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1. Observation of coherently coupled cation spin dynamics in an insulating ferrimagnetic oxide

   https://doi.org/10.1063/5.0141869  

   Klewe, C. ; Shafer, P. ; Shoup, J. E. ; Kons, C. ; Pogoryelov, Y. ; Knut, R. ; Gray, B. A. ; Jeon, H. -M. ; Howe, B. M. ; Karis, O. ; et al ( March 2023 , Applied Physics Letters)

   Many technologically useful magnetic oxides are ferrimagnetic insulators, which consist of chemically distinct cations. Here, we examine the spin dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite (Ni_0.65Zn_0.35Al_0.8Fe_1.2O₄) under continuous microwave excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance to separately probe Fe^2+/3+and Ni²⁺cations on different sublattice sites. Our results show that the precessing cation moments retain a rigid, collinear configuration to within [Formula: see text]2°. Moreover, the effective spin relaxation is identical to within <10% for all magnetic cations in the ferrite. Thus, we validate the oft-assumed “ferromagnetic-like” dynamics in the resonantly driven ferrimagnetic oxide: the magnetic moments from different cations precess as a coherent, collective magnetization, despite the high contents of nonmagnetic Zn²⁺and Al³⁺diluting the exchange interactions.
2. Emergent long-range magnetic order in ultrathin (111)-oriented LaNiO3 films

   https://doi.org/10.1038/s41535-021-00345-2  

   Kane, Margaret M. ; Vailionis, Arturas ; Riddiford, Lauren J. ; Mehta, Apurva ; N’Diaye, Alpha T. ; Klewe, Christoph ; Shafer, Padraic ; Arenholz, Elke ; Suzuki, Yuri ( May 2021 , npj Quantum Materials)

   The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO₃(LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO₆octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition att = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni²⁺ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni²⁺–Ni³⁺ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.
3. Converse magneto-electric effects in a core–shell multiferroic nanofiber by electric field tuning of ferromagnetic resonance

   https://doi.org/10.1038/s41598-020-77041-x  

   Liu, Ying ; Sreenivasulu, G. ; Zhou, P. ; Fu, J. ; Filippov, D. ; Zhang, W. ; Zhou, T. ; Zhang, T. ; Shah, Piyush ; Page, M. R. ; et al ( November 2020 , Scientific Reports)

   This report is on studies directed at the nature of magneto-electric (ME) coupling by ferromagnetic resonance (FMR) under an electric field in a coaxial nanofiber of nickel ferrite (NFO) and lead zirconate titanate (PZT). Fibers with ferrite cores and PZT shells were prepared by electrospinning. The core–shell structure of annealed fibers was confirmed by electron- and scanning probe microscopy. For studies on converse ME effects, i.e., the magnetic response of the fibers to an applied electric field, FMR measurements were done on a single fiber with a near-field scanning microwave microscope (NSMM) at 5–10 GHz by obtaining profiles of both amplitude and phase of the complex scattering parameterS₁₁as a function of bias magnetic field. The strength of the voltage-ME couplingA_vwas determined from the shift in the resonance fieldH_rfor bias voltage ofV = 0–7 V applied to the fiber. The coefficientA_vfor the NFO core/PZT shell structure was estimated to be − 1.92 kA/Vm (− 24 Oe/V). A model was developed for the converse ME effects in the fibers and the theoretical estimates are in good agreement with the data.
4. The 2022 magneto-optics roadmap

   https://doi.org/10.1088/1361-6463/ac8da0  

   Kimel, Alexey ; Zvezdin, Anatoly ; Sharma, Sangeeta ; Shallcross, Samuel ; de Sousa, Nuno ; García-Martín, Antonio ; Salvan, Georgeta ; Hamrle, Jaroslav ; Stejskal, Ondřej ; McCord, Jeffrey ; et al ( September 2022 , Journal of Physics D: Applied Physics)

   Abstract Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of today’s magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enablemore »quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton–Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light–matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17). Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future.« less
5. Design of soft magnetic materials

   https://doi.org/10.1038/s41524-021-00682-7  

   Renuka Balakrishna, Ananya ; James, Richard D. ( January 2022 , npj Computational Materials)

   We present a strategy for the design of ferromagnetic materials with exceptionally low magnetic hysteresis, quantified by coercivity. In this strategy, we use a micromagnetic algorithm that we have developed in previous research and which has been validated by its success in solving the “Permalloy Problem”—the well-known difficulty of predicting the composition 78.5% Ni of the lowest coercivity in the Fe–Ni system—and by the insight it provides into the “Coercivity Paradox” of W. F. Brown. Unexpectedly, the design strategy predicts that cubic materials with large saturation magnetizationm_sand large magnetocrystalline anisotropy constantκ₁will have low coercivity on the order of that of Permalloy, as long as the magnetostriction constantsλ₁₀₀, λ₁₁₁are tuned to special values. The explicit prediction for a cubic material with low coercivity is the dimensionless number$$({c}_{11}-{c}_{12}){\lambda }_{100}^{2}/(2{\kappa }_{1})=81$$$\left(c_{11}-c_{12}\right){\lambda }_{100}^2/\left(2{\kappa }_1\right)=81$for 〈100〉 easy axes. The results would seem to have broad potential application, especially to magnetic materials of interest in energy research.