PCSI2026 Session PCSI-SuA: Exotic Forms of Magnetism

Magnon-based hybrid quantum systems are promising candidates for quantum interconnects and quantum sensors, and they offer a rich platform for exploring nonlinear magnonics and cavity–photon interactions. Two-dimensional (2D) van der Waals magnets provide a compact, atomically flat geometry that can be easily integrated into existing quantum circuits, such as superconducting resonators and qubits. Among various 2D magnets, the magnetic semiconductor CrSBr is particularly unique due to its strong spin–exciton [1, 2], spin–lattice [3], and magnon–exciton [4] interactions. In this presentation, I will first discuss magnon-exciton coupling despite their energetical mismatch by orders of magnitude. I will then discuss our recent work demonstrating coherent coupling between antiferromagnetic magnons in CrSBr and microwave photons in a niobium-based-on-chip resonator [5]. This work demonstrates the first step toward integrating layered van der Waals 2D magnets into superconducting microwave circuits, with full access for both microwave and optical probing. Finally, I will discuss how these properties of magnetic semiconductors can be harnessed for spintronic devices and quantum information science.

[1]Wilson, Nathan P., et al. "Interlayer electronic coupling on demand in a 2D magnetic semiconductor." Nature Materials 20.12 (2021): 1657-1662.

[2]Brennan, Nicholas J., et al. "Important elements of spin-exciton and magnon-exciton coupling." ACS Physical Chemistry Au 4.4 (2024): 322-327.

[3]Bae, YounJue, et al. "Transient magnetoelastic coupling in CrSBr." Physical Review B 109.10 (2024): 104401.

[4]Bae, YounJue, et al. "Exciton-coupled coherent magnons in a 2D semiconductor." Nature 609.7926 (2022): 282-286.

[5]Tang, J., Singh, A., Brennan, N., Chica, D., Li, Y., Roy, X., Rana, F., Bae, Y.J., Coherent Magnon-Photon Coupling in the Magnetic Semiconductor, 2025, Nano Lett., 25, (2025),10912-10918.

In recent years, several groups reported theoretical predictions of collinear antiferromagnets demonstrating time-reversal symmetry breaking phenomena and spin-polarized behaviors, subsequently named altermagnets. Altermagnetism by a net zero magnetization, yet the Kramers degeneracy is lifted by a nonrelativistic, momentum-dependent spin-splitting of the electronic bands. Due to intrinsic time-reversal symmetry breaking predicted to emerge in these systems, altermagnets have been theorized to exhibit numerous exotic phenomena. Atomic‐scale spectroscopic insights into altermagnets are highly-desirable, but elusive.

Here we study a new layered triangular lattice altermagnet, Co-intercalated NbSe₂ using scanning tunneling microscopy and spectroscopy (STM/S) [1]. Spatial mapping using spectroscopic-imaging STM and spin-polarized STM further reveals emergent tri-directional charge and spin density modulations with a 2a₀ wavelength on the Se surface (Figure 1). Density functional theory (DFT) simulations suggest that these modulations reflect the underlying Co superstructure. Interestingly, we discover that out-of-plane magnetic field can serve as a knob to tune the amplitudes of the modulations as well as alter the overall electronic density-of-states in a manner that is strongly dependent on the field direction and strength. This can be attributed to the tilting of spins by the external magnetic field, which can have profound implications on the electronic properties of the altermagnet. By providing elusive atomic-scale insights, our work uncovers a magnetic-field tunable band structure in an altermagnet and highlights the crucial need to understand and quantify spin canting in altermagnets.

Magnetic monopoles in spin ice emerge as fractionalized excitations of the underlying spin configuration [1]. However, in bulk spin ice, the populations of monopoles and antimonopoles are always equal, resulting in zero net magnetic charge. Here we demonstrate a two-dimensional magnetic monopole (2DMG) gas formed at a spin ice/antiferromagnet (AFM) interface, using Monte Carlo simulation [2]. Unlike the bulk case, this interfacial monopole gas exhibits a non-zero net charge arising from the boundary conditions. We show that a singly charged monopole gas can exist in an AFM/spin ice/AFM sandwich heterostructure, as shown in Fig. 1. Although monopole motion within the spin ice layer costs no energy, the monopoles preferentially accumulate near the AFM/spin ice interface due to entropy maximization [3]. Furthermore, we demonstrate that this charged monopole gas enables novel functionalities: (1) it can be manipulated by external magnetic fields, functioning analogously to a field-effect transistor [2], and (2) engineered monopole traps can store non-volatile magnetic information, with the monopole position serving as a binary state that can be read and written magnetically [4].

[1] C. Castelnovo, R. Moessner, and S.L. Sondhi, Nature 451, 42 (2008).

[2] L. Miao, Y. Lee, A.B. Mei, M.J. Lawler, and K.M. Shen Nat. Commun. 11, 1341 (2020).

[3] P. Timsina, B. Kiefer, L. Miao, Phys. Rev. B 110, 184420 (2024).

[4] P. Timsina, A. Chappa, D. Alyones, B. Kiefer, L. Miao, arXiv: 2507.22315 (2025).

Pulsed magnetic fields provide access to extreme field regimes that are essential for probing quantum phenomena and characterizing complex material behaviors. However, their rapid field ramping introduces substantial measurement challenges, particularly the emergence of large Faraday-induced voltages in electrical transport setups. These unwanted voltages, arising from thetime derivative of themagneticflux, can exceed the intrinsicsample signal by ordersof magnitude and result in misleading asymmetries between the up-sweep and down-sweep of the magnetic field. This artifact not only distorts critical features such as quantum oscillations and resistive transitions but also complicates post-experimental analysis. To address this issue, we developed a Python-based software tool equipped with a graphical user interface(GUI) using the Tkinter library. The program enables users to automatically correct for the Faraday-induced voltage component by leveraging the inherentant isymmetry of the induced sign al between rising and falling field sweeps. It applies a least-squares fitting algorithm to extract normalization coefficients (Aₓand Aᵧ) that best describe the proportional contribution of the induced signal in each voltage channel.These coefficients are then used to reconstruct and subtract the unwanted induced voltage component, yielding clean, symmetrized transport data. The GUI design prioritizes accessibility, allowing experimentalists with no programming experience to process their data through a point-and-click interface. Applied to real datasets from pulsed high-field measurements, the tool demonstrated excellent performance in recovering the true voltage response of materials, reducing up/down-sweep discrepancies to within noise levels. By removing the inductive artifact, the program clarifies transport signatures, improves interpretability, and enables consistent analysis across datasets.This tool significantly enhances the workflow efficiency and measurement fidelity for condensed matter researchers utilizing pulsed field environments.

Helical spin structures are expressions of magnetically induced chirality, entangling the dipolar and magnetic orders in materials. The recent discovery of helical van der Waals multiferroics down to the ultrathin limit raises prospects of large chiral magnetoelectric correlations in two dimensions. However, the exact nature and magnitude of these couplings have remained unknown so far. Here we perform a precision measurement of the dynamical magnetoelectric coupling for an enantiopure domain in an exfoliated van der Waals multiferroic. We evaluate this interaction in resonance with a collective electromagnon mode, capturing the impact of its oscillations on the dipolar and magnetic orders of the material with a suite of ultrafast optical probes. Our data show a giant natural optical activity at terahertz frequencies, characterized by quadrature modulations between the electric polarization and magnetization components. First-principles calculations further show that these chiral couplings originate from the synergy between the non-collinear spin texture and relativistic spin–orbit interactions, resulting in substantial enhancements over lattice-mediated effects. Our findings highlight the potential for intertwined orders to enable unique functionalities in the two-dimensional limit and pave the way for the development of van der Waals magnetoelectric devices operating at terahertz speeds.

Reference:

Gao, F.Y., Peng, X., Cheng, X. et al. Giant chiral magnetoelectric oscillations in a van der Waals multiferroic. Nature632, 273–279 (2024). https://doi.org/10.1038/s41586-024-07678-5

Nitride antiperovskites offer a distinct and underexplored playground for uncovering spintronic and magnetic functionalities. Recently, we have synthesized polycrystalline and epitaxial films of phase pure (001) Co₃PdN for the first time. The magnetization behavior of epitaxial films exhibits a ‘two-step’ magnetization curve that is extremely sensitive to the direction of the applied magnetic field relative to high symmetry directions^1,2. In Figure 1, we show the magnetic field dependence of the Hall resistivity for fields applied in-plane. In this planar Hall configuration, a clear step-like feature emerges which is dependent on sweep direction, field magnitude, and applied field angle relative to the a-axis. Relatively small, planar magnetic fields generate an anomalous Hall response in Co₃PdN.

By combining MOKE magnetometry and magnetotransport, we identify a rotation of the net magnetization towards the (001) axis (film growth direction) for fields applied in-plane, up to 300 K. We hypothesize that distinct domain dynamics and the magnetic free energy drives the behavior^1,2.The unique tunability of the magnetization combined with a spin-polarized DOS positions Co₃PdN as a potentially powerful spintronics platform.

References:

1 H. X. Tang, R. K. Kawakami, D. D. Awschalom, and M. L. Roukes, Giant Planar Hall Effect in Epitaxial (Ga,Mn)As Devices, Physical Review Letters, 90.107201 [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.90.107201] (2003).

2 R. P. Cowburn, S. J. Gray, J. Ferré, J. A. C. Bland, and J. Miltat, Magnetic switching and in-plane uniaxial anisotropy in ultrathin Ag/Fe/Ag(100) epitaxial films, Jounral of Applied Physics, 78, 7210 [https://pubs.aip.org/aip/jap/article/78/12/7210/492641/Magnetic-switching-and-in-plane-uniaxial] (1995)

Frustrated magnets host unconventional states stabilized by degeneracy and entropy, from spin ice [1] to quantum spin liquids [2] and pyrochlore oxides [3]. Pyrochlore iridates R₂Ir₂O₇ (R = Dy, Ho) provide a platform with tunable d-f exchange interactions and multiple frustrated phases [3,4]. In these systems, competing interactions suppress long-range order, yielding emergent quasiparticles such as magnetic monopoles [1].

Using Monte Carlo simulations, we map the thermodynamic phase diagram, identifying the 2-in–2-out (2I2O) spin ice, fragmented 3-in–1-out/1-in–3-out (3I1O/1I3O) [4], and all-in–all-out (AIAO) ground states [5]. In this talk, we will investigate the two finite-temperature enhanced-entropy (EE) phases near phase boundaries, characterized by high entropy, strong susceptibility, and mixed spin configurations. These phases are found to be stabilized by entropy-driven free-energy minimization, with distinct behavior of specific heat capacity decoupling from susceptibility serving as key signatures [5] (Fig. 1 in PDF). These EE states define a new class of entropy-stabilized magnetic phases, underscoring the role of frustration in finite-temperature correlated states and offering pathways for entropy-based material design.

[1] A. P. Ramirez, A. Hayashi, R. J. Cava, R. Siddharthan, & B. S. Shastry, Nature 399, 333 (1999).

[2] C. Broholm, R. J. Cava, S. A. Kivelson, D. G. Nocera, M. R. Norman, and T. Senthil, Science 367, 263–273 (2020).
[3] J. S. Gardner, M. J. P. Gingras, and J. E. Greedan, Rev. Mod. Phys. 82, 53 (2010).
[4] E. Lefrancois, V. Cathelin, E. Lhotel, J. Robert, P. Lejay, C.V. Colin, B. Canals, F. Damay, J. Ollivier, B. Fak, L. C. Chapon, R. Ballou, and V. Simonet, Nat. Commun. 8, 209 (2017).

[5] P. Timsina, A. Chappa, D. Alyones, I. Vasiliev, and L. Miao, arXiv:2505.13352 (submitted: PRB, 2025).

⁺ Author for correspondence: lmiao@nmsu.edu