of modern spintronics and information technology. Two-dimensional (2D) magnets with stable 2D magnetism have bought new opportunities to manipulate magnetism down to the 2D limit^1,2. For instance, recently gate-induced magnetic phase transition in bilayer CrI₃^3,4 and multiferroism in monolayer NiI₂⁵ have been successfully demonstrated. Despite these progresses, the realization of electrically manipulated magnetization in other 2D magnets is still desirable. In this talk, we report the electrically tunable tunneling anisotropy magnetoresistance (TAMR) in CrPS₄-based magnetic tunnel junctions with a structure of graphite (top gate)/h-BN/graphene/few-layer CrPS₄/graphene/h-BN/graphite (bottom gate). We will first discuss the layer-dependent magnetism in few-layer CrPS₄ through the electrical tunneling magnetotransport measurements. Then, we will focus on the gate dependences of tunneling magnetoresistance and TAMR of the magnetic tunnel junctions. Strikingly, we find that the polarity of the TAMR can be successfully changed by either the applied bias or gate voltage, suggesting that the magnetic anisotropy in CrPS₄ can be modulated by electrical means. Our findings suggest that few-layer CrPS₄ may feature strong magnetoelectric coupling, thus offering a potential platform for realizing nonvolatile memory devices.  

1.        Gong, C. et al. Nature 546, 265–269 (2017).

2.        Huang, B. et al. Nature 546, 270–273 (2017).

3.        Huang, B. et al. Nature Nanotechnology 13, 544–548 (2018).

4.        Jiang, S. et al.  Nature Nanotechnology 13, 549–553 (2018).

5.        Song, Q. et al. Nature. 26, 1–17 (2021).

    

Tunning the magnetic transition temperature and spin structure in Cr-doped Co3BO5 Ludwigite

Ludwigites have formula unit M₂M'BO₅, where M and M’ are transition metals. These compounds may exhibit different physical properties ranging from structural ordering, charge ordering, coexistence of magnetic order and paramagnetism, metamagnetic transition, magnetocaloric effect, dimensional crossover, spin crossover and spin glass. This variety of physical behavior has been attributed to a combination of strong correlation and low-dimensional effects. So, these materials offer a unique opportunity to study the correlation between magnetism and dimensionality. The homometallic Co₃BO₅ ludwigite, with high-spin (HS) Co²⁺ and low-spin (LS) Co³⁺, shows an AFM ordering at 42 K. When the compound is doped with non-magnetic ions, in most cases, and contrary to expected, the magnetic interactions are strengthened and the magnetic transition temperature increases. On the other hand, when doped with magnetic ions, such as Fe³⁺, Mn³⁺, magnetic disorder leads the system to a spin glass state, except for Cr, which occupies a unique site in the structure. Particularly, when Co₃BO₅ is doped with Cr³, forming the Co_2.5Cr_0.5BO₅, the magnetic interactions are strengthened, and T_N rises to 76 K. Despite the presence of three different magnetic ions, Co²⁺, Co³⁺ and Cr³⁺, in the HS states, frustrated magnetic interactions are established. The magnetic dimensionality increases from 2D in Co₃BO₅ to 3D in Co_2.5Cr_0.5BO₅. Here we extend our study of Cr-doped ludwigite Co_3-xCr_xBO₅, comprising a broader range of Cr concentrations to understand the role of Cr, which yield changes of the magnetic structure and dimensionality in these compounds. X-ray diffraction results show that Cr³⁺ occupy a unique site and gradually expand the unit cell as the Cr content increases, thus favoring the appearance of HS Co³⁺. The magnetic transition temperature increases gradually with the increase of Cr content in the compound, reaching 115 K for x=1.0. We show that depending on the Cr concentration, the compound adopts different spin structures. It goes from AFM for low Cr content, through ferrimagnetic (FIM) for intermediate concentrations, and back to AFM for high Cr concentrations.   

X-ray absorption spectroscopy investigation of complicated magnetic anisotropy of VI3

             Enhanced quantum fluctuation in low dimensions often engenders unusual properties of two-dimensional (2D) magnets. Mermin-Wagner theorem states that any long-range magnetism is forbidden in the isotropic 2D system owing to the strong spin fluctuation. In this context, magnetic anisotropy is a critical ingredient to stabilize the magnetic orderings in 2D systems, and understanding its microscopic origin is of paramount importance in the study of 2D magnets.

             Recently, the true 2D ferromagnetism down to the monolayer limit is realized in Cr-based van-der-Waals magnet CrI₃. In this system, the ⁴A₂ ground state of Cr³⁺ with quenched orbital moment cannot contribute to the single ion anisotropy by LS coupling. Instead, the strong spin-orbit coupling of ligand 5p orbital is identified as the origin of strong c-axis magnetic anisotropy in CrI₃.

             In this work, we investigated the origin of magnetic anisotropy in recently discovered V-based 2D ferromagnet VI₃, where both single ion anisotropy of V³⁺ and spin-orbit coupling from ligand 5p orbital contribute to the magnetic anisotropy. Using the synergetic combination of X-ray absorption spectroscopy, X-ray magnetic circular dichroism, and Cluster many-body calculation, we resolved the ground state of V³⁺ ion and successfully disentangled the contributions to the magnetic anisotropy. Our study provides fresh insights into the microscopic origin of long-range magnetic orderings in 3d-transition metal-based 2D magnets.