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1. Room-temperature valence transition in a strain-tuned perovskite oxide

   https://doi.org/10.1038/s41467-022-35024-8  

   Chaturvedi, Vipul ; Ghosh, Supriya ; Gautreau, Dominique ; Postiglione, William M. ; Dewey, John E. ; Quarterman, Patrick ; Balakrishnan, Purnima P. ; Kirby, Brian J. ; Zhou, Hua ; Cheng, Huikai ; et al ( December 2022 , Nature Communications)

   Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr_1-xCa_xCoO₃). Here, we show that in thin films of the complex perovskite (Pr_1-yY_y)_1-xCa_xCoO_3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

    

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2. Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

   https://doi.org/10.1038/s41598-019-44602-8  

   Shivaram, B. S. ; Holleis, Ludwig ; Ulrich, V. W. ; Singleton, John ; Jaime, Marcelo ( June 2019 , Scientific Reports)

   Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt₃with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these phase transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which forθ = 90° (B||c-axis) and vicinity abruptly become very strong in the 24.8–30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T forθ = 0° but moves to higher fields as 1/cosθ. This merge, suggesting a tricritical behavior, occurs atθ ≈ 51° from the ab-plane. This is an unique off-symmetry angle where the length change along the c-axis is precisely zero due to the anisotropic nature of MS in UPt₃for all magnetic field values.

    

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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.

    

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4. Applying Unconventional Spectroscopies to the Single‐Molecule Magnets, Co(PPh 3 ) 2 X 2 (X=Cl, Br, I): Unveiling Magnetic Transitions and Spin‐Phonon Coupling

   https://doi.org/10.1002/chem.202100705  

   Bone, Alexandria N. ; Widener, Chelsea N. ; Moseley, Duncan H. ; Liu, Zhiming ; Lu, Zhengguang ; Cheng, Yongqiang ; Daemen, Luke L. ; Ozerov, Mykhaylo ; Telser, Joshua ; Thirunavukkuarasu, Komalavalli ; et al ( August 2021 , Chemistry – A European Journal)

   Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single‐molecule magnets (SMMs). Spin‐phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin‐phonon coupling in molecules is challenging. We have found that far‐IR magnetic spectra (FIRMS) of Co(PPh₃)₂X₂(Co‐X; X=Cl, Br, I) reveal rarely observed spin‐phonon coupling as avoided crossings between magnetic andu‐symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero‐field split (ZFS) levels of theS=3/2 electronic ground state were probed by INS, high‐frequency and ‐field EPR (HFEPR), FIRMS, and frequency‐domain FT terahertz EPR (FD‐FT THz‐EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) andgvalues. Ligand‐field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities inCo‐X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin‐phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

    

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5. Spin Transitions and Compressibility of ε‐Fe 7 N 3 and γ′‐Fe 4 N: Implications for Iron Alloys in Terrestrial Planet Cores

   https://doi.org/10.1029/2020JB020660  

   Lv, Mingda ; Liu, Jiachao ; Zhu, Feng ; Li, Jie ; Zhang, Dongzhou ; Xiao, Yuming ; Dorfman, Susannah M. ( November 2020 , Journal of Geophysical Research: Solid Earth)

   Iron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure‐induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X‐ray emission spectroscopy (XES) and X‐ray diffraction (XRD) results for ε‐Fe₇N₃and γ′‐Fe₄N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe₇N₃and γ′‐Fe₄N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe₇N₃by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe₄N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yieldsV₀ = 83.29 ± 0.03 (ų),K₀ = 232 ± 9 GPa,K₀′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe₇N₃above the spin transition completion pressure, andV₀ = 54.82 ± 0.02 (ų),K₀ = 152 ± 2 GPa,K₀′ = 4.0 ± 0.1 for γ′‐Fe₄N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe₃S, Fe₃P, Fe₇C₃, and Fe₃C based on previous XES and XRD measurements, we located the completion of high‐ to low‐spin transition at ~67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe₃S, Fe₃P, and Fe₃C induces elastic stiffening, whereas that of Fe₇C₃induces elastic softening. Changes in compressibility at completion of spin transitions in iron‐light element alloys may influence the properties of Earth's and planetary cores.

    

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