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1. Unconventional Hund metal in a weak itinerant ferromagnet

   https://doi.org/10.1038/s41467-020-16868-4  

   Chen, Xiang ; Krivenko, Igor ; Stone, Matthew B. ; Kolesnikov, Alexander I. ; Wolf, Thomas ; Reznik, Dmitry ; Bedell, Kevin S. ; Lechermann, Frank ; Wilson, Stephen D. ( June 2020 , Nature Communications)

   The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi-liquid theory. While magnetic fluctuations play an important role in the materials’ unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate its non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund’s exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets.
2. Reversal of the magnetoelectric effect at a ferromagnetic metal/ferroelectric interface induced by metal oxidation

   https://doi.org/10.1038/s41524-021-00679-2  

   Chen, Zhaojin ; Yang, Qiong ; Tao, Lingling ; Tsymbal, Evgeny Y. ( December 2021 , npj Computational Materials)

   Multiferroic materials composed of ferromagnetic and ferroelectric components are interesting for technological applications due to sizable magnetoelectric coupling allowing the control of magnetic properties by electric fields. Due to being compatible with the silicon-based technology, HfO₂-based ferroelectrics could serve as a promising component in the composite multiferroics. Recently, a strong charge-mediated magnetoelectric coupling has been predicted for a Ni/HfO₂multiferroic heterostructure. Here, using density functional theory calculations, we systematically study the effects of the interfacial oxygen stoichiometry relevant to experiments on the magnetoelectric effect at the Ni/HfO₂interface. We demonstrate that the magnetoelectric effect is very sensitive to the interface stoichiometry and is reversed if an oxidized Ni monolayer is formed at the interface. The reversal of the magnetoelectric effect is driven by a strong Ni−O bonding producing exchange-split polarization-sensitive antibonding states at the Fermi energy. We argue that the predicted reversal of the magnetoelectric effect is typical for other 3dferromagnetic metals, such as Co and Fe, where the metal-oxide antibonding states have an opposite spin polarization compared to that in the pristine ferromagnetic metals. Our results provide an important insight into the mechanism of the interfacial magnetoelectric coupling, which is essential for the physics and application of multiferroic heterostructures.
3. Field-induced quantum critical point in the itinerant antiferromagnet Ti3Cu4

   https://doi.org/10.1038/s42005-022-00901-7  

   Moya, Jaime M. ; Hallas, Alannah M. ; Loganathan, Vaideesh ; Huang, C. -L. ; Kish, Lazar L. ; Aczel, Adam A. ; Beare, J. ; Cai, Y. ; Luke, G. M. ; Weickert, Franziska ; et al ( May 2022 , Communications Physics)

   New phases of matter emerge at the edge of magnetic instabilities, which can occur in materials with moments that are localized, itinerant or intermediate between these extremes. In local moment systems, such as heavy fermions, the magnetism can be tuned towards a zero-temperature transition at a quantum critical point (QCP) via pressure, chemical doping, and, rarely, magnetic field. By contrast, in itinerant moment systems, QCPs are more rare, and they are induced by pressure or doping; there are no known examples of field induced transitions. This means that no universal behaviour has been established across the whole itinerant-to-local moment range—a substantial gap in our knowledge of quantum criticality. Here we report an itinerant antiferromagnet, Ti₃Cu₄, that can be tuned to a QCP by a small magnetic field. We see signatures of quantum criticality and the associated non-Fermi liquid behaviour in thermodynamic and transport measurements, while band structure calculations point to an orbital-selective, spin density wave ground state, a consequence of the square net structural motif in Ti₃Cu₄. Ti₃Cu₄thus provides a platform for the comparison and generalisation of quantum critical behaviour across the whole spectrum of magnetism.
4. Coexistence of metallic and nonmetallic properties in the pyrochlore Lu2Rh2O7

   https://doi.org/10.1038/s41535-019-0148-1  

   Hallas, Alannah M. ; Sharma, Arzoo Z. ; Mauws, Cole ; Chen, Qiang ; Zhou, Haidong D. ; Ding, Cui ; Gong, Zizhou ; Tachibana, Makoto ; Sarte, Paul M. ; Attfield, J. Paul ; et al ( March 2019 , npj Quantum Materials)

   Transition metal oxides of the 4dand 5dblock have recently become the targets of materials discovery, largely due to their strong spin–orbit coupling that can generate exotic magnetic and electronic states. Here, we report the high-pressure synthesis of Lu₂Rh₂O₇, a new cubic pyrochlore oxide based on 4d⁵Rh⁴⁺, and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient,γ = 21.8(1) mJ/mol-Rh K². Muon spin relaxation measurements confirm that Lu₂Rh₂O₇remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu₂Rh₂O₇is a correlated paramagnetic metal with a Wilson ratio ofR_W = 2.5. However, electric transport measurements present a striking contradiction as the resistivity of Lu₂Rh₂O₇is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu₂Rh₂O₇may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.
5. Energy gap of topological surface states in proximity to a magnetic insulator

   https://doi.org/10.1038/s42005-023-01327-5  

   Wang, Jiashu ; Wang, Tianyi ; Ozerov, Mykhaylo ; Zhang, Zhan ; Bermejo-Ortiz, Joaquin ; Bac, Seul-Ki ; Trinh, Hoai ; Zhukovskyi, Maksym ; Orlova, Tatyana ; Ambaye, Haile ; et al ( August 2023 , Communications Physics)

   Topological surface-states can acquire an energy gap when time-reversal symmetry is broken by interfacing with a magnetic insulator. This gap has yet to be measured. Such topological-magnetic insulator heterostructures can host a quantized anomalous Hall effect and can allow the control of the magnetic state of the insulator in a spintronic device. In this work, we observe the energy gap of topological surface-states in proximity to a magnetic insulator using magnetooptical Landau level spectroscopy. We measure Pb_1-xSn_xSe–EuSe heterostructures grown by molecular beam epitaxy exhibiting a record mobility and low Fermi energy. Through temperature dependent measurements and theoretical calculations, we show this gap is likely due to quantum confinement and conclude that the magnetic proximity effect is weak in this system. This weakness is disadvantageous for the realization of the quantum anomalous Hall effect, but favorable for spintronic devices which require the preservation of spin-momentum locking at the Fermi level.