Nandi, S.; Jawale, M.; Bachhar, S.; Kumar, Rahul; Schuller, M.; Bag, R.; Wilkinson, J.; Sichelschmidt, J.; Sundaresan, A.; Haravifard, S.; Büttgen, N.; Mahajan, A. V. Observation of a gapped phase in the one-dimensional S = 1/2 Heisenberg antiferromagnetic chain Cu(Ampy)ClBr Journal Article Phys. Rev. B 112, 134437 (2025). @article{nandi_observationof_2025,
title = {Observation of a gapped phase in the one-dimensional S = 1/2 Heisenberg antiferromagnetic chain Cu(Ampy)ClBr},
author = {S. Nandi and M. Jawale and S. Bachhar and Rahul Kumar and M. Schuller and R. Bag and J. Wilkinson and J. Sichelschmidt and A. Sundaresan and S. Haravifard and N. Büttgen and A. V. Mahajan},
url = {https://link.aps.org/doi/10.1103/mkdv-8m61},
doi = {10.1103/mkdv-8m61},
year = {2025},
date = {2025-10-22},
urldate = {2025-10-22},
journal = {Phys. Rev. B},
volume = {112},
number = {13},
pages = {134437},
abstract = {Spin-1/2 Heisenberg antiferromagnetic frustrated spin-chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr(Ampy=C6H8N2=2 -(aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, 1H nuclear magnetic resonance (NMR), electron spin resonance, and muon spin relaxation (𝜇SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu2+ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu2+ spins as reflected by a Curie-Weiss temperature of about −9 K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific-heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the 1H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field 𝜇SR evidences lack of any static magnetism.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Spin-1/2 Heisenberg antiferromagnetic frustrated spin-chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr(Ampy=C6H8N2=2 -(aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, 1H nuclear magnetic resonance (NMR), electron spin resonance, and muon spin relaxation (𝜇SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu2+ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu2+ spins as reflected by a Curie-Weiss temperature of about −9 K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific-heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the 1H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field 𝜇SR evidences lack of any static magnetism. |
Kovács, A.; Weber, J. T.; Charilaou, M.; Kong, D.; Prodan, L.; Tsurkan, V.; Schröder, A.; Kiselev, N. S.; Kézsmárki, I.; Dunin-Borkowski, R. E.; Tavabi, A. H.; Schäfer, S. All-optical stochastic switching of magnetisation textures in Fe3Sn2 Journal Article Commun. Mater. 6, 223 (2025). @article{kovacs_all-optical_2025,
title = {All-optical stochastic switching of magnetisation textures in Fe_{3}Sn_{2}},
author = {A. Kovács and J. T. Weber and M. Charilaou and D. Kong and L. Prodan and V. Tsurkan and A. Schröder and N. S. Kiselev and I. Kézsmárki and R. E. Dunin-Borkowski and A. H. Tavabi and S. Schäfer},
url = {https://doi.org/10.1038/s43246-025-00974-1},
doi = {10.1038/s43246-025-00974-1},
year = {2025},
date = {2025-10-14},
urldate = {2025-10-01},
journal = {Commun. Mater.},
volume = {6},
number = {1},
pages = {223},
abstract = {The all-optical control of magnetisation at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe3Sn2 in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualise the light-induced stochastic switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remains unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetisation dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The all-optical control of magnetisation at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe3Sn2 in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualise the light-induced stochastic switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remains unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetisation dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses. |
Rao, P.; Knolle, J. Order-by-disorder in magnets with frustrated spin interactions—classical and large-S limits via the spin functional integral Journal Article J. Phys.: Condens. Matter 37, 405802 (2025). @article{rao_order-by-disorder_2025,
title = {Order-by-disorder in magnets with frustrated spin interactions—classical and large-S limits via the spin functional integral},
author = {P. Rao and J. Knolle},
url = {https://doi.org/10.1088/1361-648X/ae0bdf},
doi = {10.1088/1361-648X/ae0bdf},
year = {2025},
date = {2025-10-07},
urldate = {2025-10-01},
journal = {J. Phys.: Condens. Matter},
volume = {37},
number = {40},
pages = {405802},
abstract = {We investigate spin systems with extensive degeneracies in the classical ground states due to anisotropic frustrated spin interactions, where the degeneracy is not protected by symmetry. Using spin functional integration, we study the lifting of the degeneracies by fluctuations called order-by-disorder (ObD), and the associated gap in the spin-wave spectrum. It is shown that ObD corresponds to gradient-dependent anisotropic interactions of the pseudo-Goldstone modes, which vanish for a classical uniform spin configuration. Fluctuations generate a gradient-independent effective potential which determines the ground state and the pseudo-Goldstone gap. Furthermore, we recover previous predictions for the pseudo-Goldstone gap in type-I and II ObD with two-spin interactions in the large spin-S limit or the classical small temperature limit, by computing the gap explicitly for the type-II cubic compass model and the type-I square compass model. We show that these two limits correspond to the one-loop approximation for the effective potential. We also discuss other types of order by disorder due to m-spin interactions where m > 2.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We investigate spin systems with extensive degeneracies in the classical ground states due to anisotropic frustrated spin interactions, where the degeneracy is not protected by symmetry. Using spin functional integration, we study the lifting of the degeneracies by fluctuations called order-by-disorder (ObD), and the associated gap in the spin-wave spectrum. It is shown that ObD corresponds to gradient-dependent anisotropic interactions of the pseudo-Goldstone modes, which vanish for a classical uniform spin configuration. Fluctuations generate a gradient-independent effective potential which determines the ground state and the pseudo-Goldstone gap. Furthermore, we recover previous predictions for the pseudo-Goldstone gap in type-I and II ObD with two-spin interactions in the large spin-S limit or the classical small temperature limit, by computing the gap explicitly for the type-II cubic compass model and the type-I square compass model. We show that these two limits correspond to the one-loop approximation for the effective potential. We also discuss other types of order by disorder due to m-spin interactions where m > 2. |
Shiotani, T.; Waki, T.; Tabata, Y.; Kézsmárki, I.; Nakamura, H. Novel family of near-room-temperature compensated itinerant pyrochlore ferrimagnets, RInCo4 (R = Dy-Tm) Unpublished (2025), arXiv:2510.04287. @unpublished{shiotani_novel_2025,
title = {Novel family of near-room-temperature compensated itinerant pyrochlore ferrimagnets, RInCo_{4} (R = Dy-Tm)},
author = {T. Shiotani and T. Waki and Y. Tabata and I. Kézsmárki and H. Nakamura},
url = {https://arxiv.org/abs/2510.04287},
doi = {10.48550/arXiv.2510.04287},
year = {2025},
date = {2025-10-05},
urldate = {2025-10-05},
abstract = {We successfully synthesized single crystals of a series of C15b Laves phase compounds, RInCo4 (R=Dy-Tm), with Co-pyrochlore and R-fcc sublattices, and systematically studied their magnetic properties via magnetometry measurements. These itinerant cubic compounds, with Curie temperatures above room temperature, show compensated ferrimagnetism featuring an antiferromagnetic coupling between the two sublattices. From this series, DyInCo4 exhibits the highest T_C (= 368 K) and a near-room-temperature compensation point T_cp (= 295 K). T_C does not change drastically with the R atom, whereas T_cp depends on the de Gennes factor of R^3+. Another magnetization anomaly is observed in all the compounds at low temperatures, which may be indicative of changes in the lattice or magnetic structure. The easy axis the ferrimagnetic moment of DyInCo4, ErInCo4, and TmInCo4 is found at T = 5 K to be along the [001], [111] and [110] directions, respectively. However, the simple easy-axis or easy-plane ferrimagnetic picture cannot be applied to HoInCo4. These observations suggest that the R sublattice determines magnetic anisotropy and compensation, while the Co sublattice plays a role in strong magnetic ordering. The high Curie temperature, together with the magnetization compensation point near room temperature, renders these itinerant pyrochlore magnets interesting for spintronic applications. },
note = {arXiv:2510.04287},
keywords = {},
pubstate = {published},
tppubtype = {unpublished}
}
We successfully synthesized single crystals of a series of C15b Laves phase compounds, RInCo4 (R=Dy-Tm), with Co-pyrochlore and R-fcc sublattices, and systematically studied their magnetic properties via magnetometry measurements. These itinerant cubic compounds, with Curie temperatures above room temperature, show compensated ferrimagnetism featuring an antiferromagnetic coupling between the two sublattices. From this series, DyInCo4 exhibits the highest T_C (= 368 K) and a near-room-temperature compensation point T_cp (= 295 K). T_C does not change drastically with the R atom, whereas T_cp depends on the de Gennes factor of R^3+. Another magnetization anomaly is observed in all the compounds at low temperatures, which may be indicative of changes in the lattice or magnetic structure. The easy axis the ferrimagnetic moment of DyInCo4, ErInCo4, and TmInCo4 is found at T = 5 K to be along the [001], [111] and [110] directions, respectively. However, the simple easy-axis or easy-plane ferrimagnetic picture cannot be applied to HoInCo4. These observations suggest that the R sublattice determines magnetic anisotropy and compensation, while the Co sublattice plays a role in strong magnetic ordering. The high Curie temperature, together with the magnetization compensation point near room temperature, renders these itinerant pyrochlore magnets interesting for spintronic applications. |
Heinsdorf, N.; Joshi, D. G.; Katsura, H.; Schnyder, A. P. Fate of Bosonic Topological Edge Modes in the Presence of Many-Body Interactions Journal Article Phys. Rev. Lett. 135, 146702 (2025). @article{heinsdorf_fate_2025,
title = {Fate of Bosonic Topological Edge Modes in the Presence of Many-Body Interactions},
author = {N. Heinsdorf and D. G. Joshi and H. Katsura and A. P. Schnyder},
url = {https://link.aps.org/doi/10.1103/1pty-fvpf},
doi = {10.1103/1pty-fvpf},
year = {2025},
date = {2025-10-03},
urldate = {2025-10-01},
journal = {Phys. Rev. Lett.},
volume = {135},
number = {14},
pages = {146702},
abstract = {Many magnetic materials are predicted to exhibit bosonic topological edge modes in their excitation spectra, because of the nontrivial topology of their magnon, triplon, or other quasiparticle band structures. However, there is a discrepancy between theory prediction and experimental observation, which suggests some underlying mechanism that intrinsically suppresses the expected experimental signatures, like the thermal Hall current. Many-body interactions that are not accounted for in the noninteracting quasiparticle picture are most often identified as the reason for the absence of the topological edge modes. Here we report persistent bosonic edge modes at the boundaries of a ladder quantum paramagnet with gapped triplon excitations in the presence of the full many-body interaction. We use tensor network methods to resolve topological edge modes in the time-dependent spin-spin correlations and the dynamical structure factor, which is directly accessible experimentally. We further show that signatures of these edge modes survive even when the noninteracting quasiparticle theory breaks down; we discuss the topological phase diagram of the model, demonstrate the fractionalization of its low-lying excitations, and propose potential material candidates.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Many magnetic materials are predicted to exhibit bosonic topological edge modes in their excitation spectra, because of the nontrivial topology of their magnon, triplon, or other quasiparticle band structures. However, there is a discrepancy between theory prediction and experimental observation, which suggests some underlying mechanism that intrinsically suppresses the expected experimental signatures, like the thermal Hall current. Many-body interactions that are not accounted for in the noninteracting quasiparticle picture are most often identified as the reason for the absence of the topological edge modes. Here we report persistent bosonic edge modes at the boundaries of a ladder quantum paramagnet with gapped triplon excitations in the presence of the full many-body interaction. We use tensor network methods to resolve topological edge modes in the time-dependent spin-spin correlations and the dynamical structure factor, which is directly accessible experimentally. We further show that signatures of these edge modes survive even when the noninteracting quasiparticle theory breaks down; we discuss the topological phase diagram of the model, demonstrate the fractionalization of its low-lying excitations, and propose potential material candidates. |
